WO2018207177A1

WO2018207177A1 - Method and kit for attaching metallic surfaces

Info

Publication number: WO2018207177A1
Application number: PCT/IL2018/050494
Authority: WO
Inventors: Yiftah Karni; Sagi Daren
Original assignee: Printcb Ltd.
Priority date: 2017-05-07
Filing date: 2018-05-07
Publication date: 2018-11-15

Abstract

The present invention provides a method for connecting metallic surfaces with a metal- based adhesive film, the method utilizing successive application and heating of two distinct compositions comprising metal or metal alloy powders having different melting points. Further provided is a kit for attaching metallic surfaces, the kit containing two distinct metal-based compositions. Further disclosed is an assembly of two electronic or semiconductor components comprising a continuous metal-based adhesive film there between.

Description

METHOD AND KIT FOR ATTACHING METALLIC SURFACES

FIELD OF THE INVENTION

The present invention is directed to methods for attaching metallic surfaces using highly thermally conductive metallic-based adhesive material.

BACKGROUND OF THE INVENTION

Electrical connections and packaging for electronic and semiconductor components requires either electrically or thermally conductive materials. Two major types of adhesive materials used in electronic assemblies are solder and conductive adhesives.

Solder is a fusible metal alloy used to create a permanent bond between metal workpieces. In order to adhere to and connect the pieces together, solder must be melted. Thus, a suitable alloy for use as solder will have a lower melting point than the pieces it is intended to join. Soft solder typically has a melting point range of 90 to 450 °C and is commonly used in electronics, plumbing, and sheet metal work. Alloys that melt between 180 and 250 °C are the most commonly used. Soldering performed using alloys with a melting point above 450 °C is called "hard soldering", "silver soldering", or brazing.

Whenever possible, the solder should also be resistant to short-term, as well as, long-term oxidative and corrosive effects. In order to prevent oxides from forming on the surface of the molten metal during the soldering process, flux is typically used. In high-temperature metal joining processes the primary purpose of flux is to prevent oxidation of the base and filler materials. Tin-lead solder attaches very well to copper, but poorly to the various oxides of copper, which form quickly at soldering temperatures. Flux is a substance which is nearly inert at room temperature, but which becomes strongly reducing at elevated temperatures, preventing the formation of metal oxides. Additionally, flux allows solder to flow easily on the working piece rather than forming beads as it would do otherwise.

Accordingly, the role of a flux in joining processes is typically dual: dissolving of the oxides on the metal surface, which facilitates wetting by molten metal, and acting as an oxygen barrier by coating the hot surface, preventing its oxidation. In some applications molten flux also serves as a heat transfer medium, facilitating heating of the joint by the soldering tool or molten solder. Fluxes for soft soldering are typically of organic nature, especially in electronics applications.

The first and foremost drawback of lead solders is toxicity. Due to the toxicity and the environmental impact issues, the electronic industry is replacing lead soldering at a fast pace. Additionally, during the soldering process, the assembly is subjected to very high temperatures. Some temperature sensitive components in the near vicinity might be damaged due to this high temperature exposure. Also, tin/lead solders can dissolve gold and form some brittle intermetallic compounds. In such cases, the mechanical strength of the joint is considerably reduced.

Outgassing from the solder joint is a further concern in many applications. Solder has a tendency to form pores (or voids) in the bond line, both during the soldering process and during subsequent thermal excursions. The use of organic flux in a solder increases the probability of the formation of pores due to the evaporation of the organic material. These voids create hot spots of poor thermal conductivity between the connected components.

In order to increase thermal conductivity of the adhesive layer (for example, in applications, which require heat dissipation), metals and metal alloys with high thermal conductivity should be used, such as, but not limited to, copper and silver. Melting or even sintering powder such as copper or silver on a polymeric substrate is impossible due to temperature limitations. Lowering the sintering temperature can be achieved by mixing such powder with a low melting point metal or alloy, such as a solder alloy. This approach is commonly known as Liquid Phase Sintering (LPS). However, as the sintering of the joint material in the electronics industry is typically done without applying pressure, the structure obtained is highly porous and its thermal conductivity is poor. Additionally, the use of flux induces void formation.

Conductive adhesives have been widely explored as solder replacements in electronic components attachment. From the environmental standpoint, conductive adhesives provide a lead-free alternative to conventional electronic solders. Conductive adhesives offer lower temperature processing, fewer surface insulation resistance failures and do not require extensive pre-cleaning of the metal surfaces to be electrically and mechanically joined.

Particularly in applications requiring thermal conductivity, although a metallurgical joint can provide a reliable interconnection, conductive adhesives are most often used to connect both printed circuit boards and components to heatsinks. This is because welding a component to a heat sink generally requires high temperatures, and the surfaces to be joined must be wettable by the soldering material for adhesion to occur. One of the advantages of conductive adhesives is that they do not need a solderable surface for joint formation.

Most of the thermally conductive adhesive compositions are based on thermosetting or thermoplastic resins. Thermal conductivity of pure polymer adhesives is poor. To increase the thermal conductivity of polymer adhesives, filler particles are added to produce thermally conductive compositions. These include electrically conducting as well as insulating materials such as various ceramics, metals and diamond.

The major disadvantage of the thermally conductive adhesives is that the thermal conduction is achieved through fillers which have only a few contacts with other similar particles, thereby failing to provide stable thermal conduction due to temperature and humidity fluctuations. Although the composite nature of conductive adhesives allows them to be tailored for optimum mechanical and electrical performance, robust mechanical characteristics have been achieved at the expense of reliable conductivity that only an actual metallurgical or chemical bond can achieve.

The drawbacks associated with the use of solders and conductive adhesives led to the development of Transient Liquid Phase Sintering (TLPS) conductive adhesives, which have been developed for die attachment and PCB via filling. TLPS conductive adhesive consists of a polymer flux system and a metal system, including metal fillers having a high melting point (HMP), such as, Cu and/or Ag and solder fillers with a low melting point (LMP), such as SnPb or Pb-free tin alloys. The solder refiow process first melts and coats contacting surfaces and the HMP fillers form a metal-polymer inter-penetrated network. This is followed by a heating process which causes the TLPS to sinter, where the solid metal powder particles e.g. Cu coexist with a liquid solder phase e.g. SnBi, and densification of the mixture takes place as the metals diffuse into one another. The composition of solder is therefore changed over time with inter- diffusion, causing a melting temperature shift to higher values.

This principle has been demonstrated successfully in materials joining in semiconductor device packaging. For example, US Patent No. 5,853,622 is directed to a method for electrical and thermal electronic component attachment utilizing a combination of transient liquid phase sintering (TLPS) and a permanent adhesive flux binder, which delivers electrical and thermal conduction through sintered metal joints and mechanical properties based on a tailorable polymer matrix.

US Patent Number 9,583,453 discloses an assembly comprising a semiconductor wafer metallized with a solderable metal and at least one b-staged, sinterable die-attach material disposed on at least a portion of the metallized portion of the wafer surface, wherein the die attach material comprises an organic binder comprising a flux, and a means to render the flux inert as a consequence of a die-attach process; and a filler comprising a mixture of metallic particles, which are capable of undergoing transient liquid phase sintering. Accordingly, the TLPS approach requires customized organic materials for improving the metal adhesion and/or for reducing the negative effect of the flux on the physical properties and structure of the adhesive.

There still remains a need for conductive adhesive compositions for use, inter alia, in packaging and cooling of electronic and semiconductor components. Said compositions should offer high thermal conductivity, as well as high mechanical and chemical stability without requiring elevated temperatures and/or pressures or customized materials for their proper setting.

SUMMARY OF THE INVENTION

The present invention provides a unique method for connecting metallic surfaces. The method of the invention does not involve very high temperatures and/or elevated pressure, thus being suitable for use in attaching heat-sensitive components at ambient pressure conditions. Connection between the metallic surfaces is afforded by forming a metal-based layer, thereby providing excellent thermal conductivity. Accordingly, the present method can beneficially be used in the electronic industry, including, but not limited to, for packaging and cooling the electronic and semiconductor components.

The present invention overcomes the problems associated with the Liquid Phase Sintering approach, and in particular the high porosity and insufficient thermal conductivity of the connecting layers. The present invention is based in part on an unexpected finding that using two distinct compositions comprising metal or metal alloy powders, wherein said powders have different melting points, affords for the formation of a highly stable and densely packed adhesive layer, which can efficiently connect two metallic surfaces. It was surprisingly found that applying a composition having a lower melting point metal or metal alloy powder on a sintered film of a higher melting point metal alloy not only reduces porosity of said layer but also facilitates formation of strong bonds between the film and the adjacent metallic surfaces, thereby enabling a thermally conductive and mechanically stable adhesion of elements having at least a partially metallic surface.

Further provided is a kit for the formation of the metal-based adhesive film and an assembly comprising semiconductor or electric components connected by means of said metal- based adhesive film.

Thus, in a first aspect there is provided a method for connecting metallic surfaces with a metal-based adhesive film, the method comprising:

(a) providing a first metallic surface and a second metallic surface; (b) applying a first composition comprising a first metal or metal alloy powder onto at least one of the first metallic surface and the second metallic surface;

(c) heating the first composition to a first temperature, thereby forming a first metallic layer;

(d) applying a second composition comprising a second metal or metal alloy powder onto the first metallic layer, wherein the second metal or metal alloy powder has a lower melting point than the first metal or metal alloy powder;

(e) contacting the first metallic surface with the second metallic surface, wherein the first metallic layer and the second composition are disposed between said first metallic surface and said second metallic surface; and

(f) heating the second composition to a second temperature,

thereby forming the metal-based adhesive film between said metallic surfaces.

According to some embodiments, the first metal or metal alloy powder comprises a high melting point (HMP) metal or metal alloy powder. The HMP metal or metal alloy powder can include at least one metal selected from the group consisting of Cu, Ni, Co, Fe, Mo, Al, Ag, Au, Pt, Pd, Be, and Rh. Each possibility represents a separate embodiment of the invention. In certain embodiments, the HMP metal or metal alloy powder comprises Cu.

According to some embodiments, the first metal or metal alloy powder further comprises a low melting point (LMP) metal or metal alloy powder. The LMP metal or metal alloy powder can include at least one metal selected from the group consisting of Sn, Bi, Pb, Cd, Zn, Ga, In, Te, Hg, Tl, Sb, Se, and Po. Each possibility represents a separate embodiment of the invention. In certain embodiments, the LMP metal or metal alloy powder comprises Sn.

According to some embodiments, the second metal or metal alloy powder comprises a low melting point (LMP) metal or metal alloy powder. The LMP metal or metal alloy powder can include at least one metal selected from the group consisting of Sn, Bi, Pb, Cd, Zn, Ga, In, Te, Hg, Tl, Sb, Se, and Po. Each possibility represents a separate embodiment of the invention. In certain embodiments, the LMP metal or metal alloy powder comprises Sn.

According to some embodiments, the weight ratio of said first composition to said second composition ranges between about 10:1 to about 1 :10.

In some embodiments, the first composition, the second composition or both comprise a solder flux. The solder flux can be present in a weight percent ranging from about 2% to about 15% of the total weight of the first composition and/or the second composition. Each possibility represents a separate embodiment of the invention. In particular embodiments, the solder flux is present in a weight percent ranging from about 2% to about 10% of the total weight of the first composition. In other particular embodiments, the solder flux is present in a weight percent ranging from about 2% to about 10% of the total weight of the second composition. In further embodiments, the solder flux is present in a weight percent ranging from about 2% to about 7% of the total weight of the second composition. The solder flux can include an organic material selected from the group consisting of ether, glycol diether, alcohol, polyol, phenol, carboxylic acid, fatty acid, acid anhydride, and combinations thereof. Each possibility represents a separate embodiment of the invention.

In some currently preferred embodiments, the first composition, the second composition or both are essentially free of an adhesion-promoting agent, selected from the group consisting of a polymer, a reactive monomer, a cross-linking agent, a thermosetting resin and combinations thereof. Each possibility represents a separate embodiment of the invention.

According to some embodiments, the first temperature is a sintering temperature of the first metal or metal alloy powder. In further embodiments, said first temperature ranges from about 120□ to about 270□. According to some embodiments, the second temperature is a melting temperature of the second metal or metal alloy powder. In further embodiments, said second temperature ranges from about 120D to about 270□. The heating can be performed by an external heating source selected from the group consisting of focused IR, indirect IR, halogen lamp, laser beam, hot air, and combinations thereof. Each possibility represents a separate embodiment of the invention.

According to some embodiments, the step of applying the first composition, the step of applying the second composition or both is performed by a method selected from the group consisting of dispensing, screen printing, stencil printing, doctor blading, spraying, brushing, rolling, pad transfer, additive manufacturing and manual layering. Each possibility represents a separate embodiment of the invention. In some embodiments, the step of applying the first composition comprises covering at least a portion of the first metallic surface, the second metallic surface or both by said first composition. In certain embodiments, the step of applying the first composition comprises forming a predefined pattern on the first metallic surface, the second metallic surface or both.

According to some embodiments, the first metallic surface, the second metallic surface or both comprise a metallic coating deposited on a non-metallic substrate. In certain embodiments, the metallic surface covers at least a portion of the non-metallic substrate. The non-metallic substrate can be selected from the group consisting of a semiconductor, plastic, glass, fiberglass, silicon, ceramics, and composite material. Each possibility represents a separate embodiment of the invention. In certain embodiments, the non-metallic substrate is heat-sensitive.

In some embodiments, at least one of the first metallic surface and the second metallic surface constitutes a part of an electronic or semiconductor component selected from the group consisting of a wafer, die, integrated circuit (IC), printed circuit board (PCB), epoxy substrate, passive component and LED. In some other embodiments, at least one of the first metallic surface and the second metallic surface constitutes a part of a heat dissipation component, selected from the group consisting of cooling pad, heatsink, metallic part and thick metallic coating. In additional embodiments, at least one of the first metallic surface and the second metallic surface constitutes a part of a packaging component, selected from the group consisting of a lead-frame, a laminate-based package, a ceramic-based package, surface-mount technology (SMT), surface-mount device (SMD), ball grid array (BGA), and a metal clip. Each possibility represents a separate embodiment of the invention.

In another aspect, there is provided a product obtainable by the method according to the principles and various embodiments of the invention, as described hereinabove.

In yet another aspect, the invention provides a kit for attaching metallic surfaces, the kit comprising:

(i) a first composition comprising a high melting point (HMP) metal or metal alloy powder, a low melting point (LMP) metal or metal alloy powder and a solder flux; and

(ii) a second composition comprising a low melting point (LMP) metal or metal alloy powder and a solder flux present in the weight percent ranging from about 2% to about 15% of the total weight of the second composition, wherein the metal or metal alloy powder in the second composition has a lower melting point than the metal alloy formed following liquid phase sintering of said HMP and LMP metal or metal alloy powders in the first composition.

According to some particular embodiments, the solder flux is present in the second composition in the weight percent ranging from about 2% to about 10% of the total weight of the second composition. According to further particular embodiments, the solder flux is present in the second composition in the weight percent ranging from about 2% to about 7% of the total weight of the second composition.

According to some embodiments, the solder flux is present in the first composition in the weight percent ranging from about 2% to about 15% of the total weight of the composition. According to some particular embodiments, the solder flux is present in the first composition in the weight percent ranging from about 2% to about 10% of the total weight of the composition. In further embodiments, the solder flux is present in the first composition in the weight percent ranging from about 2% to about 7% of the total weight of the composition.

The HMP metal or metal alloy powder can include at least one metal selected from the group consisting of Cu, Ni, Co, Fe, Mo, Al, Ag, Au, Pt, Pd, Be, and Rh. Each possibility represents a separate embodiment of the invention. In certain embodiments, the HMP metal or metal alloy comprises Cu. The LMP metal or metal alloy powder can include at least one metal selected from the group consisting of Sn, Bi, Pb, Cd, Zn, Ga, In, Te, Hg, Tl, Sb, Se, and Po. Each possibility represents a separate embodiment of the invention. In certain embodiments, the LMP metal or metal alloy comprises Sn

According to some embodiments, the solder flux comprises an organic material selected from the group consisting of ether, glycol diether, alcohol, polyol, phenol, carboxylic acid, fatty acid, acid anhydride and combinations thereof. Each possibility represents a separate embodiment of the invention.

In some currently preferred embodiments, the first composition, the second composition or both are essentially free of an adhesion-promoting agent selected from the group consisting of a polymer, a reactive monomer, a cross-linking agent, a thermosetting resin and combinations thereof. Each possibility represents a separate embodiment of the invention.

In still another aspect of the invention, there is provided an assembly comprising:

i. a first electronic or semiconductor component having at least one surface, wherein at least a portion of the at least one surface comprises a metal or metal alloy layer;

ii. a second electronic or semiconductor component having at least one surface, wherein at least a portion of the at least one surface comprises a metal or metal alloy layer; and iii. a continuous metal-based adhesive film, which connects the at least one surface of the first electronic or semiconductor component with the at least one surface of the second electronic or semiconductor component, the film comprising a metal alloy or intermetallic compound comprising at least one high melting point (HMP) metal and at least one low melting point (LMP) metal, wherein the metal alloy or intermetallic compound are present in the adhesive film in a weight percent of at least about 90%.

According to some embodiments, the metal-based adhesive film has a porosity of less than about 20%. In further embodiments, at least a portion of the pores of the metal-based adhesive film is filled by the LMP metal or metal alloy.

In some embodiments, the metal-based adhesive film comprises at least about 50% (w/w) LMP metal. The HMP metal can be selected from the group consisting of Cu, Ni, Co, Fe, Mo, Al, Ag, Au, Pt, Pd, Be, and Rh. Each possibility represents a separate embodiment of the invention. In certain embodiments, the HMP metal comprises Cu. The LMP metal can be selected from the group consisting of Sn, Bi, Pb, Cd, Zn, Ga, In, Te, Hg, Tl, Sb, Se, and Po. Each possibility represents a separate embodiment of the invention. In certain embodiments, the LMP metal comprises Sn.

According to some embodiments, the metal-based adhesive film further comprises a solder flux. Preferably, the solder flux is present in the metal-based adhesive film in a weight percent of less than about 10% (w/w). In further embodiments, the solder flux is present in the metal-based adhesive film in a weight percent of less than about 7% (w/w). The solder flux can include an organic material selected from the group consisting of ether, glycol diether, alcohol, polyol, phenol, carboxylic acid, fatty acid, acid anhydride and combinations thereof. Each possibility represents a separate embodiment of the invention.

In some currently preferred embodiments, the metal-based adhesive film is essentially free of an adhesion-promoting agent selected from the group consisting of a polymer, a reactive monomer, a cross-linking agent a thermosetting resin and combinations thereof. Each possibility represents a separate embodiment of the invention.

In some embodiments, the metal-based adhesive film has a thickness ranging between about 10 and about 1000 μπι.

According to some embodiments, at least one of the first electronic or semiconductor component and the second electronic or semiconductor component is selected from the group consisting of a wafer, die, integrated circuit (IC), printed circuit board (PCB), epoxy substrate, passive component and LED. In some other embodiments, at least one of the first electronic or semiconductor component and the second electronic or semiconductor component is a heat dissipation component, selected from the group consisting of cooling pad, heatsink, metallic part and thick metallic coating. In additional embodiments, at least one of the first electronic or semiconductor component and the second electronic or semiconductor component is a packaging component, selected from the group consisting of a lead-frame, a laminate-based package, a ceramic-based package, a metal clip, surface-mount technology (SMT), surface-mount device (SMD) and ball grid array (BGA). Each possibility represents a separate embodiment of the invention.

Further embodiments and the full scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1A-1D: schematically illustrate the method of the present invention comprising applying two layers of conductive metallic composition: Figure 1A depicts the deposition of the first composition layer on the top side of the substrate, prior to the first heating step; Figure IB depicts the first layer deposition following the first heating step; Figure 1C depicts the deposition of the second composition on top of the first layer, with its metallic surface on the top side of the second ink layer, prior to the second heating step; and Figure ID depicts the final structure of the conductive metallic compositions following the second heating.

Figure 2 represents a photograph of the metal-based adhesive layer prepared by a single- step process, using one composition comprising a HMP metal and a LMP metal alloy.

Figure 3 represents a photograph of the metal-based adhesive layer prepared by a two- stage method according to the principles of the invention, using a first composition comprising a HMP metal and a LMP metal alloy and a second composition comprising a LMP metal alloy.

Figure 4 represents a photograph of a glass-reinforced epoxy laminate (FR4) substrate having a pattern of copper coated with gold thereupon (bottom substrate) attached to a small piece of FR4 coated with a round copper pad on both sides (top substrate) by means of the metal- based adhesive layer prepared by the two-stage method of the invention.

Figure 5 represents a photograph of the assembly presented in Figure 4, wherein the FR4 substrate with the copper pattern and the small piece of FR4 coated with the round copper pad are being pulled apart. The photograph shows that the round copper pad remained attached to the bottom FR4 substrate (with copper traces) via the metal-based adhesive layer.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a two-stage method for connecting metallic surfaces. In particular, the method includes successive application of two distinct metal-based compositions onto a metallic surface, wherein the metals or metal alloys in said compositions have different melting points. The method of the present invention overcomes the problems associated with the Liquid Phase Sintering approach, and in particular, with the high porosity and insufficient thermal conductivity of the connecting layer. It was surprisingly found by the inventors of the present invention that applying a composition having a lower melting point metal or metal alloy powder on a sintered film of a higher melting point metal alloy (e.g. a LPS alloy) not only reduces porosity of said layer but also facilitates formation of strong bonds between the film and the adjacent metallic surfaces. Without wishing to being bound by theory or mechanism of action, densification of the sintered layer proceeds in a capillary flow manner by melting the lower melting point metal or metal alloy over the sintered porous first layer. It was further surprisingly discovered that the application of a low melting point metal or metal alloy, which typically has a lower thermal conductivity than the LPS alloys allows to increase the overall thermal conductivity of the adhesive film.

Without further wishing to being bound by theory or mechanism of action, it is contemplated that since the adhesion of the film to the metallic surfaces is achieved by the formation of the direct bonds between the metals, no polymer or resin are required to be incorporated in the compositions of the invention and/or to be present in the final product including the connecting layer. The present invention thus provides a simple and low-cost method, which involves only a limited amount of chemicals and offers a mechanically and chemically stable and long-lasting adhesion to metallic surfaces. Customary solder pastes can beneficially be used in the method of the invention, without any additional components. Furthermore, the thickness and form of the adhesive film can be easily tailored for the desired application. The present method can beneficially be used in the electronics and semiconductor industry, including, but not limited to, for packaging and cooling the electronic components.

Further provided is a kit for attaching metallic surfaces, the kit comprising: a first composition comprising a high melting point (HMP) metal or metal alloy powder, a low melting point (LMP) metal or metal alloy powder and a solder flux; and a second composition comprising a low melting point (LMP) metal or metal alloy powder, said compositions including a very low concentration of a solder flux. Said kit provides strong and stable adhesive metal- based material, which is highly thermally conductive.

The present invention further provides an assembly of semiconductor or electric components comprising a metal-based adhesive film there between, the film comprising a metal alloy or intermetallic compound comprising at least one high melting point (HMP) metal and at least one low melting point (LMP) metal, wherein the metal alloy or intermetallic compound are present in the adhesive film in a weight percent of at least about 90%. The high contents of the metal in the adhesive film enables high thermal conductivity and good mechanical and chemical stability and bonding to the metallic surfaces of the assembly components.

Thus, according to a first aspect of the invention, there is provided a method for connecting metallic surfaces with a metal-based adhesive film, the method comprising: providing a first metallic surface and a second metallic surface; applying a first composition comprising a first metal or metal alloy powder onto at least one of the first metallic surface and the second metallic surface; heating the first composition to a first temperature, thereby forming a first metallic layer; applying a second composition comprising a second metal or metal alloy powder onto the first metallic layer and/or the second metallic surface, wherein the second metal or metal alloy powder has a lower melting point than the first metal or metal alloy powder; contacting the first metallic surface with the second metallic surface, wherein the first metallic layer and the second composition are disposed between said first metallic surface and said second metallic surface; and heating the second composition to a second temperature, thereby forming the metal-based adhesive film between said metallic surfaces.

In some embodiments, the above-mentioned steps are consecutive steps. It is to be emphasized that according to the principles of the present invention, the application of the second composition is performed following heating of the first composition. Heating of the second composition is performed after contacting the first metallic surface with the second metallic surface. In some embodiments, the second metal or metal alloy powder has a lower melting point than the metal alloy formed following liquid phase sintering of the first metal or metal alloy powder.

In some embodiments, the method comprises applying the first composition onto the first metallic surface. In some embodiments, the method comprises applying the first composition onto the second metallic surface. In some embodiments, the method comprises applying the first composition onto the first metallic surface and onto the second metallic surface.

In some embodiments, the method comprises applying the second composition onto the first metallic layer, wherein the second composition covers the entire surface area of the first metallic layer. In some other embodiments, the method comprises applying the second composition onto the first metallic layer, wherein the second composition covers only a portion of the first metallic layer surface area.

According to some embodiments, the first metal or metal alloy powder comprises a high melting point (HMP) metal or metal alloy powder. Each possibility represents a separate embodiment of the invention. The term "high melting point (HMP) metal" as used herein, refers to a metal which is able to absorb more heat before softening and liquefying, compared to low melting point (LMP) metals. In further embodiments, the term "high melting point (HMP) metal", refers to a metal having a melting point above about 500□. The non- limiting examples of HMP metals include Cu, Ni, Co, Fe, Mo, Al, Ag, Au, Pt, Pd, Be, and Rh. According to some embodiments, the first composition comprises a powder comprising at least one of the HMP metals, as listed hereinabove. According to some embodiments, the first composition comprises the HMP metal alloy powder comprising at least one of said HMP metals. Said alloy can include two or more HMP metals. In certain embodiments, the first composition comprises copper, copper alloy or a combination thereof.

In some embodiments, the melting temperature of the first metal or metal alloy powder comprising a high melting point (HMP) metal is above about 750□. In further embodiments, the melting temperature of the first metal or metal alloy powder comprising a high melting point (HMP) metal is above about 1000□.

In some embodiments, the first metal or metal alloy powder further comprises a low melting point (LMP) metal or metal alloy powder. The term "low melting point (LMP) metal", as used herein, refers in some embodiments, to a metal having a melting point below about 500□. The non-limiting examples of LMP metals include Sn, Bi, Pb, Cd, Zn, Ga, In, Te, Hg, Tl, Sb, Se, and Po. According to some embodiments, the first composition comprises a powder comprising at least one of the LMP metals, as listed hereinabove. According to some embodiments, the first composition comprises the LMP metal alloy powder comprising at least one of said LMP metals. In some embodiments, the LMP metal alloy powder comprises at least one of Sn and Pb.

The low LMP metal alloy can further include one or more HMP metals, as listed hereinabove. A person skilled in the art will readily realize that the concentration of the HMP metal in the LMP metal alloy should be selected to provide an alloy having a low melting temperature. Non-limiting examples of HMP metals suitable for use in the LMP alloys include As, Sb, Ge, Cu, Au, Al, and Ag.

In some embodiments, the melting temperature of the low melting point (LMP) metal alloy of the first metal or metal alloy powder is below about 300□. In some embodiments, the melting temperature of said LMP metal alloy is below about 250□.

In certain embodiments, the first composition comprises a combination of a HMP metal or metal alloy powder and a LMP metal or metal alloy powder. In further embodiments, the first composition comprises a combination of a copper powder or copper alloy powder and a tin alloy powder. The LMP metal alloy can further include at least one of As, Sb, Ge, Cu, Au, Al, and Ag. Each possibility represents a separate embodiment of the invention. In certain embodiments, the first composition comprises a combination of a copper powder or copper alloy powder and a Sn- Pb alloy powder.

In some embodiments, the second metal or metal alloy powder comprises a low melting point (LMP) metal or metal alloy powder. The non-limiting examples of suitable LMP metals are listed hereinabove. According to some embodiments, the second composition comprises a powder comprising at least one of said LMP metals. According to some embodiments, the second composition comprises the LMP metal alloy powder comprising at least one of said LMP metals. In some embodiments, the LMP metal alloy powder comprises at least one of Sn and Pb. In certain embodiments, the second metal or metal alloy powder comprises a Sn-Pb alloy powder.

In some embodiments, the LMP metal alloy of the second composition comprises one or more HMP metals, such as, but not limited to, As, Sb, Ge, Cu, Au, Al, and Ag. Each possibility represents a separate embodiment of the invention. In certain embodiments, the second metal or metal alloy powder comprises at least one of Sn, Pb, Cu and Ag.

In some embodiments, the melting temperature of the second metal or metal alloy powder comprising the low melting point (LMP) metal or metal alloy is below about 300□. In some embodiments, the melting temperature of the second metal or metal alloy powder comprising a low melting point (LMP) metal is below about 250□.

In some embodiments, the weight ratio of said first composition to said second composition ranges between about 10:1 to about 1 :10. In further embodiments, the weight ratio of said first composition to said second composition ranges between about 9:1 to about 1 :9. In still further embodiments, the weight ratio of said first composition to said second composition ranges between about 8 :1 to about 1 :8. In yet further embodiments, the weight ratio of said first composition to said second composition ranges between about 7:1 to about 1 :7. In still further embodiments, the weight ratio of said first composition to said second composition ranges between about 6:1 to about 1 :6. In yet further embodiments, the weight ratio of said first composition to said second composition ranges between about 5 :1 to about 1 :5. In still further embodiments, the weight ratio of said first composition to said second composition ranges between about 4:1 to about 1 :4. In yet further embodiments, the weight ratio of said first composition to said second composition ranges between about 3: 1 to about 1 :3. In certain embodiments, the weight ratio of said first composition to said second composition ranges from about 10:1 to about 1 :1, from about 9:1 to about 1 :1, from about 8: 1 to about 1 :1 , from about 7:1 to about 1 :1 , from about 6:1 to about 1 :1 , from about 5:1 to about 1 :1 , from about 4:1 to about 1 :1 , or from about 3:1 to about 1 :1. In still further embodiments, the weight ratio of said first composition to said second composition ranges from about 9:1 to about 2:1 , from about 8:1 to about 3:1 , or from about 7:1 to about 4: 1. Each possibility represents a separate embodiment of the invention.

In some embodiments, the weight ratio of said first metal or metal alloy powder to said second metal or metal alloy powder ranges between about 10: 1 to about 1 :10. In further embodiments, the weight ratio of said first metal or metal alloy powder to said second metal or metal alloy powder ranges between about 9:1 to about 1 :9. In still further embodiments, the weight ratio of said first metal or metal alloy powder to said second metal or metal alloy powder ranges between about 8 :1 to about 1 :8. In yet further embodiments, the weight ratio of said first metal or metal alloy powder to said second metal or metal alloy powder ranges between about 7:1 to about 1 :7. In still further embodiments, the weight ratio of said first metal or metal alloy powder to said second metal or metal alloy powder ranges between about 6:1 to about 1 :6. In yet further embodiments, the weight ratio of said first metal or metal alloy powder to said second metal or metal alloy powder ranges between about 5:1 to about 1 :5. In still further embodiments, the weight ratio of said first metal or metal alloy powder to said second metal or metal alloy powder ranges between about 4:1 to about 1 :4. In yet further embodiments, the weight ratio of said first metal or metal alloy powder to said second metal or metal alloy powder ranges between about 3: 1 to about 1 :3. In certain embodiments, the weight ratio of said first metal or metal alloy powder to said second metal or metal alloy powder ranges from about 10:1 to about 1 :1, from about 9:1 to about 1 :1 , from about 8:1 to about 1 :1, from about 7:1 to about 1 : 1, from about 6:1 to about 1 :1 , from about 5:1 to about 1 :1, from about 4:1 to about 1 :1, or from about 3:1 to about 1 :1. In still further embodiments, the weight ratio of said first metal or metal alloy powder to said second metal or metal alloy powder ranges from about 9: 1 to about 2:1 , from about 8: 1 to about 3 :1 , or from about 7:1 to about 4: 1. Each possibility represents a separate embodiment of the invention.

In some embodiments, the first composition, the second composition or both comprise a solder flux. Each possibility represents a separate embodiment of the invention. The term "solder flux" as used herein, refers in some embodiments to a reducing, wetting and/or cleaning agent, which facilitates soldering of metallic elements. Solder flux can be used for preventing oxidation of the metallic surfaces to be joined, thus improving the wetting thereof. Solder flux can include organic or inorganic materials. In some embodiments, the solder flux comprises an organic material. Non-limiting examples of an organic materials to be used as solder flux include ether, glycol diether, alcohol, polyol, phenol, carboxylic acid, fatty acid, and acid anhydride. Non- limiting examples of a suitable glycol diether include propylene glycol dimethyl ether, diethylene glycol dimethyl ether, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol dipropyl ether, diethylene glycol diethyl ether, triethylene glycol butyl methyl ether, tetraethylene glycol dimethyl ether, dipropylene glycol dimethyl ether, and polyethylene glycol dimethyl ether. Non-limiting examples of suitable carboxylic acids include citric acid, lactic acid, malic acid, tartaric acid, glutamic acid, and phthalic acid. A non-limiting example of a suitable fatty acid is stearic acid. A non-limiting example of a suitable alcohol is isopropyl alcohol.

In some embodiments, the solder flux is present in a weight percent ranging from about 2% to about 25% of the total weight of the first composition. In further embodiments, the solder flux is present in a weight percent ranging from about 2% to about 20% of the total weight of the first composition. In still further embodiments, the solder flux is present in a weight percent ranging from about 2% to about 15% of the total weight of the first composition. In yet further embodiments, the solder flux is present in a weight percent ranging from about 2% to about 10% of the total weight of the first composition. In still further embodiments, the solder flux is present in a weight percent ranging from about 2% to about 7% of the total weight of the first composition. In some embodiments, the solder flux is present in a weight percent ranging from about 3% to about 6% of the total weight of the first composition. In some embodiments, the solder flux is present in a weight percent of about 5% of the total weight of the first composition.

In some embodiments, the solder flux is present in a weight percent ranging from about 2% to about 15% of the total weight of the second composition. In some embodiments, the solder flux is present in a weight percent ranging from about 2% to about 10% of the total weight of the second composition. In further embodiments, the solder flux is present in a weight percent ranging from about 2% to about 7% of the total weight of the second composition. In some embodiments, the solder flux is present in a weight percent ranging from about 3% to about 6% of the total weight of the second composition. In certain embodiments, the solder flux is present in a weight percent of about 5% of the total weight of the second composition.

In some embodiments, the first composition, the second composition or both are essentially free of an adhesion-promoting agent. The term "essentially free", as used herein, refers in some embodiments to the concentration of said adhesion-promoting agent of less than about 1 % (w/w) of the total weight of the first composition and/or second composition. In further embodiments, the term "essentially free", refers to the concentration of said adhesion-promoting agent of less than about 0.5% (w/w) of the total weight of the first composition and/or second composition. In still further embodiments, the term "essentially free", refers to the concentration of said adhesion-promoting agent of less than about 0.1 % (w/w) of the total weight of the first composition and/or second composition. Said adhesion-promoting agent can be an organic, organometallic, or siloxane-based agent. In certain embodiments, the adhesion-promoting agent, which preferably is not included in the first and/or second composition is selected from a polymer, a reactive monomer, a cross-linking agent and a thermosetting resin. Each possibility represents a separate embodiment of the invention.

In certain embodiments, the first composition consists essentially of the first metal or metal alloy powder and solder flux. In additional embodiments, the second composition consists essentially of the second metal or metal alloy powder and solder flux.

In certain embodiments, at least one of the first composition and the second composition are in a form of a paste. The paste can include a metal or metal alloy powder dispersed in the flux.

The first and/or the second metal or metal alloy powder can comprise particles having a mean diameter between several nanometers and about 100 micron. In further embodiments, the particles have a mean particle diameter of about 0.5 to about 50 microns.

In some embodiments, the first and/or the second metal or metal alloy powder comprises particles having a substantially spherical shape. In other embodiments, the first and/or the second metal or metal alloy powder comprises particles being flake or platelet shaped. In additional embodiments, the first and/or the second metal or metal alloy powder comprises particles having a shape selected from the group of spherical, non-spherical, dendritic, flake, platelet, spongiform, and combinations thereof.

According to some embodiments, the step of applying the first composition, the step of applying the second composition or both is performed by a method selected from the group consisting of dispensing, screen printing, stencil printing, doctor blading, spraying, brushing, rolling, manual layering, pad transfer and additive manufacturing (3D-printing). Each possibility represents a separate embodiment of the invention.

In some embodiments, the application or deposition of the first composition defines the pattern and the shape of the metal-based adhesive film. As used herein, the term "shape" refers to the geometric dimensions in a Cartesian coordinate 3-dimentional system. In some embodiments, the application of the second metallic layer does not alter the pattern and/or the shape of the first metallic layer.

In some embodiments, the step of applying the first composition comprises covering at least a portion of the first metallic surface, the second metallic surface or both by said first composition. In certain embodiments, the step of applying the first composition comprises covering substantially fully the first metallic surface, the second metallic surface or both by said first composition. In some embodiments, the step of applying the first composition comprises covering designated areas such as, but not limited to, metal contact pads, of the first and/or second metallic surfaces by said first composition.

In some embodiments, the step of applying the first composition comprises forming a predefined pattern on the first metallic surface, the second metallic surface or both.

In some embodiments, the first temperature to which the first composition is being heated is a sintering temperature of the first metal or metal alloy powder. In certain embodiments, said first temperature ranges from about 100 to about 350□. In some other embodiments, said first temperature ranges from about 120 to about 270 □. In additional embodiments, the second temperature to which the second composition is being heated is the melting temperature of the second metal or metal alloy powder. According to some embodiments, said second temperature is the eutectic temperature of the second metal alloy. According to certain embodiments, said second temperature ranges from about 100 to about 350□. According to some embodiments, said second temperature ranges from about 120 to about 270□.

In some embodiments, the first heating step, the second heating step or both are performed for about 30 sec to about 10 minutes. In further embodiments the first heating step, the second heating step or both are performed for about 1-5 minutes.

The heating can be performed in a gradual fashion. The temperature to which the first composition and/or the second composition are exposed can be increased, for example, from the room temperature to the first temperature or the second temperature, respectively. In certain embodiments, the heating time refers to the period of time at which the first composition and/or second composition are exposed to the first temperature or the second temperature, respectively.

According to some embodiments, the heating of the first and/or the second compositions is performed by an external heating source. The non-limiting examples of an external heating source include focused IR, indirect IR, halogen lamp, laser beam, hot air, and combinations thereof. Each possibility represents a separate embodiment of the invention. According to some embodiments, the heating to a first temperature, the heating to a second temperature or both decompose the solder flux components. According to further embodiments, the heating to a first temperature, the heating to a second temperature or both reduce the concentration of the solder flux in the first composition, in the second composition or both by at least about 10%, at least about 20%, at least about 30%, at least about 40%, at least about 50%, at least about 60%, at least about 70%, or at least about 80%. Each possibility represents a separate embodiment of the invention.

Without wishing to being bound by theory or mechanism of action, it is contemplated that during heating of the first metal or metal alloy powder Liquid Phase Sintering (LPS) occurs, forming a first metallic layer. It should be emphasized that the first composition is applied to either one or both of the first metallic surface and the second metallic surface and heated prior to combining the two metallic surfaces with each other. In such way, the flux components, which decompose during LPS can be released from the bulk of the metallic layer to the unconcealed surface, reducing formation of voids or cracks in the first metallic layer. Reducing the solder flux composition to levels lower than 10% (w/w) or preferably, 5% (w/w) further reduces the formation of voids, pores and/or cracks. The first metallic layer forms the initial metallic connection with the first and/or second metallic surface and defines the structure of the final adhesive layer. Application of the second composition comprising the second metal or metal alloy powder and heating thereof following the contacting with the corresponding metallic surface provides densification of the first metallic layer. Without further wishing to being bound by theory or mechanism of action, it is assumed that said densification proceeds in a capillary flow manner by melting the lower melting point metal or alloy over the sintered first metallic layer. The second metal or metal alloy powder further enhances the connection of the first metallic layer and the first and/or the second metallic surfaces. Furthermore, the second metal or metal alloy powder forms the connection with the second and/or the first metallic surfaces, respectively. Since the second metal or metal alloy powder is used to fill the pores of the existing metallic structure, the amount of the second composition can be relatively low, thereby reducing the risk of formation of additional pores resulting from flux decomposition. Furthermore, the concentration of flux in the second composition can be kept to levels lower than 10% (w/w), or preferably 7% (w/w), or even more preferably 5% (w/w), in order to form a dense and low- porosity metal-based adhesive.

The first metallic surface and the second metallic surface can be made of a metal or metal alloy, which withstands temperatures of at least about 100□. In certain embodiments, the metallic surface comprises a solderable metal. Non-limiting examples of suitable metals include Cu, Au, Ag, Fe, Ni, Co and Al.

In some embodiments, the first metallic surface, the second metallic surface or both comprise a metallic coating deposited on a metallic substrate.

In some embodiments, the first metallic surface, the second metallic surface or both comprise a metallic coating deposited on a non-metallic substrate. In certain embodiments, the metallic surface covers at least a portion of the non-metallic substrate. In additional embodiments, the metallic coating covers substantially fully the non-metallic substrate.

In some embodiments, the non-metallic substrate is selected from the group consisting of a semiconductor, plastic, glass, fiberglass, silicon wafer, ceramics, and composite materials. In certain embodiments, the non-metallic substrate is heat-sensitive.

The non-metallic surfaces can be metallized as known in the art, for example by means of electroless deposition.

The thickness of the first metallic surface and/or the second metallic surface can range from about 0.1 μπι to about 5 mm, such as, for example, from about 0.1 μπι to about 1 μπι, from about 1 μπι to about 10 μπι, from about 10 μπι to about 100 μπι, from about 100 μπι to about 1 mm, or from about 1 mm to about 5 mm.

In some embodiments, at least one of the first metallic surface and the second metallic surface constitute a part of a high-power electronic component. High-power electronic components typically require extensive cooling by an appropriate external element, as known in the art.

In certain embodiments, at least one of the first metallic surface and the second metallic surface constitute a part of an electronic component or a semiconductor component. The term "semiconductor component" as used herein, refers in some embodiments to an electronic component being used in the semiconductors industry. In some embodiments, the term "semiconductor component" refers to an electronic component that exploits the electronic properties of semiconductor materials, such as silicon, germanium, or gallium arsenide, as well as organic semiconductors. The non-limiting examples of electronic or semiconductor components include a die, silicon wafer, integrated circuit (IC), printed circuit board (PCB), epoxy substrate, passive component and/or LED.

In certain embodiments, at least one of the first metallic surface and the second metallic surface constitute a part of a wafer or a die. The surface of the wafer or die can be metallized in order to be attached to a second metallic surface. In certain embodiments, at least one of the first metallic surface and the second metallic surface constitute a part of a printed circuit board (PCB). The PCB can include active components, passive components, ICs, LEDs, silicon chips on board and/or power components. Each possibility represents a separate embodiment of the invention. The non-conducting substrate can be metallized in full or partially in order to be attached to a second metallic surface.

In some embodiments, at least one of the first metallic surface and the second metallic surface constitute a part of a heat dissipation component, selected from the group consisting of a cooling pad, locally thick metallic coating heatsink and bulk metallic layer.

In some embodiments, the method comprises applying the second composition to a portion of the first metallic layer. The larger area of the first metallic layer as compared the area of the final metal-based adhesive film can improve heat dissipation from the first metallic surface, second metallic surface or both to the outer surroundings.

In some embodiments, at least one of the first metallic surface and the second metallic surface constitute a part of a packaging component, selected from the group consisting of a lead- frame, surface-mount technology (SMT), surface-mount device (SMD), ball grid array (BGA), a laminate-based package, a ceramic-based package and a metal clip.

In some embodiments, the method of the present invention, as presented hereinabove, is utilized to attach various electric components and/or semiconductor components to a metal contact pad on top of a PCB. In some other embodiments, the method of the present invention, as presented hereinabove, is utilized to attach heatsinks to a PCB. In certain embodiments, the method of the present invention, as presented hereinabove, is used to directly attach a heatsink to an electric component, which is mounted on top of a PCB. In further embodiments, the method is used to attach a wafer or a die comprising a cooling pad to a PCB.

Reference is now made to Figures 1A-1D, which schematically illustrate the two-stage attaching method of the present invention. In accordance with some embodiments, Figure 1A depicts substrate 101 with conductive coating 102 (also termed herein "first metallic surface"). The first composition comprising HMP metal powder 111, LMP metal powder 112 and flux 113, is applied onto conductive coating 102. According to some embodiments, the first layer of the first conductive composition is sintered and transforms into first metallic layer 115, as illustrated by Figure IB. In some embodiments, first metallic layer 115 defines the physical dimensions of the adhesion metallic layer. This layer dimensions can be optimized to yield a layer with specific dimensions for different application and for different connection between various electronic or semiconductor components. According to some embodiments, during sintering by means of an external heating source, organic gases are formed and released from the first conductive layer, leaving pores in the layer, which can be illustrated by white spaces in the first composition (Figure IB). According to some embodiments, in order to lower the porosity to a minimum, a second composition comprising LMP metal powder 121, and flux 122, is applied directly on top of first metallic layer 115. Conductive coating 131 which contacts component 132 (second metallic surface), is then placed on top of the second composition (as illustrated by Figure 1C). In accordance with some embodiments, Figure ID illustrates the result of the second heating step (heating of the second composition to a second temperature). The second composition can melt as a result of heating, thus filling the pores of the first layer, which reduces porosity, and increases wetting between the first and second metallic surfaces. As a result of the application of said two distinct compositions and their respective sintering and melting, continuous conductive metallic adhesive layer 140 is formed. According to some embodiments, the first conductive composition defines the physical dimensions of the adhesion metallic layer, and the second composition provides the high thermal conductivity properties and enhanced adhesion while maintaining the structure defined by the first layer. In certain such embodiments, the combination of the two compositions and the two-stage application and heating method forms a continuous metallic adhesive layer with reduced porosity, high thermal conductivity, mechanical and chemical stability and strong adhesion to the two metallic surfaces.

Additional beneficial feature of the two-stage connection method of the invention as compared to a single-stage process is that the second composition assists the alignment of the two metallic surfaces. According to some embodiments, the second composition has a certain degree of tackiness.

According to another aspect of the invention, there is provided a product obtainable by the method of the present invention, as described hereinabove. Said product can include, inter alia, an electric component and/or semiconductor component comprising a metal-based adhesive film between two communicating metallic surfaces.

According to yet another aspect of the invention, there is provided a kit for attaching metallic surfaces, the kit comprising: a first composition comprising a high melting point (HMP) metal or metal alloy powder, a low melting point (LMP) metal or metal alloy powder and a solder flux; and a second composition comprising a low melting point (LMP) metal or metal alloy powder and a solder flux present in the weight percent ranging from about 2% to about 15% of the total weight of the second composition, wherein the metal or metal alloy powder in the second composition has a lower melting point than the metal alloy formed following liquid phase sintering of said HMP and LMP metal or metal alloy powders.

In some embodiments, the solder flux is present in the second composition in the weight percent ranging from about 2% to about 10% of the total weight of the second composition. In further embodiments, the solder flux is present in the second composition in the weight percent ranging from about 2% to about 7% of the total weight of the second composition. In still further embodiments, the solder flux is present in the second composition in the weight percent ranging from about 3% to about 6% of the total weight of the second composition.

In some embodiments, the solder flux is present in the second composition in the weight percent of below about 10% of the total weight of the second composition, below about 9%, blow about 8%, below about 7%, below about 6% or below about 5%. Each possibility represents a separate embodiment of the invention.

In some embodiments, the solder flux is present in the first composition in the weight percent ranging from about 2% to about 20% of the total weight of the composition. In further embodiments, the solder flux is present in the first composition in the weight percent ranging from about 2% to about 15% of the total weight of the composition. In still further embodiments, the solder flux is present in the first composition in the weight percent ranging from about 2% to about 10% of the total weight of the composition. In yet further embodiments, the solder flux is present in the first composition in the weight percent ranging from about 2% to about 7% of the total weight of the composition. In still embodiments, the solder flux is present in the first composition in the weight percent ranging from about 3% to about 6% of the total weight of the composition.

In some embodiments, the solder flux is present in the first composition in the weight percent of below about 10% of the total weight of the first composition, below about 9%, blow about 8%, below about 7%, below about 6% or below about 5%. Each possibility represents a separate embodiment of the invention.

In some embodiments, the HMP metal or metal alloy powder comprises at least one metal selected from the group consisting of Cu, Ni, Co, Fe, Mo, Al, Ag, Au, Pt, Pd, Be, and Rh. In some embodiments, the LMP metal or metal alloy powder comprises at least one metal selected from the group consisting of Sn, Bi, Pb, Cd, Zn, Ga, In, Te, Hg, Tl, Sb, Se, and Po. In certain embodiments, the LMP metal alloy further comprises one or more HMP metals, such as, but not limited to, As, Sb, Ge, Cu, Au, Al, and Ag.

In some embodiments, the weight ratio of the metal or metal alloy powder in the first composition to the metal or metal alloy powder in the second composition ranges between about 10: 1 to about 1 :10. In further embodiments, the weight ratio of the metal or metal alloy powder in the first composition to the metal or metal alloy powder in the second composition ranges between about 9:1 to about 1 :9. In still further embodiments, the weight ratio of the metal or metal alloy powder in the first composition to the metal or metal alloy powder in the second composition ranges between about 8:1 to about 1 :8. In yet further embodiments, the weight ratio of the metal or metal alloy powder in the first composition to the metal or metal alloy powder in the second composition ranges between about 7:1 to about 1 :7. In still further embodiments, the weight ratio of the metal or metal alloy powder in the first composition to the metal or metal alloy powder in the second composition ranges between about 6:1 to about 1 :6. In yet further embodiments, the weight ratio of the metal or metal alloy powder in the first composition to the metal or metal alloy powder in the second composition ranges between about 5:1 to about 1 :5. In still further embodiments, the weight ratio of the metal or metal alloy powder in the first composition to the metal or metal alloy powder in the second composition ranges between about 4:1 to about 1 :4. In yet further embodiments, the weight ratio of the metal or metal alloy powder in the first composition to the metal or metal alloy powder in the second composition ranges between about 3:1 to about 1 :3. In certain embodiments, the weight ratio of the metal or metal alloy powder in the first composition to the metal or metal alloy powder in the second composition ranges from about 10:1 to about 1 : 1, from about 9:1 to about 1 : 1, from about 8 :1 to about 1 : 1, from about 7:1 to about 1 : 1, from about 6:1 to about 1 : 1, from about 5:1 to about 1 :1 , from about 4:1 to about 1 :1 , or from about 3 :1 to about 1 :1. In still further embodiments, the weight ratio of the metal or metal alloy powder in the first composition to the metal or metal alloy powder in the second composition ranges from about 9:1 to about 2:1 , from about 8:1 to about 3:1 , or from about 7:1 to about 4: 1. Each possibility represents a separate embodiment of the invention.

In some embodiments, the solder flux comprises an organic material selected from the group consisting of ether, glycol diether, alcohol, polyol, phenol, carboxylic acid, fatty acid, and acid anhydride. In some embodiments, the first composition, the second composition or both are essentially free of an adhesion-promoting agent, selected from a polymer, a reactive monomer, a cross-linking agent and a thermosetting resin. Each possibility represents a separate embodiment of the invention.

In accordance with another aspect of the present invention, there is provided an assembly comprising: a first electronic or semiconductor component having at least one surface, wherein at least a portion of said at least one surface comprises a metal or metal alloy layer; a second electronic or semiconductor component having at least one surface, wherein at least a portion of said at least one surface comprises a metal or metal alloy layer; and a continuous metal-based adhesive film, which connects the at least one surface of the first electronic or semiconductor component with the at least one surface of the second electronic or semiconductor component, the film comprising a metal alloy or intermetallic compound comprising at least one high melting point (HMP) metal and at least one low melting point (LMP) metal, wherein the metal alloy or intermetallic compound are present in the adhesive film in a weight percent of at least about 90%.

In some embodiments, the metal-based adhesive film has a porosity of less than about 20%. In further embodiments, the metal-based adhesive film has a porosity of less than about 15%. In still further embodiments, the metal-based adhesive film has a porosity of less than about 10%. In some embodiments, at least a portion of the pores of the metal-based adhesive film is filled by the LMP metal or metal alloy. In some embodiments, the metal-based adhesive film comprises at least about 20% (w/w) LMP metal. In some embodiments, the metal-based adhesive film comprises at least about 30% (w/w) LMP metal. In some embodiments, the metal-based adhesive film comprises at least about 40% (w/w) LMP metal. In some embodiments, the metal- based adhesive film comprises at least about 50% (w/w) LMP metal. In further embodiments, the metal-based adhesive film comprises at least about 60% (w/w) LMP metal. In still further embodiments, the metal-based adhesive film comprises at least about 70% (w/w) LMP metal. In yet further embodiments, the metal-based adhesive film comprises at least about 80% (w/w) LMP metal. In still further embodiments, the metal-based adhesive film comprises at least about 90% (w/w) LMP metal.

In some embodiments, the HMP metal is selected from the group consisting of Cu, Ni, Co, Fe, Mo, Al, Ag, Au, Pt, Pd, Be, and Rh. In some embodiments, the LMP metal is selected from the group consisting of Sn, Bi, Pb, Cd, Zn, Ga, In, Te, Hg, Tl, Sb, Se, and Po.

In some embodiments, the metal-based adhesive film further comprises a solder flux. In some embodiments, the solder flux is present in the metal-based adhesive film in a weight percent of less than about 10%. In further embodiments, the solder flux is present in the metal- based adhesive film in a weight percent of less than about 9%. In still further embodiments, the solder flux is present in the metal-based adhesive film in a weight percent of less than about 8%. In yet further embodiments, the solder flux is present in the metal-based adhesive film in a weight percent of less than about 7%. In still further embodiments, the solder flux is present in the metal-based adhesive film in a weight percent of less than about 6%. In certain embodiments, the solder flux is present in the metal-based adhesive film in a weight percent of less than about 5%. In some embodiments, the solder flux comprises an organic material selected from the group consisting of ether, glycol diether, alcohol, polyol, phenol, carboxylic acid, fatty acid, and acid anhydride.

In some embodiments, the metal-based adhesive film is essentially free of an adhesion- promoting agent, selected from a polymer, a reactive monomer, a cross-linking agent or a thermosetting resin.

In some embodiments, the metal-based adhesive film has a thickness ranging between about 5 and about 5000 μπι. In further embodiments, the metal-based adhesive film has a thickness ranging between about 10 and about 1000 μιη. In still further embodiments, the metal- based adhesive film has a thickness ranging between about 50 and about 500 μιη. According to some embodiments, the at least one surface of the first electronic or semiconductor component and the at least one surface of the second electronic or semiconductor component are essentially parallel

The metal or metal alloy layer of the at least one surface of the first electronic or semiconductor component can be made of a metal or metal alloy, which withstands temperatures of at least about 100D. The metal or metal alloy layer of the at least one surface of the second electronic or semiconductor component can be made of a metal or metal alloy, which withstands temperatures of at least about 100□. In certain embodiments, said metal or metal alloy layer comprises a solderable metal. The non-limiting examples of suitable metals include Cu, Au, Ag, Fe, Ni, Co and Al. The thickness of said metal or metal alloy layer can range from about 0.1 μπι to about 5 mm, such as, for example, from about 0.1 μπι to about 1 μπι, from about 1 μπι to about 10 μπι, from about 10 μπι to about 100 μπι, from about 100 μπι to about 1 mm, or from about 1 mm to about 5 mm.

The electronic or semiconductor component can include a metallic or a non-metallic substrate. Each possibility represents a separate embodiment of the invention. The first electronic or semiconductor component and the second electronic or semiconductor component can be of the same type or different types. Each possibility represents a separate embodiment of the invention.

In some embodiments, at least one of the first electronic or semiconductor component and the second electronic or a semiconductor component is selected from the group consisting of a die, silicon wafer, integrated circuit (IC), printed circuit board (PCB), epoxy substrate, passive component and LED.

In some embodiments, at least one of the first electronic or a semiconductor component and the second electronic or semiconductor component is a heat dissipation component, selected from the group consisting of cooling pad, locally thick metallic coating heatsink and bulk metallic layer.

In some embodiments, at least one of the first electronic or semiconductor component and the second electronic or semiconductor component is a packaging component, selected from the group consisting of a lead-frame, surface-mount technology (SMT), surface-mount device (SMD), ball grid array (BGA), a laminate-based package, a ceramic-based package and a metal clip.

As used herein and in the appended claims the singular forms "a", "an," and "the" include plural references unless the content clearly dictates otherwise. It should be noted that the term "and" or the term "or" is generally employed in its sense including "and/or" unless the content clearly dictates otherwise. As used herein, the term "about", when referring to a measurable value such as an amount, a temporal duration, and the like, is meant to encompass variations of +/-10%, more preferably +1-5%, even more preferably +/- 1 , and still more preferably +/-0.1 % from the specified value, as such variations are appropriate to perform the disclosed methods.

The following examples are presented in order to more fully illustrate some embodiments of the invention. They should, in no way be construed, however, as limiting the broad scope of the invention. One skilled in the art can readily devise many variations and modifications of the principles disclosed herein without departing from the scope of the invention.

EXAMPLES Example 1: Attaching two metal surfaces utilizing one layer of conductive ink (Comparative Example)

An experiment was performed to determine the ability of the first composition to act as a single layer of a conductive adhesive, and to attach two metal surfaces in a one-step process. In order to easily visualize the formed metallic film, the experiment was performed on two glass slabs instead of two metallic surfaces.

The first composition included a copper powder and/or copper alloy and a solder paste. The first composition further included a commercial organic solder flux in a weight percent of about 5% of the total weight of the first composition.

The components were blended until a uniform mixture was achieved. 0.02 gr of the first composition were applied by screen printing onto a first glass slab and a second glass slab was placed on top of the first composition. The screen-printed first composition was cured under 200°C for 2 minutes at atmospheric pressure.

During sintering, the heated flux releases gases which diffuse outside the first composition metallic layer, thereby disrupting the uniform and continuous form of the layer. Following sintering, the first composition transforms into a discontinuous metallic layer, containing many pores, voids and cracks as a result of the gas release during sintering.

The obtained assembly is shown in Figure 2. It can be seen that only limited number of the metal aggregates in the film are connected and no continuous film is formed. The large voids evident from the photograph, disrupt the thermal conductivity of the metallic layer and reduce the adhesive ability thereof.

Example 2: Structure of the metal-based adhesive film prepared by the two-stage method with two distinct metal-based compositions

An experiment was performed to determine the ability of the first composition and the second composition to act as a metal-based adhesive film. In order to easily visualize the formed metallic film, the experiment was first performed on two glass slabs.

The fabrication of the first composition was identical to the composition presented at Example 1. The second composition included Sn-based alloys and a solder flux (the same flux as in the first composition) in a weight percent of about 10% of the total weight of the second composition. The components were blended until a uniform mixture was achieved.

About 0.02 gr of the first composition, as the first layer, were applied by screen printing onto a first glass slab (as illustrated by Figure 1 A, while using a glass slab instead of a metallic surface). The screen-printed first composition was sintered under 200°C for 2 minutes at atmospheric pressure (as illustrated by Figure IB) and a first metallic layer was formed. When the first composition is sintered without a second glass slab (or metallic surface) being placed on top of it, the gasses generated from the decomposition of flux are released through the top surface of the deposited first composition, thus keeping the first metallic layer intact, and preventing formation of voids and cracks in the layer. In order to further lower the porosity of the first metallic layer, the second step was applied: about 0.005 gr of the second composition was applied directly on top of the first metallic layer, and the second glass slab was placed on top of the second composition (as illustrated by Figure 1C). The screen-printed second composition was heated at 200°C for 2 minutes at atmospheric pressure. Upon heating, the second composition melts and is absorbed into the pores of the first metallic layer to form a continuous metallic layer with the outer dimensions determined by the first layer (as illustrated by Figure ID).

The obtained assembly is shown in Figure 3. It can be seen that the resulting conductive adhesive metallic film, comprising the first composition and the second composition, between the two mating surfaces contains significantly smaller amount of pores and voids as compared to the one layer of the first composition (as presented in Example 1). Accordingly, the combination of the two compositions provides improved adhesion and thermal contact between the two metallic surfaces.

Example 3: Attaching metallic surfaces utilizing the two-stage method with two distinct metal-based compositions

The experiment presented in Example 2 was performed with two metallic surfaces instead of glass slabs. The fabrication and compositions of the first composition and the second composition were identical to those presented in Example 2.

About 0.003 gr of the first composition, as the first layer, were applied by screen printing onto an FR4 substrate with a pattern of copper coated with gold thereupon. The first composition was sintered under 200°C for 2 minutes at atmospheric pressure, forming the first metallic layer.

About 0.001 gr of the second composition were applied directly on top of the first metallic layer, and a small piece of FR4 coated with a round copper pad (on both sides) was placed on top of the second composition. The screen-printed second composition was heated at 200°C for 2 minutes at atmospheric pressure. The connected assembly of the two FR4 substrates is shown in Figure 4.

Example 4: Adhesive properties of the metal-based adhesive layer

Physical properties of adhesive are typically measured by pulling apart two substrate materials held together by an adhesive. The adhesive ability of the metal-based adhesive layer was tested by pulling apart the two FR4 pieces of the assembly prepared in Example 3. As can be seen from Figure 5, showing the separated FR4 substrates, the copper pad, which was coated on the small piece of FR4, was detached from said FR4 piece (top FR4 in the figure) and remained attached to the metal-based adhesive layer. Hence, the adhesion of the copper pad to the adhesive layer was strong enough to withstand pulling apart of the two metallic surfaces and the adhesive layer has high shear strength.

Example 5: Attaching metallic surfaces utilizing the two-stage method with a second composition having low flux content

The experiment presented in Example 3 is performed with a similar first composition and a different second composition, which includes Sn-based alloys and a solder flux (the same flux as in the first composition) in a weight percent of about 5% of the total weight of the second composition. The components of the second composition are blended until a uniform mixture is achieved. About 0.003 gr of the first composition, as the first layer, are applied by screen printing onto an FR4 substrate with a pattern of copper coated with gold thereupon. The first composition is sintered under 200°C for 2 minutes at atmospheric pressure, forming the first metallic layer.

About 0.001 gr of the second composition are applied directly on top of the first metallic layer, and a small piece of FR4 coated with a round copper pad (on both sides) is placed on top of the second composition. The screen-printed second composition is heated at 200°C for 2 minutes at atmospheric pressure.

While certain embodiments of the invention have been illustrated and described, it will be clear that the invention is not limited to the embodiments described herein. Numerous modifications, changes, variations, substitutions and equivalents will be apparent to those skilled in the art without departing from the spirit and scope of the present invention as described by the claims, which follow.

Claims

1. A method for connecting metallic surfaces with a metal-based adhesive film, the method comprising:

(a) providing a first metallic surface and a second metallic surface;

(b) applying a first composition comprising a first metal or metal alloy powder onto at least one of the first metallic surface and the second metallic surface;

(f) heating the second composition to a second temperature,

thereby forming the metal-based adhesive film between said metallic surfaces.

2. The method according to claim 1 , wherein the first metal or metal alloy powder comprises a high melting point (HMP) metal or metal alloy powder.

3. The method according to claim 2, wherein the HMP metal or metal alloy powder comprises at least one metal selected from the group consisting of Cu, Ni, Co, Fe, Mo, Al, Ag, Au, Pt, Pd, Be, and Rh.

4. The method according to any one of claims 2 and 3, wherein the first metal or metal alloy powder further comprises a low melting point (LMP) metal or metal alloy powder.

5. The method according to any one of claims 1 to 4, wherein the second metal or metal alloy powder comprises a low melting point (LMP) metal or metal alloy powder.

6. The method according to any one of claims 4 and 5, wherein the LMP metal or metal alloy powder comprises at least one metal selected from the group consisting of Sn, Bi, Pb, Cd, Zn, Ga, In, Te, Hg, Tl, Sb, Se, and Po.

7. The method according to any one of claims 1 to 6, wherein the weight ratio of said first composition to said second composition ranges between about 10:1 to about 1 :10.

8. The method according to any one of claims 1 to 7, wherein the first composition, the second composition or both comprise a solder flux.

9. The method according to claim 8, wherein the solder flux is present in a weight percent ranging from about 2% to about 15% of the total weight of the first composition and/or the second composition.

10. The method according to claim 9, wherein the solder flux is present in a weight percent ranging from about 2% to about 7% of the total weight of the second composition.

11. The method according to any one of claims 8 to 10, wherein the solder flux comprises an organic material selected from the group consisting of ether, glycol diether, alcohol, polyol, phenol, carboxylic acid, fatty acid, acid anhydride, and combinations thereof.

12. The method according to any one of claims 1 to 11, wherein the first composition, the second composition or both are essentially free of an adhesion-promoting agent, selected from the group consisting of a polymer, a reactive monomer, a cross-linking agent, a thermosetting resin and combinations thereof.

13. The method according to any one of claims 1 to 12, wherein the first temperature is a sintering temperature of the first metal or metal alloy powder.

14. The method according to any one of claims 1 to 13, wherein said first temperature ranges from about 120D to about 270□.

15. The method according to any one of claims 1 to 14, wherein the second temperature is a melting temperature of the second metal or metal alloy powder.

16. The method according to any one of claims 1 to 15, wherein said second temperature ranges from about 120D to about 270□.

17. The method according to any one of claims 1 to 16, wherein the heating is performed by an external heating source selected from the group consisting of focused IR, indirect IR, halogen lamp, laser beam, hot air, and combinations thereof.

18. The method according to any one of claims 1 to 17, wherein the step of applying the first composition, the step of applying the second composition or both is performed by a method selected from the group consisting of dispensing, screen printing, stencil printing, doctor blading, spraying, brushing, rolling, pad transfer, additive manufacturing and manual layering.

19. The method according to any one of claims 1 to 18, wherein the step of applying the first composition comprises covering at least a portion of the first metallic surface, the second metallic surface or both by said first composition.

20. The method according to any one of claims 1 to 19, wherein the step of applying the first composition comprises forming a predefined pattern on the first metallic surface, the second metallic surface or both.

21. The method according to any one of claims 1 to 20, wherein the first metallic surface, the second metallic surface or both comprise a metallic coating deposited on a non-metallic substrate.

22. The method according to claim 21, wherein the metallic surface covers at least a portion of the non-metallic substrate.

23. The method according to any one of claims 21 and 22, wherein the non- metallic substrate is selected from the group consisting of a semiconductor, plastic, glass, fiberglass, silicon, ceramics and composite material.

24. The method according to any one of claims 21 to 23, wherein the non- metallic substrate is heat-sensitive.

25. The method according to any one of claims 1 to 24, wherein at least one of the first metallic surface and the second metallic surface constitute a part of an electronic or semiconductor component selected from the group consisting of a wafer, die, integrated circuit (IC), printed circuit board (PCB), epoxy substrate, passive component and LED.

26. The method according to any one of claims 1 to 24, wherein at least one of the first metallic surface and the second metallic surface constitute a part of a heat dissipation component, selected from the group consisting of cooling pad, heatsink, metallic part and thick metallic coating.

27. The method according to any one of claims 1 to 24, wherein at least one of the first metallic surface and the second metallic surface constitute a part of a packaging component, selected from the group consisting of a lead-frame, a laminate-based package, a ceramic-based package, surface-mount technology (SMT), surface-mount device (SMD), ball grid array (BGA), and a metal clip.

28. A product obtainable by the method according to any one of the preceding claims.

29. A kit for attaching metallic surfaces, the kit comprising:

(ii) a second composition comprising a low melting point (LMP) metal or metal alloy powder and a solder flux present in the weight percent ranging from about 2% to about 7% of the total weight of the second composition, wherein the metal or metal alloy powder in the second composition has a lower melting point than the metal alloy formed following liquid phase sintering of said HMP and LMP metal or metal alloy powders in the first composition.

30. The kit according to claim 29, wherein the solder flux is present in the first composition in the weight percent ranging from about 2% to about 15% of the total weight of the composition.

31. The kit according to claim 30, wherein the solder flux is present in the first composition in the weight percent ranging from about 2% to about 7% of the total weight of the composition.

32. The kit according to any one of claims 29 to 31, wherein the HMP metal or metal alloy powder comprises at least one metal selected from the group consisting of Cu, Ni, Co, Fe, Mo,

Al, Ag, Au, Pt, Pd, Be, and Rh.

33. The kit according to any one of claims 29 to 32, wherein the LMP metal or metal alloy powder comprises at least one metal selected from the group consisting of Sn, Bi, Pb, Cd, Zn, Ga, In, Te, Hg, Tl, Sb, Se, and Po.

34. The kit according to any one of claims 29 to 33, wherein the solder flux comprises an organic material selected from the group consisting of ether, glycol diether, alcohol, polyol, phenol, carboxylic acid, fatty acid, acid anhydride and combinations thereof.

35. The kit according to any one of claims 29 to 34, wherein the first composition, the second composition or both are essentially free of an adhesion-promoting agent selected from the group consisting of a polymer, a reactive monomer, a cross-linking agent, a thermosetting resin and combinations thereof.

36. An assembly comprising:

ii. a second electronic or semiconductor component having at least one surface, wherein at least a portion of the at least one surface comprises a metal or metal alloy layer; and

iii. a continuous metal-based adhesive film, which connects the at least one surface of the first electronic or semiconductor component with the at least one surface of the second electronic or semiconductor component, the film comprising a metal alloy or intermetallic compound comprising at least one high melting point (HMP) metal and at least one low melting point (LMP) metal, wherein the metal alloy or intermetallic compound are present in the adhesive film in a weight percent of at least about 90%.

37. The assembly according to claim 36, wherein the metal-based adhesive film has a porosity of less than about 20%.

38. The assembly according to any one of claims 36 and 37, wherein at least a portion of the pores of the metal-based adhesive film is filled by the LMP metal or metal alloy.

39. The assembly according to any one of claims 36 to 38, wherein the metal-based adhesive film comprises at least about 50% (w/w) LMP metal.

40. The assembly according to any one of claims 36 to 39, wherein the HMP metal is selected from the group consisting of Cu, Ni, Co, Fe, Mo, Al, Ag, Au, Pt, Pd, Be, and Rh.

41. The assembly according to any one of claims 36 to 40, wherein the LMP metal is selected from the group consisting of Sn, Bi, Pb, Cd, Zn, Ga, In, Te, Hg, Tl, Sb, Se, and Po.

42. The assembly according to any one of claims 36 to 41, wherein the metal-based adhesive film further comprises a solder flux.

43. The assembly according to claim 42, wherein the solder flux is present in the metal-based adhesive film in a weight percent of less than about 10%.

44. The assembly according to any one of claims 42 and 43, wherein the solder flux comprises an organic material selected from the group consisting of ether, glycol diether, alcohol, polyol, phenol, carboxylic acid, fatty acid, acid anhydride and combinations thereof.

45. The assembly according to any one of claims 36 to 44, wherein the metal-based adhesive film is essentially free of an adhesion-promoting agent selected from the group consisting of a polymer, a reactive monomer, a cross-linking agent a thermosetting resin and combinations thereof.

46. The assembly according to any one of claims 36 to 45 wherein the metal-based adhesive film has a thickness ranging between about 10 and about 1000 μιη.

47. The assembly according to any one of claims 36 to 46, wherein at least one of the first electronic or semiconductor component and the second electronic or semiconductor component is selected from the group consisting of a wafer, die, integrated circuit (IC), printed circuit board (PCB), epoxy substrate, passive component and LED.

48. The assembly according to any one of claims 36 to 46, wherein at least one of the first electronic or semiconductor component and the second electronic or semiconductor component is a heat dissipation component, selected from the group consisting of cooling pad, heatsink, metallic part and thick metallic coating.

49. The assembly according to any one of claims 36 to 46, wherein at least one of the first electronic or semiconductor component and the second electronic or semiconductor component is a packaging component, selected from the group consisting of a lead-frame, a laminate-based package, a ceramic-based package, a metal clip, surface-mount technology (SMT), surface- mount device (SMD) and ball grid array (BGA).