WO2016087613A1

WO2016087613A1 - Conductive adhesive composition

Info

Publication number: WO2016087613A1
Application number: PCT/EP2015/078580
Authority: WO
Inventors: Manuel Arturo LOPEZ QUINTELA; Senén PAZ ABUÍN
Original assignee: Nanogap Sub Nm Powder, S.A.; Galega De Impermeabilizaciones Y Revestimientos Especiales, S.A. (Gairesa)
Priority date: 2014-12-04
Filing date: 2015-12-03
Publication date: 2016-06-09
Also published as: EP3227355A1

Abstract

The present invention refers to a water base epoxy resin formulation which comprises the combination of a liquid epoxy resin of Bisphenol A diglycidylether and an aqueous dispersion of a liquid epoxy resin of Bisphenol A diglycidylether, a glycidyl ether resin and a latent curing agent, said formulation being suitable for the elaboration of a single-component conductive adhesive having conductive fillers evenly dispersed within the epoxy resin.

Description

CONDUCTIVE ADHESIVE COMPOSITION

FIELD OF THE INVENTION

The present invention relates to single component epoxy based conductive adhesive compositions, and more particularly to a room stable conductive adhesive composition containing silver nanofibers as conductive additive.

BACKGROUND

Electrically conductive adhesives (ECAs) are an environmentally friendly alternative to tin/lead (Sn/Pb) solders for surface mount applications. Compared to Sn/Pb solders, conductive adhesive technology offers numerous advantages including environmental friendliness (no lead), low temperature processing conditions, fewer processing steps, low stress on the substrates, and fine pitch interconnect capability.

Basically, there are two types of ECAs, isotropic conductive adhesive (ICA) and anisotropic conductive adhesive (AC A) (Gilleo, K., Soldering & Surface Mount Technology, 1995, 7(1), 12-17). Although the concepts of these materials are different, both materials are composite materials consisting of polymer matrix containing conductive fillers. Typically, ICAs contain conductive filler concentrations between 60 and 80% wt%, and the adhesives are conductive in all directions. In ACAs, the volume fractions of conductive fillers are normally between 5 and 10 wt% and the electrical conduction is generally achieved only in the pressurization direction during curing.

ICAs materials consist of a polymer matrix containing electrically conductive fillers. Traditionally, bisphenol-A based epoxies have been widely used in the electronic packing industry due to their excellent reliability, good thermal stability and high Young's modulus. Silver is a widely used conductive filler, as it is the most conductive metal found in nature. Combinations of both components exist in a number of commercially available products, although they show some drawbacks as: the polymer matrix has to be loaded with large amounts of silver, often exceeding 60 wt% in order to achieve low resistivity values (<1χ10^~2 Ωαη). Large amounts of silver impact on the physical-chemical and mechanical properties of the adhesive; most commercial available adhesives are sold as two component systems, usually based on Bisphenol A resins, that require blending at point of use, thus final properties of the conductive adhesive depend on the quality and effectiveness of mixing procedure prior to use. those commercially available products that are sold as a single component must be stored at low temperature, typically -40°C, in order to prevent the onset of resin curing. In view of that, there exist the need to provide conductive adhesive compositions which try to overcome the drawbacks mentioned above.

BRIEF DESCRIPTION OF THE INVENTION

The authors of the present invention have developed a new water base epoxy resin formulation suitable for preparing a single-component isotropic conductive adhesive with improved properties based on Bisphenol A resin.

The formulation is characterized for comprising a mixture of two epoxy resins based on Bisphenol A diglycidylether, one is a liquid resin and the other one a waterborne liquid resin, in addition to an epoxy diluent and a latent curing agent. The presence of water in the formulation allows the subsequent entire and homogeneous dispersion of conductive fillers within the resin formulation and also the incorporation of the curing agent therein, thus avoiding the use of a separate formulation containing the curing agent which has to be mixed-in prior to use, as well as preventing any potential quality problems associated with the effectiveness and accuracy of said mixing.

Furthermore, said formulation also allows the incorporation of relatively low concentrations of conductive fillers in comparison to existing products, there being less detrimental impact on the mechanical and physical properties of the resin. Therefore, the first aspect of the present invention refers to a water base epoxy resin formulation comprising: a) the combination of a liquid epoxy resin of Bisphenol A diglycidylether of formula (I) and an aqueous dispersion of a liquid epoxy resin of Bisphenol A diglycidylether of formula (I):

(I)

wherein n is a number ranging from 0 to 1 ; b) a glycidyl ether of formula (II):

(II)

wherein:

R is an aliphatic or cycloaliphatic radical of valency m;

R' is selected from H and methyl; and m is an integer from 1 to 3; c) a latent curing agent.

In a particular embodiment, the water base epoxy resin formulation further comprises conductive fillers.

Another aspect of the invention relates to a process for the preparation of a water base epoxy resin formulation as defined above, said process comprising: a) mixing a liquid epoxy resin of Bisphenol A diglycidylether of formula (I) with a glycidyl ether of formula (II); b) adding the mixture resulting from step a) to an aqueous dispersion comprising an epoxy resin Bisphenol A diglycidylether of formula (I); c) adding the latent curing agent to the mixture resulting from step b).

A further aspect of the invention refers to a water base epoxy resin formulation obtainable by the process as defined above.

In a particular embodiment, the process of the invention further comprises the addition of conductive fillers to the mixture resulting from step c). Accordingly, another aspect of the invention relates to a water base conductive epoxy resin formulation obtainable by this process.

In another particular embodiment, the process of the invention further comprises the evaporation of at least 99.8 ± 0.2 wt% of the water contained in the resulting water base conductive epoxy resin formulation mentioned above, thus providing a single- component isotropic conductive epoxy adhesive. Therefore, a further aspect of the invention relates to the single-component isotropic conductive adhesive obtainable by this process.

With respect to other single-component adhesives which require to be stored at low temperatures, the conductive adhesive of the invention is characterized in that it can be stored at room temperature for prolonged periods.

As mentioned before, due to the fact that the curing agent is included in the formulation, there is no need to use a separate hardener prior to its use, thus preventing any problem derived from an inadequate and inaccuracy mixing of this component with the resin. Furthermore, since relatively low concentration of conductive fillers is enough to impart a good electrical conductivity, the impact on the physical, chemical and mechanical properties of the conductive adhesive is minimized.

Another aspect of the present invention relates to the use of the single-component isotropic conductive adhesive for surface mount applications and for bonding electronic components.

It is a further aspect of the invention a method for the surface mounting of electronic parts on a substrate circuit of an electronic device, said method comprising the application of a single-component isotropic conductive adhesive as defined above. Finally, another aspect of the invention relates to a method for bonding electronic components, said method comprising the application of a single-component isotropic conductive adhesive as defined above.

DETAILED DESCRIPTION OF THE INVENTION

According to the first aspect of the invention, this refers to a water base epoxy resin formulation comprising: a) the combination of a liquid epoxy resin of Bisphenol A diglycidylether of formula (I) and an aqueous dispersion of a liquid epoxy resin of Bisphenol A diglycidylether of formula (I):

(I)

(II)

wherein:

R is an aliphatic or cycloaliphatic radical of valency m;

By the term "water base epoxy resin" it is understood a formulation based on epoxy resins which is dispersed in an aqueous medium. A) Combination of a liquid epoxy resin and an aqueous dispersion of a liquid epoxy resin of Bisphenol A diglycidylether

The water base epoxy resin formulation of the invention comprises the combination of two epoxy resins based on Bisphenol A diglycidyl ether (BADGE) of formula (I):

(I)

wherein n is a number ranging from 0 to 1.

One of the epoxy resins is the liquid resin as such, whereas the other epoxy resin is based on an aqueous dispersion of the liquid resin. The Bisphenol A diglycidyl ether, as previously defined, may be obtained by reaction between bisphenol A and epichlorohydrin according to any procedure known by a skilled person. The reaction between bisphenol A and epichlorohydrin can be controlled to produce different molecular weights. Low molecular weight molecules tend to be liquids, whereas higher molecular weight molecules tend to be more viscous liquids or solids. This reaction can be carried out either continuously or discontinuously, and in the presence of an alkali metal hydroxide in the amount of two moles, or about two moles, for each mole of Bisphenol A (US2801227 and US3069434).

Alternative routes make use of allyl acetate as allylating agent in a Pd-catalyzed formation of the biallyl ether of Bisphenol A (WO96/20232), whereas other procedures use allyl halides and base instead of allyl acetate (US4740330). Chlorine-free and salt- free processes have also been proved to be effective via the direct nucleophilic substitution of allyl alcohol.

In any event, the Bisphenol A diglycidyl ether useful for the formulation of the present invention is liquid under ambient conditions, thus having a low molecular weight. In a particular embodiment, n ranges from 0 to 1, being preferred a Bisphenol A diglycidylether with a molecular weight lower than 700.

The liquid Bisphenol A diglycidyl ether is also commercially available, being supplied, for example, by Huntsman, Momentive or Dow Chemical. The aqueous dispersion of the liquid resin is simply prepared by dispersing the liquid resin in water. In a particular embodiment, the resin is dispersed in a proportion ranging from 40 to 65 wt% with respect to the total weight of the aqueous dispersion.

B) Glvcidyl ether

The glycidyl ether of formula (II) is an epoxy diluent which is added to the formulation to impair flexibility, to reduce viscosity and improve adhesion.

(II)

wherein: R is an aliphatic or cycloaliphatic radical of valency m;

R' is selected from H and methyl; and m is an integer from 1 to 3;

By the term "aliphatic radical" should be understood a straight or branched, saturated or unsaturated, hydrocarbon chain, interrupted in the chain by one or more oxygen or sulfur atoms or contains one or more carbonyl groups.

By the term "cycloaliphatic radical" should be understood a saturated or unsaturated hydrocarbon cycle, that contains one or more ring systems and may contain a carbonyl group, wherein said rings may be substituted by and/or linked through alkyl groups.

In a preferred embodiment, R is an aliphatic radical of formula:

wherein R" is H or a C₁-C3 alkyl group and n is an integer from 1 to 20.

More preferably, R" is H or methyl, and n ranges from 1 to 3.

In another preferred embodiment, the integer "m" is 1 or 2, more preferably Even more preferably is the use of a glycidyl ether of formula (Ha):

(Ila) wherein: R" is H or a C1-C3 alkyl group; and n is an integer from 1 to 20.

The term "C1-C3 alkyl group" refers to a linear or branched alkyl group having from one to three carbon atoms, such as methyl, ethyl, propyl and iso-propyl.

More preferably, R" is H or methyl, and n ranges from 1 to 3. It is preferred the use of poly ethylenegly col diglycidyl ether or polypropylenglycol diglycidyl ether as epoxy diluent, even more preferably polypropylenglycol diglycidyl ether.

The polypropylenglycol diglycidyl ether is an aliphatic epoxy resin compatible with Bisphenol A-based epoxy resins at all proportions and, upon curing, becomes an integral part of the cured resin.

In a preferred embodiment, the glycidyl ether of formula (Ila) has a molecular weight ranging from 145 to 1200, even more preferably n ranges from 1 to 20.

Such glycidyl ether is liquid under ambient conditions.

The glycidyl ether of formula (II) useful for the purpose of the present invention can be obtained by reaction of the corresponding alcohol [R-(OH)_m] with an epihalohydrin of formula (III):

(Ml)

in the presence of a catalyst, such as a Lewis acid, until a halohydrin ether intermediate is obtained. hen, the dehydrogenation of said intermediate is effected by contact with an inorganic base, generally an alkali metal hydroxide, such as sodium hydroxide. The glycols useful for this purpose are chosen among ethyleneglycol, propylenglycol, butylenglycol, etc.

However, the glycidyl ether can also be commercially available. For example, a polypropylenglycol diglycidyl ether is manufactured under the name DER 732 by the Dow Cheical Company. This liquid epoxy resin is a reaction product of epichlorohydrin and polypropylene glycol. It is a flexible, low viscosity, light color, epoxy resin which, in combination with Bisphenol A based epoxy resins, imparts flexibility, elongation and improves impact resistance. C) Latent curing agent

The term "latent curing agent" is widely used in the art to define a curing agent which is active at high temperatures, generally at a temperature higher than 50°C. This component is the responsible of the subsequent curing of the epoxy resins. The term "curing" has its ordinary meaning as known to those skilled in the art and may include polymerization and/or cross-linking of the epoxy resins which, in the case of the present invention, is performed by heating.

In the present document, the terms "curing agent" and "hardener" are used indistinctly.

In a preferred embodiment, the curing agent used in the formulation of the present invention is characterized for being water soluble as the formulation is a water base epoxy resin.

Examples of hardeners that can be used in the present invention include low molecular weight polyamide- and polyamine-based compounds, aromatic amines, solid epoxy amine adducts, and solid anhydrides.

Examples of low molecular weight polyamide- and polyamine-based compounds include dicyanodiamide (DICY), isophthalic dihydrazide, and adipic dihydrazide, BC1₃ amine complex or BF₃ amine complex.

Suitable aromatic amines include 4,4' diaminodiphenylsulfone and 3,3' diaminodiphenylsulfone.

Representative candidates for suitable solid anhydrides include trimel l it ic anhydride, pyromellitic dianhydridc 3,3',4,4'-Benzophenonetetracarboxylic dianhydride. maleic anhydride, giutaric anhydride, cis- 1 ,2-cyc!ohexane dicarboxylic anhydride, and 3,4,5,6 tetrahydrophtha! ic anhydride and mixtures thereof.

Other representative latent curing agents are solid epoxy amine adducts as those commercially available under the trade name AJICURE PN-23 and MY-24 from AJIMOTO or EPIKURE P-101 and P- 103 from MOMENTIVE which also act as curing accelerators.

In a preferred embodiment, the latent curing agent is a low molecular weight polyamide- and polyamine-based compound or an aliphatic amine curing agent. The choice of the proper curing agent is determined by its solubility in water and by its low reactivity at low temperature (i.e., a long pot life).

In a preferred embodiment, the latent curing agent is dicyanodiamide which has the following formula:

It is a dimer of cyanamide and it can be produced by treating cyanamide with a base. Formulations comprising dicyanodiamide as latent curing agent have excellent storage stability, but they should be heated at high temperature to allow the curing of the epoxy resin.

To improve or accelerate the progress of the subsequent curing reaction of the epoxy compounds, conventional accelerators such as ureas, imidazoles, urethans, biurets, allophanates, amides and lactames can be added to the formulation, said accelerators also may impair storage stability.

The amount of the cure accelerator to be added is selected to be within the range of the catalytic quantity to the epoxy components. In a particular embodiment, the amount of the cure accelerator to be added is desirably selected to be within the range of 0.01 to 1 moles per epoxy equivalent of the formulation.

The cure accelerator that may be used in the present invention remains intact in the cured conductive product obtained after heat curing. The amount of the epoxy-functional resins in the epoxy resin formulation can vary depending in part upon the intended application. In a typical embodiment, the epoxy- functional resins are present in an amount ranging from 45 to 95 percent by weight, based on the total weight of the components of the formulation excluding water. In the water base epoxy resin formulation according to the present invention, the ratio of the epoxy groups to the curing agent is selected so that the curing agent is within the equivalents ratio range between 0.7 and 1.2, more preferably between 0.8 and 1.1 equivalents, with respect to the epoxy equivalent of the epoxy resins. Thereby, the amount of unreacted curing agent remaining in the final cured product can be kept in a very low level.

Conductive fillers

In a particular embodiment of the invention, the water base epoxy resin formulation of the invention further comprises conductive fillers. Examples of conductive fillers may include, but are not limited to, metals and metal alloys, metal-coated particles, surface functionalized metals, conductive veils, non-metals, polymers and nano-scale materials. The morphology of the conductive fillers may include one or more of flakes, powders, particles, fibers and the like.

Metals and their alloys may be employed as effective conductive fillers, owing to their relatively high electrical conductivity. Examples of metals and alloys for use with embodiments of the present disclosure may include silver, gold, nickel, copper, aluminium and alloys and mixtures thereof. In certain embodiment, the morphology of the conductive filler may include one or more of flakes, powders, fibers, wires, microspheres and nanospheres, singly or in combination. In a particular embodiment, metals such as gold and silver are preferred due to their stability and effectiveness. Even more preferable is the use of silver.

In other embodiments, the conductive fillers may comprise metal coated particles. Examples of metal-coated particles include metal coated glass balloons, metal coated graphite, and metal-coated fibers. Examples of metals which may be used as substrates or coatings may include silver, gold, nickel, copper, aluminum, and mixtures thereof. Embodiments of non-metals suitable for use as conductive fillers may include conductive carbon black, graphite or carbon fiber.

Embodiments of nanomaterials suitable for use as conductive fillers may include carbon nanotubes, carbon nanofibers, metal nanofibers or metal nanowires, metal-coated nanofibers, metal nanoparticles and graphite in the form of nanoplatelets.

In a preferred embodiment of the invention, the conductive fillers are nanowires or nanofibers. Both terms are used indistinctly in the present disclosure. Examples of nanowires or nanofibers suitable for use as conductive fillers may include nickel, iron, silver, copper, aluminum and alloys thereof. More preferably, the conductive fillers are silver nanowires, more preferably anisotropic silver nanowires as silver is the most conductive metal.

The length of the nanowires may be greater than about 1 μιη, greater than about 5 μιη, greater than about 10 μιη, and about 10 to 25 μιη. The diameter of the nanowires may range from 20 to 150 nm. In a particular embodiment, said diameter is greater than about 10 nm, greater than about 40 nm, greater than about 70 nm, and about 80 to 100 nm.

Examples of silver nanowires may include SNW-A60, SNW-A90, SNW-A300 and SNW-A900 from Filigree Nanotech, Inc.

Due to the high aspect ratio of the nanowires and therefore low percolation threshold, a low concentration of nanowires is required to achieve good electrical conductivity in comparison to micro sized flakes and particles.

Furthermore, the presence of water in the epoxy resin formulation of the invention allows the entire and homogeneous dispersion of the conductive fillers within the resin formulation, thus also contributing to the reduction in the content of these fillers. In a preferred embodiment, the proportion of conductive fillers in the water base epoxy resin formulation ranges from 0.1 to 75 wt% with respect to the total weight of the resin formulation excluding water, more preferably from 0.5 to 45 wt%, even more preferably from 5 to 30 wt%. Other ingredients

The curable compositions of the present invention can include a variety of optional ingredients and/or additives that are usually employed in epoxy adhesives composition, such as pigments including carbon black or graphite, reinforcements, thixotropes, accelerators, surfactants, plasticizers, extenders, oligomers such as urethane and acrylates stabilizers, corrosion inhibitors, diluents, antioxidants, and chemical blowing agents.

Process Another aspect of the present invention relates to a process for the preparation of the water base epoxy resin formulation as defined above, said process comprising: a) mixing the liquid epoxy resin of Bisphenol A diglycidylether of formula (I) with the glycidyl ether of formula (II); b) adding the mixture resulting from step a) to an aqueous dispersion comprising a liquid epoxy resin Bisphenol A diglycidylether of formula (I); c) adding the water soluble hardener to the mixture resulting from step b).

Step a) of the process of the invention involves the mixture of the liquid epoxy resin of Bisphenol A diglycidylether of formula (I) with the glycidyl ether resin of formula (II). In a preferred embodiment, the glycidyl ether is mixed in a proportion ranging from 0.5 to 50 wt% with respect to the weight of the liquid epoxy resin of Bisphenol A diglycidyl ether (I).

The mixture is allowed to stir at room temperature untill a homogeneous blend is formed.

To the resulting mixture, an aqueous dispersion of a liquid epoxy resin Bisphenol A diglycidylether of formula (I) is added. Said aqueous dispersion is prepared by dispersing the liquid epoxy resin Bisphenol A diglycidylether in water in the amounts mentioned hereinbefore. In particular, the liquid epoxy resin is added to the water in a proportion ranging from 40 to 65 wt% with respect to the total weight of the dispersion. In a particular embodiment, the aqueous dispersion of the liquid epoxy resin of Bisphenol A diglycidylether of formula (I) is added to the resulting mixture so as the proportion of the liquid epoxy resin dispersed in the water ranges from 20 to 150 parts in weight per 100 parts of the liquid epoxy resin Bisphenol A diglycidylether of formula (I) used in step a) of the process of the invention, more preferably from 50 to 100 parts in weight.

The resulting mixture is also allowed to stir at room temperature till a homogenous aqueous dispersion is formed.

Subsequently, to the resulting mixture, the latent curing agent is added. In a particular embodiment, when the latent curing agent is water soluble, such as dicyandiamide, it is first dissolved in water and the aqueous solution formed is added to the mixture resulting from step b) of the process of the invention. In a preferred embodiment, the latent curing agent is added to the water in a proportion ranging from 10 to 50 wt% depending on the solubility of the curing agent. In a particular embodiment, the aqueous dispersion of the latent curing agent is added to the resulting mixture so as the proportion of the latent curing agent ranges from 5 to 70 parts in weight per 100 parts of the resins contained in the mixture obtained after conducting step b) of the process of the invention, more preferably from 10 to 50 parts in weight. In both particular cases, the mixture resulting from step c) is also stir till a homogenous aqueous dispersion of all components is obtained.

In the particular case when a cure accelerator is used, this can be added to the aqueous solution where the curing agent is dissolved, provided that said cure accelerator has an activation temperature higher than 100°C. If the cure accelerator has an activation temperature lower than 100°C, this is added in a subsequent step, once the water has been evaporated.

This process provides a water base epoxy resin formulation which is also a further aspect of the present invention.

In a particular embodiment, the process of the invention further comprises the addition of conductive fillers to the water base epoxy resin formulation obtained according to the process mentioned above to form a conductive water base epoxy resin formulation wherein the conductive fillers are homogeneously dispersed in said formulation.

In a preferred embodiment, the conductive fillers are previously dispersed in water and subsequently, the aqueous dispersion formed is added to the water base epoxy resin formulation.

Even more preferably, the conductive fillers are dispersed in the water in a proportion ranging from 2 to 25 wt%, more preferably at 15 wt%, with respect to the total weight of the aqueous dispersion.

In another preferred embodiment, the conductive fillers are nano wires or nano fibers selected from materials such as nickel, iron, silver, copper, aluminum and alloys thereof. In a more preferred embodiment, the conductive fillers are silver nanowires.

The use of the water base epoxy resin formulation of the invention as precursor of a conductive adhesive allows the entire conductive fillers to be evenly distributed within the epoxy resin, thus avoiding the formation of agglomerates and, therefore, enhancing the physical contact between said fillers even when lower amount of fillers is added.

Therefore, another aspect of the invention refers to the conductive water base epoxy resin formulation obtainable by the process as defined above, wherein the conductive fillers are homogeneously distributed within the formulation.

In another particular embodiment, the process of the invention further comprises a step of evaporation of at least 99.8 ± 0.2 wt% of the water contained in the resulting conductive water base epoxy resin formulation to form a single component conductive adhesive.

The water can be evaporated by any method known by a skilled person. Preferably, said method includes the use of rotatory vacuum evaporators, so as a more homogenous evaporation is obtained.

Actually, another aspect of the invention relates to the single component conductive adhesive obtainable by the process as defined above in which the water has been evaporated in the proportions mentioned above. The term "single-component" is used to refer to a formulation which comprises therein the hardener or curing agent and, therefore, it does not require the addition of the hardener or curing agent to be mixed- in just prior its use.

The single component conductive adhesive according to the present invention is a dispersion-type conductive adhesive wherein the conductive fillers, used as a conductive medium, are evenly dispersed in a single-component epoxy resin composition used as a binder resin component. The electric conduction paths between conductive fillers are constructed by the formation of dense physical contact between the fillers being bound to each other with the epoxy resin. The amount of the epoxy- functional resins in the curable composition can vary depending in part upon the intended application of the composition. In a typical embodiment, the epoxy-functional resins are present in an amount ranging from 45 to 95 percent by weight, based on the total weight of the curable composition.

In the conductive adhesive according to the present invention, the ratio of the epoxy groups to the curing agent is selected so that the curing agent is within the equivalents ratio range between 0.7 and 1.1 , more preferably between 0.8 and 0.95 equivalents, with respect to the epoxy equivalent of the epoxy resins. Thereby, the amount of unreacted curing agent remaining in the obtained cured product can be kept in a very low level.

In a particular embodiment, the content of the conductive fillers in the single component conductive adhesive of the invention is lower than 45 wt%, more preferably ranges from 0.5 to lower than 45 wt%, even more preferably from 5 to 30 wt%. Existing products load over 50% of silver fillers to achieve comparable results.

As mentioned before, the conductive fillers are preferably nanowires or nanofibers. More preferably, the conductive fillers are silver nanowires, more preferably anisotropic silver nanowires.

The use of silver nanowires in the proportions mentioned above provides formulations with conductivities ranging from 1 to lxlO^"3 ohm cm. As relatively low concentrations of nanowires are used in comparison to existing products, there is less detrimental impact on the mechanical and physical properties of the epoxy resins. Applications

The conductive adhesive according to the present invention is used by applying it onto the surface of one metal surface in a predetermined thickness, when a junction is formed therewith. Therefore, when applying, it is required to adjust the viscosity of the conductive adhesive within a predetermined viscosity range in order to maintain the film thickness of the applied film. For example, in such a case that the target thickness of the applied film is within the range of 100 μιη or less, as the applying method, a printing method using a metal mask or dispense applying method may be used, and its viscosity should be adjusted within the range suited to each of the applying method. After the conductive adhesive according to the invention is applied in a predetermined film thickness onto one surface of a metal forming junction, the other metal surface is arranged on the upper surface thereof, and then treatment for pressure welding is performed so as to achieve a dense contact between the metal surface and the conductive filler dispersed in the conductive adhesive. After that, heating treatment is carried out to cure the epoxy resin contained therein. In such a process, the adhesion between the metal surface and the obtained cured conductive layer, and the binding between the conductive fillers, are accomplished by the cured product of the epoxy resin that is formed therein.

The conductive joint fabricated using the conductive adhesive according to the present invention is excellent in long-term reliability in a high-temperature environment of at least 150°C, long-term reliability under high humidity conditions and long-term reliability in cooling-heating temperature cycle environment.

Therefore, the conductive adhesive of the present invention can be used as means for the surface mounting of electronic parts on a substrate circuit which are intended for in- vehicle electronic devices, modules and the like. It can also be used as a conductive adhesive suited for the purpose of substituting soldering, being capable of forming a conductive joint under a low-temperature condition, at least not exceeding 200°C, in the fabrication of electronic parts. Examples

Example 1

A mixture of 30 pbw (parts by weight) of liquid epoxy resin of Bisphenol A and 6 pbw of polypropyleneglycol diglycidyl ether was prepared. The resulting mixture was added to a reactor equipped with mechanical stirring and containing 30 pbw of an aqueous dispersion of a liquid epoxy resin of Bisphenol A (54% content of resin). Subsequently, and under stirring, 34.2 pbw of an aqueous solution of dicyanodiamide (10 wt%) were added to the reactor.

After homogenization of the resulting mixture, an aqueous dispersion of silver nano fibers was added. Then, the water was evaporated resulting in a nano fibers concentration of 44.6 wt% in the final formulation.

The evaporation of water was conducted in a rotary evaporator at a temperature of 45± 2 °C and at a pressure from 10 to 110 bar.

The resulting conductive formulation has a density of 2.1 g/cm³ and a viscosity at room temperature of 22000 ± 3000 Pa.s (at 0.01s^"1), 2000 ± 200 Pa.s (at 1 s^"1) and 400 ± 40 Pa.s (at 10 s^"1). After subjecting the formulation to a curing process during 30 minutes at 180°C, the resulting conductive adhesive has an electrical conductivity (bulk) of 2 ± 1 xlO^"3 ohm cm, a glass transition temperature of 80 ± 5 °C, thermal conductivity of 4.5 ± 0.5 W/mK, Young modulus of 4.5 ± 0.5 GPa and a coefficient of thermal expansion (CTE) of 75 ppm/°C (T<Tg) and of 152 ppm/°C.

Claims

1. A water base epoxy resin formulation comprising: a) the combination of a liquid epoxy resin of Bisphenol A diglycidylether of formula (I) and an aqueous dispersion of a liquid epoxy resin of Bisphenol A diglycidylether of formula (I):

(I)

(II)

wherein:

R is an aliphatic or cycloaliphatic radical of valency m;

The formulation according to claim 1, wherein the Bisphenol A diglycidylether of formula (I) has a molecular weight lower than 700.

The formulation according to any of claims 1 to 2, wherein the glycidyl ether of formula (II) is polypropylenglycol diglycidylether.

The formulation according to any of claims 1 to 3, wherein the latent curing agent is dicyandiamide.

5. The formulation according to any of claims 1 to 4, which further comprises conductive fillers.

6. The formulation according to claim 5, wherein the conductive fillers are silver nano wires. 7. The formulation according to any of claims 1 to 6, wherein the proportion of conductive fillers in the water base epoxy resin formulation ranges from 0.1 to 75 wt% with respect to the total weight of the resin formulation excluding water.

8. A process for the preparation of a water base epoxy resin formulation as defined in any one of claims 1 to 7, said process comprising: a) mixing a liquid epoxy resin of Bisphenol A diglycidylether of formula (I) with a glycidyl ether of formula (II); b) adding the mixture resulting from step a) to an aqueous dispersion comprising an epoxy resin Bisphenol A diglycidylether of formula (I); c) adding the latent curing agent to the mixture resulting from step b). 9. The process according to claim 8, which further comprises the addition of conductive nanoparticles to the mixture resulting from step c) to obtain a water base conductive epoxy resin formulation.

10. The process according to claim 9, which further comprises evaporation of at least 99.8 ± 0.2 wt% of the water contained in the resulting water base conductive epoxy resin formulation.

11. A water base epoxy resin formulation obtainable by the process as defined in claim 8.

12. A water base conductive epoxy resin formulation obtainable by the process as defined in claim 9. 13. A single-component conductive adhesive obtainable by the process as defined in claim 10.

14. The conductive adhesive according to claim 13, wherein the content of conductive fillers ranges from 5 to 45 wt% with respect to the total weight of the adhesive. Use of the conductive adhesive as defined in any of claims 13 to 14 for surface mount applications and for bonding electronic components.