WO1999003632A1 - A coating used for manufacturing and assembling electronic components - Google Patents

A coating used for manufacturing and assembling electronic components Download PDF

Info

Publication number
WO1999003632A1
WO1999003632A1 PCT/FI1998/000594 FI9800594W WO9903632A1 WO 1999003632 A1 WO1999003632 A1 WO 1999003632A1 FI 9800594 W FI9800594 W FI 9800594W WO 9903632 A1 WO9903632 A1 WO 9903632A1
Authority
WO
WIPO (PCT)
Prior art keywords
bismuth
solder
solder balls
interconnection method
contact areas
Prior art date
Application number
PCT/FI1998/000594
Other languages
French (fr)
Inventor
Jorma Kivilahti
Original Assignee
Jorma Kivilahti
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Jorma Kivilahti filed Critical Jorma Kivilahti
Publication of WO1999003632A1 publication Critical patent/WO1999003632A1/en

Links

Classifications

    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K3/00—Apparatus or processes for manufacturing printed circuits
    • H05K3/30—Assembling printed circuits with electric components, e.g. with resistor
    • H05K3/32—Assembling printed circuits with electric components, e.g. with resistor electrically connecting electric components or wires to printed circuits
    • H05K3/34—Assembling printed circuits with electric components, e.g. with resistor electrically connecting electric components or wires to printed circuits by soldering
    • H05K3/341—Surface mounted components
    • H05K3/3431—Leadless components
    • H05K3/3436—Leadless components having an array of bottom contacts, e.g. pad grid array or ball grid array components
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K35/00—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting
    • B23K35/02—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting characterised by mechanical features, e.g. shape
    • B23K35/0222—Rods, electrodes, materials, or media, for use in soldering, welding, or cutting characterised by mechanical features, e.g. shape for use in soldering, brazing
    • B23K35/0244—Powders, particles or spheres; Preforms made therefrom
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L23/00—Details of semiconductor or other solid state devices
    • H01L23/48—Arrangements for conducting electric current to or from the solid state body in operation, e.g. leads, terminal arrangements ; Selection of materials therefor
    • H01L23/488—Arrangements for conducting electric current to or from the solid state body in operation, e.g. leads, terminal arrangements ; Selection of materials therefor consisting of soldered or bonded constructions
    • H01L23/498—Leads, i.e. metallisations or lead-frames on insulating substrates, e.g. chip carriers
    • H01L23/49811—Additional leads joined to the metallisation on the insulating substrate, e.g. pins, bumps, wires, flat leads
    • H01L23/49816—Spherical bumps on the substrate for external connection, e.g. ball grid arrays [BGA]
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L23/00—Details of semiconductor or other solid state devices
    • H01L23/48—Arrangements for conducting electric current to or from the solid state body in operation, e.g. leads, terminal arrangements ; Selection of materials therefor
    • H01L23/488—Arrangements for conducting electric current to or from the solid state body in operation, e.g. leads, terminal arrangements ; Selection of materials therefor consisting of soldered or bonded constructions
    • H01L23/498—Leads, i.e. metallisations or lead-frames on insulating substrates, e.g. chip carriers
    • H01L23/49866—Leads, i.e. metallisations or lead-frames on insulating substrates, e.g. chip carriers characterised by the materials
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F2998/00—Supplementary information concerning processes or compositions relating to powder metallurgy
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B23—MACHINE TOOLS; METAL-WORKING NOT OTHERWISE PROVIDED FOR
    • B23K—SOLDERING OR UNSOLDERING; WELDING; CLADDING OR PLATING BY SOLDERING OR WELDING; CUTTING BY APPLYING HEAT LOCALLY, e.g. FLAME CUTTING; WORKING BY LASER BEAM
    • B23K2101/00—Articles made by soldering, welding or cutting
    • B23K2101/36—Electric or electronic devices
    • B23K2101/38—Conductors
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L2924/00—Indexing scheme for arrangements or methods for connecting or disconnecting semiconductor or solid-state bodies as covered by H01L24/00
    • H01L2924/0001—Technical content checked by a classifier
    • H01L2924/0002—Not covered by any one of groups H01L24/00, H01L24/00 and H01L2224/00
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K2201/00—Indexing scheme relating to printed circuits covered by H05K1/00
    • H05K2201/10—Details of components or other objects attached to or integrated in a printed circuit board
    • H05K2201/10613—Details of electrical connections of non-printed components, e.g. special leads
    • H05K2201/10954—Other details of electrical connections
    • H05K2201/10992—Using different connection materials, e.g. different solders, for the same connection
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K2203/00—Indexing scheme relating to apparatus or processes for manufacturing printed circuits covered by H05K3/00
    • H05K2203/04—Soldering or other types of metallurgic bonding
    • H05K2203/041—Solder preforms in the shape of solder balls
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K2203/00—Indexing scheme relating to apparatus or processes for manufacturing printed circuits covered by H05K3/00
    • H05K2203/04—Soldering or other types of metallurgic bonding
    • H05K2203/0435—Metal coated solder, e.g. for passivation of solder balls
    • H—ELECTRICITY
    • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
    • H05K3/00—Apparatus or processes for manufacturing printed circuits
    • H05K3/30—Assembling printed circuits with electric components, e.g. with resistor
    • H05K3/32—Assembling printed circuits with electric components, e.g. with resistor electrically connecting electric components or wires to printed circuits
    • H05K3/34—Assembling printed circuits with electric components, e.g. with resistor electrically connecting electric components or wires to printed circuits by soldering
    • H05K3/3457—Solder materials or compositions; Methods of application thereof
    • H05K3/3463—Solder compositions in relation to features of the printed circuit board or the mounting process
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
    • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

  • the invention relates to metallic coating that can be employed for manufacturing electronic components and joining of the components fluxlessly onto various substrates.
  • electrically conductive contact areas (Cu and/or Ni) are coated first with 10-50 ⁇ m thick tin or with an appropriate tin-based alloy coating (undercoating). On this coating a thin 1-10 ⁇ m, preferably 1-3 ⁇ m thick layer of pure bismuth (over or topcoating) is deposited by chemical or electrochemical process.
  • the oxygen-protected irreversible melting which initiates at the metallic interface between top and bottom coatings and the transfusion of the liquids is achieved and thereby fluxless soldering of the materials of the joinable components which may be, for exmaple, active- or passive components, printed wiring boards (PWB) or their parts.
  • the amount of the liquid as well as the composition of the joint is controlled by adjusting the thickness of the coating (Bi) and the temperature of joining.
  • Bismuth which is itself a weak oxidizer and can form the water-soluble oxide protects effectively the contact surfaces to be joined against oxidation and so makes possible the fluxless soldering.
  • This protective charasteristic of bismuth has been also employed as an additional benefit among its other useful effects.
  • a thin oxide layer on the top of bismuth can be removed, for example, by washing it in water or steam. It's also essential that the interface between thin topcoating (Bi) and undercoating (Sn, SnPb, eutectic SnAg or SnAgCu) is oxide- free (i.e. metallic).
  • Joining technologies in which the fluxlessness would be especially useful are for example the joining of active and passive components with a paste printing technique on printed circuit boards, the attachment of bare chips with flip chip technique or the joining of solder balls of the surface mount packages (e.g. BGA- or CSP-package) on the interposer and the attachment of the package itself on the substrate, which is generally printed wiring board.
  • SMT surface mount
  • solder balls in the BGA-packages is generally 500-600 ⁇ m.
  • Solder balls can be used also for interconnecting bare chip to the interposer of surface mount package (BGA or CSP) or directly on substrate. Then the diameter of solder balls can be about 100 ⁇ m or even smaller.
  • the drawback of the interconnection methods according to the above mentioned technology of the known art is the need to use flux in soldering, and especially in the case of so-called high temperature solders (like SnAg, SnAgSb, SnAgCu and Pb-based alloys) the need to employ unconventionally high soldering temperatures as well as to use expensive protective metallisations which produce known problems and additional costs.
  • so-called high temperature solders like SnAg, SnAgSb, SnAgCu and Pb-based alloys
  • the object of the present invention is to provide a new fluxless interconnection technique which is based especially on the utilization of solder balls and which is without the drawbacks of the interconnection techniques of the known art.
  • the technique can be used also for interconnecting passive and active components on substrate even without solder balls, when the corresponding coatings are deposited directly on to the mating contact areas.
  • the solder balls are transferred to the contact areas mixed with thermoplastic or thermosetting polymer as so-called fluxless paste or adhesive. If the adhesive in question is spread over the whole substrate with a silk-print-technique or as a tape, it will be a new kind of anisotropically conducted adhesive.
  • solder balls which are either pure tin or so-called low temperature alloys like SnPb-alloys or high temperature alloys like the eutectic SnAg, SnAgSb or SnAgCu and the electrically conductive contact areas, which are precoated with the corresponding solder ball material are coated with a thin bismuth layer.
  • Bismuth layer can be produced with a simple chemical coating method and the price of bismuth is only a fraction of the price of gold. Furthermore, one of the advantages of bismuth is its lower degree of nobility in relation to the noble metal coatings. Then, for example, the scratches penetrating the topcoatings do not cause marked galvanic corrosion of the contact surfaces during the storage. Even a minor amount (about 0.5 wt-%) of bismuth dissolved from the coating eliminates the possibility of tin pest and increases the strength of tin.
  • Figure 1(a) presents the phase diagram of the ternary SnPbBi system at the temperature of 91°C and Figure 1(b) presents the phase diagram of the ternary SnAgBi system at 150 °C.
  • the contact lines are depicted in both phase diagrams. These lines connect pure bismuth to the binary solder alloy with which it is in contact; the low temperature eutectic SnPb solder alloy in Figure 1(b) and the high temperature eutectic SnAg solder alloy in Figure 1(b).
  • the contact line points out how the relative amounts of different components will alter when bismuth dissolves into either of the solder alloys at the temperatures represented by the phase diagrams.
  • Figure 2 presents schematically with the help of the cross-sectional images the different stages of the formation of the joints produced with the technique of the present invention.
  • the interconnection technique of the present invention it is essential for successful application of the interconnection technique of the present invention to use - in addition to the controlled bismuth deposition - the right thickness of the bismuth layer and the temperature of joining with which one controls the amount and composition of so-called primary liquid produced by transfusing bismuth coating and the underlying solder alloy and thereby also mechanical properties of the interconnections. If the bismuth layer is in the case of small solder balls too thick, too much bismuth which may dissolve during the soldering and especially during possible subsequent solderings will change mechanical and thermodynamic properties, for example, the melting point of solder balls in an undesirable manner. If the solder ball is e.g.
  • the critical content of bismuth which depends on the Sn/Pb-ratio, can be read from the ternary phase diagram at the temperature of 91°C shown in Figure 1(a). If the nominal composition point of the alloy locates inside the so-called four-phase triangle ABC in Figure 1(a), the solder alloy begins to melt during the heating already above about 91°C. This may cause problems in use due to the risk of melting of the interconnections. A problem may also arise from the reduced toughness of the SnPbBi alloy owing to too high bismuth content.
  • the composition of the solder ball does not exceed the dotted line DE denoted by the arrows. Then at this temperature the microstructure of the solder alloy is composed of tin- and lead-rich crystals, which contain different amounts of Bi-precipitates.
  • solder ball is, for example, the eutectic SnAg alloy, whose melting point is 221°C
  • the coating thickness corresponding to the critical bismuth content of the joint can be calculated in the same manner by using, for example, the phase diagram of Figure 1(b), which represents the equilibrium structures of SnAgBi alloy system at the temperature of 150°C.
  • the bismuth content of the SnAgBi joint should be kept below six atomic percentages (i.e. 10 wt-%) for the risk of weakening mechanical properties.
  • r s b is the radius of solder ball
  • tni is the thickness of bismuth coating
  • x B j is the nominal bismuth content (mole-fraction) of an alloy.
  • the Sn/Pb-ratio of an alloy affects through the molar volumes of the elements relatively weakly the predicted thickness of bismuth layer. Instead, the Sn Pb-ratio of the solder alloy influences the value of the maximum allowed bismuth content - in accordance with the Figure 1(a).
  • the thickness of bismuth layer can be at most 15 ⁇ m according to the Figure 1(a).
  • the bismuth layer can be at most about 2.5 ⁇ m thick.
  • the bismuth coating can be at most about 8 ⁇ m thick. Since the solubility of bismuth in both Sn- and Pb-rich crystals changes with temperature, the thickness of bismuth coating changes also with temperature. Correspondingly, the information included in Figure
  • 1(b) can be used for calculating maximum allowed thicknesses of bismuth coatings, for example, in the case of Sn3.5Ag solder balls.
  • Figure 2 (a) presents the situation before heating to so-called lower joining temperature T a , at which the interfacial regions between the bismuth coatings and the underlying tin alloys start to melt and liquids to fuse.
  • T a i.e. above the melting point of "surface alloy” formed by bismuth and tin alloy.
  • the liquid alloy layer has formed around the core of solid tin alloy as well as on the top of contact area; the amounts that can form from bismuth coating (t Bi ) of a definitive thickness.
  • the thickness of the liquid layer Tu q (per unit area) can be calculated by using the following approximate formula
  • t B j is the thickness of solid bismuth coating
  • v B j and x Sn are the partial molar volumes of bismuth and tin, respectively
  • x B j and x Sn are the mole-fractions of bismuth and tin in the liquid.
  • the mole-fractions of the components depend strongly on temperature. If, for example, the bismuth thickness on tin-rich alloy is 1 ⁇ m, then the thicknesses of the liquid layers at 180°C and 200°C are about 3.5 ⁇ m and 5 ⁇ m, respectively.
  • composition wt-% variation preferably
  • the amounts of the metanosulphonic acid and solderon BR-starter depend on the surface to be coated as well as its state of oxidation. Bismuth and metanosulphonic acid are mixed and let to stay for 2-10 days for producing Bi-ions from the metal. The other substances are mixed in the order giving in Table 1, and the electrolyte is allowed to age for a few days.
  • the contact surface to be coated is cleaned before the coating, for example, with acetone, after which it is emersed into the coating solution for about 0.5 - 1 minutes. After the coating the object is cleaned with distilled water.

Abstract

The invention relates to a metallic coating, which can be used, for example, for manufacturing electronic components and for interconnecting irreversibly the components into various substrates by employing either low temperature or high temperature solders. It is characteristic for the invention that the coating enables to interconnect electronic components fluxlessly. The invention relates especially to the interconnection technique which is used for example for interconnecting bare chips to the interposer of surface mount package or directly on substrate like printed wiring board or interconnecting surface mount package on printing wiring board. The object of the present invention is in particular the interconnection technique in which the electrolytically deposited solder bumps on electrical contact pads are replaced with self-contained solder balls which make the alloys selection of the bumps more versatile, simpler and cheaper to accomplish. It is in accordance with the present invention to coat solder balls and tin ortin-alloy precoated contact surfaces with the bismuth layer of appropriate thickness. It is also in accordance with the present invention to place bismuth-coated solder particles to contact areas mixed in organic solvent as a paste or as an anisotropically conducted adhesive.

Description

A coating used for manufacturing and assembling electronics components
The invention relates to metallic coating that can be employed for manufacturing electronic components and joining of the components fluxlessly onto various substrates.
Manufacturing of electrically functional and reliable solder joint implies that a molten metal or alloy wets adequately metallic contact surfaces to be joined. Almost all the metals and alloys form immediately a thin and chemically stable oxide layer on their surfaces. It is known that this oxide layer makes it more difficult for solder connection to form by weakening the wettability of the conductor surfaces by molten solder and thereby the chemical bonding of the solder to the etanols that are to be connected. For improving the wettability the contact surfaces are generally processed with a chemically reactive flux, which at the bonding temperature reduces oxide layers and so improves the wettability. For reducing the oxide layers of the strongly oxidizing connector metals such as Al, Ni and Cu and also those of solder alloys such as Sn-based alloys strong fluxing is usually needed. The residues of the fluxes have to be washed away after joining, but even after the cleaning operation there are small amounts of residues that increase the risk of corrosion. The situation has become more complicated due to the fact that in the cleaning process one has to abandon the usage of the efficient fluoridated hydrocarbon solvents for environmental reasons. Therefore one attempts to utilise increasingly milder, so-called no-clean fluxes, even though they also leave harmful comtamination on contact areas. The oxidization of the contact areas to be joined is attempted to prevent by coating them with noble metals like gold or palladium.
In the Finnish patent application 945101 is presented the method which enables to produce soldered joints fluxlessly in microelectronics. In the method electrically conductive contact areas (Cu and/or Ni) are coated first with 10-50 μm thick tin or with an appropriate tin-based alloy coating (undercoating). On this coating a thin 1-10 μm, preferably 1-3 μm thick layer of pure bismuth (over or topcoating) is deposited by chemical or electrochemical process. In this manner the oxygen-protected irreversible melting which initiates at the metallic interface between top and bottom coatings and the transfusion of the liquids is achieved and thereby fluxless soldering of the materials of the joinable components which may be, for exmaple, active- or passive components, printed wiring boards (PWB) or their parts. The amount of the liquid as well as the composition of the joint is controlled by adjusting the thickness of the coating (Bi) and the temperature of joining. Thus it is necessary for fluxless joining that the melting begins below the bismuth coatings and proceeds rapidly by the transfusion of the liquids which are protected against oxidation.
Bismuth, which is itself a weak oxidizer and can form the water-soluble oxide protects effectively the contact surfaces to be joined against oxidation and so makes possible the fluxless soldering. This protective charasteristic of bismuth has been also employed as an additional benefit among its other useful effects. A thin oxide layer on the top of bismuth can be removed, for example, by washing it in water or steam. It's also essential that the interface between thin topcoating (Bi) and undercoating (Sn, SnPb, eutectic SnAg or SnAgCu) is oxide- free (i.e. metallic).
In electronics there are several applications, in which the achievement of fluxless soldering would be a significant improvement as compared with known assembly technologies. Joining technologies, in which the fluxlessness would be especially useful are for example the joining of active and passive components with a paste printing technique on printed circuit boards, the attachment of bare chips with flip chip technique or the joining of solder balls of the surface mount packages (e.g. BGA- or CSP-package) on the interposer and the attachment of the package itself on the substrate, which is generally printed wiring board. In the attachment of surface mount (SMT) components electrical contact areas are often protected against oxidation by coating them with a thin noble metal layer, but the attachment itself occurs with the help of flux-bearing paste, which implies careful post-cleaning.
In addition to the benefits of fluxless soldering the difficulties related especially to the usage of so called high temperature solders (SnAg, SnAgSp, SnAgCu, Pb-based alloys, etc) are diminished due to the irreversible decrease of the melting temperature of the interface between the bismuth coating and undercoating. Owing to the irreversible nature of the phenomenon operational temperatures can be higher than the joining temperatures without the formation of liquid in the joint. This is useful, for example, for different applications in automobile industry, where the operational temperatures of electronics can be high. The latest development in the above mentioned production and interconnection methods of component packages is to transfer solder metal into contact areas as separate solder balls instead of previously used electrolytically deposited solder bumps. Then, in relation to the packaged surface mount components, one speaks, for example, about BGA- (Ball Grid Array), micro-BGA- or CSP (Chip Scale Package)-techniques and in the case of bare chip C4- or Flip Chip-technique. For example, the diameter of solder balls in the BGA-packages is generally 500-600 μm. Solder balls can be used also for interconnecting bare chip to the interposer of surface mount package (BGA or CSP) or directly on substrate. Then the diameter of solder balls can be about 100 μm or even smaller.
The drawback of the interconnection methods according to the above mentioned technology of the known art is the need to use flux in soldering, and especially in the case of so-called high temperature solders (like SnAg, SnAgSb, SnAgCu and Pb-based alloys) the need to employ unconventionally high soldering temperatures as well as to use expensive protective metallisations which produce known problems and additional costs.
The object of the present invention is to provide a new fluxless interconnection technique which is based especially on the utilization of solder balls and which is without the drawbacks of the interconnection techniques of the known art. The technique can be used also for interconnecting passive and active components on substrate even without solder balls, when the corresponding coatings are deposited directly on to the mating contact areas. It is also the embodiment of the present invention that the solder balls are transferred to the contact areas mixed with thermoplastic or thermosetting polymer as so-called fluxless paste or adhesive. If the adhesive in question is spread over the whole substrate with a silk-print-technique or as a tape, it will be a new kind of anisotropically conducted adhesive.
The present invention are characterised by what is stated in the novelty part of the first and sixth claims.
In the interconnection technique of the present invention solder balls which are either pure tin or so-called low temperature alloys like SnPb-alloys or high temperature alloys like the eutectic SnAg, SnAgSb or SnAgCu and the electrically conductive contact areas, which are precoated with the corresponding solder ball material are coated with a thin bismuth layer.
Bismuth layer can be produced with a simple chemical coating method and the price of bismuth is only a fraction of the price of gold. Furthermore, one of the advantages of bismuth is its lower degree of nobility in relation to the noble metal coatings. Then, for example, the scratches penetrating the topcoatings do not cause marked galvanic corrosion of the contact surfaces during the storage. Even a minor amount (about 0.5 wt-%) of bismuth dissolved from the coating eliminates the possibility of tin pest and increases the strength of tin.
Figure 1(a) presents the phase diagram of the ternary SnPbBi system at the temperature of 91°C and Figure 1(b) presents the phase diagram of the ternary SnAgBi system at 150 °C. The contact lines are depicted in both phase diagrams. These lines connect pure bismuth to the binary solder alloy with which it is in contact; the low temperature eutectic SnPb solder alloy in Figure 1(b) and the high temperature eutectic SnAg solder alloy in Figure 1(b).
The contact line points out how the relative amounts of different components will alter when bismuth dissolves into either of the solder alloys at the temperatures represented by the phase diagrams. Figure 2 presents schematically with the help of the cross-sectional images the different stages of the formation of the joints produced with the technique of the present invention.
In the following different ways of applying the interconnection method of the present invention is described in detail with the reference to Figures 1 and 2.
It is essential for successful application of the interconnection technique of the present invention to use - in addition to the controlled bismuth deposition - the right thickness of the bismuth layer and the temperature of joining with which one controls the amount and composition of so-called primary liquid produced by transfusing bismuth coating and the underlying solder alloy and thereby also mechanical properties of the interconnections. If the bismuth layer is in the case of small solder balls too thick, too much bismuth which may dissolve during the soldering and especially during possible subsequent solderings will change mechanical and thermodynamic properties, for example, the melting point of solder balls in an undesirable manner. If the solder ball is e.g. eutectic tin-lead alloy, the melting point of which is 183°C and which is suitable for relatively low temperature applications, the critical content of bismuth, which depends on the Sn/Pb-ratio, can be read from the ternary phase diagram at the temperature of 91°C shown in Figure 1(a). If the nominal composition point of the alloy locates inside the so-called four-phase triangle ABC in Figure 1(a), the solder alloy begins to melt during the heating already above about 91°C. This may cause problems in use due to the risk of melting of the interconnections. A problem may also arise from the reduced toughness of the SnPbBi alloy owing to too high bismuth content. Therefore, it is beneficial that the composition of the solder ball does not exceed the dotted line DE denoted by the arrows. Then at this temperature the microstructure of the solder alloy is composed of tin- and lead-rich crystals, which contain different amounts of Bi-precipitates.
If solder ball is, for example, the eutectic SnAg alloy, whose melting point is 221°C, the coating thickness corresponding to the critical bismuth content of the joint can be calculated in the same manner by using, for example, the phase diagram of Figure 1(b), which represents the equilibrium structures of SnAgBi alloy system at the temperature of 150°C. On the other hand, the bismuth content of the SnAgBi joint should be kept below six atomic percentages (i.e. 10 wt-%) for the risk of weakening mechanical properties.
For the determination of the critical bismuth layer thickness or preferable maximum layer thickness one can use the following approximate equation:
LBi - 1
1 - x lsb
Bi
where rsb is the radius of solder ball, tni is the thickness of bismuth coating and xBj is the nominal bismuth content (mole-fraction) of an alloy.
In the case of SnPb solder the Sn/Pb-ratio of an alloy affects through the molar volumes of the elements relatively weakly the predicted thickness of bismuth layer. Instead, the Sn Pb-ratio of the solder alloy influences the value of the maximum allowed bismuth content - in accordance with the Figure 1(a).
In order to prevent the melting point of SnPb particle of the diameter 600 μm from decreasing to the lowest possible melting point of the SnPbBi alloy (about 91°C), the thickness of bismuth layer can be at most 15 μm according to the Figure 1(a). Correspondingly, on the top of the particle having the diameter of 100 μm the bismuth layer can be at most about 2.5 μm thick.
More accurate analysis gives only slightly smaller value. If it is wanted that there is no intermetallic τ-phase in a 600 μm particle due to the dissolved bismuth at the temperature of
Figure 1(a), the bismuth coating can be at most about 8 μm thick. Since the solubility of bismuth in both Sn- and Pb-rich crystals changes with temperature, the thickness of bismuth coating changes also with temperature. Correspondingly, the information included in Figure
1(b) can be used for calculating maximum allowed thicknesses of bismuth coatings, for example, in the case of Sn3.5Ag solder balls.
Figure 2 (a) presents the situation before heating to so-called lower joining temperature Ta , at which the interfacial regions between the bismuth coatings and the underlying tin alloys start to melt and liquids to fuse. In the case of Figure 2 (b) the temperature has increased to Ta, i.e. above the melting point of "surface alloy" formed by bismuth and tin alloy. At this temperature the liquid alloy layer has formed around the core of solid tin alloy as well as on the top of contact area; the amounts that can form from bismuth coating (tBi) of a definitive thickness. The thickness of the liquid layer Tuq (per unit area) can be calculated by using the following approximate formula
-Liq )
Figure imgf000008_0001
where tBj is the thickness of solid bismuth coating, vBj and xSn are the partial molar volumes of bismuth and tin, respectively, and xBj and xSn are the mole-fractions of bismuth and tin in the liquid. Among these quantities especially the mole-fractions of the components depend strongly on temperature. If, for example, the bismuth thickness on tin-rich alloy is 1 μm, then the thicknesses of the liquid layers at 180°C and 200°C are about 3.5 μm and 5μm, respectively.
During the melting of surface alloys the liquids fuse rapidly - first locally over relatively small contact area and then over rapidly expanding area driven by the pressure difference of curved interfaces. If the temperature of the particle-substrate system is now allowed to decrease, the liquid joint region solidifies and forms solid joint. In the case of Figure 2 (c) temperature has been increased (from the case of Figure 2 (b)) to the so-called upper joining temperature, Tu (above the melting point of the alloy particle of the original composition), at which the whole particle as well as the coating on the contact area are molten. Then the liquid particle spreads rapidly over the surface of molten contact area.
When deposited correctly chemically or galvanically an oxide-free metallic interface is formed between the bismuth coating and the undercoating. The melting initiates immediately there, when the temperature is higher than the melting point of the region. If, for example, BGA- package, to which the solder balls have been connected fluxlessly, will be interconnected fluxlessly also, for example, to the printed wiring board, the BGA-bumps can be recoated with bismuth. The same procedure can be employed, for example, for bumping unpackaged integrated circuit (IC) and for interconnecting bumped bare IC fluxlessly on substrate (e.g. PWB) e.g. with the flip chip technique.
It's according to the present invention to use water-based electrolyte in the coating process, the composition of which is given in Table 1 :
Table I
Composition, wt-% variation preferably
Bi 0.5- 3 2
Bi2θ3 0.5- 4 3
Metanosulphonic acid 3-25 20
Gelatine 1-10 5
Metanol 2-10 ml/1 10 ml/1
Solderon BR-starter 2-20 ml/1 10 ml/1
The amounts of the metanosulphonic acid and solderon BR-starter depend on the surface to be coated as well as its state of oxidation. Bismuth and metanosulphonic acid are mixed and let to stay for 2-10 days for producing Bi-ions from the metal. The other substances are mixed in the order giving in Table 1, and the electrolyte is allowed to age for a few days. The contact surface to be coated is cleaned before the coating, for example, with acetone, after which it is emersed into the coating solution for about 0.5 - 1 minutes. After the coating the object is cleaned with distilled water.

Claims

Claims:
1. Fluxless interconnection method of electronic active and passive components, where the joining occurs with the help of solder balls, which are placed between electrical contact areas and which are melted them by heating to the soldering temperature wherein the interconnection method comprises that both the solder balls and the solder precoated contact areas are coated with a thin layer of bismuth.
2. An interconnection method according to the claim 1 wherein the thicknesses and joining temperatures of the bismuth coatings of solder balls and those of solder precoated contact areas are such that the bismuth content of the solder (or solders) does not exceed its critical value - depending on the the composition and and the diameter of solder ball - when the bismuth coatings dissolve during soldering either into the liquid surface layer of each solder ball having solid interior or into fully molten solder balls.
3. An interconnection method according to one of the claims 1 - 2 wherein the bismuth coating is produced in the bath having the following chemical composition:
Composition, wt-% range of variation
Bi 0.5- 3
Bi2╬╕3 0.5- 4
Metanosulphonic acid 3-25
Gelatine 1-10
Metanol 2-20 ml/1
Solderon BR-starter 2-20 ml/1
4. An interconnection method according to one of the claims 1-2 wherein the bismuth coating is produced in the bath the chemical composition of which is the following Composition. weight- %
Bi 2
Bi2╬╕3 3
Metanosulphonic acid 20
Gelatine 5
Metanol 10 ml/1
Solderon BR-starter 10 ml/1
5. An interconnection method according to one of the claims 1-2 wherein bismuth-coated solder balls are transferred to contact areas mixed with organic solvent (matrix) as an paste or as an anisotropically conducted adhesive.
6. A fluxless interconnection method of electronic components, where the joining occurs with the help of solder balls, which are placed between electrical contact- areas and which are melted by heating them to the soldering temperature wherein high temperature solder balls, for example eutectic SnAg-, SnAgSb-, SnAgCu-alloy or lead- rich alloy, and solder pre-coated contact areas have been coated with a thin layer of bismuth and soldered together at the temperatures which are significantly lower than the melting tempratures of the alloys.
7. An interconnection method according to the claim 6 wherein the thicknesses of the bismuth coatings of solder balls and those of solder precoated contact areas are such that the bismuth content of the solder does not exceed its critical value - depending on the the composition and the diameter of the solder ball - when the bismuth coatings dissolve during soldering either into the liquid surface layer of each solder ball having solid interior or into fully molten solder balls
PCT/FI1998/000594 1997-07-14 1998-07-14 A coating used for manufacturing and assembling electronic components WO1999003632A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
FI972982 1997-07-14
FI972982A FI972982A (en) 1997-07-14 1997-07-14 A coating used in the manufacture and assembly of electronic components

Publications (1)

Publication Number Publication Date
WO1999003632A1 true WO1999003632A1 (en) 1999-01-28

Family

ID=8549252

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/FI1998/000594 WO1999003632A1 (en) 1997-07-14 1998-07-14 A coating used for manufacturing and assembling electronic components

Country Status (2)

Country Link
FI (1) FI972982A (en)
WO (1) WO1999003632A1 (en)

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US7276296B2 (en) * 2003-05-01 2007-10-02 International Business Machines Corporation Immersion plating and plated structures

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3429040A (en) * 1965-06-18 1969-02-25 Ibm Method of joining a component to a substrate
WO1996013353A1 (en) * 1994-10-28 1996-05-09 Jorma Kivilahti A method for joining metals by soldering
EP0747954A2 (en) * 1995-06-07 1996-12-11 International Business Machines Corporation Reflowed solder ball with low melting point metal cap

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3429040A (en) * 1965-06-18 1969-02-25 Ibm Method of joining a component to a substrate
WO1996013353A1 (en) * 1994-10-28 1996-05-09 Jorma Kivilahti A method for joining metals by soldering
EP0747954A2 (en) * 1995-06-07 1996-12-11 International Business Machines Corporation Reflowed solder ball with low melting point metal cap

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US7276296B2 (en) * 2003-05-01 2007-10-02 International Business Machines Corporation Immersion plating and plated structures

Also Published As

Publication number Publication date
FI972982A (en) 1999-01-15
FI972982A0 (en) 1997-07-14

Similar Documents

Publication Publication Date Title
US6307160B1 (en) High-strength solder interconnect for copper/electroless nickel/immersion gold metallization solder pad and method
Kang et al. Lead (Pb)-free solders for electronic packaging
EP0629463B1 (en) Lead-free, high-temperature, tin based multi-component solder
US6010060A (en) Lead-free solder process
US5411703A (en) Lead-free, tin, antimony, bismtuh, copper solder alloy
EP0629467B1 (en) Lead free, tin-bismuth solder alloys
US5730932A (en) Lead-free, tin-based multi-component solder alloys
US5414303A (en) Lead-free, high tin, ternary solder alloy of tin, bismuth, and indium
US5075965A (en) Low temperature controlled collapse chip attach process
KR100321837B1 (en) Pb-in-sn tall c-4 for fatigue enhancement
JP2960017B2 (en) Solder paste composition
US7344061B2 (en) Multi-functional solder and articles made therewith, such as microelectronic components
JPH0788680A (en) Composition of high-temperature lead-free tin based solder
US6127731A (en) Capped solder bumps which form an interconnection with a tailored reflow melting point
US6214131B1 (en) Mixed solder pastes for low-temperature soldering process
GB2297507A (en) Electrical interconnection assembly
WO1999003632A1 (en) A coating used for manufacturing and assembling electronic components
JPH0985484A (en) Lead-free solder and packaging method using the same and packaged articles
Korhonen et al. Interconnections based on Bi-coated SnAg solder balls
JP3592054B2 (en) Electronic circuit and manufacturing method thereof
JP3460442B2 (en) Lead-free solder and mounted products using it
Kulojärvi et al. A new under bump metallurgy for solder bump flip chip applications
WO1997047425A1 (en) Lead-free, high tin ternary solder alloy of tin, silver, and bismuth

Legal Events

Date Code Title Description
AK Designated states

Kind code of ref document: A1

Designated state(s): CN JP US

AL Designated countries for regional patents

Kind code of ref document: A1

Designated state(s): AT BE CH CY DE DK ES FI FR GB GR IE IT LU MC NL PT SE

121 Ep: the epo has been informed by wipo that ep was designated in this application
122 Ep: pct application non-entry in european phase
NENP Non-entry into the national phase

Ref country code: JP

Ref document number: 1999506504

Format of ref document f/p: F