Most electrical conductors used in electronic devices are made of metals, such as copper, aluminum, gold, silver, lead/tin (solder), molybdenum and others. Solder connection technology using lead/tin alloys plays a key role in various levels of electronic packaging, such as flip-chip connections (i.e., C4), solder-ball connection in ball-grid-arrays (BGA), and IC package connection to a printed circuit board (PCB). Solder joints produced in the electronic packages serve both as electrical interconnections and mechanical/physical connections. When either of those functions is not achieved, the solder joint is considered to have failed.
When microelectronic packages are assembled to a printed circuit board, a lead-tin eutectic solder, 63% Sn-37% Pb, having the lowest melting point (183.degree. C.) among Pb--Sn alloys, is most widely used. In such applications, there are two solder connection technologies employed for mass production: plated-through-hole (PTH) and surface mount technology (SMT) soldering. The basic difference between the two technologies originates from the difference in the PCB design and its interconnection scheme.
In SMT soldering, microelectronic packages are directly attached to the surface of a PCB. A major advantage of SMT is high packaging density, which is realized by eliminating most PTHs in the PCB as well as by utilizing both surfaces of the PCB to accommodate components. In addition, SMT packages have a finer lead pitch and a smaller package size compared to traditional PTH packages. Hence, SMT has contributed significantly to reducing the size of electronic packages.
In SMT soldering, solder paste is applied to a PCB by screen printing. Solder paste consists of fine solder powder, flux, and organic vehicles. During a reflow process, solder particles are melted, flux is activated, solvent materials are evaporated and simultaneously, molten solder coalesces and is eventually solidified. In contrast, in the wave soldering process, a PCB is first fluxed and components are mounted thereon. Then the PCB is moved over a wave of molten solder.
The soldering process is usually completed by subjecting the solder joints to a cleaning step to remove residual flux materials. Due to environmental concerns, CFCs (chlorofluorocarbons) and other harmful cleaning agents used for this purpose are being eliminated. Water-soluble or no-clean flux materials are being used to minimize or eliminate the cleaning steps.
Recent advances in microelectronic devices demand a very fine pitch connection between electronic packages and a printed circuit board (on an order of a few hundred micrometer pitch). The current solder paste technology used in SMT cannot handle this very fine pitch interconnection due to occurrence of soldering defects, such as bridging or solder balling. Another technical limitation of using Pb--Sn eutectic solder is its high reflow temperature, approximately 215.degree. C. That temperature is higher than the glass transition temperature of the epoxy resin used in most polymeric printed circuit board materials. Thermal exposure at this reflow temperature produces significant thermal strains in a printed circuit board, especially in a direction perpendicular to the surface of a PCB, because no structural reinforcement is provided in that direction. Such residual thermal strains in an assembled PCB can significantly degrade reliability.
A more serious concern regarding the usage of lead (Pb)-containing solders is an environmental issue, a trend already experienced in other industries which has led to the elimination of lead from gasoline and paints.
In the electronics industry, two different groups of materials are currently under investigation as possible substitutes for Pb-containing solder materials: Pb-free solder alloys, and electrically conductive pastes (ECP). An electrically conductive paste (or adhesive) includes metallic filler particles loaded in a matrix of a polymer material. The polymer matrix can be any polymer suitable for use in a paste, for example, a thermoplastic or a thermoset. The polymer is selected preferably from the group comprising: epoxies, polyesters and polyimides.
A soluble epoxy, in particular, soluble ketal and a acetal diepoxides, as described in U.S. patent application Ser. No. 08/210,879, filed Mar. 18, 1994, the teaching of which is incorporated herein by reference, can also be used as the polymer matrix. Referring to FIG. 1, an electrically conductive paste 2 with silver-particles 4 and a fill epoxy 6, is a most common example of an electrically conductive paste (schematically shown disposed between surface 8 and surface 10). Silver particles 4 usually are in the shape of flakes and provide electrical conduction by a percolation mechanism, while epoxy fill 6 provides adhesive bonds between the components. Electrically conductive paste 2 has been long used in as a die-bonding material, where its good thermal conduction rather than electrical conduction property is utilized. However, this material has not been accepted for applications requiring high electroconduction and fine pitch connections.
A silver-filled epoxy material has several limitations, such as low electrical conductivity, increase in contact resistance during thermal exposure, low joint strength, silver migration, difficulty in rework, and others. Since a silver-filled epoxy material is electrically conductive in all directions, it is classified as "isotropic" in electroconduction.
There is another class of electrically conductive adhesives (or films), which provide electroconduction only in one direction. This class of the materials is known as "anisotropically" conductive and an example is shown as adhesive film 12 in FIG. 2A. Adhesive film 12 contains electrically conductive particles 18 in a binder or adhesive material 20. Anisotropic conductive adhesive or film 12 becomes conductive only when it is compressed between two conducting surfaces 19 and 21 as shown in FIG. 2B. This process normally requires heat and pressure.
The major application of anisotropic conductive films is for joining of liquid crystal display panels to their electronic printed circuit boards. Conducting particles 18 are usually deformable, such as solder balls, or plastic balls coated with nickel and gold. The binder or adhesive material 20 is mostly a thermosetting resin.
An ECP made of Sn-plated Cu powder and polyimide-siloxane resin is disclosed in our earlier U.S. patent application Ser. No. 08/641,406 filed May 1, 1996 and Ser. No. 08/883,188 filed Jun. 26, 1997. Such an ECP is a good candidate for high temperature solder joints such as controlled collapse chip connections (C4) and solder ball connection (SBC) to a ceramic substrate. However, for polymeric printed circuit board applications, that ECP is not adequate, because the reflow temperature (e.g., 250.degree. C.) is much higher than the glass transition temperature of the polymeric resin used therewith, e.g., FR-4. Candidates for such applications are ECP's made of Cu powder plated with Indium, tin-bismuth alloys or Indium-tin alloys, formulated with polyimide-siloxane resin. The reflow temperature of these powder pastes is expected to be between 120 and 180.degree. C., which is even lower than the reflow temperature of the Pb/Sn eutectic solder, 215.degree. C.
In an earlier U.S. patent application Ser. No. 08/414,063, filed Mar. 31, 1995, we disclosed a process to produce dendritic copper powder overcoated with Sn or Sn and BiSn coatings by electrolytic plating on a rigid inert cathode. The morphology of the powder that can be made by this technique is restricted to the dendritic shape which is not always the preferred one for all ECP applications.
A solder/polymer composite paste material is disclosed in U.S. Pat. No. 5,062,896 (Huang et. al.) and comprises a major proportion of a meltable solder powder filler, such as Bi--Sn, Pb--Sn, Bi--Sn--Pb alloys, a minor proportion of a thermoplastic polymer such as a polyimide siloxane and a minor proportion of a fluxing agent. An oxide-free, partially coalesced solder alloy connection is obtained, which is polymer strengthened and reworkable at a low reflow temperatures, per se, or in the presence of a polymer solvent.
In U.S. Pat. No. 5,286,417 (Mahmoud et. al.), a fusible conductive adhesive is disclosed, which comprises metal alloy fillers such as Sn--Au and Bi--Au, and a thermoplastic polymer having a glass transition temperature overlapping the melting temperature of the metal filler alloys. The loading of the conductive material in the polymer is in the range of about 15% to about 20% by weight.
In U.S. Pat. No. 5,136,365 (Pennisi et. al.), an adhesive material is disclosed, which contains a fluxing agent and metal particles for use in reflow soldering, such as Sn, Pb, In, Bi, Sb, Ag and others, in the matrix of an epoxy resin. Upon reflow soldering, the adhesive forms an isotropic electroconduction between an electrical component and a substrate.
In U.S. Pat. No. 5,213,715 (Patterson et. al.), a directionally conductive polymer is disclosed, which contains a metallic filler powder of Ni or Cu. The metallic powder is treated by a different polymer than the polymer used as a matrix resin. Upon compression, the coated polymer dissolves to make an electrical conductor among the filler particles.
It is an object of the present invention to coat free standing metallic powder of any desired shape, morphology and size with a surface coating of a metal or an alloy by electroplating.
It is another object of the present invention to provide a method of fabricating an electrically conductive paste material which is environmentally safe and low cost.
It is still another object of the present invention to provide a method of fabricating an electrically conductive paste material which produces a higher electrical conductivity than does silver-filled epoxies.
It is a further object of the present invention to provide a method of fabricating an electrically conductive paste material which can be processed at a lower temperature than a reflow temperature of Pb--Sn eutectic solder paste.