High conductivity, high strength, lead-free, low cost, electrically conducting materials and applications

A structure and method of fabrication are described. The structure is a combination of a polymeric material and particles, e.g. Cu, having an electrically conductive coating, e.g. Sn. Heat is applied to fuse the coating of adjacent particles. The polymeric material is a thermoplastic phenoxy polymer or a styrene allyl alcohol resin. The structure is disposed between two electrically conductive surfaces, e.g. chip and substrate pads, to provide electrical interconnection and adhesion between their pads.

FIELD OF THE INVENTION
 The present invention relates to novel interconnection materials for
 forming electroconductive connections between electroconductive members,
 and to the method for producing such electroconductive connections. In
 addition, this invention addresses environmentally-safe materials and
 processes, which can be an alternative to lead (Pb)-containing solder
 connection technology.
 BACKGROUND
 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
 connection (or C4), solder-ball connection in ball-grid-arrays (BGA), and
 IC package assembly to a printed circuit board (PCB). Solder joints
 produced in the electronic packages serve critically as electrical
 interconnections as well as mechanical/physical connections. When either
 of the functions is not achieved, the solder joint is considered to have
 failed, which can often threaten a shut-down of the whole electronic
 system.
 When microelectronic packages are assembled to a printed circuit board, the
 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 these
 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 PTH's 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 in
 reducing the size of electronic packages and thereby the volume of the
 overall system.
 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 the 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 on it. Then it
 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 (chlorofluoro carbons) and other harmful cleaning agents
 used for this purpose are being eliminated and 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 (in an
 order of a few hundred micrometer pitch). The current solder paste
 technology used in SMT can not handle this very fine pitch interconnection
 due to the soldering defects such as bridging or solder balling. Another
 technical limitation of using the Pb--Sn eutectic solder is its high
 reflow temperature, approximately 215.degree. C. This temperature is
 already 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 after soldering, especially in the direction
 perpendicular to the surface of a PCB, because no structural reinforcement
 is made in that direction. Thereby, the residual thermal strains in an
 assembled PCB could significantly degrade the reliability of an electronic
 system.
 A more serious concern regarding the usage of lead (Pb)-containing solders
 is an environmental issue, a trend already experienced in other industries
 and has led to the elimination of lead from gasoline and paints.
 In the electronic industry, two different groups of materials are
 investigated currently for the possibility of substituting the
 Pb-containing solder materials; Pb-free solder alloys, and electrically
 conductive pastes (ECP). The present invention discusses the development
 and applications of the electrically conductive paste materials. An
 electrically conductive paste (or adhesive) is made of metallic filler
 particles loaded in the matrix of a polymer material. The polymer matrix
 can be any polymer suitable for use in a paste, for example, a
 thermoplastic of thermoset. The polymer is selected preferably from the
 group comprising epoxy, polyester and polyimide. The soluble epoxy, in
 particular, soluble ketal and a acetal diepoxides, as described in U.S.
 application Ser. No. 08/210,879, the teaching of which is incorporated
 herein by reference can also be used as the polymer matrix. Referring to
 FIG. 1, silver-particle 4 filled epoxy 6 is the most common example of the
 electrically conductive pastes 2, schematically shown therein as disposed
 between surface 8 and surface 10. The silver particles usually in the
 shape of flakes provide electrical conduction by percolation mechanism,
 while the epoxy matrix provides adhesive bond between the components and a
 substrate. This silver-filled epoxy material has been long used in the
 electronic applications 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 the applications
 requiring high electroconduction and fine pitch connection. The
 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 this silver-filled epoxy material is electrically conductive
 in all the directions, it is classified as "isotropic" in
 electro-conduction. There is another class of electrically conductive
 adhesive (or film), which provides electroconduction only in one
 direction. This class of the materials is known as "anisotropic"
 conductive adhesive film 12, shown schematically in FIG. 2A, which
 contains electrically conductive particles 18 in a binder or adhesive
 material 20. The anisotropic conductive adhesive or film 12 becomes
 conductive only when it is compressed between two conducting surfaces 22
 and 24 as shown in FIG. 2B. This process normally requires heat and
 pressure. The major application of the anisotropic conductive film is for
 joining of a liquid crystal display panel to its electronic printed
 circuit board. The 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.
 The ECP made of Sn-plated Cu powder and polyimide-siloxane resin disclosed
 in our earlier U.S. patent application Ser. No. 08/641,406 filed on May 1,
 1996, the teaching of which is incorporated herein by reference, is a good
 candidate for the high temperature solder joints such as controlled
 collapse chip connections (C4) and solder ball connection (SBC) to a
 ceramic substrate. However, for the polymeric printed circuit board
 applications, this ECP is not adequate, because the reflow temperature
 such as 250.degree. C. is much higher than the glass transition
 temperature of the polymeric resin, for example, FR-4. Candidates for this
 purpose 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 on Mar. 31,
 1995, the teaching of which is incorporated herein by reference, we have
 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.), comprising 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 temperature, per se, or in the presence of
 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 said adhesive forms
 anisotropic 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 conduction among the filler particles.
 OBJECTS
 It is an object of the present invention to provide an electrically
 conductive paste material which is environmentally safe and low cost.
 It is another object of the present invention to provide an electrically
 conductive paste material which produces a higher electrical conductivity
 than the conventional silver-filled epoxy does.
 It is another object of the present invention to provide an electrically
 conductive paste material which produces a higher joint strength than the
 conventional silver-filled epoxy does.
 It is another 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 the reflow temperature of Pb--Sn
 eutectic solder paste.
 It is another object of the present invention to provide an electrically
 conductive paste material which produces a more reliable joint than the
 conventional silver-filled epoxy does, specifically, in em silver
 migration.

SUMMARY OF THE INVENTION
 A broad aspect of the present invention is an electrically conductive
 material formed from a plurality of particles in a thermoplastic phenoxy
 polymer resin, a polyimide siloxane resin or a styrene allyl alcohol
 resin, each having an electrically conductive coating which is fused to an
 electrically conductive coating on an adjacent particle to form a network
 of fused particles.
 Another broad aspect of the present invention is a paste containing
 particles having a coating of an electrically conductive material and a
 polymer material.
 Another broad aspect of the present invention is a method of forming an
 electrically conductive joint between two surfaces by forming a paste of
 particles having an electrically conductive coating and a polymeric
 material wherein the paste is disposed between two surfaces to be
 adhesively and electrically joined. Heat is provided to fuse the
 electrically conductive particles to themselves, to metallurgically bond
 them to the contact pads and to cure the polymeric material.
 DETAILED DESCRIPTION
 In one particular embodiment, we disclose a new electrically conductive
 paste material consisting of tin-coated copper powder, polyimide-siloxane,
 solvent (N-methyl pyrrolidione or NMP), carboxylic acid/surfactant. A
 joining operation can be performed near the melting point of Sn,
 230.degree. C., where a metallurgical bonding of Sn-to-Sn or Sn-to-Au is
 accomplished at the particle-to-particle as well as particle-to-substrate
 pad interfaces. The joining process can be either solid-state or
 liquid-solid reaction. The polymer curing process can be combined with the
 joining process depending on the paste formulation. Because of the
 metallurgical bonding, a higher electrical conductivity is expected with
 the joints made of the new paste material than with those of the
 silver-epoxy material. The metallurgical bonds also provide stable
 electrical conductivity of the new joints upon thermal exposure and
 cycling. It is also expected to have a higher joint strength from the
 combined effect of the metallurgical and adhesive bonds. Depending on the
 applications, the particle size of tin-coated powder, composition of the
 polymer matrix and volume fraction of the filler material can be adjusted.
 Since the present conductive paste is primarily based on the metallurgical
 bonds, the critical volume fraction of the filler material required to
 achieve acceptable conductivity levels is much less than the conventional
 Ag-epoxy paste.
 In another embodiment, we propose the use of polymer resins prepared from
 renewable resources or bio-based materials after appropriate
 functionalization to achieve the desirable thermal and rheological
 properties, see for example, the final report on NSF
 Grant # BCS 85-12636 by W. G. Glasser and T. C. Ward. Lignin (by product
 from paper manufacture), cellulose, wood or crop oils are potential
 candidates for this purpose. Use of these materials is environmentally
 preferable because they are derived from natural and renewable resources
 and can be disposed of more readily at the end of the useful life of the
 electronic assembly. This is particularly attractive because the use of
 the Cu--Sn powder eliminates the use of lead (Pb) containing solders and
 the resulting paste formulation is non-toxic and easy to dispose.
 FIG. 3 illustrates new electrically conductive paste (ECP) materials 30,
 according to the present invention, comprising particles 34 having an
 electrically conducting coating 32, as conducting filler materials, and a
 polymer matrix 36. The particles 34 are preferably Cu particles. The
 coating 32 is preferably tin, indium, and bismuth antimony or combinations
 thereof. The polymer matrix is preferably a thermoplastic, most preferably
 a polyimide siloxane or phenoxy polymer or styrene allyl alcohol resin.
 The invention will be described below in terms of the preferred
 embodiment, but it is not limited thereto.
 The first step of tin-plating on copper powder is cleaning of fine copper
 powder in a dilute sulfuric acid. The copper powder used is spherical in
 shape, having a size distribution of 2 to 8 .mu.m in diameter, which was
 obtained from Degussa Corporation, South Plainfield, N.J. Tin plating is
 performed on the clean copper powder in an immersion tin plating solution,
 TINPOSIT LT-34, from Shipley, Newton, Mass. The optimum thickness of tin
 is 0.3 to 0.5 .mu.m on 5-7 .mu.m Cu powder. After rinsing, the tin-plated
 copper powder is immediately mixed with a no-clean flux, FLUX305, from
 Qualitek International, Inc., Addison, Ill. This prevents tin-plated
 copper powder from oxidation until it is processed into a conductive
 paste. The tin-plated copper powder is formulated into a conducting paste
 by mixing with polyimide siloxane, or phenoxy polymer or styrene allyl
 alcohol, NMP solvent or ethyl benzoate, butyric acid and ethylene glycol.
 The relative amount of filler powder over the polymer matrix is varied
 from 30 to 90% in weight, depending on the applications. In general, for
 the isotropic conduction, a high filler weight percent is required, while
 a low filler weight percent is required for the anisotropic applications.
 To insure uniform dispersion of the ingredients, the mixture is processed
 in a three-roll shear mill. The viscosity is also controlled by adjusting
 the volume fraction of the filler powder in the paste. When the filler
 weight percent is low, for example, 30% in weight, a solvent drying
 process, for example, 100.degree. C., 1 hour, is required to adjust the
 viscosity of the paste before dispensing the paste on to a desired foot
 print.
 In order to characterize the electrical and mechanical properties, joined
 samples made of the tin-plated copper-filled conductive paste are
 manufactured by laminating two "L-shaped" copper coupons. The lamination
 is performed at a temperature slightly above the melting point of Sn, for
 example, 250.degree. C., at a pressure of 25 psi. In order to compare the
 conductivity values, other joined samples are also fabricated under the
 similar process by using commercial Ag-epoxy and Sn/Pb eutectic solder
 paste materials. The joined samples made of the paste according to the
 present invention showed the lowest electrical resistance value; for
 example, 2.6.times.10.sup.-5 ohm for Sn-plated Cu paste,
 4.7.times.10.sup.-5 ohm for Sn/Pb solder paste, and 7.3.times.10.sup.-5
 ohm for Ag-epoxy for a contact area of about 0.050 inch by 0.050 inch. The
 resistance of the paste according to the present invention, is even lower
 than that of the Sn/Pb solder paste. This can be attributed to the
 difference in the bulk conductivities of copper versus Sn/Pb solder.
 Measurements of the joint strength has also demonstrated that the joint
 made using the paste according to the present invention has a higher joint
 strength than that made of the Ag-epoxy paste.
 The ECP made of Sn-plated Cu powder and polyimide-siloxane resin or phenoxy
 polymer or styrene allyl alcohol resin is a good candidate for the high
 temperature solder joints such as C4 and solder ball connection (SBC) to a
 ceramic substrate. However, for the polymeric printed circuit board
 applications, this ECP is not adequate, because the reflow temperature
 such as 250.degree. C. is much higher than the glass transition
 temperature of the polymeric resin, for example, FR-4. A candidate for
 this purpose is an ECP made of Indium-plated Cu powder formulated with
 polyimide-siloxane OR phenoxy polymer or styrene allyl alcohol resin. The
 reflow temperature of the Indium-plated Cu powder paste is about
 180.degree. C., which is even lower than the reflow temperature of the
 Pb/Sn eutectic solder, 215.degree. C. Referring to FIG. 4, the paste is
 disposed between surface 40 and 42 and heated to the reflow temperature,
 which causes the conductive coating 32 of a particle 34 to fuse to the
 conductive coating 32 of an adjacent particle to form a bond 44
 therebetween. Additionally, metallurgical bonds 46 are also formed between
 the contact surfaces 42 and the particles adjacent to these surfaces.
 In light of the environmental issues, alternative polymer resins made from
 renewable or bio-based systems such as functionalized lignin, cellulose
 and wood or crop oils can be also used. These resins are biodegradable or
 made from non-fossil fuel resources and allow ease of recycling when the
 electronic assemblies are dismantled at the end of their useful life.
 FIG. 5 depicts schematically an IC package attached to a PCB 50 by using a
 conductive paste according to the present invention. The conductive paste
 is screen printed on to each copper bond pad 52 on a PCB as practiced with
 the conventional solder paste. Pad 52 typically has a Sn coating 54. The
 paste 56 is disposed between Sn 58 coated lead frame 60 which electrically
 interconnects SMT plastic package 62 to PCB 50. The fine-pitch SMT
 assembly typically uses a pad spacing of about 0.025" or less. Therefore,
 the particle size of the tin-coated powder should be in the range of 5 to
 10 .mu.m. The joining operation is combined with the polymer curing
 process at the temperature between 120 and 150.degree. C. This low
 temperature process would introduce a much less amount of thermal
 distortion to the PCB compared to the soldering process. In addition, the
 joining process is free of external fluxes and no flux cleaning step is
 required.
 FIG. 6 depicts an IC chip 60 attached to a high-density circuit card 62
 such as surface laminated circuits (SLC), where the conductive paste
 material 64, according to the present invention, is dispensed in a
 two-dimensional array matching the footprint of the chip pads 66. The
 joining metallurgy on the chip side is preferably Cr/Cu/Au, and Au-to-Sn
 bond is expected to form at this interface. The electrically conductive
 joint made of a thermoplastic polymer resin can be reworked by heating to
 about 200.degree. C. or in the presence of NMP as a solvent. In case of
 direct chip attachment using C4 solder bumps, an encapsulation process is
 employed to obtain a desired thermal fatigue resistance of the solder
 joints. In the present application, the polymer matrix serves as a
 flexible phase that allows accommodation of the thermal mismatch strains
 between the substrate and the components. Additionally, one can
 encapsulate the spaces between the paste pads with a second polymer to
 further enhance the thermal fatigue resistance if desired.
 FIG. 7 shows an application for wafer-scale burn-in of C4 chips. The
 conductive paste material 70 is dispensed on a multilayer ceramic
 substrate 72 whose pad footprint 74 is matched with the silicon wafer pad
 footprint 76 on which are disposed C4 solder mounds 78 to be tested and
 burnt-in. The MLC substrate provides interconnects required to power the
 chips up during burn-in and the external I/O through a pin grid array 80.
 The conductive paste on the substrate is cured and the Sn-coated particles
 are bonded together with the C4's on the wafers before the burn-in step.
 The burn-in operation is performed typically at 150 C, 6 hr. After
 burn-in, the substrate is separated from the wafer, and can be used again
 by etching away any residual solder transferred from the C4 bumps during
 the test, or by dissolving the pads in NMP and re-screening the paste to
 form new pads. The chip C4 pads themselves would not have changed shape or
 composition due to the limited metallurgical contact area and pressure
 between the paste and the solder. Thus one should be able to clean the
 good chips in a suitable solvent (such as NMP) and assemble them on
 substrates as per normal process without any problems or added reflow
 steps.
 FIG. 8 shows a chemical molecular structure of thermoplastic polymer resins
 useful to produce the present invention. A particular example of which is
 UCAR phenoxy polymer resins, supplied by Union Carbide Chemicals and
 Plastics Company, Inc., Danbury, Conn. The phenoxy polymer resins are
 reported to be tough and ductile thermoplastic with high cohesive strength
 and good impact resistance. The phenoxy resins are also reported to be
 thermally stable materials, having a glass transition temperature of
 98.degree. C. and a flow temperature of 180.degree. C., which can be
 processed at high temperature and high speeds. The phenoxy resins have
 ether linkages and pendant hydroxyl groups that promote wetting and
 bonding to polar substrates and fillers.
 Examples of new electrically conductive paste materials according to the
 present invention to be used for the applications of surface mount package
 assembly to a printed circuit board, direct chip attachment to a
 fine-pitch card, and wafer-scale burn-in of flip chips, in several types
 of formulations are as follows:
 copper powder coated with a thin layer of low melting point, non-toxic
 metals, such as Sn, In, Bi, Sb, and their alloys, mixed with an
 environmentally-safe fluxing agent, such as no-clean or water-soluble
 flux.
 tin-coated copper powder, mixed with polyimide siloxane, NMP solvent, and
 butyric acid and ethylene glycol or no-clean flux.
 tin-coated copper powder, mixed with phenoxy polymer, ethyl benzoate, and
 butyric acid and ethylene glycol or no-clean flux.
 tin-coated copper powder, mixed with styrene allyl alcohol resin, and ethyl
 benzoate, and butyric acid and ethylene glycol or no-clean flux.
 tin-coated copper powder, mixed with renewable or bio-based polymer resin,
 suitable solvent, and butyric acid and ethylene glycol or no-clean flux.
 indium-coated copper powder, mixed with polyimide siloxane, NMP solvent,
 and butyric acid and ethylene glycol or no-clean flux.
 indium-coated copper powder, mixed with phenoxy polymer, ethyl benzoate and
 butyric acid and ethylene glycol or no-clean flux.
 indium-coated copper powder, mixed with renewable or bio-based polymer
 resin, suitable solvent, and butyric acid and ethylene glycol or no-clean
 flux.
 bismuth/tin alloy-coated copper powder, mixed with renewable or bio-based
 polymer resin, suitable solvent, and butyric acid and ethylene glycol or
 no-clean flux.
 bismuth/tin alloy-coated copper powder, mixed with phenoxy polymer, ethyl
 benzoate, and butyric acid and ethylene glycol or no-clean flux.
 an optimized formulation for the surface mount application, comprising
 indium-coated copper powder of 30 to 90% in weight, polyimide siloxane,
 NMP solvent, and butyric acid and ethylene glycol or no-clean flux.
 an optimized formulation for the direct chip attach application, comprising
 indium-coated copper powder of 30 to 90% in weight, polyimide siloxane,
 NMP solvent, and butyric acid and ethylene glycol or no-clean flux.
 an optimized formulation for the burn-in application, comprising tin-coated
 copper powder of 30 to 90% in weight, polyimide siloxane, NMP solvent, and
 butyric acid and ethylene glycol or no-clean flux.
 phenoxy polymers have general structural formula:
 ##STR1##
 n&gt;1, preferably 2000.gtoreq.n.gtoreq.1
 m=0or 1 R.sub.o is
 ##STR2##
 or --O-- where R.sub.2 and R.sub.3 are any organic group, preferably
 consisting of H, CH.sub.3, C.sub.3 H.sub.2s 1, CF.sub.3 where S.gtoreq.1,
 preferably 6.gtoreq.S.gtoreq.1 where R.sub.4 is H or any organic radical.
 The conductive pastes according to the present invention can be used as
 conducting lines, ground planes, and via fills in the conventional printed
 circuit boards by replacing either the additive or subtractive Cu
 technology. This will facilitate the elimination of process steps and
 chemicals thus reducing cost and the environmental impact associated with
 printed circuit board manufacturing.
 While the present invention has been described with respect to preferred
 embodiments, numerous modifications, changes, and improvements will occur
 to those skilled in the art without departing from the spirit and scope of
 the invention.