Abstract:
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. The structure is disposed between two electrically conductive surfaces, e.g. chip and substrate pads, to provide electrical interconnection and adhesion between their pads.

Description:
FIELD OF THE INVENTION  
         [0001]    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  
         [0002]    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 C 4 ), solder-ball connection in ball-grid-arrays (BGA), 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 these functions fails, the solder joint is considered to have failed, which can often threaten a shut-down of the whole electronic system.  
           [0003]    Flip-chip connection is a most effective chip interconnection method currently used for high performance packaging applications, such as multichip modules, where lead (Pb)-rich solder bumps connect high performance chips directly to a ceramic substrate.  
           [0004]    Solder-ball connection in ball-grid array (BGA) packages is an extension of the flip-chip connection method, where either single-chip or multichip modules are connected to a polymeric PCB by using a grid array of large solder balls. The solder balls currently in use have the composition of a high tin content, such as Pb-10% Sn or Pb-20% Sn, in order to act as a non-melting stand-off when these modules are attached to the PCB substrate using lower melting point Pb—Sn cutechic solder.  
           [0005]    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° 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.  
           [0006]    In PTH soldering, solder joints are made by utilizing the plated-through-holes on a PCB. For example, in wave soldering, molten solder is directly applied to the area of the PTH and is further drawn in by a capillary force to fill the gap between the I/O pin and the wall of the plated-through hole.  
           [0007]    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&#39;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.  
           [0008]    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 and other harmful cleaning agents are being eliminated and replaced by either water-soluble or no-clean flux materials.  
           [0009]    Recent advances in microelectronic devices demand a very fine pitch connection between electronic packages and a printed circuit board (on the 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 or using the Pb—Sn eutectic solder is its high reflow temperature, approximately 215° 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, where no structural reinforcement is present. Thereby, the residual thermal strains in an assembled PCB would adversely affect the reliability of an electronic system.  
           [0010]    A more serious concern regarding the usage of lead (Pb)-containing solders is an environmental issue, which we have experienced already in other industries by eliminating lead from gasoline, paints, and household plumbing solders.  
           [0011]    In the electronic industry, two different groups of materials are being investigated currently for the possibility of replacing 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 electrically conducting filler particles loaded in the matrix of a polymer material. Silver-particle filled epoxy  2  is the most common example of the electrically conductive pastes, schematically shown in FIG. 1. The silver particles  4 , usually in the shape of flakes, provide electrical conduction by percolation mechanism, while the epoxy matrix  6  provides adhesive bond between the components  8  and a substrate  10 . 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.  
           [0012]    There is another class of electrically conductive adhesive (or film), which provides electroconduction only in one direction. This class of the materials is called as “anisotropic” conductive adhesive or film  12 , shown schematically in FIG. 2, contains electrically conductive particles  18  within a polymeric adhesive material  20 . The anisotropic conductive adhesive or film  12 , becomes conductive only when it is compressed between two conducting pads  14  and  16  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 are usually deformable, such as solder balls, or plastic balls coated with nickel arid gold. The binder or adhesive material is mostly a thermosetting resin.  
         OBJECTS  
         [0013]    It is an object of the present invention to provide an electrically conductive paste material which is environmentally safe and low cost.  
           [0014]    It is an 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.  
           [0015]    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.  
           [0016]    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 terms of silver migration under an application of temperature/humidity/voltage.  
         SUMMARY  
         [0017]    A broad aspect of the present invention is an electrically conductive material formed A from a plurality of particles, 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.  
           [0018]    Another broad aspect of the present invention is a paste containing particles having a coating of an electrically conductive material and a polymer material.  
           [0019]    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.  
           [0020]    The present invention relates to a new electrically conductive adhesive material which is environmentally safer than Pb and is low cost. In addition, the new Pb-free conductive adhesive material provides a higher electrical conductivity and a better joint strength than the conventional silver-filled epoxy.  
           [0021]    The new electrically conductive adhesive material comprises a conducting filler powder coated with a low melting point metal, a thermoplastic polymer resin, and other minor organic additives. A conductive filler powder, in general, is selected from the group consisting of Au, Cu, Ag, Al, Pd and Pt. The filler particles are coated with low melting point, non-toxic metals which can be fused to achieve metallurgical bonding between adjacent particles, and between the particles and the contact surfaces that are joined using the adhesive material. The coating layer is selected from the group of fusible metals such as Bi, In, Sn, Sb and Zn. The polymeric material is selected from the group consisting of polyimide, siloxane, polyimide-siloxane, polyester, and others. The relative amounts of filler powder added to the polymer matrix varies according to the applications. In general, for isotopic conduction, a high filler loading is required, while a low filler loading is required for anisotropic application. To insure uniform dispersion of the ingredients, the mixture is conveniently processed in a three-roll shear mill. The viscosity is also controlled is also controlled by adjusting the volume fraction of the filler powder in the adhesive. For a low loading formulation, a solvent drying process is required to adjust the viscosity of the adhesive before dispensing the adhesive on to a desired foot print. Depending upon the application needs, the particles size of the filler powder, the composition of the polymer resin and the volume fraction of the filler powder can be adjusted. Since the present conductive adhesive is primarily based upon particle-particle and particle-pad metallurgical bonds, the critical volume fraction of the filler material required to achieve acceptable conductivity levels is much less than the conventional silver-epoxy adhesive.  
           [0022]    [0022]FIG. 8 is an SEM micrograph showing the new Pb-free conductive adhesive material dispensed and cured at a low temperature. The conducting filler particles coated with tin (Sn) are dispersed uniformly in the matrix of polyimide-siloxane, a thermoplastic polymer.  
           [0023]    The present invention embodies filler particles having a controlled, thin layer of low melting point metal or alloy coated onto the conducting powder, such as copper particles. This thin coating layer provides metallurgical bonds among the conducting filler particles when they are subjected to heat and pressure. This provides superior electrical and mechanical properties of the joints made according to the present invention. In the prior art, using solder, the electrical conduction occurs through the joints made of lead-based solder.  
           [0024]    As noted previously, the instant invention uses a “composite paste material” comprising a coated metal powder as a filler, polymer resin used as the matrix and other organic ingredients.  
           [0025]    The composite paste material makes an electrical and mechanical connection by forming metallurgical joints among the filler materials as well as to the substrate surface, which has conductive regions. For this reason heat and pressure are required, and the joining temperature should be near or higher than the melting point of the coated layer. The method of the present invention does not involve a cold pressing or cold welding operation. The joint formed between the paste and the conductive regions is both electrically conductive and mechanically sound.  
           [0026]    Because applicants&#39; composite paste materials have two major components, the filler and the matrix, their properties can vary according to the relative composition of each constituent. Applicants refer to Tables I and II hereinafter to list and demonstrate the electrical and mechanical properties of model joints made of their Sn-coated Cu paste. The data illustrates that the electrical properties improve as the filler content increases, while the mechanical properties deteriorate as the filler content increases.  
           [0027]    As an extreme case, when the filler material is used only to form a joint by cold compaction, as taught in the prior art, both electrical and mechanical properties are not acceptable for microelectronic applications.  
           [0028]    Because of the composite paste material, the choice of polymer matrix is critical in formulating an electrically conducting paste. A thermoplastic polymer resin with a glass transition temperature lower than the melting point of the coated layer of the powder is necessary to facilitate the metallurgical bonding among the filler particles. In addition, the mechanical properties (e.g. joint strength, joint ductility) and the long term reliabilities such as thermal fatigue life are strongly dependent upon the choice of the polymer matrix resin.  
           [0029]    In order to characterize the electrical and mechanical properties of the experimental joint samples made with the new Pb-free conductive adhesive were made by joining two “L” shaped copper coupons, as shown in FIG. 9. A joining operation can be performed near the melting point of the coated layer, where a metallurgical bond is accomplished at the particle-to-particle boundaries, as well as at the particle-to-substrate interfaces. The joining process can be either a solid-state or liquid-solid reaction. The polymer curing process can be combined with the joining process depending on the formulation. In order to compare the materials properties, other joint samples were also fabricated under a similar process by using commercial Ag-epoxy and Sn/Pb eutectic solder alloy materials. FIGS.  10 - 12  illustrate a typical cross sectional view of model joints made of silver-epoxy, Pb/Sn solder and the Pb-free material of the present invention, respectively. In the joint made with the silver-epoxy as shown in FIG. 10, silver flakes of a few microns are seen, and the joint is formed by epoxy adhesive bonding  
           [0030]    In FIG. 11, a typical solder joint microstructure is observed in which a metallurgical reaction has produced a solid bond and some reaction layers on the copper substrates. For the novel Pb-free adhesive material of the present invention as shown in FIG. 12, the joint microstructure shows two distinct regions; one where the conducting particles fused to the copper substrates to form a metallurgical joint, and another region where the polymer resin forms adhesive bonding.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]    Further objects, features and advantages of the present invention will become apparent from a consideration of the following detailed description of the invention when read in conjunction with the drawing FIGS.  1 - 7  in which:  
         [0032]    [0032]FIG. 1 is a schematic illustration of an electrically conductive paste comprising silver flake particles as a filter in the matrix of epoxy resin. The conductive paste is classified as isotropic in electroconduction. (prior art)  
         [0033]    [0033]FIG. 2A is a schematic illustration of an electrically conductive adhesive which becomes electrically conductive only in the one direction when the adhesive film is compressed between two contact or bond pads as shown in FIG. 2B. The conductive adhesive (or film) is classified as anisotropic. (prior art)  
         [0034]    [0034]FIG. 3 is a schematic drawing of a paste according to the present invention.  
         [0035]    [0035]FIG. 4 is a schematic illustration of an electrically conductive paste material according to the present invention, comprising particles filled in the matrix of thermoplastic polymer resin. The particles are coated with low melting point, non-toxic metals which were fused to achieve metallurgical bonding between adjacent particles, and between the particles and the contact surfaces that are joined using the paste.  
         [0036]    [0036]FIG. 5 is a schematic cross-sectional illustration representing a surface mount interacted circuit package connected to a circuit board by an electrically conductive paste according to the invention.  
         [0037]    [0037]FIG. 6 is a schematic cross-sectional illustration representing an integrated circuit chip directly attached to a high density printed circuit board by using the electrically conductive paste according to the present invention.  
         [0038]    [0038]FIG. 7 is a schematic illustration of a multilayer ceramic substrate with the conductive paste structure matching to the C 4  bump structure on a silicon wafer. The multilayer ceramic substrate serves as a vehicle for wafer-scale burn-in and chip testing.  
         [0039]    [0039]FIG. 8 is an SEM micrograph showing the Pb-free conductive adhesive of the present invention.  
         [0040]    [0040]FIG. 9 is a schematic diagram of a model joint of two copper “L” shaped coupons for mechanical and electrical evaluation.  
         [0041]    [0041]FIG. 10 is a cross sectional view of a model joint made of conventional silver-filled epoxy.  
         [0042]    [0042]FIG. 11 is a cross sectional view of a model joint made of a Pb/Sn solder alloy material.  
         [0043]    [0043]FIG. 12 is a cross sectional view of a model joint made of the novel Pb-free adhesive material of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0044]    In one particular embodiment, we disclose a new electrically conductive paste material consisting or 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° 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.  
         [0045]    In another embodiment; we propose the use or 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.  
         [0046]    [0046]FIG. 3 illustrates new electrically conductive paste (ECP) materials  30 , according to the present invention, comprising particles  32  having an electrically conducting coating  34 , as conducting filler materials, and a polymer matrix  36 . The particles  32  are preferably Cu particles. The coating  34  is preferably tin, indium, and bismuth antimony or combinations thereof. The polymer matrix is preferably a thermoplastic, most preferably a polyimide siloxane. The invention will be described below in terms of the preferred embodiment, but it is not limited thereto.  
         [0047]    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 μm 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 μm on 5-7 μm Cu powder. After rinsing, the tin-plated copper powder is immediately mixed with a no-clean flux, FLUX305, from Qualitck 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, NMP solvent, butyric acid and ethylene glycol. The relative amount of filler powder over the polymer matrix is varied from 30 to 90% in weight, depending (in 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° C., 1 hour, is required to adjust the viscosity of the paste before dispensing the paste on to a desired foot print.  
         [0048]    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° 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, 2.6×10 −5  ohm for Sn-plated Cu paste, 4.7×10 −5  ohm for Sn/Pb solder paste, and 7.3×10 −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.  
         [0049]    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.  
         [0050]    The ECP made of Sn-plated Cu powder and polyimide-siloxane resin is a good candidate for the high temperature sokler joints such as C 4  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° 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 resin. The reflow temperature of the Indium-plated Cu powder paste is about 180° C., which is even lower than the reflow temperature of the Pb/Sn eutectic solder, 215° 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  34  of a particle  32  to fuse to the conductive coating  34  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.  
         [0051]    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.  
         [0052]    [0052]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 μm. The joining operation is combined with the polymer curing process at the temperature between 120 and 150° 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.  
         [0053]    [0053]FIG. 6 depicts an IC chip  60  attached to a high-density circuit card  52  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. Since polyimide siloxane is a thermoplastic copolymer, this joint can be reworked by heating to about 200° C. in the presence of NMP as a solvent. In case of direct chip attachment using C 4  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.  
         [0054]    [0054]FIG. 7 shows an application for wafer-scale burn-in of C 4  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 C 4  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 C 4 &#39;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 C 4  bumps during the test, or by dissolving the pads in NMP and re-screening the paste to form new pads. The chip C 4  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.  
         [0055]    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:  
         [0056]    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.  
         [0057]    tin-coated copper powder, mixed with polyimide siloxane, NMP solvent, and butyric acid and ethylene glycol or no-clean flux.  
         [0058]    tin-coated copper powder, mixed with renewable or bio-based polymer resin, suitable solvent, and butyric acid and ethylene glycol or no-clean flux.  
         [0059]    indium-coated copper powder, mixed with polyimide siloxane, NMP solvent, and butyric acid and ethylene glycol or no-clean flux.  
         [0060]    indium-coated copper powder, mixed with renewable or bio-based polymer resin, suitable solvent, and butyric acid and ethylene glycol or no-clean flux.  
         [0061]    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.  
         [0062]    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.  
         [0063]    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.  
         [0064]    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.  
         [0065]    Electrical properties of the model joints of the type depicted in FIG. 9 and specifically shown in FIGS.  10 - 12  as described above, were evaluated by measuring four-point resistance of the joints. The apparent joint area is about 50 mil×50 mil (1.25 mm×1.25 mm). For the Pb-free adhesive material, three different formulations were evaluated as a function of the filler loading level: low, medium and high loading. Table I summarizes the electrical resistance values of various joint samples. Each value represents an average value of 5-10 measurements. The electrical resistance value of the Pb/Sn joint serves as a reference for comparison. The resistance ratio is calculated by normalizing each value against that of the Pb/Sn solder joint. The resistance of the silver-epoxy joint is about 50% higher than that of the Pb/Sn solder joint. The joint resistance of the Pb-free adhesive of the present invention varies according to the level of the level of filler loading. The new Pb-free adhesive with a higher filler loading shows even a lower resistance than that of the Pb/Sn solder joint. The resistance of the Pb-free adhesive with a medium loading is close to that of the Pb/Sn solder joint.  
         [0066]    Mechanical properties of the model joints were evaluated by measuring the joint strength in a shear mode by pulling apart two legs in opposite directions a shown in FIG. 9. The average value of each sample group is listed in Table II. As expected, the Pb/Sn solder joint shows the highest joint strength and the silver-epoxy shows the lowest. The joint strength of the Pb-free adhesive materials varies again according to the level of filler loading, but in the opposite fashion, as the electrical resistance of the joint does. The joint strength of the Pb-free adhesive materials decreases as the level of the filler loading increases. In general, the Pb-free adhesive material demonstrates a better joint strength than the silver-epoxy material, and for a low loading formulation, it shows a joint strength very close to that of the Pb/Sn solder material.  
         [0067]    In order to evaluate long term reliability behavior of the Pb-free adhesive materials, the joint samples were laced in a temperature/humidity chamber (85° C./85% RH). After every 250 h interval, three to five samples were removed from the chamber to measure their electrical resistance and joint strength. The change in electrical resistance of the model joints is summarized in Table III. Most changes occurred during the initial 250 h period, and then the resistance change is not significant up to 1000 h. Both the Pb/Sn solder and the Pb-free adhesive joints show less an increase in resistance compared with the silver-epoxy joint.  
                                                           TABLE I                           Electrical Resistance of Model Joints                    Filler Loading   Resistance*               Materials   (wt %)   (10 −5  Ω)   Ratio**                            Ag-epoxy   (60-80)   7.3   1.5 5             Pb/Sn solder   &gt;90   4.7   1.0           Cu/Sn paste   90   2.6   0.5 5             Cu/Sn paste   60   6.0   1.2           Cu/Sn paste   30   8.6   1.8                                              
 
         [0068]    [0068]                                                           TABLE II                           Mechanical Strength of Model Joints                    Filler Loading   Strength*               Materials   (wt %)   (lb)   Ratio**                            Ag-epoxy   (60-80)   6.6   0.6 5             Pb/Sn solder   &gt;90   10.2   1.0           Cu/Sn paste   90   5.6   0.5 5             Cu/Sn paste   60   7.2   0.7           Cu/Sn paste   30   9.5   0.9                                                
         [0069]    [0069]                                         TABLE III                           ELECTRICAL RESISTANCE CHANGE DURING T/H TESTING*                    As-Bond   250 h   500 h   1000 h           Materials   (10 −5  Ω)   (10 −5  Ω)   (10 −5  Ω)   (10 −5  Ω)                       Ag-epoxy   7.3   15-20   18-25   16-25           Pb/Sn   4.7    8-12   15-20   11-15           Pb-free   2.6    7-11   16-20   11-15           (high)           Pb-free   6.0   10-16   17-20   21-24           (medium)           Pb-free   8.6   12-14   12-16   15-20           (low)                                    
         [0070]    A drastic reduction of electrical resistivity can be achieved in a composite material, such as gold-filled or silver filled epoxy material, by adding fine conducting metallic particles into an insulating epoxy matrix. These conductive adhesive materials may be adequate for high impedance, low current applications. However, a further improvement of the electrical resistivity is required for the low impedance, high current applications, such as flip chip joining, or SMT package assembly. In addition, silver-filled conductive material has a serious reliability concern such as silver migration, when it is considered for a fine pitch application. As demonstrated in the experimental data presented above, the Pb-free adhesive material does not contain silver particles, the concern of silver migration does not exist. In addition, the Pb-free adhesive of the present invention can be reworkable either by heating or by dissolving in a solvent, because it is formulated with a thermoplastic polymer resin.  
         [0071]    Depending upon the application, the novel Pb-free adhesive material can be tailored for isotropic or anisotropic formulation by adjusting the level of filler loading. FIG. 13 shows two different formulations: one (A) with a low loading and another (B) with a medium loading. The low loading reveals a filler particle density similar to that used in commercial anisotropic conducting films (ACF), a few thousand particles per square millimeter.  
         [0072]    The Pb-free, high conductivity reworkable adhesive materials of the present invention have been developed for device attachment. Adhesive formulations for isotropic as well as anisotropic applications are possible. Metallurgical and polymer adhesive bonding provide good joint strength, electrical conduction and contact resistance stability comparable to solder joints.  
         [0073]    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.