A-staged thermoplastic-polyimide (TPI) adhesive compound containing flat inorganic particle fillers and method of use

Abstract

A compound and method of use thereof consisting of an A-staged thermoplastic-polyimide (TPI) adhesive, a viscous uncured liquid of polyamic-acid polymer (PAA), the TPI precursor, synthesized and dissolved in a polar aprotic organic solvent, and including, as appropriate, combinations of flat particulate inorganic ceramic and/or metallic electrically insulating, and/or electrically conducting, and/or thermally conducting fillers for interface-bonding to create a robust joint between surfaces with conventional lamination processes that utilize relatively moderate temperatures and applied pressures, such particles resulting in the reduction of the occurrence and size of gas voids within the adhesive bondline.

Claims

1. The process of reducing the occurrence and size of gas voids in a bondline formed by the thermoplastic polyimide adhesive interface bonding of two surfaces, said process comprising in combination: A. providing an adhesive solution consisting of an A-staged uncured thermoplastic-polyimide (TPI) solution, said thermoplastic polyimide having the characteristic of being insoluble in an organic solvent in the fully imidized, fully cured state, in the form of a viscous liquid solution containing in combination: 1. a quantity of polar aprotic organic solvent; 2. a quantity of TPI precursor polyamic-acid polymer (PAA) synthesized and dissolved in said solvent wherein said polyamic-acid polymer comprises a mixture of diamine and dianhydride monomers, said monomers selected, in combination, to result in a thermoplastic polyimide having the characteristic of being insoluble in an organic solvent in the fully imidized, fully cured state, and 3. a quantity of flat particulate filler; said filler comprising an inorganic material having a particle size of between 0.1 and 2.0 um in thickness, and 1.0 and 20.0 um in width and wherein the width of said particles is greater than the thickness; B. applying said uncured solution to at least one of said surfaces; C. applying pressure to said bondline in a selected amount of between 0 and 100 psi; and D. applying heat to said bondline at a selected temperature of between 150 and 470 C., thereby converting said PAA to TPI, in situ, to form said bond.

2. The process of interface bonding of claim 1 wherein said diamine monomer is selected from the group consisting of 3,3-diaminobenzophenone (3,3-DABP), 3,4-diaminobenzophenone (3,4-DABP), 1,3-bis (4-aminophenoxy) benzene (TPER), 3,4-oxydianiline (3,4-ODA), 4,4-oxydianiline (4,4-ODA), 4,4-methylene dianiline (4,4-MDA), an aliphatic diamine, and a silicon-diamine; and wherein said dianhydride monomer is selected from the group consisting of 3,3,4,4-biphenyltetracarboxylic dianhydride (BPDA), 3,3,4,4-benzophenone tetracarboxylic dianhydride (BTDA), 4,4-oxydiphthalic anhydride (ODPA), pyromellitic dianhydride (PMDA), and 2,2-bis-(3,4-Dicarboxyphenyl) hexafluoropropane dianhydride (6FDA).

3. The process of interface bonding of claim 1 wherein said particulate filler comprises a quantity of thermally conducting solid particulate filler in the amount of between 5 and 98% by weight.

4. The process of interface bonding of claim 1 wherein said particulate filler comprises a quantity of electrically conducting solid particulate filler in the amount of between 5 and 98% by weight.

5. The process of interface bonding of claim 1 wherein said particulate filler comprises a quantity of electrically insulating solid particulate filler in the amount of between 5 and 98% by weight.

6. An adhesive solution for reducing the occurrence and size of gas voids in a bondline formed by the thermoplastic polyimide adhesive interface bonding of two surfaces, said adhesive solution comprising in combination: an A-staged uncured thermoplastic-polyimide (TPI) solution, said thermoplastic polyimide solution having the characteristic of being insoluble in an organic solvent in the fully imidized, fully cured state, in the form of a viscous liquid solution containing in combination: A. a quantity of polar aprotic organic solvent; B. a quantity of TPI precursor polyamic-acid polymer (PAA) synthesized and dissolved in said solvent wherein said polyamic-acid polymer comprises a mixture of diamine and dianhydride monomers, said monomers selected, in combination, to result in a thermoplastic polyimide having the characteristic of being insoluble in an organic solvent in the fully imidized, fully cured state, and C. a quantity of flat particulate filler; said filler comprising an inorganic material having a particle size of between 0.1 and 2.0 um in thickness, and 1.0 and 20.0 um in width and wherein the width of said particles is greater than the thickness.

7. The adhesive solution for interface bonding of claim 6 wherein said diamine monomer is selected from the group consisting of 3,3-diaminobenzophenone (3,3-DABP), 3,4-diaminobenzophenone (3,4-DABP), 1,3-bis (4-aminophenoxy) benzene (TPER), 3,4-oxydianiline (3,4-ODA), 4,4-oxydianiline (4,4-ODA), 4,4-methylene dianiline (4,4-MDA), an aliphatic diamine, and a silicon-diamine; and wherein said dianhydride monomer is selected from the group consisting of 3,3,4,4-biphenyltetracarboxylic dianhydride (BPDA), 3,3,4,4-benzophenone tetracarboxylic dianhydride (BTDA), 4,4-oxydiphthalic anhydride (ODPA), pyromellitic dianhydride (PMDA), and 2,2-bis-(3,4-Dicarboxyphenyl) hexafluoropropane dianhydride (6FDA).

8. The adhesive solution for interface bonding of claim 6 wherein said particulate filler comprises a quantity of thermally conducting solid particulate filler in the amount of between 5 and 98% by weight.

9. The adhesive solution for interface bonding of claim 6 wherein said particulate filler comprises a quantity of electrically conducting solid particulate filler in the amount of between 5 and 98% by weight.

10. The adhesive solution for interface bonding of claim 6 wherein said particulate filler comprises a quantity of electrically insulating solid particulate filler in the amount of between 5 and 98% by weight.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic drawing of the chemical process of the invention;

(2) FIG. 2 is a graph relating two parameters of the operation of the invention;

(3) FIG. 3 is a comparison graph relating to a first example of the invention;

(4) FIG. 4 is a cross-sectional schematic drawing of a laminate of the first example;

(5) FIG. 5 is a cross-sectional schematic drawing of an additional laminate of the first example;

(6) FIG. 6 is a top view of the experimental arrangement of the first example;

(7) FIG. 7 is a comparison graph relating to a second example of the invention;

(8) FIG. 8 is a cross-sectional schematic drawing of a laminate of the second example;

(9) FIG. 9 is a cross-sectional schematic drawing of an additional laminate of the second example;

(10) FIG. 10 is a top view of the experimental arrangement of the second example;

(11) FIG. 11 is a comparison graph relating to a third example of the invention;

(12) FIG. 12 is a cross-sectional schematic drawing of a laminate of the third example;

(13) FIG. 13 is a cross-sectional schematic drawing of an additional laminate of the third example; and

(14) FIG. 14 is a top view of the experimental arrangement of the third example.

DESCRIPTION OF THE PREFERRED EMBODIMENT

(15) TPI coatings are made by polymerizing polyamic-acid (PAA) polymer in polar aprotic solvents, such as NMP (N-methylpyrrolidone), DMAc (dimethylacetamide), and DMF (dimethylformamide). The PAA's solids concentration can be 5-40% in solution (by weight), and commonly 15-25%. TPI-PAA solutions are a one-part adhesive, and very stable when kept in a freezer or left out at room temperature for a few days.

(16) Typical TPI diamine can be, for example, one or more of the following monomers: 3,5-diaminobenzoic acid (DABA), 3,3-diaminobenzophenone (3,3-DABP), 3,4-diaminobenzophenone (3,4-DABP), diester diamine (RDEDA), 1,3-bis-(4-aminophenoxy) benzene (TPER), 3,4-oxydianiline (3,4-ODA), 4,4-oxydianiline (4,4-ODA), 4,4-methylene dianiline (4,4-MDA), an aliphatic diamine, or a silicone-diamine among others.

(17) Typical TPI dianhydride can be one or more of the following monomers: 3,3,4,4-biphenyltetracarboxylic dianhydride (BPDA), 3,3,4,4-benzophenone tetracarboxylic dianhydride (BTDA), 4,4-oxydiphthalic anhydride (ODPA), pyromellitic dianhydride (PMDA), or 2,2-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) among others. TPI-precursor solutions, polyamic-acid polymer in solution, are also available commercially, such as LARC-TPI or Fraivillig Technologies FM901 solutions.

(18) TPI coatings can be compounded with powder or particulate fillers such as ceramic, metal and pigments to tailor the properties of the bondline. On a solids basis, fillers can be compounded from 5-98% (by weight) into the TPI polymer. There are many fillers that could be used to optimize the properties of a TPI bondline, but these examples will cover a large majority of applications. Representative thermally conductive, electrically insulting inorganic fillers for loading A-staged (liquid precursor) thermoplastic polyimide (TPI) include:

(19) Boron nitride (BN) powder and flake, available from Momentive Performance Materials Inc., Strongsville, Ohio;

(20) Alumina fumed powder, available from Evonik Industries AG, Parsippany, N.J., and Cabot Corporation, Billerica, Mass.; and

(21) Boron nitride (BN) nano-tubes, available from Tekna Advanced Materials Inc., Sherbrooke, Quebec.

(22) These fillers can be combined to optimize properties, such as BN platelets (which are relatively large, a few microns) with fumed alumina (which is submicron), as this maximizes the amount of property changing ceramic. Representative thermally conductive, electrically conductive inorganic fillers include:

(23) Silver (Ag) flake, available from Metalor Technologies SA, North Attleboro, Mass.

(24) The TPI coating can be applied to surfaces to be bonded with a range of conventional technologies, even a simple wipe. The viscosity of the TPI-PAA solution is very sensitive to temperature, yet stable, a feature which can be utilized in tailoring for a specific application of the TPI coating.

(25) Pre-treatment of the surfaces to be coated, such as corona, plasma, or flame treatment, may improve the wetting of the TPI coating and eventual adhesion of the cured TPI bondline, especially to a polymer surface, but is often not required.

(26) The surfaces to be bonded are then assembled together, ensuring excellent contact between them. Pressure can be applied mechanically to ensure intimacy. It may be a goal to minimize applied pressure, as that can result in residual stress in the finished laminate.

(27) The TPI coating is tacky as a liquid at room temperature before the drying-and-curing process. As it dries, resulting in solvent evaporation, the partially dried coating will be naturally tacky at temperatures above what was the previous maximum process temperate for a short period until the solvent evaporates to its new equilibrium within the polymer matrix. This tacky feature may be advantageous in assembly operations.

(28) Since liquid TPI coatings are relatively low-solids, typically 15-25%, the initial thickness of the bondline in processing will be much greater than the finished cured bondline. Using a TPI coating solids of 20%, the final TPI bondline would be less than 1/7.sup.th the initial wet thickness. The final cured thickness of a TPI bondline can be 1-20 um. Assuming a solids-level of 20%, the initial A-staged coating/bondline would be approximately 7-140 um.

(29) Heat is then applied to drive off the solvent and cure the TPI polymer in a bondline made with TPI coating. This process can be done with conventional ovens, vacuum ovens and hot plates.

(30) Depending on the application, heat can be increased gradually over a controlled cycle or can be applied quickly, such as when placing an assembly on a hot plate.

(31) As the TPI coating within the bondline heats up, its viscosity drops significantly and the solvent begins to evaporate. These actions can facilitate surface wetting of the laminate, which can optimize the finished bondline for strength and intimacy. It is important to note that the polar aprotic solvent has relatively low surface tension, which facilitates its evacuation from a bondline as a vapor without significant bubbling as opposed to water.

(32) Before it evaporates, the activity of the aggressive polar aprotic solvent at elevated temperatures can be beneficial to the final bondline, as the solvent scours the surfaces to be bonded.

(33) As the TPI bondline approaches 100 C., the solvent begins to evaporate and evacuate the bondline. The effect escalates as the bondline temperature increases. During this time, the solvent vapor can purge the bondline of residual air.

(34) When most of the solvent has evaporated, i.e., when the bondline is at 180-200 C., the PAA polymer will start converting to TPI, which is a condensation reaction that evolves water vapor as shown in FIG. 1. This water vapor will have a very high vapor pressure, as shown in FIG. 2, which is considerably higher than applied pressure on the laminate, so the water will escape cleanly.

(35) After the conversion to TPI, there will be no additional evolution of water, and the micro-channels from which the water vapor escaped will collapse.

(36) Maximum process temperature that the TPI bondline should see is dependent on the application. For moderate temperature applications, the process temperature should be 10-20 C. above the expected maximum downstream temperature in manufacturing or use. For high-temperature applications, such as 300 C. and above, the maximum process temperature of the bondline should ensure that the TPI polymer is fully cured, as no additional water would be evolved.

(37) After the water outgassing, at or near the maximum process temperature, additional pressure can then be applied to ensure the adhesion and intimacy of the bondline. Duration of the pressure is not typically a factor with TPI bondlines, which is helpful in minimizing process time.

(38) TPI bondline assembly can be assisted with vacuum lamination, which helps the removal of evaporating solvent and water evolved from the PAA's condensation reaction to PI.

(39) An A-staged TPI coating in contact with an existing B-staged TPI surface will allow the B-staged coating to absorb a portion of the solvent in the A-stage coating, which solidifies that bondline over time, if only temporarily, until full curing at high temperature. The same solvent-absorption effect is seen with lesser B-staged TPI coating i.e., less cure, more solvent, on greater B-staged TPI coating i.e., more cure, less solvent. This mating effect of surfaces with similar chemistry, but dissimilar phase states (A-stage vs. B-stage; less B-stage vs. more B-stage), enables temporary mating of surfaces, with full lamination at the final cure at higher temperatures.

(40) As long as there is enough pressure to ensure contract between the lamination surfaces, then tooling and the applied pressure can be minimized during the lamination process. This ensures that minimal internal stresses are inherent in the laminate when it cools from the process temperature. When the laminated assembly heats back up towards its maximum process temperature during downstream processing and operation the internal stresses will be reduced.

(41) Assessing and monitoring the level of TPI cure can be critical to ensure properties and avoid further polymer reaction from causing blistering, when the part sees elevated temperature. This is especially important in applications where the expected temperature is above the final TPI-cure temperature. Cure level of the TPI polymer can be assessed accurately by monitoring the electrical-resistivity (ion-viscosity) of the bondline; the precursor PAA polymer has a low resistivity; TPI has a high resistivity.

(42) The TPI coating can be applied to one or both surfaces to be bonded. TPI coating(s) can be partially cured or B-staged, which gives the coating stability at room temperature and ensures consistent thickness with high temperature lamination (greatly reduced squeeze-out with applied pressure).

(43) B-staged TPI adhesive coatings or bondlines are stable at room temperature and have an indefinite shelf life. This facilitates the manufacturing and storage of TPI products and intermediate-process assemblies.

(44) B-staged TPI adhesive coatings and bondlines may have residual solvent (10-50%), but will act as a solid at room temperature.

(45) The effective glass-transition temperature (Tg) of B-staged TPI coatings and bondlines is the highest temperature that that polymer has experienced in previous processing. Above this temperature, the B-staged TPI will soften and become tacky again, which may assist assembly. As further solvent is lost and additional PAA polymer converted to TPI, the effective Tg of the B-staged TPI coatings and bondlines increases.

(46) Surfaces to be bonded with TPI can be pre-primed with A-staged TPI adhesive which would then be B-staged, before being bonded by additional A-staged TPI adhesive.

(47) During high-temperature TPI lamination, it is critical that the surfaces are in intimate contact, as the bondlines are relatively thin (2-10 um, typically).

(48) Pressure can be applied with hardware or platen. Less pressure locks in less inherent stress between the lamination layers. Even the lamination of surfaces with no applied pressure, i.e., just the force of gravity on the stacked parts, can be an effective bondline. Assembly clips and other hardware can apply pressures of 1-50 psi during TPI lamination. This moderate pressure allows the solvent and evolved water vapor (which has a very high vapor-pressure at high-temperature TPI lamination) to evacuate the bondline.

(49) The maximum TPI lamination curing process temperature is application-dependent. If the dielectric properties of the TPI do not require high dielectric strength or resistivity (residual PAA is low in both electrical parameters, but has good structural properties), then a maximum temperature of 150-200 C. will suffice. If the dielectric properties are critical, then a higher maximum temperature of 200-300 C. is recommended. Maximum lamination temperature should be 10-20 C. above the highest expected downstream process or application temperature. If the expected downstream process or application temperature is extremely high (300-450 C.), then it is critical that full curing of the TPI bondline is ensured, through both process temperature and cure time. If the TPI is not fully cured, then encountering higher temperatures will result in additional water outgassing from subsequent curing of PAA to TPI at very high vapor pressure, which results in blistering and delamination.

(50) Dwell time will be application-dependent. The PAA polymer cures faster to TPI at elevated temperature.

(51) Full curing of a TPI bondline can be determined with the polymer's electrical-resistivity (ion-viscosity) measurement.

Experimental Examples and Comparisons

(52) The following is a description of experimental results relating to particular fillers in the present invention which enhances the use of TPI bonding technology in the same format as conventional thermoset adhesives, such as epoxy. It has been found that when the PAA polymer solution is filled with flat inorganic particles, such as boron nitride (BN) platelets or silver (Ag) flake, the in situ TPI material within the bondline, with the proper process conditions, can outgas the solvent and evolved water vapor from the conversion of PAA to TPI cleanly from the laminate edges with no blistering. Laminate integrity with this A-staged TPI one-step method can approach that of the B-staged TPI two-step method, while streamlining the lamination process.

(53) Orientation, crystallinity and fillers are of substantial importance in SCRR TPI bondlines. As noted earlier, SCRR polyimides benefit from polymer-chain orientation, which enables crystallinity. Crystallinity enhances physical, electrical, chemical and radiation properties of the polymer. For a bondline, the application of an A-staged liquid SCRR polyamic acid precursor on a substrate surface with a coating rod or roll, extrusion-die casting, spin-coating, or even just manually smearing promotes X-Y orientation of the polymer. After the application of the PAA coating, the mere process of drying off the solvent, typically 80-90% of the PAA's volume, significantly shrinks the bondline in the Z-axis and further encourages X-Y orientation of the polymer chains.

(54) In a bondline, the inherent X-Y orientation and resultant crystallinity naturally inhibit thermal conductivity in the Z-axis, as the polymer structure tends to be laminar. The thermal conductivity of a pure TPI bondline is therefore low between bonded substrates. To enhance Z-axis thermal conductivity, very important in many electronic applications, ceramic filler, such as BN platelets, can be compounded into the precursor PAA solution for a dielectric bondline. If Z-axis electrical conductivity is desired or allowable, metallic fillers, such as Ag flake, can be compounded into the precursor PAA solution, metals having a much higher thermal conductivity than dielectric ceramics.

(55) In addition to boosting the thermal conductivity of the TPI bondlines, the flat inorganic particles assist the outgassing in the X-Y plane during lamination, which is critical to preventing blistering. The inorganic particles must be able to lie-flat during drying and curing operations, as often TPI bondlines have a thickness of only 3-5 um in the Z-axis, the distance between bonded substrates.

(56) Ag flake is an exemplary filler that is thermally and electrically conductive. While commercially available Ag flakes can range from 2-20 um in width (XY-plane), they are only 0.1-1 um thick (Z-axis). As an example, using commercially available median values of width, 7 um, and thickness, 0.5 um, for the dimension ranges, the typical XY-to-Z dimension ratio of width-to-thickness for Ag flake would be about 14-to-one.

(57) Further, BN platelets are an exemplary filler that is thermally conductive yet electrically insulating. While the BN platelets can be 1-20 um in width (XY-plane), they are only 0.1-2 um thick (Z-axis). As an example, using commercially available median values of width, 8 um, and thickness, 0.7 um, for the dimension ranges, the typical XY-to-Z dimension ratio of width-to-thickness for BN platelets would be about 11.5-to-one.

(58) In side-by-side tests as presented in detail below on silicon die bonded directly to aluminum plate with A-staged TPI, particle-filled TPI laminations had higher and more consistent shear strengths than die bonded with unfilled TPI. When these same coatings were B-staged, i.e., dried and partially cured before lamination, the unfilled and filled TPI bondlines performed equivalently.

(59) Additionally, it appears that the incorporation of flat fillers may also provide an advantage in dissipating some of the stress in TPI bondlines with severely CTE-mismatched substrates. Examples of improved A-staged TPI bondlines with flat fillers as compared to those without are as follows. These experiments also demonstrate the advantage of the use of flat inorganic fillers in A-staged TPI bonding, while there is little to no such advantage in B-staged TPI bonding.

(60) A silicon die bonded onto an aluminum substrate is the most severe CTE-mismatch in electronic packaging (2.6 ppm/ C. for silicon, 23 ppm/ C. for aluminum). After bonding, severe thermal shocks of the SiAl lamination degrade non-robust bondlines, and eliminate any added bond strength due to any fillet that might have formed with the adhesive around the edge of the silicon die during bonding. Shear strength of a bonded die is used as a proxy for bond integrity. The industry-standard minimum shear strength for most die is 5.5 lbs.

(61) Silicon die can have a very thin aluminum plating on their backside bonding surface to assist adhesion as most adhesive systems bond better to aluminum than raw silicon. Silicon die backsides can also be pre-coated with a polyimide layer for dielectric standoff with the aluminum heat sink. This experiment looks at both aluminum-plated and polyimide-coated silicon die, bonded to the aluminum with either neat, i.e., unfilled or Ag-filled A-staged TPI solution. As a comparison, silicon die bonded with partially cured or B-staged TPI coatings, both neat and Ag filled, are also evaluated.

(62) In the first example, the silicon die employed are approximately 3.6 mm-square. The A-staged TPI solution used in these tests was commercially available FM901 polyamic-acid solution from Fraivillig Technologies, Boston, Mass. Both neat and Ag-filled (67% Ag, by weight) FM901 were applied to an aluminum-plated backside die surface by placing a small amount of the liquid A-staged TPI solution on a Teflon surface and smearing the backside die surface in the drop of the solution. This deposits an estimated 25-40 um thickness of wet TPI which will correspond to about 4-6 um bondline when dry. It should be noted that the relative thickness of the TPI bonding layer has little impact on bond strength.

(63) With the A-staged TPI bonding, the wetted die were then placed directly onto an aluminum substrate or plate's surface. For test consistency, die coated with neat TPI and die coated with Ag-filled TPI were placed on the same aluminum plate, to provide the same process conditions. The assembly was then placed into a 125 C. oven for 5 minutes to partially cure and dry, or B-stage, the TPI in situ. Then, the assembly was placed on a 250 C. hot plate, with deadweight that applied 12 psi, for full or C-staged curing, again in situ. After bonding, the assembly was thermal-shocked three times from 250 C. to room temperature in a few seconds. The die were then sheared off, and the shear strength was recorded. This procedure was repeated four times using four separate aluminum substrates.

(64) FIGS. 3-6 further illustrate the process and details of the first example.

(65) FIG. 3 is a chart showing eight comparisons of lamination shear strengths of unfilled and Ag flake-filled A-staged PAA cured to C-staged TPI for backside aluminum-plated silicon die attached to an aluminum heat sink. FIGS. 4 and 6 are schematic cross-sections of the unfilled and filled laminates of the process, and FIG. 6 shows the physical placement of the unfilled and filled die on the experimental heat sink plates. This construction is useful when the die need to be electrically grounded to a heat sink. Aluminum is the most common semiconductor backside plating.

(66) The second example repeats the above described process substituting for the die backside aluminum plating with an electrically insulating pre-coating or layer of cured BN platelet-filled 10 um-thick TPI coating employing boron nitride powder. This construction is useful on die that need to be dielectrically isolated from the aluminum heat sink which the TPI coating provides. BN filling maximizes the thermal conductivity of the dielectric layer. While a TPI insulation layer is used here, the dielectric coating could be any compatible polymer, such as a traditional non-adhesive polyimide coating for semiconductors, such as Pyralin polyamic acid solution from Mitsui DuPont.

(67) FIGS. 7-10 further illustrate the process and details of the second example.

(68) FIG. 7 is a chart showing ten comparisons of lamination shear strengths of unfilled and Ag flake-filled A-staged PAA cured to C-staged TPI for backside BN filled TPI pre-coated silicon die attached to an aluminum heat sink. FIGS. 8 and 9 are schematic cross-sections of the unfilled and filled laminates of the process, and FIG. 10 shows the physical placement of the unfilled and filled die on the experimental heat sink plates.

(69) In the third example as a comparison to the A-stage first example above, the same neat and 67%-Ag filled TPI solutions were partially cured or B-staged onto the aluminum-plated die. The B-staging consisted of a five-minute bake in a 125 C. oven after coating. The die were then placed onto the aluminum plate surface, and the assembly was placed onto a 250 C. hot plate, with 12 psi of deadweight, for five minutes. The three assemblies were then thermal-shocked three times from 250 C. to room temperature.

(70) FIGS. 11-14 further illustrate the process and details of the third example.

(71) FIG. 11 is a chart showing six comparisons of lamination shear strengths of unfilled and Ag-filled B-staged PAA cured to C-staged TPI for backside aluminum-plated silicon die attached to an aluminum heat sink. FIGS. 12 and 13 are schematic cross-sections of the unfilled and filled laminates of the process, and FIG. 14 shows the physical placement of the unfilled and filled die on the experimental heat sink plates.

(72) The above examples illustrate the superior bonding strength obtained by including flat flakes or platelets of inorganic material in PAA in the initial uncured A-stage condition in a TPI bonding process as described above. As further shown, the increase disappears when compared with a partially cured/B-staged, or fully cured/C-staged TPI processes.

(73) In particular, as illustrated in FIG. 3, the Ag-filled TPI-coated die with backside aluminum plating had a considerably higher average shear strength than the neat TPI die, 16.9 lbs. for the neat and 27.6 lbs. for the filled, as well as a much lower standard deviation and data range, i.e., a narrower percentage range of 16.5-35.0 lbs. for the Ag-filled, versus 2.9-65.8 lbs. for the neat.

(74) Also, as illustrated in FIG. 7, the Ag-filled TPI die with backside BN-filled TPI pre-coating had a considerably higher average shear strength than the neat TPI die, 13.2 lbs. for the neat and 38.9 lbs. for the filled, as well as a lower data range on a percentage basis of 16.4-72.2 lbs. for the Ag-filled, versus 2.3-21.5 lbs. for the neat.

(75) The results of the B-staged TPI die for the neat and 67% Ag-filled were comparable in their shear strength's average value, standard deviation and data range.

(76) Accordingly, the invention described above is defined by the following claims.