A-staged thermoplastic-polyimide (TPI) adhesive compound 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 particulate ceramic and/or metallic thermally conducting, electrically insulating, and thermally conducting, electrically conducting fillers for interface-bonding to create a robust joint between surfaces with conventional lamination processes that utilize relatively moderate temperatures and applied pressures.

Claims

1. The process of interface bonding of two surfaces forming a bondline, said process comprising in combination: A. providing an adhesive solution comprising an A-staged uncured thermoplastic-polyimide (TPI), 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, and 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); and 3. a quantity of particulate filler, 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 particulate filler comprises a quantity of thermally conducting solid particulate filler in the amount of between 5 and 98% by weight.

3. 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.

4. 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.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

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

DESCRIPTION OF THE PREFERRED EMBODIMENT

(3) 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.

(4) 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.

(5) 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.

(6) 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:

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

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

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

(10) 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:

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

(12) 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.

(13) 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, but is often not required.

(14) 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.

(15) 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.

(16) 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 bondline would be approximately 7-140 um.

(17) 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.

(18) 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.

(19) 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.

(20) 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.

(21) 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.

(22) 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, which is considerably higher than applied pressure on the laminate, so the water will escape cleanly as shown in FIG. 2.

(23) 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.

(24) 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.

(25) 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.

(26) 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.

(27) 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.

(28) 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.

(29) 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.

(30) 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).

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

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

(33) 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.

(34) 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.

(35) 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).

(36) 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.

(37) 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, 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.

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

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

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