Surface mounting using partially cured B staged and fully cured C staged thermoplastic polyimide TPI adhesive compounds

Abstract

A process utilizing thermoplastic adhesives for surface mounting or laminating two or more substrate surfaces consisting of a combination of thermoplastic-polyimide (TPI) adhesive layers, one of which is B-staged or partially cured, and the other of which is C-Staged or fully cured, employed both as direct coatings and/or stand alone bondfilms, as well as their advantageous use in joining materials of mismatched Coefficients of Thermal Expansion (CTE).

Claims

1. A method for preparing a laminate by bonding two surfaces utilizing heat activated thermoplastic adhesive, said method comprising in combination: A. providing a first laminate surface to be bonded; B. providing a first liquid solution comprising a less than fully cured thermoplastic polyimide adhesive precursor polyamic acid polymer disposed in a solvent; C. coating a layer of said first liquid solution upon said first surface; D. fully curing said first liquid solution to form a layer of fully cured thermoplastic polyimide adhesive; E. providing a second laminate surface to be bonded; F. providing a second liquid solution comprising a less than fully cured thermoplastic polyimide adhesive precursor polyamic acid polymer disposed in a solvent; G. coating a layer of said second liquid solution upon either said second laminate surface or said layer of fully cured thermoplastic polyimide; H. partially curing said second liquid solution to a layer of partially cured thermoplastic polyimide adhesive comprising in combination: a mixture of polyamic acid polymer and thermoplastic polyimide polymer solids disposed in said solvent, said mixture containing no less than 10% and no greater than 50% thermoplastic polyimide of the total polymer mass and said mixture further containing an amount of solvent of between 20 and 60% of the total mass of the mixture; I. positioning said first and second laminate surfaces together to form an unbonded laminate wherein said partially cured layer of thermoplastic polyimide is disposed between second laminate surface and said fully cured layer of thermoplastic polyimide; and J. applying heat and pressure to said unbonded laminate to: i. outgas said solvent from said adhesive bondline; ii. substantially convert all of said polyamic-acid polymer to said thermoplastic polyimide polymer, said conversion generating a quantity of water vapor; iii. outgas said water vapor from said adhesive bondline; and iv. bond said surfaces at the completion of said outgassing.

2. The method of claim 1 wherein the coefficient of thermal expansion of one of said laminate surfaces is greater than the other.

3. The method of claim 1 further including the further step of applying additional higher pressure to said laminate to enhance said bond.

4. The method of claim 1 wherein said solvent is selected from the group consisting of N-methyl-2-pyrrolidone (NMP) and dimethylacetamide (DMAC).

5. The method of claim 1 wherein said polyamic acid polymer comprises a mixture of diamine and dianhydride monomers, said diamine monomer is selected from the group consisting of: 3,5-diaminobenzoic acid (DABA), 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 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).

6. A method for preparing a laminate by bonding two surfaces utilizing a heat activated thermoplastic adhesive coated bondfilm, said method comprising in combination: A. providing a substrate having first and second substrate surfaces; B. providing a first liquid solution comprising a less than fully cured thermoplastic polyimide adhesive precursor polyamic acid polymer disposed in a solvent; C. coating a layer of said first liquid solution upon said first substrate surface; D. fully curing said first liquid solution to form a layer of thermoplastic polyimide adhesive on said first substrate surface; E. providing a second liquid solution a less than fully cured thermoplastic polyimide adhesive precursor polyamic acid polymer disposed in a solvent; F. coating a layer of said second solution upon said second substrate surface; G. partially curing said second liquid solution to form a layer of thermoplastic polyimide adhesive on said second substrate surface comprising in combination: a mixture of polyamic acid polymer and thermoplastic polyimide polymer solids disposed in said solvent, said mixture containing no less than 10% and no greater than 50% thermoplastic polyimide of the total polymer mass and said mixture further containing an amount of solvent of between 20 and 60% of the total mass of the mixture; said coated substrate comprising said bondfilm; H. providing a first laminate surface to be bonded; I. providing a second laminate surface to be bonded; J. disposing said coated substrate comprising said bondfilm between said first and second laminate surfaces to form an unbounded laminate; and K. applying heat and pressure to said unbonded laminate to: i. outgas said solvent from said adhesive bondline; ii. substantially convert all of said polyamic-acid polymer to said thermoplastic polyimide polymer, said conversion generating a quantity of water vapor; iii. outgas said water vapor from said adhesive bondline; and iv. bond said surfaces at the completion of said outgassing.

7. The method of claim 6 wherein the coefficient of thermal expansion of one of said laminate surfaces is greater than the other.

8. The method of claim 6 further including the further step of applying additional higher pressure to said laminate to enhance said bond.

9. The method of claim 6 wherein said solvent is selected from the group consisting of N-methyl-2-pyrrolidone (NMP) and dimethylacetamide (DMAC).

10. The method of claim 6 wherein said polyamic acid polymer comprises a mixture of diamine and dianhydride monomers, said diamine monomer is selected from the group consisting of: 3,5-diaminobenzoic acid (DABA), 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 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).

Description

DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a table of materials appropriate for use in the invention;

(2) FIG. 2 is a schematic representation of one preferred embodiment of the invention;

(3) FIG. 3 is a schematic representation of an additional preferred embodiment of the invention; and

(4) FIG. 4 is a schematic drawing of the process of the embodiment of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(5) The details of the description below are presented where appropriate with reference to a bonding procedure carried out between, for example, a semiconductor and an aluminum heat sink as those shown in the table of FIG. 1 and are exemplary only as the invention is applicable to other surface materials as is stated above, such as all semiconductor, ceramic, metal, and plastic materials.

(6) TPI coatings are made by polymerizing polyamic-acid (PAA) polymer in polar aprotic solvents, such as N-methyl-2-pyrrolidone (NMP) and dimethylacetamide (DMAC). This liquid form is defined as A-staged. The PAA's solids concentration can be 5-40% by weight in solution, and commonly is 15-25%. TPI PAA solution is a one-part adhesive, and is very stable when kept in a freezer or may be left out at ambient temperatures for a few days.

(7) TPI coatings can be compounded with powder 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.

(8) 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. This feature can be utilized in carrying out specific applications of the TPI coating.

(9) 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.

(10) Since liquid TPI coatings are relatively low-solids, typically 15-25%, the initial thickness of a coating in processing will be much greater than the finished cured bondline. Using a TPI coating solids of 20%, the final TPI coating would be less than one seventh the initial wet thickness. The final cured thickness of a TPI coating can be 0.5-20 um. Assuming a solids level of 20%, the initial A-staged coating would be approximately 3-120 um before drying.

(11) The C-staged and B-staged layers can be extremely thin. The initial C-staged primer layer on the silicon when dry can be 0.1-2 um. The subsequent B-staged layer will need to be relatively thicker to provide conformance to the aluminum surface; 1-12 um when dry should generally suffice. An even thicker B-staged layer may be required in applications with very rough surfaces or where a dielectric strength of the finished TPI insulation layer is required.

(12) As shown in FIG. 2, liquid A-staged TPI is applied to the semiconductor, after which heat is applied to drive off the solvent and dry and cure the TPI coating. This process can be done with conventional ovens, vacuum ovens, hot plates and radiant heaters. To ensure the full curing or C-staging of the initial TPI coating on the semiconductor surface, a high temperature bake is required, generally at least at 230 C.

(13) A second liquid A-staged layer of TPI is then applied to the C-staged layer and then partially cured to a B-stage by baking at much more moderate temperatures, generally at between 70 and 120 C., ensuring the polymer reactivity required to adhere to the aluminum surface during lamination. Alternatively, the second liquid A-staged layer of TPI may be directly applied to the aluminum heat sink.

(14) To construct the C-staged and B-staged TPI coatings on the semiconductor surface, liquid TPI precursor, A-staged polyamic-acid polymer in solvent is applied with conventional methods, such as spin-coating, extrusion or spraying. These are the same commonplace processes already utilized to apply polyimide coatings to the topside of semiconductors for a dielectric layer, and are standard procedures in the electronics industry.

(15) Both C-staged and B-staged TPI coatings can be done on an entire semiconductor wafer before dicing or on individual die. The combined two layer TPI coated semiconductor die are placed on the aluminum surface and then laminated with relatively moderate temperature and pressure, for example, 240 C. or more and 5-200 psi. The moderate pressure allows evolved solvent and water from the polymer's condensation reaction to escape along the bondline. If the pressure is too great, and the water vapor is entrapped, blistering occurs, which will destroy the bondline.

(16) Maximum process temperature for semiconductor-aluminum laminations is application-dependent. 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 250 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. TPI bondline assembly can also be assisted with vacuum-lamination, which helps the removal of evaporating solvent and water evolved from the PAA's condensation reaction to TPI.

(17) As long as there is enough pressure to ensure contact 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. In the automated assembly of electronic packages, A-staged TPI liquid, which is tacky, can also assist in die placement when disposed on the surface of the aluminum.

(18) Referring next to FIG. 3, an additional preferred embodiment is illustrated in which a pre-prepared bondfoil having a C-staged TPI layer disposed on one side and a second B-staged on the other. A metal bondfoil substrate for this application will maximize thermal conductivity. A silicon die with its backside either metallized or TPI-primed is placed on an aluminum heat sink with a TPI bondfoil interposed. The topside of the silicon die and corresponding TPI bondfoil can be pre-adhered onto a high-temperature pressure-sensitive carrier tape, for example, Kapton tape with silicone adhesive, that both ensures the die's position on the aluminum as well as protects the die's topside during processing. The tape can be removed after lamination or can serve as a selective permanent dielectric layer on the aluminum heat sink.

(19) As shown in FIG. 4, the Si-on-Al assembly is then placed in a stack-up for the lamination between two stiff metal plates, with both a thick high-temperature silicon rubber pad (temperature-resistance may be enhanced by loading the silicone polymer with iron) and a Teflon-glass cloth (TGC) release layer (2-5 mil) on the silicon top side. If the aluminum heat sink is robust enough to spread applied pressure by itself, generally thick or more, the bottom stiff metal plate may not be required. This assembly stack-up is then subjected to consistent low to moderate pressure of about 8 to 10 psi by a clamping assembly. Even binder clips or their equivalent may be used throughout the lamination-process bake cycle. In this construction, the applied pressure on the silicon die, which rise above the overall surface, can be an order of magnitude greater than the average pressure applied by the mechanical clamping. This increased pressure assists with the silicon die-mounting without interfering with the naturally outgassed water from the condensation reaction of TPI curing at high temperature. For instance, the vapor pressure of water is around 500 psi at 250 C., much greater than the pressure that the silicon die experiences from clamping, 50 psi for example.

(20) As the TPI adhesive layers are very thin, generally 2-5 um, a pressure high-spot in one area results in a pressure low-spot in an adjacent area, and the lower-pressure region could have considerably lower or no adhesion of the TPI to the heat sink. Therefore, the clamping assembly should be configured to apply even pressure across the lamination surface, preferably towards the center of the plate, rather than just on the edges, as that would produce a cantilever effect that could lead to inconsistent pressure across the face.

(21) The clamped assembly is then placed in a constant-temperature oven at 220-300 C. or more, the maximum cure temperature depending on the application and the expected downstream exposure, a baked until the assembly reaches the maximum temperature plus an additional dwell time of at least several minutes to ensure complete TPI curing. This is generally in the range of 15 to 30 minutes. A vacuum-assist may also be employed to help with outgassing required by the H.sub.2O condensation reaction of PAA to TPI which occurs at elevated temperatures. The evolved water vapor has a very high vapor pressure at elevated temperatures and easily escapes the low to moderate pressure of the clamped bondline.

(22) A custom clipping structure could be flat on the bottom side, allowing the curing process described above to be done on a conventional hot plate, which would reduce the process time. In addition, the hot plate process facilitates continuous-processing as opposed to oven-baked batches.

(23) The clamped assembly is then removed from the oven, and allowed to cool. The use of water to cool the hot assembly reduces the cool-down time dramatically, although care must be taken to ensure that the cooling-process water does not contact the TPI bondline.

(24) When cool, the clamped stack-up can be disassembled, and the lamination process is complete. Cured TPI assemblies can be heat-conditioned at temperatures in excess of their maximum curing temperature: generally about 30-50 C. above the oven temperature. This post-process can be helpful in stress-relief, enhancing the bond's robustness, and can result in early detection of less than optimally bonded silicon die that can easily be reworked.