PROCESS FOR PREPARING HIGH TEMPERATURE POLYIMIDE FILM LAMINATES

Abstract: A process for fabricating large area void-free polyimide laminate structures wherein multiple ply polyimide film laminates may be constructed without decreasing the indi­ vidual film strength and wherein layers of metal foil may be laminated between polyimide film layers to yield a flexible high temperature resistant structure having capabilities for use as flexible electric circuits in aerospace applications and the like.

Description: PROCESS FOR PREPARING HIGH TEMPERATURE POLYIMIDE FILM LAMINATES Background of the Invention Flexible electrical circuits are presently being used commercially in computers, cameras, telephone and PBX systems, medical equipment controls and in automobile radios. These reliable interconnection devices save space and assembly time and it is anticipated that additional uses in aerospace and other applications will be forthcoming now that electronic designers no longer regard flexible circuits as experimental but accept such circuitry as reliable. One problem that has limited their use is the ability of present flexible circuits to remain reliable under extreme temperature variations. This unreliability is believed partially due to the presence of voids created in the flexible circuits during bonding because of volatiles in the adhesives and also because of the inherent rigidity of some adhesives. There is a definite need in the aerospace industry for reliable flexible electrical circuitry for temperature and pressure variable applications on missiles; aircraft and spacecraft. Linear aromatic polyimides are excellent candidates for such applications because they are thermally stable at temperatues between 477K (4000F) and 589K (6000F), flexible, and light in weight. Thus, a process for laminating large areas of void-free high temperature polyimide film is needed for aerospace applications. A problem exists, however, in using a linear aromatic polyimide adhesive to bond polyimide film to itself or to metal surfaces, such as is desired for various circuitry applications, due to the evolution of unwanted volaties. Such volatiles can be the result of condensation reaction by-products andlor excess solvent, both of which cause difficulties in the preparation of large-area, void-free laminates. Several years ago, a particular need for a flexible, high temperature adhesive to bond ultrathin polyimide film was presented by the proposed NASA Solar Sail Program. A flexible adhesive was needed for joining strips of 0.002 ijm (0.08 mil) DuPont polyimide Kapton B film at intervals across each sail blade measuring 8 meters in width by 7350 meters in length. Kapton is available in various thicknesses and is a trademark of the DuPont Company for a linear polyimine film formed from pyromellitic dianhydride (PMDA) and 4,4'-oxydianiline (ODA). Several linear polyimide adhesives were developed for this application using the following monomers: EMI2.1 BTDA PMDA DABP The adhesives indentified as LARC-2, 3, and 4 and formulates as shown in Table I, were prepared in a nontoxic ether solvent and proved successful in bonding 1/4 inch overlaps of thin polyimide Kapton film. The adhesives themselves showed good thermal stability when subjected to temperatures of 575K in excess of 500 hours and the adhesive bonds also showed excellent retention of strength after aging 6000 hours at elevated temperature. Although these 1/4 inch polyimide bonds were not entirely "voidfree," the adhesives were far stronger than the films they joined. A more complete description of these adhesives is found in the January 1979 issue of Adhesive Age, pages 35-39, which is incorporated herein by reference. Table I Kapton Adhesives Amic Acid T g Resin Formulation ninth K(OF) LARC-2 BTDA 0.70 520 3,3'-DABP (477) LARC-3 2 BTDA/1 PMDA 0.63 542 3,3'-DABP (516) LARC-4 3 BTDA/1 PMDA 0.50 570 4,4'-DABP (567) The 1/4 inch overlap of the polyimide bonds described above is small enough in area to provide escape for a large portion of volatiles. The difficulty arises in bonding larger areas. A definite need still exists for a method for joining larger areas of polyimide film, so as to produce 100% void-free laminates. Void-free laminates of thin polyimide film are deemed useful in high temperature packaging or encapsulating for the purpose of environmental protection. The "through-the-thickness" (TTT) strength of thick commercial polyimide film, Kapton for example, is very poor and prohibits the use thereof in certain applications. However, by use of the present invention, thick laminates of the commercially thin film can be produced without decreasing the individual layer (TTT) strength and thereby yield a thick polyimide laminate having improved physical property characteristics. It is also anticipated that polyimide film and laminates thereof constructed according to the present invention will prove useful as coatings wherein the polyimide film is primed with an adhesive, the adhesive B-staged and the film bonded to different substrates to serve as a protective high temperature resistant coating thereon. It is therefore an object of the present invention to provide an improved process for bonding high temperature polyimide film. Another object of the present invention is a method for preparing flexible, large-area, 100% void-free laminates from polyimide film. A further object of the present invention is an improved method for laminating polyimide film to itself and to metal surfaces for ultimate flexible circuit applications. An additional object of the present invention is the use of a thermoplastic bonding laminating process free of solvent and void-free to recover high temperature resistant polyimide laminate structures. Brief Description of the Invention According to the present invention, the forgoing and additional objects are attained by coating at least one surface of the polyimide film to be laminated with a thin layer of a linear polyamic acid adhesive solution, subjecting the coated film to temperature to effect imidization of the adhesive, and thermoplastically bonding or laminating the coated film with another coated or uncoated polyimide film using temperature and pressure. As an alternate process, polyamic acid adhesive film may be B-staged by preheating prior to being placed between the polyimide film layer. When using either process to produce flexible circuits, a conductive metal film layer can be interposed between layers of the polyimide film. Brief Description of the Drawings Fig. 1 is a graphic illustration of the thermal stability of the adhesives employed in the present invention; Fig. 2 is a graphic illustration of the adhesive strength exhibited by the adhesive bonds of the present invention when exposed to elevated temperatures; Fig. 3 is a schematic illustration of the liquid adhesive applied to one sheet of a pair of polyimide film sections to be bonded according to the present invention; Fig. 4 is a view similar to Fig. 3 illustrating the use of a B-staged adhesive film as the bonding agent between two sheets of polyimide film; Fig. 5a is a schematic illustration of a metal layer positioned between two sheets of adhesive coated polyimide films prior to pressure molding thereof to encase the metal film; Fig. 5b is an illustration similar to Fig. Sa illustrating the use of B-staged polyimide adhesive film as the adhesive agent; Fig. 6 is a schematic illustration of the moldassem- bly for laminating two layers of polyimide film according to the present invention; and Fig. 7 is a schematic illustration of multiple layers of polyimide film in position for laminating according to the present invention. Detailed Description Referring now to the drawings, Fig. 1 shows a graphic illustration of the excellent thermal stability properties of the various adhesives employed in the present invention when subjected to temperatures of 575K for an excess of 500 hours. Fig. 2 shows a graphic illustration of the retention of strength for the various adhesives after 6000 hours exposure at elevated temperature. One process for producing large area polyimide film laminates according to the present invention involves the following steps: (1) the polyimide film to be laminated is primed with a thin coat of a linear polyamic acid adhesive solution; (2) the primed polyimide film is then B-staged by preheating to 493K (4280F) in air for one hour to remove excess solvent and convert the polyamic acid adhesive to the more stable polyimide; (3) the B-staged film is then thermoplastially bonded to another sheet of primed film or unprimed film, as shown in Fig. 3, in a steel mold (Fig. 6) at 616K (6500F) for five minutes under 50-300 psi pressure; and (4) the polyimide film laminate is cooled under pressure before removing from the mold. The resulting laminate is clear yellow, extremely flexible, and 100t void-free. It cannot be peeled and attempts at peeling the laminate cause failure to occur in the film itself. A second process for producing large area polyimide film laminates according to the present invention involves the following steps: (1) a thin film of the polyamic acid adhesive is fabricated by casting the solution on a flat surface; (2) the polyamic acid adhesive film is thermally imidized to the polyimide by heating in air to 493K (428 F) for one hour; (3) the adhesive film is then stripped from the casting surface and placed between two sheets of polyimide film to be laminated, as shown in Fig. 4; (4) the sandwich prepared as in step 3 is placed in a steel mold (Fig. 6) and thermoplastically laminated at 616K (6500F) for five minutes under 50-300 psi pressure; and (5) the polyimide laminate is cooled under pressure. Successful laminates prepared by the processes described in the present invention were made from polyimide film using the following film thicknesses: 0.013 mm (0.5 mil), 0.025 mm (1 mil), 0.076 mm (3 mil), and 0.127 mm (5 mil). However, other film thicknesses should also produce good laminates. The film was cleaned prior to bonding with a cloth containing ethanol. Any other solvent useful in removing dust particles, fingerprints, etc., should be acceptable in preparing the film for lamination. Although DuPont's Kapton H film was used to produce the polyimide ¯ film laminates described in most of the Examples herein, other linear aromatic polyimide films may also be used for lamination by the process of the present invention. For example, polyimide film which may be employed in the lamination process of the present invention are selected from the group of polyimides having the following structure: EMI6.1 where Ar is either of EMI6.2 where Z is selected from EMI7.1 and Ar' is either of EMI7.2 where Z' is selected from -0-, EMI7.3 -S-, -SO , -CH2-, -C(CH3)2-, and -CliOll-. -O-, -C-, -S-, -SO2-, -CH2-, -C(CH3)2-, and -CHOH-. The thermosplastic polyimide adhesives used for fabricating the laminates described in the present invention were prepared using the linear aromatic LARC polyamic acid solutions in Table I. The LARC-2, 3, and 4 solutions were used at a concentration of 15% solids in N,N-dimethylacetamide (DMAc) or bis(2-methoxyethyl)ether (diglyme). Other useful solvents may include amide-type solvents such as N,N-dimethylformamide, N-methyl-2-pyrrolidone, and dimethylsulfoxide or ether-type solvents such as tetrahydrofuran, m- and p-dioxane, and 1,2-bis(methoxyethoxy) ethane. The B-stage step should be varied depending upon the adhesive and solvent used. When DMAc was used as a solvent for the adhesive, B-staging was achieved by heating the adhesive film or polyimide film coated with adhesive directly in air to 493K (4280F) and holding at temperature for one hour. However, when diglyme was used as the adhesive solvent, it was necessary to carry out the B-stage more slowly so as not to foam the adhesive. The B-stage for diglyme-containing adhesives consisted of heating in air 1/2 hour at 373K (212 F), one hour at 453K (3560F), and one hour at 4930K (4280F). Ultimate success in obtaining a void-free laminate depends on the B-stage step for complete removal of excess solvent and the thermal conversion of the adhesive polyamic acid to the polyimide prior to laminating. Although the LARC polyamic acids were used as adhesives for the specific example illustrations of the present invention, other linear aromatic polyamic acids/polyimides may be employed as long as they are Bstaged correctly to ensure solvent removal and imidization. Also, a candidate adhesive solution for the lamination process described herein must produce a polyimide with a glass transition temperature (or softening temperature) low enough so as to remain processable during the lamination. Although 616K (650 F) was specified as the lamination temperature in the above processes, longer time at a lower temperature or shorter time at a higher temperature could also be used to prepare a laminate of the same quality. When a laminating temperature of 644K (7000F) was used for five minutes, the resulting laminate was very dark indicating that decomposition of the polymers had begun to occur. Best results were obtained when the press was preheated to 505K (4500F) before inserting the laminate mold. The temperature of the press platens was then raised directly to 616K (650 F) and held for five minutes. The platens were air-cooled to within 373K (212 F) before opening. Successful lamination of the polyimide film occurred using pressures of 50 psi, 150 psi, and 300 psi. Lower pressures might be used if pressure were applied evenly. Pressures as high as 1000 psi were attempted, but with no visible improvement in the quality of the resulting laminate. For best results, the pressure was applied as soon as the laminate mold was placed between the preheated platen; and that same pressure was maintained until after the platens had cooled. Bumping (release of pressure followed by reapplying the same pressure) was attempted on several of the laminates after the maximum temperature was reached. The bumping technique often used to allow the escape of excess volatiles proved to be unnecessary in the preparation of 100Z void-free laminates and was discarded. The ultimate success in achieving a well consolidated polyimide laminate was found to be dependent upon attaining an even distribution of pressure. The use of several layers of woven glass fabric (Fig. 6) between the polyimide film laminate and the steel mold served to evenly distribute the pressure on the laminate. The laminating process was also carried out with a vacuum bag surrounding the laminate mold, but evacuating the system did not produce better laminates. The above described processes for laminating polyimide film are also used to prepare metal-containing laminates for use as flexible circuits and the like. The first process may be adapted by placing a conductive metal sheet or foil between two sheets of polyimide film that has been primed with adhesive and B-staged as shown in Fig. 5a. The second described process may be modified by placing a sheet of B-staged adhesive film and polyimide film to be laminated on both sides of the metal forming a sandwich as shown in Fig. Sb. Although aluminum, brass, copper and stainless steel have been successfully laminated to polyimide film using this process, other metals or metal-coated film may be employed and are considered within the scope of this invention. The metals need only to be degreased with a solvent such as ethanol or methylethylketone prior to laminating, although a more strenuous surface preparation may be used when desired. Suitable flexible electrical circuits are cut or stamped from the polyimide-metal laminates. Polyimide film laminates have been prepared using the above described processes that vary in size from 77.4 cm (12 inê) to 645 cmê(100 inê) inê). Larger area laminates can be made using the present processes as long temperatureas the entire area of lamination receives even temperature and pressure. The thickness of the laminate may also be varied. An 8-ply polyimide film laminate (Fig. 7) was successfully prepared that was 100: void-free and flexible. Thicker laminates can be fabricated as long as the temperature and pressure demands are met. Specific Examples Example 1. A 7.6 cm(3 in.) x 10.2 cm(4 in.) polyimide film laminate was prepared from 0.076 mm (3 mil) Kapton H film using LARC-2 (Table I) polyamic acid in diglyme as an adhesive. The Kapton film was brush-coated at room temperature with the LARC-2 adhesive solution and placed in a low humidity environment (approximately 20%) for fifteen minutes. The Kapton film primed with adhesive as then B-staged in a forced air oven for 1/2 hour at 373K (2120F), one hour at 453K (3560F), and one hour at 493K (4280F). The primed and B-staged film was cooled to within 373K (212 F) before removing from the oven. At this stage the adhesive coating was free of excess solvent, thermally imidized, and measured approximately 0.013 mm (.5 mil) in thickness. The laminate was then assembled as shown in Fig. 6 in a steel mold previously sprayed with a suitable release agent (Teflon, Freekote or the like), and inserted between press platens preheated to 505K (450cm). A pressure of 300 psi was applied and maintained. The mold was heated directly to 616K (6500F) and the temperature held for five minutes. The laminate was cooled under pressure to 373K (2120F). The resulting laminate was clear yellow, entirely flexible, and 100% void-free. Peeling was attempted but failure occurred first in the polyimide film. Example 2. A 7.6 cm(3 in) x 10.9 cm(4 in) polyimide film laminate was prepared from 0.025 mm (1 mil) Kapton film using LARC-2 adhesive in DMAc. An adhesive film was prepared by casting a 15% DMAc solution of LARC-2 onto a glass plate and B-staging in a forced air oven at 493K (4280F) for one hour. The oven was cooled to within 373K (212 F) before removing the glass plate. The plate was then immersed under tap water and the 0.025 mm (1 mil) thick adhesive film was removed from the plate and dried. The adhesive film was sandwiched between two sheets of Kapton film (Fig. 4) with two layers of cloth on each side of the laminate and placed in a steel mold as illustrated in Fig. 6. The laminating process and results were the same as for Example 1. Example 3. A Kapton laminate was successfully prepared as in Example 1 using LARC-3 in diglyme as the adhesive solution. Example 4. A Kapton laminate was prepared as in Example 1 combining a sheet of unprimed Kapton with a sheet of adhesive-primed Kapton. Coating only one of the Kapton sheets with adhesive made no difference in tie resulting laminate, which was flexible and void-free. Example 5. A large area, 25.4 cm(10 in) x 25.4 cm (10 in), Kapton laminate was successfully prepared as in Example 1. Example 6. A Kapton laminate was succesflfully prepared as in Example 1 using a pressure of 50 psi throughout the lamination. Example 7. A Kapton laminate was súccessfully prepared as in Example 1 using 0.013 mm (0.5 #il) tÅapt;on. Example 8. A Kapton laminate was successfully prepared as in Example 1 using 0.127 mm (5 mil). Example 9. A Kapton laminate comprised of eight plies was successfully prepared as in Example 1. The eight plies were assembled as shown in Fig. 7. Example 10. A Rapton/aluminum/Kapton laminate was successfully prepared as in Example 1. The laminate was assembled according to the diagram in Fig. 5a using thin commercial aluminum foil as the metal. Example 11. A Kapton/brass/Kapton laminate was successfully prepared as in Example 10. Example 12. A Kapton/copper/Kapton laminate was successfully prepared as in Example 10. Example 13. A Kapton/stainless steel/Kapton laminate was successfully prepared as in Example 10. Example 14. A 7.6 cm (3 in) x 10.2 cm (4 in) polyimide film laminate was prepared using 0.025 mm (1 mil) BTDA + ODA polyimide film with LARC-2 polyamic acid in diglyme as an adhesive. The BTDA + ODA polyimide film to be laminated was prepared by reacting a 1:1 stoichiometric ratio of 4,4'-benzophenone tetracarboxylic dianhydride (BTDA) and 4,4'-oxydianiline (ODA) in DMAc at 15% solids by weight. The resulting high molecular weight polyamic acid which gave an inherent viscosity of 1.14 was cast onto a glass plate and heated in a forced air oven one hour at 3000C. The polyimide (BTDA + ODA) film which was formed on curing was removed from the plate and used as an adherend for the LARC-2 adhesive. LARC-2 adhesive was brush-coated onto the BTDA + ODA polyimide film and B-staged in a forced air oven at 493K (4280F) for one hour. Two sheets of BTDA + ODA film primed with LARC-2 were then assembled as in Fig. 6 and laminated as described in Example 1. The resulting laminate was clear yellow, flexible, and 100% void-free. The laminate could not be peeled.

Claims: Claims: 1. A process for preparing high temperature resistant polyimide film comprising the steps of: providing at least two layers of a thin polyimide film; providing at least one layer of polyimide adhesive on one of said at least two layers of polyimide film; assembling said two layers of polyimide film to form a sandwich; and subjecting the film-adhesive sandwich to heat and pressure to thermoplastically bond the sandwich into a void-free laminate structure. 2. The process of claim 1 wherein the polyimide adhesive is in liquid form and is brush coated on only one layer of polyimide film and this coated layer is heated to an elevated temperature to B-stage the adhesive layer prior to assembly of the sandwich, or a film of polyimide adhesive is prepared and B-staged prior to being used to bond the polyimide film. 3. The method of claim 1 wherein the polyimide film layers are selected from the group consisting of polyimides formed as the reaction product of (a) PMDA and ODA and (b) BTDA and ODA. 4. The process of claim I wherein a layer of conductive metal film is also provided between each two layers of polyimide film and a layer of polyimide adhesive is provided adjacent each surface of the metal film layer. 5. The process of claim 4 including: assembling the polyimide film, metal foil and adhesive so as to give a film-adhesive-metal-adhesive-film sandwich; and subjecting the sandwich assembly to heat and pressure to thermally bond the sandwiched assembly into a void-free laminate strucutre. 6. The process of claim 4 or claim S wherein the metal film layer is selected from the group consisting of aluminum, brass, copper and stainless steel and each member of the group being in the thickness range of 1 mil and 30 mil. 7. The process of claim 5 wherein the polyimide adhesive is applied to the laminate surfaces in liquid form and consists of a polyamic acid dissolved in a' solvent or the polyimide adhesive is a thin film of a polyimide that has been previously B-staged. 8. The process of claim 1 or 5 wherein the polyimide adhesive is a linear polyamic acid adhesive solution selected from the group consisting of: (a) BTDA and 3,3'-DABP; (b) two parts BTDA to one part PMDA and 3,3'-DABP; and (c) three parts BTDA to one part PMDA and 4,4'-DABP, each group member being dissolved in a solvent. 9. The process of claim 8 wherein the adhesive solid concentration is approximately 15% and the solvent is selected from the group consisting on N,N-dimethylacetamide, bis(2-methoxyethyl)ether, N,N-dimethylformanide, N-methyl-2-pyrrolidone, dimethylsulfoxide, tetrahydrofuran, m-dioxane, p-dioxane and 1,2-bis(2-methoxyethoxy)ethane. 10. The process of claim 8 wherein the solvent is N,N'-dimethylacetamide and the adhesive is B-staged prior to lamination or the solvent is bis(2-methoxyethyl)ether and the adhesive is B-staged by applying a thin coat of the liquid adhesive onto a smooth surface and heating in air sufficiently to recover a thin film of polyimide adhesive.

Inventors: ST. CLAIR, ANNE KING, ST. CLAIR, TERRY LEE
Applicants: NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
Application year: 1981
Main classification: B32B27
IPC codes: H05K3, B32B38, B32B27, B05D7, B32B15, C09D179, B32B7, H05K1