聚酰亚胺薄膜和尺寸稳定及其制备工艺.doc

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The polyimide film is composed of an aromatic polyimide obtained from a solution of a polymer formed by polymerization of a biphenyl-tetracarboxylic acid and a phenylene diamine, wherein the average linear expansion coefficient of the polyimide film in the temperature range of from 50.degree. C. to 300.degree. C. is 0.1.times.10.sup.-5 to 2.5.times.10.sup.-5 cm/cm.degree.C., the ratio of the linear expansion coefficient in the machine direction of the film to the linear expansion coefficient in the transverse direction of the film is in the range of from 1/5 to 4, and the thermal dimension stability expressed by the ratio of the change of the size of the film at normal temperature after the heat treatment where the temperature is elevated to 400.degree. C. from normal temperature and the film is maintained at 400.degree. C. for 2 hours is less than 0.3%. Inventors: Kumagawa; Kiyoshi (Ube, JP), Kuniyasu; Kenji (Ube, JP), Nishino; Toshiyuki (Ube, JP), Matsui; Yuji (Ube, JP) Assignee:Ube Industries, Ltd. (Yamaguchi, JP) Appl. No.: 06/864,299Filed: May 16, 1986Foreign Application Priority DataMay 17, 1985 JP60-103674May 17, 1985 JP60-103675Current U.S. Class:428/220 ; 428/473.5; 428/901; 528/353Current International Class: C08J 5/18(20060101); B32B 027/28(); B32B 027/34(); C08G 069/26()Field of Search: 428/473.5,220,901 528/353 264/212 References Cited Referenced ByForeign Patent Documents007805Jan., 1980JP0007805Jan., 1980JPPrimary Examiner: Herbert; Thomas J. Attorney, Agent or Firm: Burgess, Ryan & Wayne ClaimsWe claim:1. A dimensionally stable polyimide film composed of an aromatic polyimide obtained from a solution of a polymer formed by polymerization of a biphenyl-tetracarboxylic acid and a phenylene diamine, wherein the average linear expansion coefficient of the polyimide film in the temperature range of from 50.degree. C. to 300.degree. C. is 0.1.times.10.sup.-5 to 2.5.times.10.sup.-3 cm/cm.degree. C., the ratio (MD/TD) of the linear expansion coefficient in the machine direction (MD direction) of the film to the linear expansion coefficient in the transverse direction (TD direction) of the film is in the range of from 1/5 to 4, and the thermal dimension stability expressed by the ratio of the change of the size of the film at normal temperature after the heat treatment where the temperature is elevated to 400.degree. C. from normal temperature and the film is maintained at 400.degree. C. for 2 hours is less thar 0.3%. 2. A polyimide film as set forth in claim 1, which is flexible and has a thickness of 1 to 150 .mu.m. 3. A polyimide film as set forth in claim 1, wherein the average linear expansion coefficient is 0.5.times.10.sup.-5 to 2.3.times.10.sup.-5 cm/cm.degree. C. 4. A polyimide film as set forth in claim 1, wherein the MD/TD ratio is from 1/3 to 3.0. 5. A polyimide film as set forth in claim 1, wherein the thermal dimension stability is less than 0.25%. 6. A polyimide film as set forth in claim 1, which is composed of a high-molecular-weight aromatic polyimide comprising at least 90 mole % of recurring units represented by the following general formula (I): #STR3# 7. A polyimide film as set forth in claim 6, wherein the high-molecular-weight aromatic polyimide comprises at least 95 mole % of said recurring units. 8. A polyimide film as set forth in claim 1, wherein the content of volatile components is lower than 1% by weight. 9. A polyimide film as set forth in claim 8, wherein the content of volatile components is lower than 0.5% by weight. DescriptionBACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a process for preparing an aromatic polyimide film having excellent thermal dimension stability from a solution of a polyimide precursor (for example, an aromatic polyamic acid) obtained from a biphenyl-tetracarboxylic acid and a phenylene diamine by a solution casting method or the like. More particularly, the present invention relates to a thermally stable aromatic polyimide film having a high thermal dimension stability (or a low thermal size change) and an average linear expansion coefficient which is substantially equal to the linear expansion coefficient of a ceramic or metal, and a process for the preparation thereof. This polyimide film is very valuable as a base film of a printed circuit board ordinarily used for an electric or electronic part. 2. Description of the Related Art An aromatic polyimide film obtained from pyromellitic dianhydride and an aromatic diamine is used as a typical instance of the aromatic polyimide film. Since the average linear expansion coefficient of this polyimide film in the unoriented state is as large as about 3.5.times.10.sup.-5 to about 4.5.times.10.sup.-5 cm/cm.degree. C., when the film is bonded to a copper foil or the like at a high temperature, a large amount of curling occurs. As means for solving this problem, GB-A-No. 1098556 proposes a method in which the drawing operation is carried out at the film-forming step to reduce the average linear expansion coefficient of the film. However, the film obtained according to this method is defective in that the ratio of the change of the size at normal temperature after the heat treatment where the temperature is elevated to about 400.degree. C. from normal temperature and the film is maintained at this temperature for 2 hours (thermal dimension stability) is very bad, and therefore, the film cannot cope with increase of the wiring density in a wiring pattern. SUMMARY OF THE INVENTION Research was conducted with a view to developing a process for preparing an aromatic polyimide film having such an average linear expansion coefficient that when the film is bonded to a ceramic or metal (for example, a copper foil or copper alloy foil) at a high temperature, no large curling occurs, and also having a good thermal dimension stability. As the result, it was found that if a thin layer of a solution of an aromatic polyamic acid or the like obtained by polymerization of a biphenyl-tetracarboxylic acid and a phenylene diamine is formed on the surface of a support and the thin layer is dried in two stages, that is, on the support and under a low tension in the state of a solidified film member peeled from the support, an excellent aromatic polyimide film having a specific low linear expansion coefficient and a high thermal dimension stability can be obtained. We have now completed the present invention based on this finding. More specifically, in accordance with the present invention, there is provided a dimensionally stable polyimide film composed of an aromatic polyimide obtained from a solution of a polymer formed by polymerization of a biphenyl-tetracarboxylic acid and a phenylene diamine, wherein the average linear expansion coefficient of the polyimide film in the temperature range of from 50.degree. C. to 300.degree. C. is 0.1.times.10.sup.-5 to 2.5.times.10.sup.-5 cm/cm.degree. C., the ratio (MD/TD) of the linear expansion coefficient in the machine direction (MD direction) of the film to the linear expansion coefficient in the transverse direction (TD direction) of the film is in the range of from 1/5 to 4, and the thermal dimension stability expressed by the ratio of the change of the size of the film at normal temperature after the heat treatment where the temperature is elevated to 400.degree. C. from normal temperature and the film is maintained at 400.degree. C. for 2 hours is less than about 0.3%. Furthermore, in accordance with the present invention, there is provided a process for the preparation of a dimensionally stable polyimide film, which comprises preparing a solution of a polymer formed by polymerization of a biphenyl-tetracarboxylic acid and a phenylenediamine in an organic polar solvent, forming a thin layer of the solution on the surface of a support, first drying the thin layer to form a solidified film member in which the solvent and formed water are present in an amount of 27 to 60% by weight, peeling the solidified film member from the surface of the support, second drying the film member under a tension lower than 100 g/mm.sup.2 at a temperature of 80.degree. to 250.degree. C. to form a solidified film which contains the solvent and formed water in an amount of 5 to 25% by weight, and drying and heat-treating the solidified film at a temperature of 200.degree. to 500.degree. C., which is higher than said second drying temperature, in the state where at least one pair of both the opposite edges of the film are fixed, to form a polyimide film. The dimensionally stable polyimide film of the present invention may also be prepared by another process which comprises preparing a solution of a polymer formed by polymerization of a biphenyl-tetracarboxylic acid and a phenylene diamine in an organic polar solvent, forming a thin layer of the solution on the surface of a support, drying the thin layer to form a solidified film in which the solvent and formed water are present in an amount of 20 to 60% by weight, peeling the solidified film from the surface of the support, drying and heat treating the solidified film at a temperature of 200.degree. to 500.degree. C. in the state where at least one pair of both the end edges are fixed to form and aromatic polyimide film, and again heat treating the aromatic polyimide film under a tension lower than 400 g/mm.sup.2 at a temperature of 250.degree. to 500.degree. C. DESCRIPTION OF THE PREFERRED EMBODIMENTS The aromatic polyimide film of the present invention is a novel, dimensionally stable, heat-resistant, aromatic polyimide film which has an average linear expansion coefficient which is approximately equal to those of various inorganic materials including ceramic materials, electroconductive metals and magnetic metal alloys customarily used for electronic materials. The film also has such a property that the thermal dimension stability expressed by the ratio of the change of the size after a high-temperature heat treatment is very good. Since the average thermal expansion coefficient of the aromatic polyimide film of the present invention is very close to that of an electroconductive metal and the aromatic polyimide film of the present invention is excellent in the thermal dimension stability, when a film (foil) of an electroconductive metal is formed on or bonded to the film of the present invention, curling or the like is not caused even on exposure to a high temperature. Moreover, when a composite material (laminate material) comprising an electroconductive metal and a layer of the film of the present invention undergoes a high-temperature heat history at such processing as etching of the metal layer or soldering, curling or degradation of the properties of the processed product is not caused. The processes of the present invention are excellent film-forming processes in which a dimensionally stable aromatic polyimide film having all of the above-mentioned excellent properties can be prepared intermittently or continuously and a film which is stable in the foregoing excellent properties can be prepared with good reproducibility on an industrial scale. The processes of the present invention are most characterized by the specific two-staged drying or heat treating where a thin layer is prepared from a solution of a specific polyamic acid or the like on the surface of a support by the solution casting method, the thin layer of the solution is dried on the support to form a solidified film member containing a solvent at a high ratio, the solidified film member is peeled from the support and the film member is dried or heat treated under a low tension to form a solidified film having a low solvent content. It has not been known that an aromatic polyamide film having excellent properties can be prepared according to a process as provided by the present invention. The polyimide film of the present invention is a film of an aromatic polyimide obtained, by using as a film-forming dope a polymer solution of a high-molecular-weight polymer (polyimide precursor) formed by polymerizing a biphenyl-tetracarboxylic acid (aromatic tetracarboxylic acid component) such as 2,3,3, 4-biphenyl-tetracarboxylic acid, 3,3,4,4-biphenyl-tetracarboxylic acid or a derivative thereof, for example, an acid anhydride, an acid ester or an acid halide, and a phenylene diamine (aromatic diamine component) such as o-phenylene diamine, m-phenylene diamine or p-phenylene diamine, homogeneously dissolved in an organic polar solvent. Accordingly, the film of the present invention is composed of a high-molecular-weight aromatic polyimide comprising at least 90 mole %, especially at least 95 mole %, of recurring units represented by the following general formula (I): #STR1# The polyimide film of the present invention is composed of the above-mentioned polyimide and the average linear expansion coefficient of the film is very small and 0.1.times.10.sup.-5 to 2.5.times.10.sup.-5 cm/cm.degree. C., preferably 0.5.times.10.sup.-5 to 2.3.times.10.sup.-5 cm/cm.degree. C. Moreover, the ratio (MD/TD) of the linear expansion coefficient in the machine direction (MD direction) of the film to the linear expansion coefficient in the transverse direction (TD direction) of the film is from 1/5 to 4, preferably from 1/3 to 3.0. Furthermore, the value of the thermal dimension stability is less than 0.3%, preferably less t