A process for producing a flexible printed base

Abstract: In a process for producing a flexible printed base by directly coating a copper foil with a polyimide precursor, followed by heating, drying and curing the precursor; the curing is effected under an inert gas, under a tension of 0.02 to 0.2 Kg/cm, and at a temperature of 200°C to 450°C.

Description: This invention rleates to a process for producing a flexible printed base comprising a copper foil and having good heat resistance and improved folding endurance. Flexible printed bases are used for producing flexible printed circuits, and in recent years, there has been an increasing tendency to reduce the size of electronic equipment, make the base thinner and make the density of the circuits higher. Conventional polyimide flexible bases have mainly been produced by applying a polyimide film onto a copper foil using an adhesive. However, since such bases use an adhesive, problems have been raised in the aspect of heat resistance, electrical characteristics, etc.; hence such bases have not fully realized their potential. In order to overcome these problems, a process of subjecting a polyimide film to hot-melt adhesion onto a copper foil is disclosed in Japanese laid-open patent application No. Sho 57-181,857/1982 and a process of directly coating a copper foil with a polyimide precursor, followed by heating and drying treatment (hereinafter referred to as a "direct coating process") is disclosed in Japanese patent publications Nos. Sho 61-111,359/1986 and Sho 63-69,634/1988. In these improved processes, since no adhesive is used, the heat resistance, electric characteristics, etc. are improved, but since the former process employs steps of production of film, its hot-melt adhesion, etc., it requires the same number of steps, or more, as that of conventional products (using adhesives). In the direct coating process, since it involves no film production, simplicity of steps is possible and also the heat resistance and electric characteristics of the resulting flexible printed base are improved, but the process cannot give a high folding endurance to the base. Further, for end uses requiring a high folding endurance, rolled copper foil rather than electrolytic copper foil has often been used as copper foil. However, in general, roller copper foil is not only more expensive than electrolytic copper foil, but also the peel strength of the resulting flexible printed base is reduced. The present invention proposes a process for producing an improved flexible printed base. It has been disclosed that when a flexible printed base is heat-treated at 100°C to 200°C, its folding endurance is generally improved by about 1.5 times (Japanese laid-open patent applications Nos. Sho 54-110,466/1979 and Sho 53-17,764/1978), but when an electrolytic copper foil is used for the base, the folding endurance does not reach that of rolled copper foil and is insufficient. Further, when an electrolytic copper foil was subjected to heat curing in the atmosphere at 200°C to 450°C, the fodling endurance was reduced. Whereas, when the above heat curing was carried out in an inert gas having an oxygen concentration of 0.5% or lower, preferably 0.2% or lower and under the tension of 19.6 to 196 N/m (0.02 to 0.2 Kg/cm,) it has been found surprisingly enough that the folding endurance of the resulting flexible printed base is improved up to a similar folding endurance to that of rolled copper foil. This improvement in the folding endurance has been found, from the X-ray diffraction pattern of the copper foil, to originate from inhibition of thermal cleavage reaction of high molecules in the above-mentioned atmosphere are at the same time the crystalline configuration of the copper foil. The present invention provides a process for producing a flexible printed base by directly coating a copper foil with a polyimide precursor, followed by heating, drying, and curing the precursor; in which process the curing is carried out in an inert gas, under a tension of 19.6 to 196 N/m (0.02 to 0.2 Kg/cm), and at a temperature of 200°C to 450°C. The inert gas atmosphere referred to herein means an atmosphere formed by purging the inside of a drying furnace with an inert gas such as nitrogen gas, etc. under pressure. As the inert gas, nitrogen gas having an oxygen concentration of 0.5% or less, preferably 0.2% or less is preferred. Of course, it is also possible to use rare gases such a argon, etc. and an inert gas such as CO₂ gas. The flexible printed base in the present invention may be prepared by directly apply a polyimide precursor having a thermal expansion coefficient the same as that of copper onto a copper foil, followed by heating, drying and curing. The polyimide precursor applied to the copper foil may, for example, be a polymer having repeating units of the formula: (wherein R₁ represents a tetravalent aromatic group and R₂ represents a divalent aromatic hydrocarbon radical). Examples of the aromatic tetracarboxylic acid dianhydride (affording the group R₁) used in the preparation of the above precursor are pyromellitic acids 2,3,3′,4′-tetracarboxydiphenyl, 3,3′,4,4′tetracarboxydiphenyl, 3,3′,4,4′-tetracarboxybenzophenone, etc. Examples of aromatic diamines (affording the group R₂) are p,m-phenylenediamine, 4,4′-diaminodiphenyl ether, 3,4′-diaminodiphenyl ether, 3,3′-diaminodiphenyl ether, 4,4′-diaminodiphenyl ketone, 3,4′-diaminodiphenyl ketone, 4,4′-diaminodiphenyl sulfone, 4,4′-diaminobiphenyl, 3,4′-diaminobiphenyl, 4,4′-diaminodiphenylmethane, 2,2′-bis(4-aminophenyl)propane, 1,4-di(4-aminophenyl)phenyl ether, 1,3′-di(4-aminophenyl)phenyl ether, and diaminosiloxanediamines of the formula (wherein R₃ represents a C₁-C₃ aliphatic hydrocarbon radical or a C₆-C₉ aromatic hydrocarbon radical; R₄ represents a C₃-C₅ divalent aliphatic hydrocarbon radical or a C₆-C₉ divalent aromatic hydrocarbon radical; and ℓ is an integer of 3 to 150). Examples of the organic solvent used for preparing these polyimide precursors are polar organic solvents such as N-methyl-2-pyrrolidone, N,N′-dimethylacetamide, N,N-dimethylformamide, dimethylsulfoxide, cresol, etc. and these may be used alone or in admixture. The reaction is carried out in the range of 0°C to 80°C. As to the process for forming a polyimide film on a copper foil in the present invention, firstly a solution containing 10 to 30% by weight of a polyimide precursor is coated onto the surface of the copper foil by means of Comma-Coater, doctor knife or the like, followed by removing the solvent contained in the solution by heating to 100°C to 200°C. The resulting material is then heat-cured in an inert gas having an oxygen concentration of 0.5% or less, under a tension of 0.02 to 0.2 Kg/cm and at 200°C to 450°C. In the process of the present invention, it is important to optimize the atmosphere and tension during the heat-curing step at 200°C to 450°C. Namely, according to X-ray diffraction, crystals of copper of a copper foil heat-cured at 300°C to 450°C in the atmosphere contain copper oxide having a different crystal lattice from that of copper and being hetero-generously coexistent with copper crystals ro reduce the folding endurance of the base. Further, as a first element of the present invention, when heat-curing is carried out in an inert gas having an oxygen concentration of 0.5% or less, preferably 0.2% or less at a temperature of 200°C to 450°C, the resulting copper foil exhibits the same lattice constant as that of untreated copper foil and the crystal size increases uniformly to about three times the original size, and as a second element of the present invention, when the above heat-curing is carried out under a tension of 19.6 to 196 N/m (0.02 to 0.2 Kg/cm), the reflection of the copper foil on the surface of (2,2,0) becomes weak while the reflection on the surface of (1,1,1) becomes strong. Namely, according to the heat-curing of the present invention, the resulting copper foil not only has an increased crystal size of copper in terms of single crystal lattice constant, but also the selective orientation of the crystal surface is carried out to greatly contribute to improvement in the folding endurance. Under a tension exceeding 196 N/m (0.2 Kg/cm), the resulting copper foil is elongated to raise a problem of causing anisotropies in MD (machine direction) and TD (transverse direction). The present invention will be described in more detail by way of Examples and Comparative examples, but it should not be construed to be limited thereto. Preparation example Into a 10 ℓ capacity glass reactor fixed with a thermometer, a stirrer and a nitrogen gas-introducing tube were introduced p-phenylenediamine (240.8 g), 4,4'-diaminodiphenyl ether (111.4 g) and N-methyl-2-pyrrolidone (7 ℓ) in nitrogen gas current, followed by stirring these and dissolving together, gradually adding to the resulting solution, 3,3',4,4'-biphenyltetracarboxylic acid dianhydride (818.9 g) with stirring, and reacting these at 20°C or lower for 5 hours to obtain a polyamic acid solution of a polyimide precursor. The resulting polyimide precursor had a logarithmic viscosity of 1.8 as measured in N'-methyl-2-pyrrolidone at a concentration of the precursor of 0.5 g/dℓ at 30°C. Example 1 The polyimide precursor solution prepared above was coated onto one roughened surface of the two surfaces of an electrolytic copper foil of 18 µ thick so as to give a coated thickness of 400 µm by means of a coater, followed by drying the copper foil having the polyimide precursor coated thereon in a hot-air drying furnace at 100°C and 200°C, each for 10 minutes to remove the solvent contained in the solution and curing the resulting material in nitrogen gas containing an oxygen concentration of 0.3%, under a tension of 98 N/m (0.1 Kg/cm) and at 250°C and 350°C, each for 10 minutes to obtain a flexible printed base of the polyimide of 25 µ. This flexible printed base exhibited a folding endurance (MIT) of TD 57,000 times and MD 56,000 times. The measurement of MIT was carried out by forming a conductor of one reciprocation at a width of 1.5 mm and an interval between circuits of 1.0 mm by etching, flexing the conductor at a curvature radius of 0.8 mm, a flex rate of 180 times/min. and a strength of 500 gf/cm² and seeking the flex times at which complete disconnection of the conductor circuit occurred (JIS-C-P8115). Example 2 Example 1 was repeated except that an electrolytic copper foil of 35 µ thick was used as copper foil to obtain a flexible printed base. The base exhibited a folding endurance of TD 800 times and MD 790 times. When the results are compared with those of Comparative example 5 (using a commercially available product), the folding endurance is superior. Comparative example 1 Example 1 was repeated except that heat curing was carried out in the atmosphere, to obtain a flexible printed base. This base was considerably oxidized and exhibited a folding endurance of TD 2,800 times and MD 3,300 times. Comparative example 2 Example 1 was repeated except that a copper foil of rolled copper of 18 µ was used, to obtain a flexible printed base. This base exhibited a folding endurance of TD 42,000 times and MD 60,000 times. Comparative example 3 Example 1 was repeated except that heat curing was carried out under no tension, to obtain a flexible printed base. This base exhibited a folding endurance of TD 35,000 times and MD 32,000 times. Comparative example 4 Example 1 was repeated except that heat curing was carried out under a tension of 450 N/m (0.5 Kg/cm), to obtain a flexible printed base. This base exhibited TD 51,000 times and MD 35,000 times. Further, longitudinal wrinkles were formed on the base so that the printed base raised a problem in the aspect of processing step and could not be used. Comparative example 5 A commercially available flexible printed base (electrolytic copper product) obtained by conventional lamination with an adhesive was subjected to measurement of its folding endurance in the same manner as in Example 1, to exhibit TD 288 times and MD 330 times. As described in detail, according to the process of the present invention, when a flexible printed base is produced according to a direct coating process of directly coating a polyimide precursor onto a copper foil, followed by heat curing at a high temperature, it has become possible to produce a flexible printed base having a far superior folding endurance.

Claims: A process for producing a flexible printed base by directly coating a copper foil with a polyimide precursor, followed by heating, drying, and curing the precursor; in which process the curing is carried out in an inert gas, under a tension of 19.6 to 196 N/m (0.02 to 0.2 Kg/cm), and at a temperature of 200°C to 450°C. A process according to claim 1 wherein the inert gas is nitrogen gas having an oxygen concentration of 0.5% or less. A process according to claim 1 or claim 2 wherein the copper foil is an electrolytic copper foil.

Inventors: FUKUNAGA SHOUICHI, KIKUTA KAZUTSUNE, KONOTSUNE SHIRO, TAKAHASHI ATSUSHI, FUKUNAGA, SHOUICHI, KIKUTA, KAZUTSUNE, KONOTSUNE, SHIRO, TAKAHASHI, ATSUSHI
Applicants: CHISSO CORP, CHISSO CORPORATION
Application year: 1990
Main classification: H05K3
IPC codes: B05D3, H05K3, B32B15, H05K1