Transfer sheet for making printed-wiring board by injection molding and method for producing same

Abstract: The present invention provides a transfer sheet for making a printed-wiring board by injection molding which is free from damage of circuit at injec­ tion molding and is excellent in releasability and which comprises a carrier film, a copper foil circuit and a releasing layer provided between the carrier film and the copper foil circuit. Preferably, said releasing layer provides a T-type peeling strength between the carrier film and the copper foil circuit of 0.025-0.25 kg/25 mm. A method for producing such transfer sheet is also provided which comprises coating a releasing layer on a carrier film, then contact-bonding a copper foil thereto with heating, pattern-printing an etching resist on the resulting laminate film and subjecting it to etching to form a copper foil circuit.

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to a process for producing a transfer sheet for making an injection molded printed-wiring board and to a construction of the transfer sheet. RELATED ART STATEMENT As processes for obtaining injection molded printed-wiring board by transfer method, there is a process which uses a transfer sheet comprising laminated circuit units formed by laminate-printing using electroconductive and insulating pastes [Japanese Patent Kokai (Laid-Open) No. 63-219189] and a process which uses a transfer sheet in which are provided a peeling layer comprising a material corroded with a solvent for electroconductive ink, a protective layer and printed circuit comprising an electroconductive ink [Japanese Patent Kokai (Laid-Open) No. 63-257293]. Furthermore, as process which uses vapor deposition method, there is a method of obtaining a metallic pattern printed board by providing a metallic film on an electroconductive primary film by plating, forming a pattern with a masking material, then further depositing a metallic film thereto to obtain a metallic pattern and subjecting it to etching [Japanese Patent Kokai (Laid-Open) No. 63-274194]. However, these transfer sheets suffer from various problems since electroconductive paste or metallic conductor obtained by vapor deposition is used as circuit conductor. When electroconductive paste is used, circuit can be conveniently made by printing and drying, but it is higher in conductor resistance than metallic copper and the resulting circuit lacks reliability. Furthermore, in the case of obtaining copper circuit by vapor deposition, conductor resistance has no problems, but it is difficult to obtain thickness of conductor necessary for printed-wiring board. Furthermore, the resulting metal surface is flat and so anchoring effect cannot be exhibited in adhering a substrate thereto and adhesion to substrate is inferior. Further problem is very high cost. On the other hand, copper foil generally used for printed-wiring board is high in electrical reliability as circuit and besides is roughened in surface and thus sufficient adhesion can be expected. However, in order to make a transfer sheet using this copper foil, it is necessary to laminate a carrier film and copper foil to each other with a suitable adhesive to form a laminate sheet. The above adhesive is required to have an adhesive activity high enough to inhibit damages such as shifting or peeling of circuit which may occur at production of circuit pattern or injection molding and releasing power for easy peeling of carrier film after molding which are contrary to each other in nature. It is very difficult for general adhesives to have both the properties. Sufficient solution has not yet been made in this respect. SUMMARY OF THE INVENTION The present invention has been accomplished as a result of intensive research conducted by the inventors in an attempt to solve the above problems and obtain a transfer sheet of high reliability. That is, it has been found that a transfer sheet free from damages of circuit which may occur at injection molding and excellent in releasability of carrier film can be obtained when adhesion strength in peeling the laminated carrier film and copper foil by pulling ends of them in the direction of 180° to each other (hereinafter referred to as "T-type peeling strength") which is measured in accordance with JIS K6854 is adjusted to 0.025-0.25 kg/25 mm. A transfer sheet according to the present invention is defined in claim 1, a method for producing such a transfer sheet is defined in claim 3. DETAILED DESCRIPTION OF PREFERRED EMBODIMENT Adjustment of the strength for peeling the contact-bonded carrier film and copper foil to the above range can be attained by providing a specific releasing layer between the carrier film and the copper foil. In order to adjust the T-type peeling strength to the above range by using the releasing layer, adhesion and releasability must be balanced. When substrate resins are those which are low in molding temperature such as ABS, adhesives mainly composed of polyvinyl formal resin having a relatively high heat resistance may be used as the releasing layer. However, by use of such adhesive alone, it is difficult to obtain releasability of the substrate or copper foil after molding, and releasability imparting agent such as silicone must be used in combination. When substrate resins are those which are molded at higher than 300°C such as polycarbonate resins (PC) or polyether imides (PEI), polyether sulfone (PES), and polyphenylene sulfide (PPS) which are called super engineering plastics, use of polyvinyl formal resin and silicone in combination causes damage of circuit due to softening and flowing of releasing layer. Therefore, when the above resins are used as substrate resins, a mixture of an epoxy resin, a hardener and an adhesive is used as releasing layer. The polyvinyl formal resins used as releasing layer for resins of low molding temperature such as ABS include those which have an average polymerization degree of about 200 or more, preferably about 400-450 and these are dissolved in an amount of about 5-20 (wt/wt)% in a solvent such as chloroform and this is coated. If the average polymerization degree is less than 200, heat resistance decreases and the releasing layer may be softened at injection molding to cause damage of circuit. In order to improve heat resistance of polyvinyl formal resin, isocyanate resin, melamine resin, phenolic resin, etc. may be added. Coating amount of the releasing layer (solid content excluding solvent, which will be applied to the coating amounts referred to hereinafter) is usually 1-30 g/m², preferably 5-15 g/m². If the coating amount is less than 1 g/m², lamination of carrier film and copper foil cannot be satisfactorily performed and there may occur problems such as formation of bubbles. As the releasability imparting agent, there may be used various silicone releasing agents and/or acrylic oligomers. The silicone releasing agents include dimethyl silicone oil and α-methylstyrene-, α-olefin-, polyether-, epoxy-, or carboxyl-modified silicone oils which are generally used as releasing agents. With reference to amount of the silicone releasing agent, a solution of the releasing agent at a concentration of 0.01-0.5% (wt/wt), preferably 0.05-0.2% (wt/wt) in a solvent such as toluene, chloroform or MEK is coated on a polyvinyl formal resin or on copper foil in an amount of 5-15 g/m². Alternatively, the releasing agent may be previously blended with polyvinyl formal resin. As the acrylic oligomers, there may be suitably used butyl acrylate oligomers having a molecular weight of 3000-10000. For example, toluene, chloroform or MEK may be used as solvent. A solution of the oligomers at a concentration of 0.0001-0.01% (wt/wt) may be coated in an amount of 5-15 g/m² as in the case of silicone. In the case of using a substrate resin necessary to be molded at high temperature, a mixture of an epoxy resin, hardener and an adhesive is used as releasing layer. The epoxy resin includes, for example, bisphenol type epoxy resins, novolak-type epoxy resins, and alicyclic epoxy resins and especially preferred are bisphenol type epoxy resins. The hardeners for epoxy resin, may be made of, for example, aliphatic amines such as ethylenediamine, diethylenetriamine, triethylenetetramine, and hexamethylenediamine, imidazoles such as 2-methylimidazole, 2-ethylimidazole, and 2-ethyl-4-methylimidazole and aromatic amines such as m-phenylenediamine, diaminodiphenylmethane and diaminodiphenylsulfone. Aliphatic amines and imidazoles are preferred from the point of hardenability. As the adhesives, there may be used thermoplastic resins such as thermoplastic polyester resins, polyvinyl formal resins, polyvinyl butyral resins, and EVA (ethylene-vinyl acetate copolymers). Especially preferred are polyvinyl formal resins which are the same as those referred to hereinabove. In use, 100 parts by weight of epoxy resin, a usual amount of hardener and 1-10 parts by weight, preferably 2-5 parts by weight of thermoplastic resin are blended and the resulting blend is coated in an amount of 1-30 g/m², preferably 5-15 g/m². That is, in the case of "epoxy releasing layer", the desired T-type peeling strength which is a balanced power between adhesion and releasability can be obtained by adding small amount of an adhesive to an epoxy resin and by adjustment of hardening degree of the epoxy resin and addition amount of the adhesive. The transfer sheet for making injection molded printed-wiring board and process for producing it according to the present invention will be explained below. First, a copper foil is laminated by a heat pressure roll onto a carrier film on which a releasing layer has been formed by drying or hardening and drying the above-mentioned materials. Thus, a releasing layer is formed which is adjusted to provide a T-type peeling strength of 0.025-0.25 kg/25 mm, preferably 0.04-0.10 kg/25 mm. Metallic conductors include electrolytic copper foil for printed-wiring board, rolled copper foil, aluminum foil for shielding, and other metallic foils and may be optionally selected according to the objects. Preferred is an electrolytic copper foil having roughened surface. Examples of carrier film are polyester (PET) film and polyimide (PI) film. When the electrolytic copper foil is used, the roughened surface should be the front surface. Then, onto the copper foil of the resulting laminate sheet is provided an etching resist by printing method, photographic method, etc. and circuit is formed by conventional substrative process. The thus obtained transfer sheet is high in heat resistance of releasing layer, can retain circuit even if resin flows at injection molding, is excellent in releasability of carrier film at peeling and shows stable transfer moldability. Then, process for making an injection molded printed-wiring board using the transfer sheet according to the present invention will be explained below. An adhesive is coated on the above transfer sheet on which a copper foil circuit has been formed and is dried to form an adhesive layer. The adhesive can be suitably chosen depending on the kind of substrate resins used. The transfer sheet is placed in a cavity of an injection mold comprising a stationary platen side and a moving platen side so that the carrier film is on the front surface side of molded product and the mold is clamped. Then, substrate resin is injected into the cavity and simultaneously with formation of a molded product, the circuit on the transfer sheet is transferred onto the surface of the molded product. After cooling, the molded product is taken out from the mold and the carrier film and the releasing layer of the transfer sheet are peeled from the molded product to obtain an injection molded product having circuit conductor. As the substrate resin, there may be used general-purpose resins which are generally used for electric insulation such as ABS resin, polycarbonate (PC) and polybutyl terephthalate (PBT), and super engineering plastics such as polyether sulfone (PES) and polyether imide (PEI) for uses which require heat resistance. For some uses, fillers such as glass fiber may be incorporated into the substrate resin. It is considered that damage of circuit is caused by poor interfacial adhesion between releasing layer and copper foil, and low heat resistance of releasing layer. Influence of heat resistance of releasing layer is that the releasing layer of transfer sheet placed in a mold is heated by the mold or injected resin and is molten and softened and cannot resist the flowing pressure of the resin injected, resulting in shifting or breaking of circuit. Furthermore, with reference to releasability after injection molding, if the interfacial adhesion is too strong, there occurs film breaking or releasing-layer retention (retention of the releasing layer on the molded product side) to cause product failures. These problems have been solved by improvement of heat resistance and adjustment of adhesion to proper level according to the present invention. The present invention will be explained by the following examples. Example 1 On a PET film of 50 µm thick as a carrier film was coated a chloroform solution containing 10% by weight of DENKAFORMAL #20 (commercially available polyvinyl formal resin manufactured by Denki Kagaku Kogyo Co.) in an amount of 10 µg/m² (solid content). Furthermore, in order to obtain releasability from copper foil, epoxy-modified silicone KF-103 (manufactured by Shinetsu Chemical Co., Ltd.) was coated on an electrolytic copper foil of 35 µm thick in an amount of 10 g/m² with silicone concentration being 0.1%. After drying, the carrier film and the copper foil were laminated with heating at 160°C by a heating pressure roll. Speed of the pressure roll was 1 m/min and pressure was 1 kg/cm². Then, pattern was formed with etching resist by screen printing, followed by subjecting to etching with ferric chloride solution to obtain a transfer sheet having copper foil circuit. This transfer sheet was placed in an injection mold and a molded product having circuit on the surface was obtained by conventional method using ABS resin as a substrate resin. Example 2 Transfer sheet and molded product were obtained in the same manner as in Example 1 except that a chloroform solution containing 0.001% by weight of MACROMONOMER CB-6 (manufactured by Toagosei Chemical Industry Co., Ltd.) as an acrylic oligomer was used in place of the epoxy-modified silicone KF-103 as a releasability imparting agent. Comparative Example 1 Transfer sheet and molded product were obtained in the same manner as in Example 1 except that concentration of silicone KF-103 was 0.005%. Comparative Example 2 Transfer sheet and molded product were obtained in the same manner as in Example 1 except that concentration of silicone KF-103 was 0.8%. The transfer sheets and molded products obtained in the above Examples 1 and 2 and Comparative Examples 1 and 2 were evaluated and the results are shown in Table 1. That is, results of evaluation on T-type peeling strength between carrier film and copper foil, damage of circuit and releasability are shown in Table 1. T-type peeling strength was measured according to JIS K-6854 using tensile strength tester (AUTOGRAPH manufactured by Shimadzu Seisakusho Ltd.). Damage of circuit was judged by visual evaluation of shifting or breaking of circuit transferred to molded product. Releasability was evaluated on occurrence of breakage of carrier film or occurrence of releasing-layer retention (on molded product) when carrier film was peeled from the molded product obtained by the injection molding. Example 3 A transfer sheet was prepared in the same manner as in Example 1 except that ARALDITE CY-232 (manufactured by Ciba Geigy Co.) as epoxy resin, aliphatic amine type hardener HY-956 (manufactured by Ciba Geigy Co.) as hardener and DENKAFORMAL #20 as adhesive were used at a weight ratio of 100:30:3 in place of materials used as releasing agent in Example 1; coating amount (solid content) was 10 g/m²; hardening and drying were carried out at 160°C for 1 minute; and heat lamination conditions were temperature: 200°C, roller speed: 1 m/min and roller pressure: 1 kg/m². The transfer sheet was subjected to injection molding by conventional method using polycarbonate (PC) in place of ABS resin used in Example 1 to obtain a molded product having a circuit. Examples 4, 5 and 6 Transfer sheet and molded product were obtained in the same manner as in Example 3 except that thermoplastic polyester resin VYLON-200 (manufactured by Toyobo Co., Ltd.) was used as adhesive. Separately, molded products were obtained in the same manner as above except that PES and PEI were used as substrate resin (Examples 5 and 6). Results of evaluation of these transfer sheets and molded products are shown in Table 2. Comparative Example 3 Transfer sheet and molded product were obtained in the same manner as in Example 3 except that weight ratio of materials for releasing layer was 100:30:15. Comparative Example 4 Transfer sheet and molded product were obtained in the same manner as in Example 3 except that weight ratio of materials for releasing layer was 100:30:0 and hardening and drying conditions were 160°C and 5 minutes. These were evaluated in the same manner as above and the results are shown in Table 2 in accordance with the same criteria as in Table 1. Results of evaluation on circuit damage and releasability in Comparative Example 3 are both "X" in Table 2. This is because ratio of the thermoplastic resin to the epoxy resin was high and hence adhesion increased to cause deterioration in releasability and furthermore, heat resistance decreased to cause readily the damage of circuit. As explained above, the molded product obtained according to the present invention is high in reliability of circuit and can be used for housings or connectors of various electronic equipments. According to the method of the present invention, conductor circuits in which copper foil of high electric reliability may be used in molded products can be easily produced and so such effects as sharp reduction in use of rigid or flexible printed-wiring board employed at present and simplification of setting up of parts and saving of space can be expected and the present invention makes a great contribution to miniaturization and weight saving of electronic equipments and reduction of cost.

Claims: A transfer sheet for making a printed-wiring board by injection molding which comprises a carrier film, a copper foil circuit and a releasing layer provided between the carrier film and the copper foil circuit, wherein the releasing layer comprises a combination of a polyvinyl formal resin and a releasability imparting agent or a combination of an epoxy resin, a hardener for the epoxy resin and an adhesive. A transfer sheet according to claim 1, wherein the releasing layer is formed so that a T-type peeling strength between the carrier film and the copper foil circuit is 0.025-0.25 kg/25 mm. A method for producing a transfer sheet according to claim 1 for making an injection molded product which comprises coating the releasing layer on the carrier film, then contact-bonding the copper foil on the releasing layer with heating, pattern-printing an etching resist on the copper foil of the resulting laminate film and subjecting it to etching to form the copper foil circuit.

Inventors: INOGUCHI HIROKAZU, KANNO NAOTO, NUMAZAKI YUUKI, WATANABE AKIHIKO, INOGUCHI, HIROKAZU, KANNO, NAOTO, NUMAZAKI, YUUKI, WATANABE, AKIHIKO
Applicants: NITTO BOSEKI CO LTD, NITTO BOSEKI CO., LTD.
Application year: 1990
Main classification: H05K3
IPC codes: B29C45, H05K3