Process for repairing circuit connections

Abstract: Circuit connections can be repaired with high accuracy and high reliability by peeling apart mutually connected circuit portions and placing a porous sheet of predetermined shape on the peeled circuit surface, impregnating the porous sheet with a peeling liquid, removing the porous sheet containing unnecessary adhesive dissolved by the peeling liquid, while cleaning the circuit surface contacted with the peeling liquid, and connecting the cleaned circuit surface to a circuit surface via an adhesive.

Description: BACKGROUND OF THE INVENTION The present invention relates to a process for repairing circuit connections. As electronic parts have become smaller and thinner, the circuits formed therein have become a higher density and finer. In connecting these fine circuits, the use of conventional connecting material such as solder, rubber connector or the like is inappropriate, so that in recent years, an adhesive or a film (these are hereinafter referred to as a connecting member) both of anisotropic electric conductivity has come to be used widely. In connecting fine circuits with this connecting member, a connecting member layer consisting of an adhesive containing a given proportion of an electric conductive material is provided between two circuit groups opposed to each other; pressure or pressure and heat are applied to establish electrical connection between each two opposing circuits of the two circuit groups and simultaneously impart electrical insulation between adjacent circuits; thus, the two circuit groups opposed to each other are adhered and fixed. As the film-shaped material having anisotropic electric conductivity only in the thickness direction according to the prior art, there is a sheet-shaped structure having electric conductivity in the thickness direction via electric conductive particles, obtained by dispersing electric conductive particles in a non-conductive resin so that the particles make no contact with each other and then shaping the dispersion into a sheet having substantially the same thickness as the diameter of the electric conductive particles, as disclosed in, for example, Japanese Patent Kokai No. 51-21192. Also, there is an attempt of connecting two opposing circuit groups by adhering and fixing them in a contact state by the use of an insulating adhesive containing no conductive particles (e.g. Japanese Patent Examined Publication No. 46-43732). In connection by these connecting members, when electrical connection is poor or when electronic parts or circuits become faulty after the connection, there is employed a process comprising tearing up each two connected circuits, removing the connecting member remaining on the circuits with a solvent or the like, and reconnecting the cleaned circuits with a fresh connecting member of the same type. Also, in adhering a housing or other parts to an electronic part having unconnected circuits, with an adhesive, when the adhesive has been mistakenly adhered to the unconnected circuits and consequently the later connection has become difficult, there is employed a process comprising wiping off the adhesive adhering to the unconnected circuits, with a cloth or a swab both impregnated with a solvent or the like. In the above-mentioned processes for removing the adhesive, however, the partial removal of the adhesive is extremely difficult. For example, a connecting member of anisotropic electric conductivity is also used in connecting a number of electronic parts to a large-sized electronic part having a number of circuits; when part of the electronic parts connected to the large-sized electronic part is repaired, there are problems in that the above removal of the adhesive adversely affects the other connecting member(s), resulting in incurring poor connection and reduced reliability. For example, in connection of electronic parts generally used in a liquid crystal display material, as shown in Fig. 1(a), wherein ITO (indium tin oxide) circuits on a glass substrate 1 are connected with a number of FPC (flexible printed circuit) substrates 2 by the use of a connecting member of anisotropic electric conductivity, there are used very fine circuits of, for example, 100 µm in width of each conductor (circuit) and 100 µm in distance between two adjacent conductors; further, the distance between the two adjacent FPC circuits is as small as several hundreds of microns in some cases as a result of recent trend of lighter weight, smaller thickness and smaller size in electronic parts. In order to remove one of the FPC substrates 2 shown in Fig. 1(a) and effect reconnecting, that FPC circuit is peeled and the connecting member remaining on the glass substrate is removed by the above-mentioned conventional method of, after decomposition or dissolution, wiping off with a swab impregnated with a peeling solution, infiltration of the peeling solution to the adjacent FPC circuits is unavoidable, that is, even the connecting members of the adjacent FPC circuits are decomposed or dissolved and there are invited an increase in connection resistance and reduction in reliability of connection. In the above-mentioned conventional methods for removal of adhesive or connecting member, it is possible to use, in combination, a method of using an adhesive masking tape or the like to protect the portion not to be removed, from the peeling solution. In this case, however, the adhesive in the masking tape is decomposed or dissolved by the peeling solution, resulting in unavoidably increasing in connection resistance and reducing in reliability of connection. SUMMARY OF THE INVENTION The present invention has been made in order to overcome such drawbacks of the prior art. The present invention provides a process for repairing electrical circuit connections between circuit substrates facing each other and having circuits connected via an anisotropic adhesive, which process comprises: peeling apart a circuit substrate to be replaced, together with the circuit thereon from the other substrate, placing a porous sheet of predetermined shape on the surface of adhesive remaining on the said other substrate, impregnating the porous sheet with a peeling liquid so as temporarily to contact the remaining adhesive with the peeling liquid, then removing the porous sheet containing the peeling liquid and cleaning the remaining adhesive from the surface of said other substrate to leave a cleaned circuit on said other substrate and thereafter connecting the cleaned circuit to a circuit of a new substrate with an anisotropic adhesive. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1(a) to 1(f) are schematic views explaining the process of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention resides in a process for repairing circuit connections, which enables reconnection with significantly improved reliability by allowing a peeling liquid to act only on the areas requiring the removal of adhesive and thereby preventing the other areas from being impregnated with the peeling liquid. The present invention is hereinafter described in detail. The porous sheet used in the present invention is a sheet having fine pores. The porous sheet has no particular restriction as long as it allows the infiltration of a peeling liquid into its whole portion by a capillary action in a short period of time. The surface of the porous sheet is contacted with the adhesive remaining on the circuits of circuit connections to be repaired, whereby the peeling liquid contained in the pores of the porous sheet is infiltrated into the adhesive; therefore, the large diameters of the pores make it difficult to accurately set the contact area between the porous sheet and the adhesive. Hence, the diameters of the pores are preferably 100 µm or less. If necessary, the surface area of the porous sheet opposite to a surface to be contacted with the remaining adhesive can be sealed in order to ensure no leakage of the peeling liquid from the sealed area, whereby the area of the adhesive to be removed can be limited accurately. The sealing of the porous sheet surface can be effected, for example, by heat-melting the porous sheet surface or adhering to the porous sheet surface a coating or a resin film both not attacked by the peeling solution, to seal the surface area of the porous sheet not to be contacted with the adhesive. Specific examples of the porous sheet include fibrous materials such as paper, glass and the like; a non-woven fabric; a porous polytetrafluoroethylene; porous sheets obtained by sintering a thermoplastic resin (e.g. polyethylene); an urethane foam having fine open celled pores, etc. Of these, the porous sheet obtained from a thermoplastic resin is preferable due to easy surface coating mentioned above by melting with heating or melt adhesion of film with heating. The peeling liquid used in the present invention can be any one as long as it can decompose or dissolve the adhesive. As the peeling liquid, there can be mentioned, for example, organic solvents such as toluene, alcohols, ketones and the like; organic acids such as formic acid, acetic acid and the like; inorganic acids such as hydrochloric acid, sulfuric acid and the like; and alkali compounds such as sodium hydroxide, potassium hydroxide and the like. They can be used singly or in combination, or in a solution in an appropriate solvent. The use of a peeling liquid of low viscosity and good wettability against porous sheet is preferable because such a peeling liquid has good infiltrability into porous sheet. The adhesive used in the present invention has no particular restriction as long as it can be decomposed or dissolved by the peeling liquid. The present invention is applicable to, for example, thermoplastic resins such as vinyl acetate resin, polypropylene and the like; highly heat-resistant resins such as polyethersulfone, polyimide, polysulfone and the like, and thermosetting resins such as epoxy resin and the like. Among these adhesives, the present invention is particularly effective to an adhesive having a cross-linked structure, because other methods are difficult to apply to such an adhesive while in the present invention the time of contact of peeling liquid with adhesive can be made long. In the present invention, the adhesive which has been decomposed or dissolved by the peeling liquid, is infiltrated into the porous sheet and removed. When the removal of the decomposed or dissolved adhesive is not sufficient, the complete removal can be effected by wiping off the residual adhesive with a dry cloth, a dry paper, a dry swab or the like, or with an appropriate material impregnated with a solution incapable of decomposing or dissolving the adhesive. In the step of mounting or placing the porous sheet used in the present invention accurately on the site requiring adhesive removal, coating of a part of the porous sheet surface with an adhesive can impart temporary adhesion to the porous sheet, whereby the repair operator can easily confirm the site for mounting and operation efficiency is improved. It is preferable to use an adhesive which can be decomposed or dissolved by the peeling liquid, because such an adhesive can be removed together with the adhesive remaining on circuit surface by the peeling liquid infiltrated into the porous sheet. As the adhesive, there can be used, for example, a rubber-based adhesive, an acrylic resin-based adhesive, or a water-based adhesive. During or after removing the porous sheet containing the peeling liquid dissolving the adhesive, the circuit surface contacted with the peeling liquid is cleaned by a conventional process. The repair is completed by connecting, to the thus cleaned circuit surface, a cleaned circuit surface of the other circuit substrate or a circuit of a new-substrate, with an adhesive. In this case, when the adhesive contains hard electric conductive particles, the electric conductive particles can pierce the residual adhesive layer and thereby can establish electrical conduction; accordingly, the circuit substrates separated by mechanical tear-up in the first step can be used in reconnection as they are. This is effective for reduction in repairing cost. According to the present invention, the peeling liquid contained in the pores of the porous sheet infiltrates into the portion of the adhesive with which the porous sheet surface is contacted, and dissolves or decomposes the adhesive portion, so that only the desired adhesive portion can be removed accurately and the above-mentioned problems are solved. When the pcrous sheet is sealed at the end and/or the surface opposite to the surface to be contacted with the adhesive, the spreading of the peeling liquid to the adjacent areas can be prevented. Moreover, the porous sheet has excellent workability because it is a sheet, enables adhesive removal operation in a short period of time, and makes easy the automation of removal operation. The present invention is hereinafter described specifically by way of Examples, in which all parts and percents are by weight, unless otherwise specified. Example 1 On one side of a sintered sheet (thickness: 0.4 mm, pore diameter: 30 µm) made from polyethylene particles of 50 µm in particle diameter was melt-laminated a polyethylene film. The other side of the sintered sheet was coated with an acrylic resin type adhesive to obtain a porous sheet. The porous sheet was cut by a hot knife of 200°C to prepare a porous sheet of 10 mm in width and 20 mm in length. On the other hand, three FPC substrates 2 (each having circuit width of 100 µm and circuit distance of 100 µm) having a connecting width of 10 mm were electrically connected to terminal portions of ITO circuits 11 formed on a glass substrate 1 using an adhesive film of anisotropic electric conductivity [Anisolm AC-6071 (trade name), an epoxy type product manufactured by Hitachi Chemical Co., Ltd., 20 µm in thickness] as shown in Fig. 1(a). Fig. 1(b) is a cross-sectional view of the thus obtained connection wherein the ITO circuits 11 on the glass substrate 1 are connected to the circuits 22 on the FPC substrate 2 with the adhesive 3 of anisotropic electric conductivity so as to establish electrical conduction. Among the three FPC substrates, the center FPC substrate was mechanically peeled off. Fig. 1(c) shows the state, wherein the cured adhesive 3 was solidly adhered onto the ITO circuits. The glass substrate having no center FPC substrate was used for a test according to the process of the present invention. Next, as shown in Fig. 1(d), the porous sheet 4 prepared as above and having a coated adhesive layer at the bottom was temporarily adhered onto the adhesive to be removed. Then, as shown in Fig. 1(e), a glass plate 6 was placed on the porous sheet 4, and a weight 7 was placed thereon for fixing. In this condition, the end portion 5 of the porous sheet 4 in a distance of about 3 mm from the end [see Fig. 1(d)] was immersed in a peeling liquid [Sun Econ G-430 (trade name), a product of Taiyo Kako K.K.] capable of decomposing the adhesive, whereby the peeling liquid was infiltrated into the whole of the porous sheet. In this condition, the peeling liquid was infiltrated from the porous sheet into the adhesive for about 10 seconds; then, the porous sheet was removed; the decomposed adhesive was appropriately wiped off with a Teflon®-made spatula; the residual adhesive was wiped off with a swab impregnated with methanol, whereby as shown in Fig. 1(f), the adhesive portion to be removed was completely removed and the ITO circuits 11 therebeneath were completely exposed. Since the pores at the sides 8 of the porous sheet 4 were sealed by the hot knife, there was no leakage of the peeling liquid from the porous sheet and accordingly there was seen no infiltration of the peeling liquid into the adjacent circuits. After the above removal of the adhesive, new FPC substrates were connected to the cleaned ITO circuits of the glass substrate with Anisolm AC 6071. After the reconnection, the connection resistance of each FPC circuit was measured. The difference in connection resistance between FPC circuits after reconnection and corresponding FPC circuits before adhesive removal, and the difference in connection resistance between adjacent FPC circuits after reconnection and corresponding adjacent FPC circuits before adhesive removal were ±0.5 Ω or less; thus, there was no increase in resistance by reconnection. Example 2 The same procedure as in Example 1 was repeated except that the peeling liquid used in Example 1 was changed to a mixture consisting of 74 parts of methylene chloride, 20 parts of formic acid, 5 parts of chloroacetic acid and 1 part of glycerine. There was no increase in connection resistance, either. Example 3 The same procedure as in Example 1 was repeated except that there was used, as the porous sheet of 0.5 mm in thickness, a laminate obtained by melt-adhering a polyethylene film to one side of a cellulose-made filter paper specified by JIS P 3801, or 5 µm in retained particle diameter and coating the other side of the filter paper with an acrylic resin-based adhesive. There was no increase in connection resistance, either. Comparative Example The adhesive removal in the above Examples was effected by sliding a swab impregnated with Sun Econ G-430, over the adhesive to decompose the adhesive, then appropriately wiping off the decomposed adhesive with a Teflon-made spatula, and wiping off the residual adhesive with a swab impregnated with methanol. Then, as in the above Examples, new FPC substrates were reconnected to the cleaned ITO circuits, and the connection resistance of each FPC circuit was measured. As a result, the difference in connection resistance between reconnected FPC circuits and corresponding FPC circuits before adhesive removal was ±0.5 Ω or less, and there was no increase in resistance. However, the connection resistances of FPC circuits adjacent to reconnected FPC circuits increased from the terminals adjacent to reconnected FPC circuits and, out of the total circuits of two FPC substrates adjacent to reconnected FPC circuits, 25 circuits showed a resistance increase of 100 Ω or more. As described in detail above, the present invention can conduct removal of adhesive of strictly specified area and accordingly enables repair of circuit connections with high accuracy and high reliability.

Claims: A process for repairing electrical circuit connections between circuit substrates (1,2) facing each other and having circuits (11,22) connected via an anisotropic adhesive (3), which process comprises: peeling apart a circuit substrate (2) to be replaced, together with the circuit thereon from the other substrate (1), placing a porous sheet (4) of predetermined shape on the surface of adhesive remaining on said other substrate, impregnating the porous sheet (4) with a peeling liquid so as temporarily to contact the remaining adhesive with the peeling liquid, then removing the porous sheet (4) containing the peeling liquid and cleaning the remaining adhesive from the surface of said other substrate to leave a cleaned circuit on said other substrate, and thereafter connecting the cleaned circuit to a circuit of a new substrate with an anisotropic adhesive. A process according to claim 1, wherein the surface of the porous sheet (4) opposite to the surface for contacting the remaining adhesive, is a sealed surface. A process according to claim 1 or claim 2 wherein the edges (8) of the porous sheet (4) are sealed edges. A process according to any one of claims 1 to 3 wherein one or both surfaces of the porous sheet (4) has a partial or complete coating of an adhesive.

Inventors: GOTO YASUSHI, NAKAJIMA ATSUO, OHTA TOMOHISA, TSUKAGOSHI ISAO, YAMAGUCHI YUTAKA, GOTO, YASUSHI, NAKAJIMA, ATSUO, OHTA, TOMOHISA, TSUKAGOSHI, ISAO, YAMAGUCHI, YUTAKA
Applicants: HITACHI CHEMICAL CO LTD, HITACHI CHEMICAL CO., LTD.
Application year: 1991
Main classification: H05K3
IPC codes: H05K3, H01R43