A COPPER-FOILED FLEXIBLE BASE FOR PRINTED CIRCUIT BOARD

Abstract: The copper-foiled flexible base for printed circuit board of the invention has remarkably improved adhesive bonding strength between the copper foil and the base sheet or film of a plastic resin to withstand a much larger number of bending than conventional similar products. In the inven­ tion, the plastic base sheet or film is, prior to bonding with a copper foil using an adhesive, subjected to a liquid honing treatment to be imparted with roughness of the sur­ face. The highest effect of improvement can be obtained when the microcavities or pits formed by honing each have a spe­ cified depth and a specified width at the half depth there­ of.

Description: BACKGROUND OF THE INVENTION The present invention relates to a copper-foiled flexi­ ble base board for printed circuit BOARD or, more particu­ larly, to a copper-foiled flexible base board for printed circuit board having excellent adhesive bonding strength be­ tween the copper foil and the base board. As is known, an increasing proportion of printed cir­ cuit boards in recent years are prepared using a flexiblea copper-foiled base board and flexible copper-foiled printed circuit boards are required more and more to withstand many times of repeated bending in use. Accordingly, it is eager­ ly desired to develop a copper-foiled base board for printed circuit board having good flexibility and a high adhesive bonding strength between the copper foil and the base board to which the copper foil is bonded. Conventional copper-foiled flexible base borads for printed circuit board in the prior art are mostly prepared by directly bonding an electrolytic or rolled copper foil to the surface of a film of a plastic resin such as polyesters, polyimides and the like using an adhesive. The copper-foiled flexible base board prepared in this manner has a problem in respect of adhesive bonding that the type of usable adhesive is limited and no sufficient adhesive bonding strength can be obtained between the copper foil and the base board even by use of an adhesive known to give the highest adhesive bonding strength. Indeed, the inventors have conducted investigations on the manufacturing process of copper-foiled flexible base boards for printed circuit board of the additive type using a plasitc film of a polyester, polyimide and the like only to know that it is extremely difficult to obtain an adhesive bonding strength of 2.3 kg/cm or larger between the copper foil and the base board and the adhesive bonding strength can hardly be maintained in the repeated bending test. SUMMARY OF THE INVENTION An object of the present invention is therefore to pro­ vide a novel and improved copper-foiled flexible base board for printed circuit board having excellent adhesive bonding strength between the copper foil and the base as the sub­ strate to which the copper foil is adhesively bonded so as to be able to withstand many times of repeated bending test. Thus, the copper-foiled flexible base for printed cir­ cuit board of the invention comprises a plastic film having flexiblity of which at least one surface has an increased roughness imparted by a honing treatment using a liquid dis­ persion of an abrasive powder and a copper foil adhesively and integrally bonded to the thus roughened surface of the plastic film with an intervening layer of an adhesive therebetween. In particular, it has been established that the best results of adhesive bonding between the copper foil and the plastic film can be obtained when the microscopic cavities or pits formed on the surface of the plastic film by honing with an abrasive powder have a depth in the range from 1 to 5 µm and a width in the range from 0.5 to 2 µm at a half depth thereof. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the present invention summarized above, the copper-foiled flexible base for printed circuit board is imparted with a greatly increased adhesive bonding strength between the copper foil and the base of a plastic film and hence with remarkably improved resistance against bending by virtue of the greatly increased surface area as a result of the surface roughening of the film and the anchoring effect of the microcavities or pits formed by the honing treatment for the adhesive. The base film of the inventive copper-foiled flexible base for printed circuit board should be a film of a plastic resin having flexibility and high heat resistance such as polyesters, polyimides and the like conventionally used in copper-foiled flexible bases for printed circuit board. The film should have a thickness in the range from about 12 to about 125 µm though not particularly limited thereto. The base film is subjected to the surface roughening treatment by honing using a liquid dispersion of an abrasive powder. The apparatus for honing may be a conventional one for liquid honing equipped with a liquid supply tank and a pressurized jet nozzle. The liquid abrasive dispersion is prepared by dispersing a fine abrasive powder such as alumi­ na, silicon carbide, emery, fine glass beads and the like in water by use of a surface active agent. The particle size of the abrasive powder should preferably be in the range from #280 to #500 according to the definition specified in JIS and the surface active agent is used usually in a concentra­ tion of about 3 % by weight in water. In practicing the surface roughening treatment of the base film, the liquid tank of the honing apparatus is filled with the liquid dispersion of an abrasive prepared in the above described formulation and pressurized by means of a compressor to have a hydraulic pressure of 2 kg/cm²G or, if necessary, higher. The thus pressurized liquid abrasive dispersion is blown in a jet through the jet nozzle against the surface of the film under running so that the surface of the film is roughened. This honing treatment may be per­ formed either continuously or in a batch-wise manner. The film after completion of the honing treatment is then washed with water, preferably, under pressurization followed by drying in a stream of hot air or other suitable means. The conditions of the honing treatment should be controlled so that the base film having a roughened surface after the hon­ ing treatment is provided with microcavities or pits having a depth in the range from about 1 to about 5 µm and a width at a one-half depth, referred to as a median width herein­ below, in the range from about 0.5 to 2 µm. The inventive copper-foiled flexible base for printed circuit board is prepared by adhesively and integrally bond­ ing a copper foil to the thus roughened surface of the base film. The copper foil may be an electrolytic copper foil or a rolled copper foil having a thickness in the range from about 25 to about 125 µm. The adhesive used in this case may be a conventional one mainly composed of an epoxy resin, phenolic resin and the like selected with consideration of the compatibility or affinity with the base film. The adhe­ sive is uniformly applied to the surface of the base film roughened by the honing treatment in a thickness of 10 to 25 µm and the copper foil is laid thereon and bonded together under pressure so as to give an inventive copper-foiled flexible base for printed circuit board. In the following, the copper-foiled flexible base for printed circuit board of the invention is described in more detail by way of examples. Example 1. A polyester film having a thickness of 0.1 mm was cut into cut sheets each of dimensions of 300 mm by 600 mm and each of the cut sheets was subjected to a honing treatment in a batch-wise manner. Namely, the cut sheet was set in a liquid honing apparatus and the abrasive liquid in the liq­ uid holder tank of the apparatus prepared by dispersing a #300 emery powder containing 3 % of a surface active agent in water was pressurized up to a pressure of 2 kg/cm²G by running the compressor. Then, the abrasive liquid was eject­ ed out of the nozzle by opening the valve and blown at the surface of the polyester sheet under movement which was then washed with pressurized water and dried in a stream of hot air. The polyester sheet after the honing treatment in the above described manner had a roughened surface of which the microcavities or pits each had a depth of about 2 µm and a median width of about 0.7 µm. The thus roughened surface of the polyester sheet was coated with an epoxy-based adhesive and an electrolytic copper foil having a thickness of 35 µm was bonded thereto to give a copper-foiled flexible base for printed circuit board, of which the adhesive bonding strength between the polyester sheet and the copper foil was determined to give a value of 2.9 kg/cm. This value of the adhesive bonding strength was retained without decrease even after 1000 times repetition of bending at a bending curvature of 5 mm radius. Comparative Example. A copper-foiled flexible base for printed circuit board was prepared in the same manner as in Example 1 excepting the omission of the honing treatment with the abrasive liq­ uid dispersion of the emery powder. The adhesive bonding strength between the polyester sheet and the copper foil was only 1.7 kg/cm. In the bending test, bulging was found at portious after 500 times of repeated bending with a decrease of the adhesive bonding strength to 0.8 kg/cm. When the bending test was further continued, the copper foil became broken after 650 times of repeated bending. Example 2. A polyester film of 300 mm width having a thickness of 0.1 mm in a roll was subjected to a continuous honing treat­ ment. The procedure for the honing treatment was substan­ tially the same as in Example 1 except that the polyester film was moved continuously and the abrasive liquid was an aqueous dispersion of a #320 alumina abrasive powder con­ taining 3 % of a surface active agent. The thus roughened surface of the polyester film had microcavities or pits each having a depth of about 27 µm and a median width of about 0.7 µm. An electrolytic copper foil having a thickness of 35 µm was adhesively bonded to the thus roughened surface of the polyester film by use of a phenolic adhesive to give a cop­ per-foiled flexible base for printed circuit board, of which the adhesive bonding strength between the polyester film and the copper foil was 2.8 kg/cm. This value of the adhesive bonding strength was retained without decrease even after 1000 times of repeated bending test undertaken in the same manner as in Example 1.

Claims: 1. A copper-foiled fliexible base for printed circuit board which comprises a plastic film having flexiblity, of which at least one surface has an increased roughness im­ parted by a honing treatment using a liquid dispersion of an abrasive powder, and a copper foil adhesively and integrally bonded to the thus roughened surface of the plastic film with an intervening layer of an adhesive therebetween. 2. The copper-foiled flexible base for printed circuit board as claimed in claim 1 wherein the surface of the plas­ itc film having an increased roughness has microcavities each having a depth in the range from 1 to 5 µm and a width in the range from 0.5 to 2 µm at a half depth thereof as produced by the honing treatment.

Inventors: IIDA, TAMAKI, INOUE, KANAME, KITAMURA, HAJIME
Applicants: SHIN-ETSU CHEMICAL CO., LTD.
Application year: 1985
Main classification: H05K3
IPC codes: B32B15, H05K3, H05K1