1. FIELD OF THE INVENTION
The present invention relates to a process for producing printed circuit boards and more particularly to a process for producing printed circuit boards, for which fine, high-density patterns of circuits are required today to have.
2. DESCRIPTION OF THE PRIOR ART
Conventionally, printed circuit boards with high-density patterns of circuits, such as double-sided printed circuit boards and multilayer printed circuit boards, have been fabricated mainly in the manner that copper-clad laminates are used as starting materials, through holes electroplated with copper are formed in the laminates, and the copper claddings except their circuit-corresponding portions are removed by etching. This conventional process is unfit for the fabrication of fine, high-density circuits because the application of copper electroplating results in large variation in plating thickness and the dimensional accuracy of circuits depends upon both the exactness of etching-resist formation and the exactness of copper etching. Therefore, various methods have been proposed for the fabrication of finer, higher-density circuits.
One of these methods is the chemical (electroless) copper plating pattern (selective electroless copper plating of conductive patterns) process. In this process, copper-clad laminates are used, the copper claddings except their predetermined circuit forming portions are masked with a photosensitive plating-resist, the non-masked portions of copper claddings are selectively chemical-plated with copper, the plating-resist is then removed, and the unmasked portions, i.e. the copper claddings except their predetermined circuit forming portions are removed, thereby making up circuits.
However, this process still involves the problem that the plating-resist is liable to peel off during the copper plating hence the formation of good circuits being impossible. This resist peeling is remarkable in the chemical copper plating, which gives little variation in the thickness of plating. In the copper electroplating, the resist peeling is limited, raising no significant problem in many case. That is whereas the copper electroplating proceeds quickly and is finished in a short time (in one hour), the chemical copper plating, proceeding slowly, requires a long time. for instance, the chemical copper plating to a thickness of 30 .mu.m requires from 20 to 30 hours, which are at least 10-fold as long as the time required for the copper electroplating. The plating of such long duration damages the plating-resist. For example, it is said (IBM J. RES. DEVELOP, Vol. 29, No. 1 (1985), Cathodic delamination of methyl methacrylate-based dry film polymers on copper) that a portion of the chemical copper plating solution permeates along the interface between the plating-resist and the conductive substrate or through the resist film, and causes the following reaction: EQU 2H.sub.2 O+O.sub.2 +4e.fwdarw.4OH.sup.-. . . (1)
at the interface, forming hydroxy ions; which result in the rupture of interfacial bond. In particular, the chemical copper plating bath, having an alkalinity as high as about 12 in pH, tends to cause the interfacial destruction.
In order to solve the above noted problem, a measure is proposed which, as described in the above IBM journal, comprises polishing the surface of copper cladding with pumice or the like to smooth the surface, and treating the polished surface with benzotriazole or some other reagent, followed by applying a photosensitive plating-resist to mask the copper cladding except its predetermined circuit forming portion. About the case where copper electroplating is employed instead of chemical copper plating, Japanese Patent Publication No. Sho. 50-9177 proposes the measure of adding benzotriazole or the like to a photosensitive plating-resist. These measures have greatly overcome the problem of the plating-resist delamination from substrates.
Sometimes, the use of benzotriazole as stated above achieves insufficient effect or in the chemical copper plating, produces adverse effects on plating performance. More specifically, the presence of benzotriazole may lower the rate of plating locally or throughout the whole plating surface and/or may result in plating films of inferior properties. This is assumed to be caused by the elusion of benzotriazole into the chemical copper plating solution. When such adverse effects are produced, no sufficiently reliable circuit will be obtainable or in certain cases the circuit formation itself will be impossible. It is considered that the proper control of benzotriazole concentrations in the resist and on the substrate surface is indispensable in order to preclude such difficulties as stated above, but in practice this control is difficult.