Flexible metal-clad laminate and process for preparing the same

Abstract

Disclosed are a flexible metal-clad laminate comprising a metal foil and a heat-resistant resin film layer formed on one side of the metal foil, the heat-resistant resin film layer comprising a crosslinked condensation polymer and having an N-methyl-2-pyrrolidone-insolule content of at least 1%, and a method for producing the flexible metal-clad laminate comprising the steps of applying a heat-resistant resin solution to a metal foil; predrying the metal foil until the heat-resistant resin layer has an residual solvent content of 10 to 40% by weight; and carrying out solvent removal and heat-treatment while controlling the crosslinking reaction of the resin.

Description

[0001] The present invention relates to a flexible metal-clad laminate which is for use in flexible printed boards and has high dimensional stability, heat resistance, chemical resistance (especially alkali resistance), adhesion and the like, and to a method for producing the same. More specifically, the present invention relates to a flexible metal-clad laminate produced by continuously applying a heat-resistant resin solution to a metal foil and then subjecting the laminate to predrying and heat-treatment, the metal-clad laminate having high dimensional stability, heat resistance, chemical resistance and adhesion, and to a method for producing the same.[0002] In the present specification and claims, the term "flexible metal-clad laminate" means a laminate comprising a metal foil and a resin layer, for example, a laminate which is useful for producing flexible printed wiring boards and the like.PRIOR ART[0003] Conventional flexible metal-clad laminates for flexible printed boards c...

AI Technical Summary

Benefits of technology

[0011] (b) The thus obtained crosslinked condensation polymer layer has excellent characteristics such as high heat resistance, dimensional stability, chemical resistance, adhesion, etc., as well as high alkali resistance.

[0012] (c) Since internal stress is inhibited or reduced in the resulting metal-clad laminate, the flexible metal-clad laminate does not curl and a heat-resistant resin film obtained by etching and removing the metal foil from the flexible metal-clad laminate does not curl either. Therefore, a flexible metal-clad laminate from which circuit patterns are formed by etching the unnecessary metal foil does not curl. Thus, a flexible metal-clad laminate can be produced economically.

Problems solved by technology

These flexible printed boards bonded by a thermosetting adhesive, which have thermal characteristics much lower than those of polyimide films, have the problems of limited application to chip-on-flex and blisters, peeling and other problems associated with soldering.

Such flexible printed boards also have the drawback of curling and twisting in the boards caused by the thermal hysteresis resulting from processing such as thermocompression bonding.

This curing and twisting makes the punching process to be carried out afterward difficult.

However, all of these methods have the problem of high production costs and insufficient adhesion between the polyimide film and conductor.

Specifically, such methods are disadvantageous in that electroplating on patterns to increase their strength may tear them off and exposure to heat at about 100.degree. C. may lower the adhesion between the conductor and polyimide film.

However, the flexible metal-clad laminates produced by such methods are problematic because a decrease in the volume of the solvents or a difference between the coefficients of thermal expansion of the resin and the copper foil, etc., induce internal stress which makes the metal-clad laminate curl toward the resin layer side.

However, none of these methods can straighten the curled laminates satisfactorily.

These methods also have the problem that the films obtained by etching the metal foil curl.

In addition, continuous production of the laminates entails lower productivity or require expensive equipment, and thus these methods have the problem of high production costs.

Further, a laminate produced by the above method comprising directly applying a polyimide-based solution onto a substrate and drying it has insufficient alkali resistance.

Accordingly, in various applications, such laminates are not suitable for producing flexible printed wiring boards for use in products which involve the use of alkaline substances, such as ink jet printers (which usually use alkaline inks).

A resin with an inherent viscosity not higher than 0.3 dl / g has insufficient mechanical characteristics such as bendability and initiation tear strength of the laminate.

On the other hand, an inherent viscosity not lower than 2.0 dl / g results in reduced adhesion and increased solution viscosity, making forming and processing difficult.

Therefore, reducing curling in the metal-clad laminate is generally made difficult and heat-resistant resin film obtained by removing the metal foil by etching tends to curl, making circuit processing difficult.

Although a higher residual solvent content causes less curling in a metal-clad laminate, excessively high residual solvent content is not favorable since the resin layer formed by coating may undergo sagging and becomes adherent and thus the processability in the form of a roll is lowered.

In particular, since the residual solvent content in the resin surface layer is reduced, the reduction of internal stress in later steps becomes insufficient and thus curling is likely to occur in the metal-clad laminate.

A drying temperature lower than (Tb-130).degree. C. prolongs the drying time and thus may lower productivity.

Conducting the above heat-treating and solvent removing step in air causes deterioration of the resin layer and / or excessive crosslinking, which increases curling in the substrate and lowers the mechanical characteristics of the resin layer.

When the insoluble content is lower than 1%, solder heat resistance and like heat resistance and chemical resistance, especially alkali resistance become insufficient.

However, an excessively high temperature induces excessive crosslinking and degradation in the resin.

This increases internal strain and curling in the metal-clad laminate.

Further, when the heat-treating and solvent removing temperature is lower than (Tg-250).degree. C., it takes a long time to carry out the crosslinking reaction to correct the curl in the laminate and to increase the insoluble content, lowering productivity.

Otherwise, especially when the residual solvent content is high, the coated surface and uncoated surface will come into contact, thereby lowering productivity.

The width of 100 mm or more lowers the efficiency of solvent removal and degrades the appearance of the coated surface since the area which contacts the coated surface increases (or a wider uncoated surface is required).

This results in a lowered yield.

When the average surface roughness is greater than 0.4 .mu.m, the resin film layer after the metal foil is removed by etching curls, thereby making circuit processing difficult.

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