Process for producing polyimide/metallic foil composite film

DETAILED DESCRIPTION OF THE INVENTION The process of the present invention can produce a polyimide/metallic foil composite film having a curvature radius of 25 cm or more which is substantially free of curling in both the lengthwise and widthwise directions.
The reasons that such a composite film can be produced are that the polyimide coating prepared from the above-specified diamine and aromatic tetracarboxylic acid components has a coefficient of linear thermal expansion which is nearly equal to that of a metallic foil comprising, for example, copper or aluminum, and that, in the formation of the polyimide coating on the metallic foil, a heat-drying process and a high-temperature heating process including imidation are carried out in the state that the metallic foil is fixed; in particular, the high-temperature heating process releases the stress formed in the polyimide layer.
The composite film produced by the process of the present invention can be used as substrates for the production of electric circuit boards without any problems.
The composite film has the advantage that the handling properties in processing the circuits are excellent.
Another advantage is that the electric circuit boards obtained are resistant to curling due to temperature changes, and therefore the boards have excellent dimensional stability.
The term "curvature radius" as used herein is described below.
The drawing shows a cross-sectional view of a polyimide/metallic composite film 3 comprising a metallic foil 1 and a polyimide coating 2.
This composite film 3 has a length of 10 cm and a width of 10 cm.
When the composite film 3 curls in the widthwise (or lengthwise) direction, the curvature radius is defined as a radius r, a distance from a central point P.
Assuming that a distance from a point R to the center of the composite film 3 when it curls is h, if the distance h is equal to or larger than the radius r, i.
e.
, h.
gtoreq.
r, the curvature radius is determined by actually measuring the radius r.
The symbol M indicates a horizontal line connecting the both extremities of the composite film 3 in the curled condition, and the length of the horizontal line M is represented by a