Patent ID: 7303811

Claim:
A porous insulating film consisting essentially of a highly heat resistant polyimide resin film having a fine porous structure wherein: a) fine continuous channels reaching to both surfaces of the insulating film in a nonlinear fashion have a mean pore size of 0.01-0.2 μm in the center of the film and 0.4-0.9 μm at both surfaces of the film and a porosity of 15-80%; b) the polyimide resin film is prepared from a polyimide precursor solution and consists essentially of a polyimide obtained from the combination of at least one tetracarboxylic acid component and a diamine component; and c) the insulating film has a thickness of 5-150 μm, a resistance to passage of air of from 30 sec/100 cc to 2000 sec/100 cc, and a heat shrinkage of not greater than about ±1% after being heat-treated at 105° C. for 8 hours, and does not contain a dense layer on either of the surfaces.