Polyimide-containing cover layer for a printed circuit element

Claim 1

A printed circuit element comprising: at least one metallic conductor layer, said conductor layer having an outer portion; at least one insulating layer joined to said metallic conductor layer; and at least one cover layer joined to at least one said outer portion of said conductor layer or electrical components contained on said outer portion of said conductor layer, said cover layer comprising: a laminate of an intractable fully aromatic polyimide, said aromatic polyimide being insoluble in phenolic solvents; and a heat sealable high temperature adhesive arranged between said laminate and at least one of said outer portion of said conductor layer and said electrical components, wherein said aromatic polyimide is obtained by the process comprising the steps of: reacting an aromatic tetracarboxylic acid or the dianhydride thereof and an aromatic diamine in the molar ratio of said tetracarboxylic acid or dianhydride to said diamine in the range of 0.95 to 1.05 in a polar organic solvent to form a polymeric composition comprising a polymeric acid having the formula: ##STR10## wherein R is an aromatic tetravalent radical, R' is a divalent aromatic radical and n has a value sufficient for the formation of a polyamide acid with a η red of 0.5 or higher; extruding a film of said polyamide acid onto a metallic foil; removing the solvent from said film in a first stage, said first stage having a temperature in the range of from 100° to 200° C., whereby substantially all of the solvent is removed and said polyamide acid is partially cured to polyimide; further curing said partially cured polyamide acid to form an insoluble, intractable polyimide in a second stage, said second stage having a temperature of at least 200° C., whereby at least 95% of said polyamide acid is converted to said polyimide, without the application of a roller on said film to remove the solvent; and removing the metallic foil by etching.