An electronic assembly containing an active circuit, an insulating layer and a heat sink. The insulating layer comprises an unfilled thermoplastic sheet having adhesive layers on opposite sides of the sheet. The adhesives are filled with thermally conductive material while the film is unfilled. The properties of the insulating layer are such that the thermal impedance is low while dielectric strength is high. These improved films reduce costs, improve thermal and electrical properties. The layers of the invention are particularly useful for single or multiple layer printed circuit boards having surface mount components in conjunction with a sheet like aluminum heat sink.