Electrical aging of adhesive

Insulating reliability between adjacent leads formed on organic material is an important problem for printed circuits. This problem was investigated in connection with insulator deterioration between copper conductors exposed to high temperature and high humidity under fairly high electric fields in comparison with actual operating conditions. The investigation was performed on a copper foil laminated to a thermoset adhesive coated on polyimide film. Abrupt and intermittent changes in leakage current were found to occur at high humidity and high temperature under field application. Before the abrupt change occurred, the corrosion expanded toward the cathode from the anode. Thereafter a copper component migrated to the cathode from the corrosion zone. The copper migration and the corrosion progressed through the adhesive, not along the surface. A similar failure phenomenon was observed for adhesives made of polyamide, epoxy, and phenolic resins, but their times to failure were quite different. This was attributed to their hydrolysis.<>