Ion migration in a resistor network

After a short time in service, resistor networks (R-pacs) in high current regulator circuits experienced decreasing resistance causing some power supplies to latch off. The root cause of the resistance decrease was determined to be ion migration on the R-pac alumina substrate, under the passivation oxide layer. The main corrective action was the replacement of the relatively porous passivation oxide layer with an essentially nonporous and acid resistant passivation oxide layer.