Somehow the collector mounting flanges of the transistors were leaking through the mica insulators to the chassis. I had known there was a small crease where the insulators had been bent, but that should not have caused any problem even if the cracks did penetrate completely through the insulators because the thermal grease is non-conductive – the data sheet said so.
Took another look at the data sheet. The thermal grease is non-conductive if smeared on a pcb. It is intended for heat sinks attached to CPUs; our staff uses this grease when mounting heatsinks onto BGAs. In this use, even if it was conductive it would not matter.
But the data sheet says nothing about what happens when the grease is compressed under the much higher pressure of a screw holding a transistor flange to a heatsink. Remember the grease contains microscopic silver particles immersed in an oil base – were these silver particles coming in contact with each other when squeezed tightly? And if so, when squeezed into a crack that fully penetrates a mica insulator this grease could cause exactly the problem I was seeing.
I emailed the Arctic Silver company and received a phone call a couple hours later. Yes, they confirmed that their product would become electrically conductive when under pressure.
I then obtained two more intact mica insulators and some real thermal grease that did not contain silver particles – you know, that white stuff everyone refers to as bird-poop. No more problems after that.
About the author
Glen Chenier has spent most of his engineering career in the communications field with such companies as Fujitsu, Fitel Photomatrix, and Gandalf Data. He has also developed small-motor electronic drive systems for the hobby market. From his new home in Allen, Texas, he now spends time working on EDN projects and reverse-engineering legacy telecom systems to facilitate continuing