[Herald1360]

Tin whiskers is a problem which afflicts some transistors which have tin plated cans. (And it's a problem in other areas too- just do a google search on it!) AF11x devices are notorious, but as devices get older, more types seem to be succumbing.

In brief, little whiskers of tin literally grow from the inside tin surface until eventually they hit a sensitive point on the transistor chip and short it to the can. They can mostly be cleared by splatting them using a PSU and/or charged capacitor connected between bce legs in parallel and the case. How long before the next one that hasn't quite got there yet makes it is a random period between hours and years.

Insulating your devices from the heatsink will work for a random time period.