[Boatanchors] CMOS Failure

[Paul Christensen]

While repairing items from the 1980s decade, I've found several instances of
failed CMOS logic gates.  Just recently, I repaired an Orban 9100B audio
processor that had three bad CMOS chips.  

Each CMOS device is in an intra-stage part of the processor, meaning they
are not exposed to the outside environment as is the case when connected to
an input or output jack where conditioning is minimal or non-existent. The
power supply tests fine.  Regulated voltages are spot-on and supply ripple
is less than 1mV RMS.  Internal temperatures are warm, but not excessively
hot.

This is only the most recent occurrence.  I've seen similar failures in the
last few years.  Does anyone on the list have an understanding of the
failure mechanism?  

Paul, W9AC