On 03/03/2017 08:36, N_Cook wrote:
Anyone any insight on this. Failure of 34 year old TTL chip, in
unpowered storage. Just 1 of 50 chips on a board, TTL,4000, analogue,
memory etc.
Only this one had silver tarnish looking appearance of black silver
sulphide on the pins. This make of TTL did for a while have silver
plating aded to the pin frame , for a period.
I cracked open the device and no obvious tarnish on internal frame
metal. Anyone know what the failure process was? was silver plating
found to be problematic?

Silver tarnish appearance of that sort of granular/cellular/mottling
effect of silver tarnish rather than the very black soot-like appearance
of CuS.