[Amps] Good engineering

[Steve Thompson]

>> Some semiconductors I worked with years ago were rated for 25 years 
>> continuous operation at maximum temperature.  It was said that the 
>> tracks inside the part migrated to the next nearest track over time 
>> and stress.
> I doubt the tracks themselves migrate.

The metal does migrate, dependent on temperature and current 
density. Aluminium is much worse than gold (these are the two 
metals mostly used in rf semiconductors). Amongst others, TRW 
published some work on this which was carried as a App note in the 
Motorola RF data books after the takeover.

There's also something known as 'purple plague' - not migration, 
but reaction between gold and aluminium that showed up when gold 
metallised die were bonded with aluminium wires.

Steve