I hadn't powered on my Apple-1 reproduction in a while. I wanted to try out Uncle Bernie's Apple II keyboard adapter, so I turned it on again. Alas: the display was not showing what it was supposed to show, as if it didn't synchronize. I tried a couple of monitors, cables, and eventually zeroed in on the 74175: swapping it for another chip, the issue was resolved. I tested the 74175 in my chip tester, and it showed as failed there too, unsurprisingly.
I realize that chip was made in 1973, over half a century ago. But curiously, it must somehow have gone bad between the last time I had the Apple-1 on, a few months ago, and now. I wonder if others have had chips fail seemingly at random like that.
The bad chip:
After replacement:
... there are many different failure mechanisms. Some failure mechanisms are common to both pure bipolar (i.e. TTL) ICs and PMOS/NMOS/CMOS ICs, such as faulty passivation allowing corrosion of the aluminum interconnects, or the inevitable forming of intermetallic compounds on the bond wire / aluminum bond pad interface, while other mechanisms involve contamination issues (such as mobile ions) which mostly affect NMOS transistors - PMOS transistors are a bit more robust against mobile ions. This is why CMOS was not feasable despite it has been demonstrated in the mid 1960s, and why the first commercially viable MOS ICs (they really took off beginning with Y1968) all were PMOS.
My take is that the worst enemy for old ICs is the thermal cycles when powered up and down, combined with the embrittlement of the bond wire / aluminum bond pad interface caused by the intermetallic compounds.
To find the real causation you would need to decap the die in your failed IC and take some microphotographs to hunt for signs of deterioration.
But it's probably not worth the time and effort invested.
This example debunks the myth that solid state electronics (ICs) live forever. They don't. And even when sitting somewhere in a tube, New-Old-Stock, the formation of these intermetallic compounds goes on, and any mobile ions can move around, although slower than under an electrical field. And corrosion is quite common, too. Not so much an issue with hermetic packages, but with plastic packages, all bets are off.
- Uncle Bernie