MODCOMP II Revisited...

[Brent Hilpert]

dwight elvey wrote:
> 
> >From: Lyle Bickley <lbickley at bickleywest.com>
> ---snip---
> >
> >Factoid: Many TI chips produced during 1975-77 period had very thin "legs"
> >and
> >poor tinning. When they were wave soldered, the solder offered them
> >protection. However, when they were socketed (as in the MODCOMP) they had
> >very little protection from the environment. They would rust and eventually
> >fail.
> >
> ---snip---
> 
> The problem was that the outside was the silver plating ( the blackened
> color is the
> oxidized silver ) and the material inside would react electrolytically and
> rot out
> with even the slightest dampness. I've seen them where the only material
> left was the plating. The slightest touch and the leg would just be gone :\
> Dwight

I have seen this with TI ICs from that period where the failure is not apparent
to visual inspection: the pins look fine but when you 'scope the pin at the
solder cusp where it goes through the board hole you see one signal, 'scope it
up where it enters the IC body and see a different signal. Very perplexing
to diagnose the first time as it's so unexpected to see a simple conductor
fail like that. Can show up as intermittents as well, as a little flexing of
the board can change the conduction characteristics at the failure.