[sdiy] advice please -- aging chips and caps

[Magnus Danielson]

From: James Patchell <patchell at cox.net>
Subject: Re: [sdiy] advice please -- aging chips and caps
Date: Sat, 09 Aug 2003 14:30:24 -0700

> Personally, I wouldn't worry about the IC's.  Especially, the 4000 
> series.  By todays standards, the 4000 series is a very crude IC (large 
> features).  I have no idea what process is used to make them now, or even 
> then, but somehow I just don't seem to think of metal migration and 4000 
> series in the same thought (I could be wrong).
> 
>          My understanding of metal migration is that you need a high 
> current density flowing through the conductor (how high?? don't know) to 
> make this happen.  Since the 4000 series is both crude and low current, I 
> would put this at the bottom of things to worry about...
> 
>          I would be more inclined to worry about certain vendors parts (as 
> was discussed a couple of months ago)...

As I recall things, there was an issue about with metal migration early out in
semiconductor manufacturing. The trouble was too high current densities. As I
recall those problems was avoided by design rules which should still be in use
(in technology adapted from naturally).

What basically happends is that when the current density is to high (i.e. too
much amperes per square meter) the current is so strong it pulls the metal
atoms along for a ride. This will slowly thin out the point where current
density is strong (maybe from a small defect), which has the effect of reducing
the cross-section area and for the same current increase the current density
and thus accelerating the effect until failing catastrophically as a burnt
fuse.

Design rules helps to keep this not happening for times similar to that of when
the diffusions have migrated into each other sufficiently for the chip to fail
for that reason. This will also happend eventually. There is nice little tables
to look into for getting to know what is the probable lifetime of a certain
component, assuming it is correctly used.

I am sure our semiconductor-friends can shed more light over this...

Cheers,
Magnus