On Sun, 7 Feb 2010, MCH wrote:

> Maybe it's due to "Stress" on the leads of components?
> (on the line of: if you bend a lead too often it will break)

FYI this phenomenon is known as "work hardening". The metal gets harder 
and harder until it is no longer able to bend and it breaks.

> Maybe the ultrasonics weaken the leads.

I thought putting a name to this might help you do a little more research 
if you're interested.

Regards
Sergio Masci
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