My experience with assembled board (and repaird boards) at a really larger computer
manufacturer is all ESD.
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Example board fails or deemed failed (board swapping) returned ot Depot to test and repair.
Tested board is repackaged and eventually to the field for use.... only to be returned again
now failed.? With thousands of said boards repair costs were a factor so investigation found
ESD (we are talking 74xxxx class TTL)? was the cause.? Adding tracking to the system? found
where and even who so inquiry was done to find out.??
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End result was ESD at the site end point, in dry locations, and Field Engineer not using?
ESD handling technique.? Or the external system had poor grounding and introducing?
transients (traceable to board that did external interfacing to the machines.? Generally
reinforcing ESD procedure in the field and insuring they had needed kit had a marked?
improvement in dead on arrival (or shortly after) boards.
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Likely cause would be during last parts stuffing (end user) the board suffered ESD hits
likely un-noticed while handling.? So that might answer shipped good failed in field.
Those would not be guarenteed immune as its cmos in a cmos environment unpowered.
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Also that would be seasonal, aka worse in dry weather (desert southwest usa)?
or during winter (heated dry air).
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I'd add ESD warning during assembly with suggesting mats and all.
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--
Allison
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Post online only,?
direct email will go to a bit bucket.
Karl