Bourdon, Bruce wrote:
>
> Thanks to Alexandre, Mike and Russell - sorry I missed you guys the first
> time; the "EE" portion of the subject had dropped.
>
> I too suspect ESD; I have images of their bench tech tossing these things
> together without wearing ground straps or taking any precautions for ESD.


Hi Bruce, when a silicon junction is damaged it
generally goes low ohms, due to fusing of the
differing doped layers into a homogenous mass.

In severe cases the entire die will burn through
and the thing will only have a handful of ohms
from + to gnd, like a dead FET.

With a complex die (like a micro) you may get
one or more silicon junctions fail, more like the
reduced ohms from Vdd to Vss that you mention.

Having seen ESD in the arcade machines I used to
repair it always attacks the pins affected, so it
was common to see a 7401 with one nand gate shorted,
where that one gate went to the joystick control.

I would bet that since your faults seem all related
to the Vdd pins that you can rule out ESD and look
toward overcurrent through the Vdd path, ie excess
current from the psu...

Note ESD on the Vdd pin is the LEAST likely situation
as the power pins have large capacitors on board
and ground planes etc. I really doubt you have to
worry about board handling!

Try replacing the psu in the unit giving problems,
or add a 3.7v 1W zener and prove to the customer
that the psu is going overvoltage. :o)
-Roman

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