A couple of things come to mind.

First, check to make sure no solder bridges have formed.  You didn't
mention if this happened to all assembled boards or just some.

Second, make sure the screw heads don't make contact with any
copper under them.  The solder mask will not prevent electrical contact
through the screw head if there is a trace running tangent or under the
screw head.  Put some kapton tape or non-cunductive material under the
screw head.

Lastly, make sure the copper at the mounting holes is not up to the edge
of the hole.  If the copper comes to the hole edge, the screw may make
contact with it and short to ground, or any other trace that may also
come to the hole edge.

Hopefully, it is something minor which will surface quickly.  If the short is
in every assembly I would expect it to be a layout problem.  If it is in
some, or most assemblies, it could be soldering, assembly, or even still a
layout problem.

Larry Campbell
BFGoodrich, Avionics Systems

>>> Bock Ming Gai Joanne <[log in to unmask]> 09/01/98 03:17am
>>>
Hi all,

Currently I'm facing a problem at work. I have a few 6 layers of PCBs
that are perfectly alright, meaning that they passed the ET. However,
when our engineers did some testing after board assembly, problem
surface. When they tighten the board with mounting screws, they found
that one of the supply trace was shorted to the Gnd. This was not
shown
during the ET because PCB suppliers only did the test on the bare
boards. So now I am looking into what could possibly cause them to
short
together.

I appreciate anyone who can help to provide me with a solution.

Thanks & Best Regards!
Joanne Bock

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