A customer of ours has experienced a very odd failure on 3 assemblies
and we can not determine the cause for these failures. Perhaps, someone
has had a similar experience, or can present a plausible explanation.

The failure is a burning pinhole short from the surface layer power
plane to layer 2 ground plane. The power plane on layer one is about
2.5" x 0.75" and is positioned at a corner of the board. The actual
board size is 7.0" x 4.5." The board has ten layers, 2 ounce copper on
all layers and 0.060" overall thickness. The assembly runs at 290 volts.

So far, three assemblies have failed in this manner during the
customer's testing of the assembly. The first two failures were from one
lot of Rev H, the third failure was from a lot built 3 weeks previous
and was Rev F.

The dielectric between layer one and two consists of 2 plys of 1080 and
measures approx. 0.005" thick. Microsections through the pinhole areas
reveal considerable melting and vaporization of the epoxy, resulting in
a void between layers, much larger than the pinhole through the surface
copper.

At first, I thought perhaps a void or hole in the prepreg was the cause.
But on further thought, this seemed impossible considering the
likelihood of voids in the prepreg occurring in the same general area on
three different boards and the fact that this is two plys of prepreg,
and that any holes or voids in one sheet of prepreg would undoubtedly
fill in during lamination.

We have supplied the customer with over 12,000 pieces of this part
number, on over 6 revisions, with no similar failures. Since this
problem, we have Hi-Pot tested the last two lots (600 boards) at 900
volts, 10 micro-amps leakage, with no failures.

The customer is convinced that it must be a bare board problem because
they have made no changes in their processes. The customer is most
concerned that a board will have this failure in the field, which could
have very bad consequences. So, it is very important that we determine
the cause of this problem. Any ideas?

-Tom

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