Hi Steve,
You have gotten good advise.
You did noy say how dense the board is. You can get CAF (conductive anodic
filaments) forming along the glass fibers that were cut in drilling
neigboring PTHs. This can particularly happen when the drilling was rough
ripping the glass fibers partially out of the epoxy matrix. As Susan says,
with moisture and contamination (in this case from the plating process hiding
in the capillaries of the ripped glass fiber interfaces), a path is created
that on application of a voltage causes electromigration along these glass
fiber surfaces and creates shorts. A cross-section of some PTHs should show
you how good, or bad, the drilling was; if its bad, you should actually see
Cu plated between glass fibers into the epoxy matrix.

Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL  32174  USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask], Website: www.engelmaier.com

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