Hi Stephen,
Your problem is probably caused by leakage paths along reinforcement glass
fibers in FR-4 from one PTH to the next. Drilling can rip the glass fibers
out of the polymer matrix around the PTH hple perimeter and leakage paths can
form along the interfaces of the glass fibers with the polymer. Relatively
high vapor pressure, but not current is necessary for the formation of these
paths. Upon application of voltage potential, you have current leakage paths.
This problem typically does not exist for PTHs in a 100 mil grid, but moving
them closer together, (Bellcore starts worrying at 50 mils) can be
problematic.

Werner Engelmaier
Engelmaier Associates, Inc.
Electronic Packaging, Interconnection and Reliability Consulting
23 Gunther Street
Mendham, NJ  07945  USA
Phone & Fax: 973-543-2747
E-mail: [log in to unmask]