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]