Hi Thomas,
What you have observed sounds like a classical conductive anodic filament
(CAF). You did not say what the humidity was, whether the assembly has been
baked prior to functional testing, or what the potential difference was. At
120C the cross-linking of the FR-4 epoxy definitely is changing (depends on
Tg). CAF formation certainly can occur in about 150 hours, and at 120C you do
not need much moisture in the PCB to build up very high vapor pressures. All
you need then is some contamination, fiber, hair, less than perfect bond to
together with some voltage differential to form CAFs.

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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