Hi Calvin,
You indicate that this problem is now occurring on PCBs that previously
worked. Thus, it is likely a change in materials or processing--my guess is
materials, specifically the Cu foil. You may have E1 instead of E3 foil, or
maybe the 3-sigma tail of the E3 quality distribution.
Another possibility is a change in the soldering processes, particularly
manual soldering. That maybe discernible from where the layers with the
problems are in your stack-ups.

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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