Years ago at another employer we found "tunnels" in the laminate that contacted
adjacent traces (or even skipped a trace). They sometimes filled up with flux or
water. Prior to processing, we could not detect the defect. If the panel was
baked dry, it no longer shorted.

These are real hard to detect, but you can look for them.
______________________________ Reply Separator _________________________________
Subject: [TECHNET] Long Term Failures in Multilayer PCBs
Author:  "LANE; Stephen (Collroad)" <[log in to unmask]> at INTERNET