Hi Tom,
Unfortunately, the info you provided is not complete enough to be more 
definitive, however...
(1) Your failures are definitely caused by the thermal expansion mismatch of 
the copper barrels and the surrounding reson volume;
(2) The affected vias were more suspectible to failure because they may
   (a) have seen higher and/or more often T-excursions [manual soldering];
   (b) had localized stress-riser defects [remember, a X-section sees only 2 
points of the 360°       barrel circumference], that started cracks during 
soldering proces[ses] which propagated to completion in subsequent burn-in 
and operational use---the stated "the epoxy appeared to
have a small crack also in the same plane as the barrel crack" indicates a 
possible drilling quality problem causing glass-fiber rip-out which than 
causes a localized plating defect which raises the effective maximum strain 
on the Cu barrel [see IPC-D-279];
  (c) significant dog-boning has occurred;
  (d) the physical properties of the ED barrel copper is inadequate.

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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