Hi John, 
You have already received some good advice from Joe; here are some additional 
inputs.
Barrel fractures typically occcur near the center of the vias at a prepreg 
layer [higher z-CTE than laminate duue to higher resin content and less complete 
cure].
Drilling: The first set could have been drilled with better/newer drills than 
the second [this is unlikely, however] If this is the case you see very rough 
hole walls and non-uniform barrel plating with lots of stress concentrations.
Plating: You Fab shop may be patch, rather than continuous, recondition the 
plating solution. The consequence would be more dog-boning of the pating 
[thinner in the middle] as well as lower strength and ductility of the copper 
deposit.
Overall lower plating thickness may also be an issue.
Assembly: Higher peak reflow temperatures as well as a larger number of 
solder process excursions promote via fractures.
From what you said, the via fractures were not complete [or the fracture 
surfaces made contact] so bare board testing did not show anything. As soon as the 
PCBs get warmer the fracture surfaces separate and an electrical open occurs.

Regards,
Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL 32174 USA
Phone: 386-437-8747, Cell: 386-316-5904
E-mail: [log in to unmask], Website: www.engelmaier.com



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