Hi Ryan,
What you describe is the classical via barrel failures resulting from the 
stresses generated due to the thermal expansion mismatch of the resin surrounding 
the via and the copper plated into the via during the soldering process(es). 
This has gotten significantly worse with the much higher soldering 
temperatures required for lead-free solders.
For the ED copper in the via, the following could be the cause either 
individually or in combination:
(1) Cu plating too thin,
(2) Cu plating not ductile enough,
(3) Non-uniform hole walls due to drilling problems, i.e. dull drill.
For the PCB, the following could be the cause either individually or in 
combination:
(1) Choice of resin system with too low a glass transition temperature;
(2) Choice of resin system with too high a post-glass transition temperature 
CTE;
(3) Prepreg layers too resin-rich and/or undercured (I bet you, that crack is 
at a prepreg layer, not a laminate layer).

First, good X-sections properly etched are a must.
Second, from your description you have 'dog-boning' plating in your via [
35-40um at the knee redced to 20um in the center]. This means you have at the 
location of greatest stress the smallest cross-section; it also means, that the 
strength and ductility near the PCB center is significantly reduced from what 
you are getting at the surface.
Third, you have a likely drilling defect that locally reduced the plating 
thickness to 10um—a stress concentration (it is not likely that this is caused by 
some necking phenomenon).
You either have a PCB design inadequate for the soldering temperatures used 
and/or low copper quality in the vias.

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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