Hi Gaby and other "Void" Afficionados,
To my knowledge, there have been no studies made public that showed that 
voids are a real reliability problem. I certainly would not like grossly 
large voids, and voids do reduce the heat transfer capability of a joint, if 
that is important. Thus, the argument, whether the "20-30% criterion" is by 
3-D volume or 2-D cross-section is purely arbitrary; a "20-30% criterion" by 
2-D cross-section is however more conservative. Originally, the "20-30% 
criterion" came from the void content seen of what was deemed ordinary 
typical BGA solder joints, and the inference was drawn that anything larger 
is an indication of a processing/material problem.
There is indeed evidence, that voids slow down crack propagation, but this 
crack propagation retardation is localized around the void and rather 
temporary—sorry, not an increase in reliability.
Therefore, you should NOT think in terms of "that although cross sectioning 
is very accurate and the surest method to see the distribution of the voids 
and their position, it has many drawbacks if you don't have an X-ray machine, 
the biggest being that you never know which solder ball row to cut in order 
to see a defect."
I could not justify the purchase of a $ 500,000 X-ray machine in this basis, 
and you should not attach any significance to a difference of void contents 
of 25, 30 or 35%.
No data support such a conclusion or the effort required to get there.

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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