Hi Peter,
Following the TechNet, I am sure you are aware that you are not alone with
problems with BGAs.
First, BGAs are not created equal. Some are symmetrically constructed given
to little warpage, others are very asymetrical (SBGAs) and warp during both
assembly because of the high soldering temperatures and funtional operation
because of the power dissipated within. Then, of course, CBGAs have a high
CTE-mismatch with the PCB. CSP-BGAs are very much different because of their
small solder balls, and the different  ways they can be constructed [I would
use only CSPs designed under license from Tessera] that affect solder joint
reliability.
Second, because the SJs are hidden by the BGA body, getting adequate heat to
the solder joints is not easy--it takes a higher reflow machine profile
recipe.
Third, the metallizations chosen for BGAs are in many cases suboptimum and
the cause of failures.
So your concern regarding Class 3 usage is not misplaced.
I have tried to get a SJ reliability workshop talking about issues like these
going in Singapore, but so far with no success.

Werner Engelmaier

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