OK, now you guys are starting to move me back around to where I started.

(thinking out loud here) When we first started this over a year ago, we were 
having lots of problems with fractures, we had analysis done and found some 
pretty high P enrichment at the cracks.  After adjusting the reflow profile and 
paste, we started having success with the boards.   The new SEM and EDX 
images look much better, no more than 10% P near the joints (starting with 6-
8%P Ni bath).  We've got about 600 of these in QA and field testing and, 
there have been about 3% failures from intermittent or open circuits in the 
BGA.  They are all happening in the same spot, the corner of one of the two 
BGA on the board, in  corner of the board near a few connectors.  The boards 
I had tested were 2 built a year apart at the same vendor, and 1 built 
recently from a different vendor.  I had 3 complete fractures and one laminate 
crater under a solid joint in a common location, and 17 partial fractures in 
other corners of the BGAs.  

Reading some of the flex tests, I was a bit surprised at the amount of flex the 
boards were being subjected to.  But I measured that against watching first 
hand some of the brutal treatment our boards get.  I guess all that is what 
has me focused on the mechanical stress.  I know our joints could be better, 
but not much with the equipment, environment, and operators we have.  
That's unlikely to change.

Maybe I should request some flex testing, and compare that to what ought to 
be expected?

It's really not the best way to approach this - fix what we can and try to 
figure out how good is good enough.  But that's kind of what I'm stuck with.  

Just be glad we're not building aircraft.

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