Hi Richard,
The improved reliability should not have come as a surprise. Consider what is 
happening: 
(1) the solder balls are at their full diameter when BGA is placed,
(2) the epoxy fixes the distance between PCB and BGA at that diameter value,
(3) during reflow, the epoxy expands opening the solder gap further,
(4) on cooling, that solder balls are prevented by the epoxy from collapsing,
(5) you have the equivalent reliability of using non-melting 10Sn90 Pb solder 
balls,
and (6) this results in the reliability improvement observed.
I have been pushing this for   ceramic chipp components for some time.



Werner



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