0.5 mm pitch BGA, board has via-in-pad.

On 5 of 7 via-in-pad locations, the balls separated in the intermetallic
zone at the substrate interface.  All other balls look ugly, but not out
of the usual ugliness when the BGA is SAC305 and we are attaching with
Sn/Pb, and they seem well attached.  The ugliness of the bulk of the
balls is very uniform, indicating thorough mixing of the alloys.  The
separated intermetallic zone is lumpy gray, not like cases of black pad
I've seen.

Boards had ENIG, then had SnPb SIPAD done on them (requires 2 reflow
cycles).  After that, the BGA's were attached and then the other side
was stenciled, populated, and reflowed.  So the ENIG interface had been
reflowed a total of 4X.

The surface of the via-in-pad locations is fairly lumpy, like is common
with CB100 via fill.  Perhaps this lumpiness was responsible for too
much gold in the joint?  I can't think of any other reason why these
locations are where the problems are.

Additional insight would be very much appreciated.

Thanks,

Wayne Thayer


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