You can calculate whether the gold content exceeds the 3 w/w%Au in the
via filling.
Over 6w/w% the liquidus of such SnAu alloy will be over 250dC. The
attacked Au metallisation than have a gray appearance.

Peer Langeveld
Consultant Soft Soldering Processes
5502 VH8  The Netherlands



2007/10/1, Thayer, Wayne <[log in to unmask]>:
> 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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