Hi,
We have non-conductive resin filled and capped micro-via (via-in-pad) through the complete PWB thickness (1.6mm FR4) 8-layer, we have seen considerable voiding in the BGA balls following assembly:
 
Q1) What is the chance of out-gassing from the PWB board (or specifically the filled capped via) going into the BGA ball and creating a void during reflow. 
 
Q2) What is adequate flatness on the PWB surface considering “dish-down” of the filled resin during via filling and before Cu plating.  I think the IPC spec.  is 70% but does anyone have differing experience of this.
 
Q3) Would a fairly “rough” topology on the surface of the capping plating (i.e. not totally flat, bumps etc) cause any difference in the solderability,
 
I believe that:
·         The via drilled diameter is 0.25mm
·         PWB finish ENIG,
·         Cu/Ni/Au plating thickness is 20um
·         The PWB thickness is 1.6mm
·         The PWB Tg is approx 170 deg-C
·         The paste is Pb-free,
·         The reflow profile is good to the paste suppliers recommended profile,
·         The PWB’s have been baked-out 130 deg-c 24-hours before assembly,
·         Reflow peak reached 242 deg-C
·         Reflow soak 170 deg-C for 170 seconds,
 
There are large and small voids in the BGA balls during X-Ray and cross sectioning. Some of the BGA balls apear slightly elongated not correct shape.
 
In essence would you think that it would be a PWB issue or assembly problem.
 
Any feedback would be much appreciated.
 
Thanks,
D. McCall.
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