you're thinking too hard.

A cross-section of via and voided solder joint will tell the story.

At 04:06 AM 10/13/2009, Douglas McCall wrote:
>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.
>mailto: 
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