Are the dots delam or prepreg dryness?  Perhaps you have forced most of the resin away from the BV's and when you heat them up you get a measles pattern.  I get a picture in my head of the vias standing taller than the surrounding board and
plunging the resin to the side.  This may not be a true failure.  Can you have a micro section run to see if there is an actual void.

Gfranck wrote:

> Technetters,
>
> We have a lot of a 12 layer PWB made from a High Tg Epoxy material, some 75 mil thick overall.  The board has 3 sequential laminations, and four drill operations to get the three Blind vias (BV) and thru holes in the board.  The BVs from
> layer 1 - 10 terminations are visible thru the material from the Layer 12 side.  During the assembly prebake  process the tops of these little BVs delaminate.  A patteren of little white dots which closely matchs the 1-10 BV drill pattern
> emerges.  I believe the BV drill size was 10 mil
>
> Boards are 20 months old.  Assemblier was aware of the age of the boards, and the delamination occured during the pre-bake process, and thermal profile set up.
>
> Would anyone want to discuss why a BV would be more prone to delamination than the rest of the board?
>
>                          *** Warning: the follow contains sarcasm.***
> *** For those unable or unwilling to understand sarcasm, please read no further.***
>
> Current corrective action proposals:
> 1) Don't assembly old PWBs.
> 2) Add a ground plane between layer 11 and 12 to prevent the layer 10BV  terminations (and delaminations) to be visible from the outside of the board.
> 3) Blame the PWB supplier, and get them to replace the boards.
> 4) Willing to entertain other options.
>
> Thanks,
>
> George Franck
>
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