George,
You are trying to fill a large column (via hole) with very little resin.
Remember, the resin in the dielectric between lyr 10 and 11 will fill the
vias during the lamination of layers 11 & 12 to the sub assembly 1 - 10;
however, since the volume of resin is low compared to what is needed to
fill the dielectric and the vias, as there is only so much resin in a given
area of the vias, this creates a lot of stress on the resin particularly at
the knee of the via. I have seen where the resin does, under thermal
excursions, "delaminate", looks more like stress cracks along the knee,
possibly due to a CTE mismatch of the pure resin in that area or a lack of
resin density.
At 08:49 03/03/2000 -0500, you 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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--
Regis Froats <[log in to unmask]>
Process Engineer - Herco Technology Corp.
13330 Evening Creek Drive North
San Diego, Ca. 92128
Pho:858.679.2800 - Fax:858.679.7565
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