Susan,
You say "But they are blind vias and only go down 3 layers" How deep is
three layers? One could expect substantial solder voiding if you rob the
connection to fill the blind hole. Sounds like a thin core sequntial lam
design vs. controlled depth drilling and plating.

If sequential lam is required one can build near planar blind vias quite
easily. By using a high resign content pre-preg at the join layers it is
possible to back fill the blind vias very close to the surface (pacovia
backing in press) or no pacovia and a mechanical "scrub" to remove the
oozed (very technical term) pre-preg from the external layer. This
second process described gives a very nice full pad with a hidden blind
via. But also is a yield issue if you scrub away your foil. Ouch!

Regards,
Shawn

p.s. both ways it is possible to plate over the hole and end up with a
small dimple worst case.
I am not suggesting a change, especially if is working well for you.
Just wanting to share experience.

Susan Clayton wrote:
>
> We use vias in our BGA pads. But they are blind vias and only go down 3 layers.  This works fine for us.
> Susan Clayton
> VMI

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