My answer si:  It depends :)  (been lurking on TechNet too long)

We normally encroach the vias with soldermask on BGAs with greater than 1mm
ball pitch, allowing the actual drilled holes to remain free from any
masking.

On 1mm or below we both encroach the vias and apply capping under the BGA,
even where the wave fixture is covering the opposite side from the BGA.

Our normal process in no-clean unless cleaning is required for the circuit
to function correctly (mainly RF boards, so we don't worry as much about
flux residue and such on most of our assemblies.

-----Original Message-----
From: DesignerCouncil
To: [log in to unmask]
Sent: 9/29/2004 5:14 PM
Subject: [DC] Via protection beneath a fine pitch BGA...

Hi All!

This is my first post to the Designers Council. I have a question about
solder masking via's in a BGA pattern, and want to see if I'm thinking
correctly.
I'd like your opinions about this.

I think it's a bad idea to leave the via's free from solder mask on the
topside of the board beneath the BGA, and then tent (or plug) the via's
from the
bottomside. To me, you're just asking for trouble down the road because
you're
 creating a little "cup" if you will, that willl trap flux residue and
all
kinds of stuff in the via's that won't be able to be cleaned out well.
This is
a  676 ball 1.0mm pitch BGA pattern.

This assembly will be put together using double-sided reflow, there will
be
no wave soldering. All through-hole is going to be hand-soldered.

My opinion is that either you leave the via's free from solder mask on
both
sides, or cover them on the topside only, or ideally, plug them
completely.

Am I wrong about this?

Thanks!

-Steve Gregory-


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