Steve,

You're dead on.

Either soldermask over both sides, or no sm on both sides.

Some like to use a reduced sm on both sides which leaves the hole open
but partially covers the via pad (example: pad .020", drill .010", sm
.014").  Idea is that it provides a solder dam to prevent solder flowing
thru the via from the ball / paste on the bga pad.

Dave

-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of
Steve Gregory
Sent: September 29, 2004 7:14 PM
To: [log in to unmask]
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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