Welcome to the Designers Council Steve. I have visited your website many
times.

My approach to vias under BGAs has always been to fill the vias with solder
mask. We never had any failures due to solder ball shorting under the part,
although there have been some occasions where loose balls of solder were
left under the part and had to be cleaned out.

I'm not familiar with this method of encroaching the solder mask to the edge
of the hole and then leaving the hole open... does that not make an
opportunity for loose solder balls to get trapped under the part at the via
hole edge or in the barrel? Although it may prevent trapped moisture from
exploding under the part when out gassing I still really see no advantage to
that method as yet... It just seems like any flux or other liquid materials
used in processing would get trapped in the vias and present a cleaning
challenge.

Maybe someone could explain why they would do that so I can understand what
the goal is there. Glad to see your comments on the DC list server.

Best regards,


Bill Brooks
PCB Design Engineer , C.I.D., C.I.I.
Tel: (760)597-1500 Ext 3772 Fax: (760)597-1510
http://pcbwizards.com


-----Original Message-----
From: Steve Gregory [mailto:[log in to unmask]]
Sent: Wednesday, September 29, 2004 5:14 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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