I was told by one of our populators that the vias should be open on the
bottom side and
closed on the top. If both sides are sealed then one will blow out during
heating of the
PCB in reflow. This could be the top side and throws debris under the BGA. I
seal the
top and leave the bottom open to 'breathe'
Jim Thornhill
----- Original Message -----
From: "Mark Larson" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Thursday, September 30, 2004 2:51 PM
Subject: Re: [DC] Via protection beneath a fine pitch BGA...
>I think the answer depends on other factors, for instance it would
> definitely not be a good idea to leave the vias open and uncovered if
> wave soldering due to solder wicking
>
> At the very least the top side via pads should be covered in any case in
> my opinion, then plugging the via and covering the bottom side as well
> depending on the other factors
>
> it may be nice to leave the bottom side uncovered for probing but then
> the vias should be plugged or a top side mask scheme employed that does
> not allow solder to wick through the via must be used
>
> mark
>
> -----Original Message-----
> From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of
> Steve Gregory
> Sent: Wednesday, 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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