One thing I have seen in X-ray inspection of BGA parts is small solder balls
where the paste has reflowed but not attached to the pad or part and becomes
a loose 'B-B' (ammo for an air rifle) under the part that could become a
potential short if not cleaned out from under it. Usually the technician can
used compressed air to blow the loose solder ball out from under the part
before running the parts through the aqueous wash. If you did not tent the
vias under the part they might be potential shorting points for solder balls
or other debris so as a rule I always opt for tenting the vias, unless
specific ones are used as test points like Andy points out.
Bill Brooks
PCB Design Engineer , C.I.D., C.I.I.
-----Original Message-----
From: Kowalewski, Andy [mailto:[log in to unmask]]
Sent: Friday, October 24, 2003 8:49 AM
To: [log in to unmask]
Subject: Re: [DC] Cleaning of via holes under components
Debris is one issue, but possibly a more important one is the via filling
with solder during wave soldering, forming a 'crown' on the primary side and
possibly touching a metal part of the component over the via hole, causing a
short circuit. That's particularly a problem with a lot of small chip
components, as well as a lot of RF metal-cased components, that are mounted
over the via.
Via tenting is an established practice, and there are few reasons not to
tent. It's foolproof tenting both sides, but might need via plugging if the
solder mask is a thin photoimagable type and the via hole is bigger than
about 0.3mm.
Of course the picture gets a little more complicated if the via is used as a
test point for probing. I know some people tent only the non-probed side,
but I don't like doing that - it becomes a trap for all sorts of nasty
chemicals which can give long term reliability problems.
Andy K.
Sychip Inc
Office (972) 202 8852
-----Original Message-----
From: Chris Robertson [mailto:[log in to unmask]]
Sent: Friday, October 24, 2003 10:01 AM
To: [log in to unmask]
Subject: [DC] Cleaning of via holes under components
There as been many an occasion debris or foreign matter in a via causing
problems.
I personally mask over all vias, and have never had a problem, but have a
question.
Yes, debris in a via and being under a pad, in encapsulated in solder or
plating etc.
is a reasonable question to ask. Could there be a problem, but my question
is;
Isn't this a cleanliness issue?
Is it just a fact that regardless of the manufacturer that debris or such is
a fact and not
just a manufacturer issue.
Is it that, yes it may be just a manufacturer issue, but there is no
accountability
before assembly and we can't just count on the board to be clean.
Or is it the time between manufacturing and assembly that we have to worry
about
debris?
I've never had an issue with debris in a via, and I don't know if I have
good manufacturers
or that my traces/circuits have never been that critical.
Chris Robertson
[log in to unmask] <mailto:[log in to unmask]>
----- Original Message -----
From: Paul <mailto:[log in to unmask]> Baine
To: [log in to unmask] <mailto:[log in to unmask]>
Sent: Friday, October 24, 2003 9:30 AM
Subject: [TN] Cleaning of via holes under components
Hello,
One of our Design Engineers is designing a surface mount PCB
assembly and would prefer not to have the via holes solder mask
tented. However, he is concerned that, if he does not call up
solder mask tenting on his bare PCB drawing, foreign matter could
find its way into a via hole and, if a component (e.g. an IC) was
mounted over the via hole, the foreign matter might not be removed
during the cleaning process.
Has anyone had experience with this situation? I would appreciate
any help you could give me.
Thanks very much in advance.
Paul Baine
Q.A. Manager
C-Tech Ltd.
Cornwall, Ontario, Canada
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