Hi, Steve... Welcome to "the other side of the fence"!
I'm pretty sure we were getting the "volcano effect" using the bottom-mask
only.
What you are seeing now is probably the result of a customer who first
tried
masking BOTH sides, had problems, and decided to uncover one of them.
You're probably not in too much trouble if you are using no-clean, though.
Does anyone disagree?
As a matter of fact, I've recently been asked to uncover both sides
completely.
Kind of a shock since I have been tenting so long (either completely or at
least
encroaching up to the maximum hole size).
For whoever was asking about the theory behind encroachment, here's as
brief
a summary as I can write:
Plugging is a good idea, but difficult and expensive to do correctly.
Plugging is great if you can get 100% plugs and planarity too.
BUT...
Studies have proved (ask Werner!) that a partially plugged hole is far less
reliable than either a completely plugged or completely unplugged hole.
I assume that is a conductive plug.
We used to plug with LPI mask which never gave us any problems but left
partially uncured epoxy in the hole (only fully cured on the surface by the
UV light or whatever) So the advantage of allowing the mask to flow up onto
the pad, but not down the hole seems ideal, no junk in the hole, no issues
with
the two-sided or one-sided tenting debate, the ability to put vias very
close to
pads without the solder wicking away down the hole (because we have a wider
dam of mask between the pad and the hole)
We were always taught to NOT put exposed vias under components because
of the cleaning issue, but now we use no-clean. Now we are being asked to
clear the whole via pad (for accessibilty issues like test points) and the
only
down side that I can think of is having to move the via out a little more
from
the pad to get enough mask between them. no problemo, mon!
Am I missing something?
Any thoughts?
Jack
Steve Gregory
<[log in to unmask]
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09/29/2004 07:14
PM
Subject
Please respond to [DC] Via protection beneath a fine
"(Designers pitch BGA...
Council Forum)"
<DesignerCouncil@
IPC.ORG>; Please
respond to
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Caterpillar: Confidential Green Retain Until: 10/29/2004
Retention Category: G90 -
General
Matters/Administration
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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