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February 2000

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Subject:
From:
"Stephen R. Gregory" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 15 Feb 2000 12:14:38 EST
Content-Type:
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In a message dated 02/15/2000 10:29:12 AM Central Standard Time,
[log in to unmask] writes:

<< From my experience with troubleshooting client's processes, it is a very
bad
 idea.  You have too great a chance of mixing curing or outgassing products
 from the adhesive with flux residues from the paste.  The two residues can
 interact with each other to form a detrimental combination.  I published two
 case studies on a common detrimental effect in my Process Rx column in
 Circuits Assembly magazine - "Microcanyons" and "Become one with the Glue".

 Doug Pauls
 Technical Director, CSL >>

I have to agree with Doug. We had a board at a past employer some years back
which had some problems with the lay-out of components on the
bottomside...the SOT's in particular. We were experiencing a lot of defects
(no solder mostly) on the leads of the SOT's because of the orientation.

Well, it was decided to print solderpaste, dispense glue, place the parts,
and cure and reflow at the same time, with the thought being that we would
make sure that there would be a solder fillet at those troublesome SOT's and
not have to depend on the wave to form them...seemed like a good idea at the
time.

But as we discovered, it was a really BAD idea for just the reasons Paul
talks about. Another thing, which what really caught our customers eye and
got everybody looking at the issue, was that during reflow, during the hot
slump of the paste (and all pastes do hot slump to some degree) the paste
would slump into the epoxy and embed itself into the epoxy...you would
wind-up with all these little solderballs in the epoxy. That was the first
thing that was noticed and started everybody looking at the whole issue.

As a result, the customer rejected all the boards we built doing that, rework
was out of the question, so we had to eat 750 boards...wasn't too tasty
either.

-Steve Gregory-

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