Hello Richard,
The fact that you have to re-adjust the reflow profile for a specific board
is not unusual, different designs have different thermal profiles and need
more or less heat for each design.
I'm not a big fan of using masking tape in board process, but for conformal
coating application it shouldn't represent much of a problem as long as your
coating doesn't react with the tape.
You did not specify the type of conformal coating you use and most
importantly the curing method for this coating. That may have an impact on
the answers you get.
You most likely culprit though is the boards themselves. the fact that you
see copper shows poor adhesion of the solder to the copper and that is more
then 99% of the time due to a problem between copper plating and HASL (or
whatever other coating covers and protects the copper. Have you audited your
board vendor? Not just a paper and meeting audit, but an actual tour of
their manufacturing facilities. You'd be surprise what you see happens in
some shops.
Hope I could have been more helpful,
Alain Savard, B.Sc.
Chemical Process Analyst
CAE Electronics Ltd.
e-mail: [log in to unmask]
-----Original Message-----
From: Richard Tilbrook
Subject: Re: [TN] scary tin problems ???!!!
My problem is on a subject about which I know little, and I write to you on
behalf
of a friend upon whose project the following has manifested itself.
We have a line upon which we manufacture simple through-hole PCBs. We are
using
externally supplied multi-layer boards with nothing high-density or high
tech about
them. They come in looking nice and well tinned. For about six months they
have
been flowing OK, with only one problem - a large ground plane caused a few
problems
in one area for a little while and we had to re-adjust the profile on the
flow
machine (this is, I am sure, irrelevent to the problem). We like to
manufacture to
a very high standard, so it was concerning when some PCBs recently exhibited
bare
copper in various places. We use a low tack professional masking tape to
mask two
planes on the board during conformal coat - on removal, the tinning came
away,
exposing bare copper! Even more seriously, at many places solder had not
fully
wicked to the top of the through-hole, meaning very suspect joints. In
various
places, solder had wicked away from pads, again exposing copper.
Is it possible that we are doing something wrong without realising, or shoud
we be
chatting to our board suppliers?
What do you folks feel is the problem, and what do you recommend we look at?
Richard Tilbrook
Junior Engineer
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