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November 1999

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Subject:
From:
"Collins, Graham" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 25 Nov 1999 13:44:56 -0400
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Ian
Likely you will be deluged by responses tomorrow from those out eating
turkey today.

Anyway, to the matter at hand.  I have to wonder if your coating is not
unusually sensitive to some tape residue.  The reason I say that is because
we use an very similar process, and have no problems.  Kapton tape is used
for masking during wave soldering, and we then coat the boards with one of
three possible conformal coatings: 2 urethane, 1 acrylic.  We don't always
let the boards cool before the tape is removed.
Most boards have a dry film solder mask surface, a couple have no solder
mask and a couple have LPI.

1) is this a new problem, and if so has something changed in your process
(wave soldering flux, or solder mask)?
2) if an ongoing issue, can you try a different coating material?

good luck!

Graham Collins
Process Engineer,
Litton Systems Canada, Atlantic Facility
(902) 873-2000 ext 6215

-----Original Message-----
From: Ian Slater [mailto:[log in to unmask]]
Sent: Thursday, November 25, 1999 12:44 PM
To: [log in to unmask]
Subject: [TN] Board Contamination


We're having contamination problem which is impacting our
conformal coating process.  Here's the story:

We use  "Kapton" style tape to guard the edges of a circuit board
while the board is processed in our wave solder machine.   The
board/tape reaches a peak temperature of 220C for three seconds.
The temperature ramp keeps the board above 120C for
approximately three minutes.

After passing through the wave solder machine, the board is allowed
to cool to room temperature.  The tape is then removed.  No visible
residue is left on the board.  However, when we attempt to conformal
coat the board, the coating is repelled from areas where the tape
was applied.  No manner of cleaning (alcohol, water, acetone...) will
remove the contamination and allow the conformal coat to bond to
the board.

Are you aware of what the source of contamination could be?  We
have managed to rule out flux. Boards have been taped and tested
prior to heating and no residue was found.  Four different brands of
tape were tested with identically poor results.

Ceramic edge stiffeners have been tried.  The edge stiffeners clamp
mechanically over the edges of the board.  Some contamination was
found when edge stiffeners were used, though not as much as with
the tape.

Boards run without protection show no contamination.

I would appreciate any insight you could provide into the nature of

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