Kapton tape. I have had the same problem. We used Kapton for wave solder masking and had the coat bead up in the areas that had been masked. Note the tape was not on my boards when sprayed. I found that a good cleaning with alcohol and disposable wipes helped considerably. Also by spraying the first coat very light and letting it dry for a few minutes would let the coating cover those areas.
Charles Caswell
Process Lead, PCB
Frontier Electronics Systems
-----Original Message-----
From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Tuesday, July 23, 2002 5:43 AM
To: [log in to unmask]
Subject: [TN] Conformal Coating repelled by masking
Hi, Folks,
I have a theory about one phenomenon out of two that I have just witnessed,
but seek your sage thoughts on the matter.
Our fabrication shop was spray coating an FR4 / HASL-finished board with
Humiseal 1B31, which was masked with some very expensive Kapton tape. The
tape is meant to be a no-static, no-residue type. We noticed that, under
the UV inspection lamp in the spray booth, the coating appears to recede
from the edge of the Kapton tape - at least there is a "line" 1 or 2 mm
wide at the edge of the masking, that indicates (to me) a different
thickness of coating in that zone compared with the rest of the board. We
tried over-spraying the affected areas again once the initial coat had set
a bit, but the line reappeared. It only happens where there is Kapton tape.
At the same time, the coating on the solder side of the board is a nice
shiny finish, while the component side has quite large areas that have a
more matte, slightly "rough" appearance. I doubt there is a connection
between the two phenomena, but you never know.
My theory about coating recession from the tape is that there is a static
charge on the Kapton tape, in spite of the "no-static" presumption, and
that the coating particles in the spray are likewise charged from their
passage through the spray gun. The two like charges are then repelling one
another. I noticed that neither board nor spray gun are grounded, and think
that they should be (comments, anyone?).
My secondary theory is that the coating is reacting with the tape adhesive
at the edge of the tape, creating a line of contamination that the coating
doesn't stick to properly. Kapton tape is a commonly used masking material,
though, and I haven't heard of this problem occurring with it before.
I haven't come up with an explanation for the matte areas of coating on the
component side of the board. The board had just come from the customary
cleaning and drying that I give all boards just prior to coating.
Can you come up with possible explanations and cures for what is happening
here?
MTIA
Peter
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