In message  <[log in to unmask]> [log in to unmask] writes:
> 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

Most Kapton masking tapes use a silicone based pressure sensitive adhesive,
because of its high temperature resistance (good for soldering). I suspect that
this silicone is what you are seeing. The residue will vary from tape
manufacturer to manufacturer.

Try a silicone-free tape. Alternatively, go for a liquid based mask.

I do not know why you are getting contamination from a ceramic stiffener though!

Peter
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Peter Swanson                                              Oxfordshire, England
INTERTRONICS
[log in to unmask]                  http://www.intertronics.co.uk

Suppliers of materials and consumables to the electronics & related industries
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