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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