Dip coatings are pretty good at penetrating into everything, so i agree that
you probably have uncured coating beneath these parts,  and the trapped
solvent bubbles at temperature.   What I would look at is using a small
brush and unthinned coating (so it hopefully won't run under the parts, if
you truly don't want it under there) and paint around the base of the parts
during the pre-coat prep and mask operation.   Most of us oven dry the
boards before coating to ensure moisture removal and cure the maskant.  This
oven bake might cure the thick brushed-on material and prevent intrusion of
the dip material.

Ralph Vaughan
Boeing-Atlanta

> ----------
> From:         Lum Wee Mei[SMTP:[log in to unmask]]
> Reply To:     TechNet E-Mail Forum.;Lum Wee Mei
> Sent:         Monday, August 31, 1998 10:26 PM
> To:   [log in to unmask]
> Subject:      [TN] Bubbles from conformally coated board
>
> The test engineer came back to us to complain that bubbles are found
> along the edge of the power modules after going through the temperature
> cycling tests. This problems are found on both coated boards. The power
> modules are mounted completely flushed onto the board and subject to
> conformal coating using the diping method. The coated board undergo QC
> to ensure all components are completely coated, inlcuding the edges
> around the modules.
>
> What could be the problem?
>
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