I do not have the IPC-A-610B document in front of me but I believe that
it must be free of bubbles to meet class 3 requirements.  More
importantly, you have parts that are not the highest possible quality.
The cause I believe is one of 2 things if you are using standard
acrylic, epoxy or polyurethane conformal coating.
1. The PWB has moisture in it.  An hour vacuum back at 170F will dry it
properly.
2. The PWB is warm when conformal coating is applied.  This could happen
if you bake after 1 side is sprayed or between coats.  If the PWB is
warm you will get numerous small air bubbles.
If you have further questions you may contact me off-line.

Jon Hemstad
[log in to unmask]

> ----------
> From:         <Dominick S. Scarano>[SMTP:[log in to unmask]]
> Sent:         Thursday, April 09, 1998 8:07 AM
> To:   [log in to unmask]
> Subject:      [TN] conformal coat bubbles
>
> Dear Technet,
>
>         We'd like some clarification (interpretation) of the
>         requirements noted in:
>
> J-Std-001B      pg.27   paragraph 10.1.2.2 - Coating Coverage...
> IPC-A-610B      pg.74   paragraph 7.1.2 - Coating Coverage...
>
> d. Be free of voids, BUBBLES, or foreign material which EXPOSE
> component conductors, printed wiring conductors (including ground
> planes) or other conductors, and/or violate design electrical spacing.
>
>
>         Does the word "expose" in (d.) above mean;
>
>  - No voids, bubbles, or foreign material "over" the runs (that the
>  bubbles may open over time due to temperature and pressure variations
>  and "expose" the conductors)?
>
>  - That voids, bubbles, or foreign materials indicate an absence of
> the
>  minimum required thickness of conformal coating?
>
>  - Or the literal meaning of "expose" - to leave bare, endanger?
>
>
>         We have received some PWAs with conformal coating that have
>         bubbles between and over conductor runs and around and on
>         components.  The bubbles over the runs are not broken open
> (not
>         literally exposing the conductor).  The coating when viewed
>         with the unaided eye appears hazy due to the multitude of tiny
>         bubbles throughout the coating.  There are larger bubbles and
>         small bubbles between runs that when added together would
>         violate the minimum spacing requirements.  As I mentioned
>         before, the bubbles are of various size and numerous enough to
>         make the coating appear porous (spongy looking) mostly tiny
>         bubbles throughout.  We are required by contract to meet
>         J-Std-001B class 3.
>
>         We would appreciate your help in sorting through this
> confusion.
>
> Thanks,
>
> Dominick Scarano
> Lockheed Martin
> Mfg. Training
>
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