Alexis

IMHO, the only conformal coating likely to meet your requirements is
paraxylylene (Trade mark Parylene). This is vacuum deposited and
requires very expensive equipment, although there are jobbing shops
which can do it for you without the investment. It has an added
advantage that a plasma is drawn in the vacuum chamber before the
monomer is evaporated and this "primes" the surface to ensure maximum
adhesion of the deposited polymer. It penetrates extremely well under
large components, into the bargain. The layer thickness is very uniform,
even over points, and is typically in the order of micrometres.

That having been said, you should be aware that, if you have
transmission lines, the characteristic impedance will change slightly
with ANY coating, as the dielectric constant of any polymer is higher
than unity. The conductor widths should be altered accordingly.

Hope this helps.

Brian

Alexis Meehan wrote:
>
> TechNetters;
> We are in the process of investigating conformal coating processes.
> What are your experiences with current conformal coatings? Has anyone
> used something called Halar? We are hoping to find a coating that has the
> following elements:
> * Must not affect thermal properties or electrical properties (2 gigabit
> circuitry).
> * Dry rather than liquid.
> * Minimal physical impact (our boards slide into a card cage; therefore
> the board height should not be affected more than +/- .010").
>
> Any feedback at all would be greatly appreciated; please respond to my
> email address. Thanks in advance.
>
> ********************
> Alexis Meehan
> PCB CAD Mgr.
> 408-328-4595
> [log in to unmask]
> LuxN, Inc.
> 570 Maude Court
> Sunnyvale, CA 94086
>
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--
Brian Ellis
Protonique SA
PO Box 78
CH-1032 Romanel-sur-Lausanne, Switzerland
Voice: +41 21-648 23 34 Fax: +41 21-648 24 11
E-mail: [log in to unmask]
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