gEDA-user: Conformal coatings, PCA water-proofing

[Colin D Bennett <colin@xxxxxxxxxxx>]

On Fri, 10 Jun 2011 08:03:35 -0400
Bob Paddock <graceindustries@xxxxxxxxx> wrote:

> On Thu, Jun 9, 2011 at 9:13 PM, Kai-Martin Knaak <kmk@xxxxxxxxxxxx>
> wrote:
> > Ethan Swint wrote:
> >
> >> I wonder what materials are available for soldermask and
> >> if they have higher moisture resistance?
> >
> > Maybe "conformal coating" fits your needs.
> > Â http://en.wikipedia.org/wiki/Conformal_coating
> 
> A very common misconception is that Conformal Coating is a Hermetic
> Seal. It is used a lot in the Coal Mines, and the Electronic Industry
> in general, to keep the caustic dust etc. off circuit boards.
> 
> As Conformal Coating is not a hermetic seal, what real happens is the
> impurities in the water are kept away from the circuit, but the water
> itself reaches the traces. Since the water is now fairly devoid of
> contaminates the water acts more like a dielectric insulator. You
> never notice it in a low impedance digital circuit, but unless
> debugging is an obsession don't let it get near a RF tuning circuit or
> a high impedance sensor.  For example Analog Devices for their
> (obsolete?) electrostatic grade OpAmps say to "expose to air" rather
> than coat.
> 
> Conformal Coating has lots of applications, however the application
> and environment that the product is going to be used in has to be
> understood when the system is being designed.

It is quite surprising that coating a PCA in silicone would not provide
a water proof seal.  But based on what I have recently read, you are
right that conformal coatings are permeable to water.

What about potting the circuit rather than applying a conformal coating?
Could that provide a true water proof hermetic seal?  For instance,
suppose I wanted to immerse a temperature measurement circuit in a
container of water for extended periods.  Could it be potted in
silicone or some other commonly available substance to protect it from
water?

Regards,
Colin


_______________________________________________
geda-user mailing list
geda-user@xxxxxxxxxxxxxx
http://www.seul.org/cgi-bin/mailman/listinfo/geda-user