On Fri, 10 Jun 2011 08:03:35 -0400 Bob Paddock <[email protected]> wrote:
> On Thu, Jun 9, 2011 at 9:13 PM, Kai-Martin Knaak <[email protected]> > 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. > > 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 [email protected] http://www.seul.org/cgi-bin/mailman/listinfo/geda-user
