After
having thought a little more on this, I will expound my answer. It seems that
all assume that high permeability is a bad thing but in my opinion it is not
that simple and a high permeability may even be preferable. If you have adequate
cleanliness on the assembly prior to applying the coating, the moisture
penetrating the coating will not be a problem and the permeability will then not
be an issue. It is when you have to much hygroscopic and ionic contamination
beneath the coating that you may get problem with delamination and
electrochemical migration. Even if you use Parylene, quite a lot of moisture
will be picked up by hygroscopic contamination beneath it in a few days in a
humid environment. If you then turn the euipment on and the board is heated due
to power dissipation, it will take much longer time to dry up the assembly if
you have a coating with low water permeability. And it is during this time
period electrochemical migration will occur. Therefore, a silicone coating may
actually be better than Parylene in this case.
Per-Erik Tegehall
IVF
Steve,
It
depends on what you mean with good moisture resistance. All conformal coatings
absorbs moisture but the diffusion rates of water in the coatings and the
saturation level varies. Water molecules have a high diffusion rate in
silicone coatings but the saturation level is lower than in most other
coatings. Which is to be prefered? It probably depend on the
application.
However, a general statement of good moisture
resistance, I interpret as that the coating properties are not degraded by
moisture.
Per-Erik Tegehall
IVF
Hi All!
I know when you
look up properties for Silicone conformal coating, it usually states that it
has good moisture resistance, but is it really that good?
I've heard
that it is somewhat permeable to moisture, that if you were worried about
moisture, and the assembly were going to see the outside environment, you
would be better off with an acrylic, or urethane coating.
Any
thoughts?
Thanks!
-Steve Gregory- ---------------------------------------------------
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