Per-Erik
An interesting idea. Briefly, you are proposing a more permeable coating
so that humidity can leave, as well as enter. I suggest that if these
conditions apply, then the best coating would be the most permeable.
Probably 90% or more of electronic assemblies use such a coating. It is
called air. It is extremely successful for the conditions under which
such circuits are normally used. Conformal coating is usually used when
air-coating is unsuitable, for whatever reason. Therefore, we need
something as far removed from the conditions of air as possible, such as
a theoretically impervious skin. The nearest we have to this in our
armoury of weapons of mass construction is para-xylylene
(Paralene/Parylene). This is near-ideal, but expensive and difficult to
apply, so we usually look to other materials. I would suggest that
acrylics are closest to this ideal, except that they have poor
temperature and chemical resistances. So the choice, IMHO is either air,
with maximum permeability, or a product with minimal permeability.
Compromises between the two are probably second-best.
Brian
Tegehall Per-Erik wrote:
> 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
>
> -----Ursprungligt meddelande-----
> Från: Tegehall Per-Erik
> Skickat: den 5 september 2003 08:38
> Till: [log in to unmask]
> Ämne: Re: [TN] Silicone conformal coating
>
> 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
>
> -----Ursprungligt meddelande-----
> Från: Steve Gregory [mailto:[log in to unmask]]
> Skickat: den 4 september 2003 21:35
> Till: [log in to unmask]
> Ämne: [TN] Silicone conformal coating
>
> 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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