TECHNET Archives

September 2003

TechNet@IPC.ORG

Options: Use Monospaced Font
Show Text Part by Default
Show All Mail Headers

Message: [<< First] [< Prev] [Next >] [Last >>]
Topic: [<< First] [< Prev] [Next >] [Last >>]
Author: [<< First] [< Prev] [Next >] [Last >>]

Print Reply
Subject:
From:
Brian Ellis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Sat, 6 Sep 2003 10:48:02 +0300
Content-Type:
text/plain
Parts/Attachments:
text/plain (142 lines)
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- ---------------------------------------------------
>         Technet Mail List provided as a free service by IPC using
>         LISTSERV 1.8e
>         To unsubscribe, send a message to [log in to unmask] with
>         following text in the BODY (NOT the subject field): SIGNOFF Technet
>         To temporarily halt or (re-start) delivery of Technet send
>         e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
>         To receive ONE mailing per day of all the posts: send e-mail to
>         [log in to unmask]: SET Technet Digest
>         Search the archives of previous posts at:
>         http://listserv.ipc.org/archives
>         Please visit IPC web site
>         http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional
>         information, or contact Keach Sasamori at [log in to unmask] or
>         847-509-9700 ext.5315
>         -----------------------------------------------------
>
>     ---------------------------------------------------
>     Technet Mail List provided as a free service by IPC using LISTSERV 1.8e
>     To unsubscribe, send a message to [log in to unmask] with following
>     text in the BODY (NOT the subject field): SIGNOFF Technet
>     To temporarily halt or (re-start) delivery of Technet send e-mail to
>     [log in to unmask]: SET Technet NOMAIL or (MAIL)
>     To receive ONE mailing per day of all the posts: send e-mail to
>     [log in to unmask]: SET Technet Digest
>     Search the archives of previous posts at:
>     http://listserv.ipc.org/archives
>     Please visit IPC web site
>     http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional
>     information, or contact Keach Sasamori at [log in to unmask] or
>     847-509-9700 ext.5315
>     -----------------------------------------------------
>
> ---------------------------------------------------
> Technet Mail List provided as a free service by IPC using LISTSERV 1.8e
> To unsubscribe, send a message to [log in to unmask] with following text
> in the BODY (NOT the subject field): SIGNOFF Technet
> To temporarily halt or (re-start) delivery of Technet send e-mail to
> [log in to unmask]: SET Technet NOMAIL or (MAIL)
> To receive ONE mailing per day of all the posts: send e-mail to
> [log in to unmask]: SET Technet Digest
> Search the archives of previous posts at: http://listserv.ipc.org/archives
> Please visit IPC web site
> http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional
> information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700
> ext.5315
> -----------------------------------------------------

---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
-----------------------------------------------------

ATOM RSS1 RSS2