TECHNET Archives

March 2009

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 <[log in to unmask]>, Brian Ellis <[log in to unmask]>
Date:
Thu, 5 Mar 2009 18:34:19 +0200
Content-Type:
text/plain
Parts/Attachments:
text/plain (129 lines)
0.1 mm steel or other rolled metal is better than 100 km or more of 
silicone or epoxy!!!!!

Water does not pass through polymers but water vapour does; in fact, 
they absorb them!

Many years ago (in the 1970s) I did an experiment mentioned in Ch. 30 
(protective coatings) of a well-known book. I constructed a Plexiglass 
box with two compartments with a 20 mm thick separator with a 100 mm 
diam hole. I cast 5 mm thick samples of different coating materials 
(silicone, epoxy, polyurethane) by building them up over several layers 
and clamped them over the whole with an O-ring to ensure a seal. I 
maintained a 95% 40°C atmosphere on one side and a <5% RH on t'other 
side, with the help of a phosphorus pentoxide desiccant. After 10 days, 
to ensure equilibrium, I weighed the desiccant then again after 5 more 
days. The silicone allowed about 1 g of water vapour to pass per day, 
the epoxy about 0.022 g and the polyurethane about 0.046 g. I did not do 
a control with a metal disc, but I'm sure it would have yielded 0.00 
g/day. Note that if I had filled the "wet" compartment with liquid 
water, the result would also have been as near 0.00 g as dammit (surface 
tension is the secret!).

Brian

Robert Kondner wrote:
> Brian and Blair,
> 
>  From what I hear a sealed metal box will keep out water. A layer of steel
> or glass does not allow water to pass.
> 
>   Is this not the case for a "Thick" layer of silicone or epoxy? 
> 
>   What does "Thick" mean for different materials. How think does a silicone
> adhesive need to be to equal the water penetration resistance of 1/16 (or
> 1/8) inch of steel?
> 
>   How do folks measure how water (or even any ionic material) passes through
> a substance?
> 
> Thanks,
> Bob Kondner
> 
> 
> 
> 
> 
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Blair Hogg
> Sent: Thursday, March 05, 2009 8:38 AM
> To: [log in to unmask]
> Subject: Re: [TN] Comformal Coating vs Potting
> 
> Bob,
> 
> Most conformal coatings can and will pass moisture through to the assembly
> in varying degrees, Thus, if there are any contaminants on the assembly
> prior to coating that can activate with the addition of moisture the coating
> will only slow down the effect. Also, if the environment in which your
> module will be placed is humid for a significant portion of the time, it
> will not have the opportunity to dry out, and its performance may be
> affected.
> 
> Blair Hogg 
> 
> On Wed, 4 Mar 2009 10:50:38 -0500, Robert Kondner
> <[log in to unmask]> wrote:
> 
>> Hi,
>>
>>  I have been playing with some conformal coating material and I have a
>> couple of general questions.
>>
>> 1.	Coatings get VERY thin around sharp corners. Using a number of coats
>> seems like the only way to build up any serious thickness.
>>
>> 2.	Potting seems like another name for "Very Thick Conformal Coating".
>>
>> I am looking as an application where a sensitive electronic module will be
>> placed in a wet humid environment. It is a sensor going into the cooling
>> tower of a HVAC system.
>>
>>  After reading about conformal coating and playing with some sprays and
>> brush on material I think thick potting is the only option to keep moisture
>> out of circuits. We need at least 20 Meg ohm between circuits spaced 10
> mils
>> on FR4 material.
>>
>> Can anyone verify I am on the correct track and can they point me towards
>> selecting the proper materials and process?
>>
>> Thanks,
>>
>> Bob Kondner
> 
> ---------------------------------------------------
> Technet Mail List provided as a service by IPC using LISTSERV 15.0
> 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
> 847-615-7100 ext.2815
> -----------------------------------------------------
> 
> ---------------------------------------------------
> Technet Mail List provided as a service by IPC using LISTSERV 15.0
> 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 847-615-7100 ext.2815
> -----------------------------------------------------
> 

---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 15.0
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 847-615-7100 ext.2815
-----------------------------------------------------

ATOM RSS1 RSS2