Steve,
Thanks. Thanks for the ASTM references, I will look at getting those.
Yes, everything I have seen about sealants and epoxies bring into question
"How Much Over Time" water is transported.
IO was wondering if that is any different for thermoplastics like poly
carbonate and ABS.
What is it about metals and glass that stops water in its tracks? Any
ideas?
Thanks,
Bob Kondner
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steven Creswick
Sent: Friday, March 06, 2009 10:03 AM
To: [log in to unmask]
Subject: Re: [TN] Comformal Coating vs Potting
Bob,
Others have already responded regarding transmission rates of metals and
glass [none] - this is what a hermetic package is. Metal or ceramic
sidewalls with glass or brazed seals. Ideally, one welds it shut and what
is on the outside stays there - as opposed to plastic encapsulated packages.
Still used in implantable medical and high rel stuff. Parts like this are
on Voyager I and II - still tick'en, and Cassini, circling Saturn ...
How do people measure the transmission rate? I believe that ASTM F732,
E96B, or E96BW covered this. As I recall, it was either in terms of
gm/cm2/cm or gm/m2/24 hr. In the first case it is grams of water gained
through a path which was cm2 in area by cm in length. In the second case it
is grams of water gained through a m2 area over a 24 hour period. I believe
the latter is used more for plastic films, etc.
Steve
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert Kondner
Sent: Thursday, March 05, 2009 9:17 AM
To: [log in to unmask]
Subject: Re: [TN] Comformal Coating vs Potting
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
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