[volt-nuts] PCB cleaning and sealing

shalimr9 at gmail.com shalimr9 at gmail.com
Sun Sep 26 12:23:42 UTC 2010


Yes, many silicones are hygroscopic. We use a variety of silicone based potting compounds where I work that are slightly hygroscopic. Yet, they work well to contain water into a fish tank :)

I know silicone based conformal coatings are used in the automotive industry. I can tell you from experience that they are not moisture proof. I had a height sensor fail just for that in my Ford Expedition. The PWB was badly corroded under the conformal coating and traces were shorted. Once the coating was removed and the PWB cleaned, it worked again.

I understand that some silicone based oils are not.

Didier 
Sent from my Verizon Wireless BlackBerry

-----Original Message-----
From: "Alan Scrimgeour" <scrimgap at blueyonder.co.uk>
Sender: volt-nuts-bounces at febo.com
Date: Sun, 26 Sep 2010 13:00:03 
To: Discussion of precise voltage measurement<volt-nuts at febo.com>
Reply-To: Discussion of precise voltage measurement <volt-nuts at febo.com>
Subject: Re: [volt-nuts] PCB cleaning and sealing

Do silicone based conformal coatings allow water in too?

Alan

----- Original Message ----- 
From: "Bob Paddock" <bob.paddock at gmail.com>
To: "Discussion of precise voltage measurement" <volt-nuts at febo.com>
Sent: Sunday, September 26, 2010 12:23 PM
Subject: Re: [volt-nuts] PCB cleaning and sealing


> On Sun, Sep 26, 2010 at 3:20 AM, Dr. Frank Stellmach
> <drfrank.stellmach at freenet.de> wrote:
>
>> sealing to avoid corrosion, oxydation, leakage currents.
>
> A very common misconception is that Conformal Coating is a Hermetic
> Seal. It is used a lot in the Coal Mines [Where I've done many
> designs], and the Electronic Industry in general, to keep the caustic
> dust off circuit boards.
>
> As Conformal Coating is not a hermetic seal, what real happens is the
> impurities in the water are kept away from the circuit, but the water
> itself reaches the traces. Since the water is now fairly devoid of
> contaminates the water acts more like a dielectric insulator. You
> never notice it in a low impedance digital circuit, but unless
> debugging is an obsession don't let it get near a RF tuning circuit or
> a high impedance Sensor circuit.
>
> The quality of the coating determines how long it will take the water
> to reach the traces, it could be months, but it will reach the traces.
>
> -- 
> http://blog.softwaresafety.net/
> http://www.designer-iii.com/
> http://www.wearablesmartsensors.com/
>
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