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August 2008

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Subject:
From:
Hernefjord Ingemar <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Hernefjord Ingemar <[log in to unmask]>
Date:
Tue, 12 Aug 2008 10:02:49 +0200
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???????? Don't understand. We perform refurbishment of military equipment from the field, often covered by powderlike sand and dust. Only way to remove it is with water. We add some tenside to the tap water and brush the boards gently, followed by rinsing in DI and finally drying with dry nitrogen pistol. The coating used is 1B31, and we never saw any lifted or defect coating. How is that?
Inge

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Phil Nutting
Sent: den 11 augusti 2008 18:46
To: [log in to unmask]
Subject: Re: [TN] Washing of conformally coated boards

Ioan,

I agree with Graham... from personal experience.  We use an acrylic conformal coat and even a very short (30 seconds) wash at 100° F will loosen the coating.  I'd vote for the full strip if the board bust be reworked and washed.  From what I've read, doing anything else will compromise your long term reliability.

Phil

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Graham Naisbitt
Sent: Monday, August 11, 2008 11:11 AM
To: [log in to unmask]
Subject: Re: [TN] Washing of conformally coated boards

Ioan

If its acrylic, don't do it.

You would be better off stripping the coating, carrying out the re- work and re-coating.

Your cleaning the coated assembly will, almost certainly, compromise the protection it is there to provide.

The UV tracer is in the coating for inspection at the coating stage.
It is not a long-term aid and prolonged exposure to sunlight will cause it to deteriorate over time.

You say you have to use a WS flux, so it's acid content is high and it must not therefore be left on the circuit. To make sure it is gone you will have to employ a pretty effective wash process that the coating was not designed to withstand.

Stripping the coating is pretty simple as it will dissolve, but check with the manufacturer the most suitable solvent for the job. Leaving it to soak for around 20 minutes at say 40C to 60C or higher should do the trick.

Hope this helps

Graham Naisbitt


On 11 Aug 2008, at 15:39, Ioan Tempea wrote:

> Thanks Brian!
>
> It is acrylic I'm talking about. Will 140F be OK?
>
> Also, what the should worry be: any damage to the protetcive layer
> that cannot be seen under UV light or moisture entrapment?
>
> Should I consider a low temp (how low) post-wash bake?
>
> Thank you,
>
> Ioan
>
> -----Message d'origine-----
> De : Brian Ellis [mailto:[log in to unmask]] Envoyé : 11 août 2008
> 10:23 À : TechNet E-Mail Forum; Ioan Tempea Objet : Re: [TN] Washing
> of conformally coated boards
>
> Theoretically no, if it was properly applied in the first place.
> Provisos:
> a) tender coatings, such as acrylics, like neither high temps nor
> aggressive chemicals in your wash water.
> b) silicone coatings will wash OK but you may have problems with
> moisture absorption/adsorption in the drying stage.
> c) urethanes and flexibilised epoxies should normally be OK
>
> Brian
>
> Ioan Tempea wrote:
>> Dear Technos,
>>
>> A quick one : can I water wash conformally coated boards?
>>
>> This means conformally coated boards have been returned for rework,
>> we will change the components (have to use WS flux) and have to wash.
>>
>> Can regular water wash at 140F do any damage to coating?
>>
>> Thanks,
>>
>> Ioan
>
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