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March 2002

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Subject:
From:
Graham Naisbitt <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 20 Mar 2002 23:04:31 -0000
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Techies,

I know this to be a late posting but, according to the gospel of St Brian of
Cyprus - be careful with your bristles!

Regards Graham Naisbitt

[log in to unmask] <mailto:[log in to unmask]>

Concoat Limited
Alasan House, Albany Park
Camberley GU16 7PH - UK

www.concoat.co.uk <http://www.concoat.co.uk>

Phone: +44 1276 691100
Fax: +44 1276 691227
Mobile: +44 79 6858 2121


> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]]On Behalf Of Brian Ellis
> Sent: 19 March 2002 08:26
> To: [log in to unmask]
> Subject: Re: [TN] Contamination
>
>
> Ted
>
> I agree with St. Doug but ther may be some other steps you will find
> useful. Corrosion products are usually a mixture of soluble salts, like
> copper chloride, and insoluble ones, like carbonates, oxides and
> hydroxides, as well as a gunge of the original contamination. The first
> thing I would do is to immerse the products in a 2 - 3% solution of
> hydrochloric acid at, say, 40deg C. This will dissolve away the soluble
> and much of the insoluble corrosion products (a little brushing with a
> nylon-bristles brush will aid the process). I would then suggest a
> proprietary neutraliser (of the type used as a rinse-aid after soldering
> with water-soluble fluxes) containing the disodium salt of EDTA. This
> will help to chelate any remaining metal salts, especially the
> relatively insoluble lead ones. Finally, I would give them a saponifier
> wash, to make sure that all of the original flux residues are removed,
> followed by a damned good rinse cycle, ending with DI water.
>
> At this stage, you can be reasonably sure no corrosion-causing chemistry
> remains on the assembly and it will be possible to evaluate the damage
> and whether the assembly is usable/repairable/throw-awayable. Be
> particularly cautious where there are noble metals: gold-plated kovar
> leads, for example, are notorious for promoting stress corrosion on
> bends and I've seen cases where the kovar has been completely eaten away
> and the component is held on by the gold plating!
>
> Oh, BTW, your solder joints will appear matt after this treatment, but
> this does not matter: they will be perfectly OK from the chemical,
> metallurgical and mechanical points of view. It is purely a cosmetic
> dulling.
>
> Just as an additional precaution, in view of their history, I think I'd
> be inclined to give these assemblies a spray of an acrylic conformal
> coating, but this won't be strictly necessary if they meet the testing
> that Doug advocates.
>
> Brian
>
> Ted Tontis wrote:
> >
> >         We have a number of PWB's that show signs of contamination,
> > management wants to find out if there is a way to clean the
> boards. I have
> > told them once the corrosion starts there is no way to clean
> the assemblies
> > and make them dependable enough to cover our warranty. Some of
> the boards
> > show slight contamination and some more aggressive, but these
> have been in a
> > controlled environment for some time. I feel If we where to
> send them out in
> > a industrial environment the corrosion would rapidly increases.
> These boards
> > are FR-4 and not conformal coated. Am I correct on this?
> >
> > Thank you,
> >
> > Ted Tontis C.I.D.
> > Engage Networks, Inc.
> > 1320 N. Dr. Martin Luther King Jr. Drive
> > River Level
> > Milwaukee, WI 53212
> > PH 414-918-4267
> > FX 414-273-7601
> >
> >
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