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September 2005

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Subject:
From:
"Dehoyos, Ramon" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Dehoyos, Ramon
Date:
Mon, 12 Sep 2005 14:26:21 -0400
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        Hi Juan:
        There are liquid water soluble or peel able masking that can prevent the white stuff from collecting in the holes by plugging them  and avoiding the hole contamination.
        Regards,
        Ramon
	
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Paul Klasek
Sent: Wednesday, September 07, 2005 6:40 AM
To: [log in to unmask]
Subject: Re: [TN] Cleaning rosin flux residues

On the root cause Juan,
the 'white stuff' (be flux) should not have creep that far normally (granted nothing is normal) : traced this occurrence in same incidence
(test) to flux overspray (& bake); perhaps check if wave pass applies.

-----Original Message-----
From: Brian Ellis [mailto:[log in to unmask]]
Sent: Wednesday, September 07, 2005 6:24 PM
To: [log in to unmask]
Subject: Re: [TN] Cleaning rosin flux residues


IPA is one of the worst cleaning solvents: its only merit is that it is polar. If, as you surmise, the residues come from your flux, then they are probably a nasty mixture of thermally polymerised and probably hydrolysed rosin, along with some residual activators. I am making a guess that your flux is "halogen-free", because this kind of flux can be the very devil, whether rosin based or not, because the activator content is essentially some form of carboxylic acid in much higher proportions than you get in organic chloride/bromide type fluxes and some of them can polymerise at the drop of a hat.

Isn't the use of HCFC-141b as a solvent forbidden in the EU, anyway?

Now you have a double-whammy of a problem: not just nasty residues, but deep in the contacts of a female connector. In this case, my first reaction is that an ounce of prevention is worth a kg of cure: change your connector to one whose design prevents any chance of the flux wicking into the contacts (turned contacts, instead of pressed). If this is not possible, then pre-tin them using minimal quantitites of flux, such as a good W/S flux, and thoroughly clean, then use little or no flux for the assembly. Another tip is to fill the contacts with a tight PTFE plug during the tinning/soldering process, to physically prevent flux from entering where it didn't ought to.

Now, how to clean inside, where you already have contamination. Bearing in mind the blind nature of the parts, you need both chemical and mechanical help. You need an expensive solvent and HCFC-225ca/cb with an azeotropic blend of methanol will probably be your best bet (much less aggressive than HCFC-141b). For the mechanical side, think in terms of an interdental toothbrush from your local pharmacy modified to fit in one of those small, high-speed, hand-held drills. Soak the parts in the solvent for a minute or two and insert the brush at, say, 10,000 rpm in each contact, while flooding it with the solvent from a wash bottle. 
Rinse in clean solvent.

There are two problems with this: the solvent is toxic and it is environmentally harmful, so you need very good ventilation drawing air away from the operator and you need a carbon filter in the ventilation duct.

If you can't get HCFC-225 (made only by Asahi Glass in Japan under the trade name AsahiKleen), then try HFC-4310 (DuPont) or HFE (3M), also blended azeotropically with methanol. They are less aggressive than the
HCFC-225 blend, but equally expensive (or more so). Many blends of HFCs and HFE contain trans-dichloroethylene and should be avoided if you have compatibility problems with HCFC-141b as this solvent (t-DCE) is even more aggressive.

Brian

Juan T. Marugán wrote:
> Hello teachers,
> 
> we have the following problem in some class 3 units: we have detected a
> white stuff inside the contacts of female connectors. It could be the main
> cause of the failures found during testing. Our thought is this substance
> could be due to a reaction between the rosin flux used when soldered the
> connectors whit the cleaning solvent (IPA)
> 
> Two questions:
> 
> 1) Could you recommend a solvent to remove these residues. We have tried
> with 141-b, but it could damage some parts of the unit.
> 
> 2) We would like to send a contaminated connector to a lab for analisys.
> Could you recommend a technique to determine these substances?
> 
> Thank you for your help.
> 
> Juan T. Marugán
> Indra Sistemas SA
> Espańa
> 
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