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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