Simple method for removal of stubborn flux residue

[EE-digger]

While recently repairing some Keysight equipment built 5+ years ago with unleaded processes, I ran into residue, both clear and white crystalline in appearance, that will not respond to ANY solvent or flux remover.

In the past I've used MG Chemicals heavy duty flux remover and in some cases, one or more brake cleaners for spot issues.

While the clear flux residue from no-clean and unleaded will respond after keeping it damp and significant rubbing, I also ran into white residue, crystalline in appearance, which does not respond to rubbing at all.  It had responded to a wooden stick end plus q-tip with remover, but the latest didn't even budge.

Simple solution to all of this ... with a rosin flux pen, put a light coating of flux over the clear residue, white residue and this hard white residue.  With hot air tool set to 200C, played it back and forth over the new flux with some gentle rubbing from the wood stick end.  Not enough heat to melt anything, including leaded as well.  Only remained for maybe 20 sec. or so on a multi layer, heavily planed board.

A simple wipe up with clean remover and a q-tip took off everything !

Board is now pristine in appearance without any elbow grease.

[jpanhalt]

That approach sounds reasonable, but I have not tried it.

Have you seen Kester's study on white residues?  https://www.kester.com/Portals/0/Documents/FAQs/White_Residue.pdf

More recent reports from Kester strongly suggest addition of "saponifiers."  Its #5768 cleaner has a cellosolve (butoxyethoxyethanol) and ethanolamines (saponifiers).  I have been experimenting with ethyl cellosolve (ethoxyethanol), which is more volatile, and ethanolamine.  Both are readily available to consumers. The mixture is compatible with water, which helps remove salts.

Another consideration is N-methyl-2-pyrrolidone (NMP) (https://en.wikipedia.org/wiki/N-Methyl-2-pyrrolidone).  That was off the market for awhile, but appears to be available again.  It is now included in some liquid fluxes.   Maybe that's what is in your liquid flux?  It is a powerful solvent.  I would use it in a mixture, not neat.

[abdulbadii]

strong acid, or base maybe
(what thing that strong acid cannot clean out ?)

the point is not every place allowed to do buy-sell freely, considering its harmfulness

I'm lucky have been able to buy 53% HCl, alleviates all the cleaning works

[jpanhalt]

I'm lucky have been able to buy 53% HCl, alleviates all the cleaning works

Is that 53% vol/vol of concentrated HCl? Concentrated HCl is usually limited to about 35% to 38% (wt/wt) (about 12 molar).  I would not recommend using that on any electronic circuit for cleaning.

[EE-digger]

I'm lucky have been able to buy 53% HCl, alleviates all the cleaning works

Is that 53% vol/vol of concentrated HCl? Concentrated HCl is usually limited to about 35% to 38% (wt/wt) (about 12 molar).  I would not recommend using that on any electronic circuit for cleaning.

Agreed.  These are complete assemblies with very high dollar value and varied content.  Whatever you use needs to be fairly benign to all component packages as well as other hardware and connectors, some with precious plating but some without.

I read through a few of the white residue papers and the stuff I found is tough.  When it didn't even respond to the strongest flux remover with wooden stick abrasion, I knew it was something different.  Perhaps some very tough salt deposits.

I don't think lead free and no clean belongs in high end equipment, period.

[abdulbadii]

Well just representing the seller said
simply argue him not me

so after research, the actual at most 40-41% as confirmed on chemistry that's the one once opened the bitter vapor escalates forcefully right off.
surely we must use special goggle and glove to do the moving then we can easily set the concentration suitable to work by dissolve it in water

[jpanhalt]

@EE-digger
Can you tell us what flux pen/flux worked for you?  If you go into the SDS/MSDS for flux removers, you will find there is no magic involved.  THey are mostly the conventional solvents discussed with one exception: NMP  That is a relatively new and almost unique solvent IMHO.  That is, it is relatively non-caustic and will emulsify years of enamel/latex paint coatings.   Previously, one needed to use a poultice of lye to denature those paints. (I still prefer denaturing vs. emulsifying).  Its use in flux implies that it can dissolve some metal oxides.   

If you tell us the flux, I will do the leg/chemical work.

[EE-digger]

The flux pen I used for removal I think is an MG Chemical Rosin pen.

So this morning I received an order of Tech Spray G3 flux remover.  This evaporates quicker than any I've seen.

It removes the hard white residues which the (IMHO) agressive MG Heavy Duty flux remover wouldn't touch.  They don't simply wipe off but do come off with a cut short acid brush and a little scrubbing.  Looked good when done.

The difference must be the chlorinated solvent, transdichloroethylene.  This may be why my favorite brake cleaner does not work either, as it's non chlorinated.

For those who got better grades in chemistry, do any of these attack the FR4 or similar substrates over time?  The good news is that so far, none seem to harm the silk screen at all.

[jpanhalt]

Yes, I agree the greater activity was probably due to the dichloroethylene (DCE).
https://www.mgchemicals.com/downloads/msds/01%20English%20Can-USA%20SDS/sds-835-p.pdf
rosin
2-butanol (isobutyl alcohol, butan-2-ol))
ethanol

https://www.techspray.com/content/msds/1634-12S_United%20States%20(US)%20SDS%20HCS%202012_English%20(US).pdf
https://www.techspray.com/content/msds/1634-12S_Canada%20(CA)%20SDS%20HPR%202015_English%20(US).pdf
trans-dichloroethylene (50% to 70%) (https://en.wikipedia.org/wiki/1,2-Dichloroethene)
ethanol

DCE is not a saponifier as described in Kester literature.  That is why I add a little ethanolamine to my brew. 

As for safety on FR4, the DCE version does meet a Boeing specification.  Maybe that is good.  Ordinary fiberglass is affected by paint removers containing methylene chloride.  So called Aircraft Paint Remover contained methylene chloride and was carefully used in general aviation on fiberglass parts, such as wingtips.  After use, the surface was softened, but rehardened on through drying.  I have no information on long-term effects such as cracking.  I would expect DCE to be milder than methylene chloride and FR4 to be more resistant than ordinary epoxy or polyester fiberglass to chlorinated  hydrocarbons.  In sum, I don't see a problem using it as you did.  Soaking overnight might be a different story.

The brake cleaner I prefer by far contains tetrachloroethylene (aka PERC).  It's a little harder to find in the US and might be totally prohibited in other countries.  The non-chlorinated versions I seen are just acetone and xylene.  At best, they remove oils.

[whatboy]

That flux residue, is from the wave solder stencil at manufacturing process, and you will only see it on the thruhole parts, they do not remove it at factory because it only afects cosmetically, so why clean it???

[EE-digger]

Ah, that makes sense.  It is only around the through hole parts.  I'm assuming they are a secondary operation and that the white must be from sort of a "foam" or residue near the wave.  Is that true?

Depending on the part of the world and the environment the equipment works in, I worry about residues and the effect when mixed with humidity.

But I also hope that a quality, brand manufacturer like this would have done their homework.

[whatboy]

If your most powerfull solvent can't get rid of it, what makes you think water may affect it???

Just leave it like it is.

[whatboy]

Microsoft consoles and sony consoles have lots of that... i once cleaned one with a sharp knife tool, but it leaves another mark, like shadow kind of mark on where there is no mask only copper...

[jpanhalt]

If your most powerfull solvent can't get rid of it, what makes you think water may affect it???

Just leave it like it is.

Water is a very power solvent.  But like all super solvents, it has limitations.  Name a single organic solvent that dissolves NaCl.  Hot steam (>100°C) is an even better solvent.