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cleaning flux on through hole boards

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Dan Moos:
How do you clean flux off of the bottom of through hole boards? I can dissolve it with ipa, but can't find a way to actually wipe of the dissolved residue. The pointy pins of the components makes it practically impossible top wipe off. Is there an easy method besides meticulously doing it slowly? My current board is pretty big, so some sort of pro tip would be nice!

You need the IPA in a spray can or spray bottle, + a shallow tray larger than the board (or at least longer than an edge) + a pad of several sheets of kitchen paper + a brush to scrub the board with.  Use a natural bristle chip brush or acid/flux brush, with its bristles cut down to half their length or less for stiffness, and spray the area to be cleaned with IPA and scrub till all the flux is dissolved. Now tilt the board over the tray and wash off the contaminated IPA with more IPA.  Another brush with full length bristles may help encouraging the pool towards the edge. Touch the board edge to the pad of kitchen paper to adsorb any contaminated IPA that hasn't dripped off.  Wipe the brushes on the kitchen paper and re-wet them with clean IPA till they are free from flux, then if the drying board feels at all tacky to touch, repeat the scrubbing and rinsing process.   

Note that IPA alone may leave the dreaded white streaks, and you may need to use another flux remover or even very hot water with a trace of detergent to remove them.  If you use water, after getting as much of the rinse water off as possible, rinse with IPA to encourage rapid drying.

You can also wipe off the brush, and stipple a sheet of kitchen paper into the contaminated IPA to adsorb most of it and lift it off, but if you don't have enough IPA on the board, that can backfire and leave you with shreds of kitchen paper glued to the board by flux!

I use compressed air from an oil less compressor.  If you blow at an acute angle. it will blow most of the solvent off, rather than simply evaporate it in place.  If your PCB has solder resist and you use a water-based flux/water rinse remover, it comes out virtually dry because the resist is so hydrophobic.

I have no problems with a toothbrush (or any other kind of brush), paper towel, and a solvent dispenser:

- The dispenser has a pump into a dish on top, which the brush wicks up, and, I expect (hope?), with minimal backwash.
- The brush is then scrubbed over the area of interest, loosening crusty deposits and making a sticky soup.
- The brush is now soaked with concentrated flux.  Pinch and roll in dry paper towel to suck out most of it.
- Go back to start until everything of interest has been transported to the paper towels. :)

It's a dilution process, like anything else; you get the same thing with the aforementioned, where a spray bottle (or, see also a chemist's solvent / wash bottle: tight jet rather than spray) applies clean solvent, dissolving more while also washing away higher-concentration solution that was already on the board.  Compressed air, or capillary action (into a dry brush or paper towels), removes the excess, again reducing the concentration stuck to the board.  (Rubbing with paper against THT pins is ineffective as you've discovered, but you may still find value in patting dry?)

Note that it does nothing, to soak in a basin and keep on wiping; the concentration never falls, only rises, so it always leaves residue.  This is still effective for treating very thick or trapped deposits (particularly with an ultrasonic cleaner?); or, if the increase in concentration is small enough (i.e. the bath is large enough, and the added crud small enough) not to mind, it might be fine as a single step.

(Also, obviously soaking in a basin might not be applicable, especially? for a THT board, like with electrolytics where the rubber seal or labeling is not solvent resistant; or unsealed relays, etc.)

Note also that, while IPA is adequate for most (rosin based?) fluxes, it's not great with stubborn, or especially ionic deposits.  A blend of solvents, maybe surfactants, maybe base too, works best.  So, don't be afraid to order "flux cleaner" proper; it's good stuff!


m k:
One board?

Back in the day multinational pro tip.

Make tight brush type roll of blue garage paper.
Soak it and brush over the joint, if paper is not shredding you fail.
Clean leftovers.


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