cleaning flux on through hole boards - Page 1
Forum section: electronics / projects

author: Dan Moos
author_rank: Frequent Contributor
timestamp: March 07, 2022, 02:42:55 am
content: 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!

author: Ian.M
author_rank: Super Contributor
timestamp: March 07, 2022, 04:13:13 am
content: 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!

author: jpanhalt
author_rank: Super Contributor
timestamp: March 07, 2022, 11:18:04 am
content: 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 07, 2022, 12:08:55 pm
content: 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!Tim

author: m k
author_rank: Super Contributor
timestamp: March 07, 2022, 01:27:38 pm
content: 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.

author: jpanhalt
author_rank: Super Contributor
timestamp: March 07, 2022, 01:38:22 pm
content: "blue garage paper"Is it safe to assume that is like Kimberly-Clark "blue shop towels?" (attached)Many versions both blue and white available in the US.John

author: MisterHeadache
author_rank: Regular Contributor
timestamp: March 07, 2022, 02:35:31 pm
content: My experience is limited to using resin activated (RA) flux in an applicator pen for both through hole and hand-soldered SMT projects. That cleans up nicely using lacquer thinner, which is a mixture of alcohols, toluene, acetone, and other niceties. So I use it in a ventilated space (typically the garage), pour a couple tablespoons at most into an old ceramic coffee mug that I use for this purpose, hold the board in the cup with one gloved hand (my boards are small enough to fit) and scrub with a flux brush (the ones in the plumbing section of the hardware store) to remove the flux.Then because I'm environmentally conscious, I dump the mug contents on the pavement to evaporate and let the board air dry before bringing it inside.Lacquer thinner does not remove the silk screen nor the solder mask on boards that I've used it on.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 07, 2022, 02:55:20 pm
content: Be careful using aromatic and other more aggressive solvents -- the board itself is probably fine (aside from extremely volatile and aggressive CHCs like methylene chloride -- paint stripper), but components and markings can swell, dissolve, smear, etc.As they say -- do a spot check, never hurts to test first.There are broadly two types of soldermask in common use, though I don't know who actually makes them or what they're based on. The usual glossy stuff you get with cheap, rapid protos, is not too strong, and somewhat flakes off when scraped. Which honestly isn't so bad when you're working on a proto and may need to expose traces. But do beware of that when doing fine pitch SMTs, the thin web between pads can flake off and block the pad or lift the component, reducing assembly yield. The other kind is usually, seemingly thicker anyway, haven't measured it to be sure; often matte to semigloss, and MUCH stronger, hard to scrape off -- and more abrasive at that (maybe it's loaded with silica fume or something?). Usually you get this on custom builds.Anyway yeah, I've never seen either kind be sensitive to solvents. If they're both UV cure epoxy based something or other, the chemical resistance should be similar.Tim

author: TimNJ
author_rank: Super Contributor
timestamp: March 07, 2022, 09:13:46 pm
content: IPA 93 - 99%Stiff horsehair (or similar) brush - Try MG Chemicals 856 (or similar)KimwipesLoosen flux residue with IPA + brush. Then lay a Kimwipe on top of the board and then using a blotting motion, with the brush, "through the paper wipe". The residue is wicked into the paper.Good success with me. Don't need to use Kimwipes, but I like that they don't leave fibers on the board.

author: mapleLC
author_rank: Frequent Contributor
timestamp: May 23, 2022, 08:55:14 am
content: Nobody has mentioned it, so I will. For a final wipe on something that has to be really clean/dust free, I use methanol.

author: Ian.M
author_rank: Super Contributor
timestamp: May 23, 2022, 09:38:07 am
content: ... hopefully in a fume cupboard or outdoors, and with appropriate PPE to avoid skin contact, as methanol is far more toxic than IPA (isopropanol). Also you may run into compatibility issues with some plastic encapsulated parts (more so than with IPA).

author: mapleLC
author_rank: Frequent Contributor
timestamp: May 23, 2022, 12:01:34 pm
content: Quote from: Ian.M on May 23, 2022, 09:38:07 amLet me qualify that, I don't generally use it on the board, more for something glass or metallic, etc that has a special reason to do so like a lens or metal surface I am bonding (even then maybe acetone). I use it rarely and in isolation, just so I am clear.