Cleaning Flux From PCBs The Easy Way


While we’re all for building circuits on protoboard or constructing a deadbug circuit for a last minute project, it’s always nice to see a proper PCB now and again. We think that leftover flux can sometimes make even the nicest of circuit boards look a bit dingy, and Hackaday reader [RandomTask] wholeheartedly agrees. He wrote in to share a method he found online that he uses to get his PCBs squeaky clean after soldering.

The secret to his clean PCBs is a product called Poly Clens. It’s essentially a paint brush cleaner that does a great job at removing flux without having to resort to using a brush to scrub it off the board. [RandomTask] simply submerges his newly assembled board in a small container filled with Poly Clens, agitating it for about half a minute or so. After the flux has been removed he rinses it with water, pats it dry, then ensures the board is moisture-free with a few passes of his heat gun.

He says that the entire process takes him less than 5 minutes per board, which is far better than the old alcohol and stiff brush method he used in the past.

What tips or tricks do you have for getting your new projects cleaned up? Be sure to share them with us in the comments.

77 thoughts on “Cleaning Flux From PCBs The Easy Way

  1. I dip my desoldering brade in flux, then remove any additional solder.

    I dremmel down the contacts so they’re rounded and no longer pointy.

    If the board is to sit outside a project box, ill cut a piece of plexiglass to size and use some Walmart ultra clear liquid cement to fasten it. Its non conductive and allows you to see without risk of shorting.

  2. Another cheap and effective method is simply isopropyl alcohol and an old toothbrush. The excess alcohol can be blotted off with a paper towel, or for that extra shiny finish blown off with canned or compressed air.

  3. I worked in the electronics industry for a while, and you’re all over thinking it. We used to throw assembled boards into one of several off the shelf dishwashers and ran them through a regular wash cycle. None of the boards cleaned this way had anything that breaks down in water such as cardboard sleeves in the transformers, nor any component that might accumulate excess water in deep recesses. I believe we even used regular liquid dishwasher soap, just a very small amount of it. We let the boards sit for a couple days air drying before sending them on for burn in testing.

  4. i’ve been using a “water soluble” flux pen from kester (#2331-zx) in combo with some rat shack rosin core silver solder and all I can say is it is no fun.

    as hard as ive tried (hot water and toothbrush) the flux does not seem to be water soluble at all. my proto-boards are dirty and its makin’ me crazy… I have lots of trim-pots that I dont want to get wet either… I looked on sparkfun (where I got the flux) and it says clean with alcohol, so I’ll try that i guess…

    but beyond that, has anyone had any luck with “no clean” flux/solder. kester seems to make a few of these products and I am thinking of switching to kester 44 solder [63/37 .031″] which is not listed as no-clean but cleaning is not required… and a no clean flux pen. anyone use this stuff? whats your opinion?

    HAD should do a post about solder/flux in general…

  5. @adam outler: I wouldn’t Dremel the contacts. The mechancial vibration of grinding them down will dramatically reduce the quality of the joint. If there’s a specific and dramatic need to not have pointy bits, some really good clippers and a coating of rubberized weatherproofing material will do it.

    I’m surprised the editors here mention the only reason to remove flux is to make a PCB prettier. The main reason to do it is because flux is pretty corrosive and will, over time, damage the board.

    I’m a big fan of alcohol and a stiff brush, then a flush with alcohol and air dry. Don’t need to rinse with water since even purified water will pick up mineral deposits from the board and leave them where they can cause the most damage. Going back over the board with a heat gun again seems like stressing the board and asking for trouble.

  6. @Brett: That’s nice if you happen to have a dishwasher sitting around just to wash PCBs in. I doubt that’s the case for most of us though :)

    Personally I just use some alcohol and a toothbrush, works fine for me.

  7. @Brett – Imagine my surprise being shown the high-tech board cleaning system when touring an assembly house I hired. Kenmore.

    I shouldn’t have been surprised because when I was just a young engineer and had a board come back from the prototype house non-functional, an older engineer took all my boards into the company lunchroom and ran them through the dishwasher. They worked perfectly after that.

  8. @Juan Cubillo

    I use rubbing alcohol and an old toothbrush. I’ve tried acetone before, and it worked pretty well, but it seems pretty nasty (managed to get a drip on my sink and dissolve a nice hole from the outer coating!) Alcohol is not nearly as corrosive.

  9. I’m using an ultrasonic cleaner with heated up isopropyl alcohol. no work besides putting them in and out(forget the cheapo 30 USD ones off ebay, the cheapest chinese enay cleaners that work cost 120USD and up, best of course is medical cleaners for 600USD used)

    i love the dishwasher idea though, gonna test that:-)

  10. @ibedazzled

    I prefer the “lead free” flux pen from Circuitworks:

    They’re about $9 but last a while. It’s a no-clean flux that actually works really well, much better than the no-clean Kester pens I’ve used. They work great with regular tin/lead solder, despite the name. I still clean it off of my boards if I’m using it to assemble the whole board, but it’s not necessary (and I usually won’t bother for spot repairs.) Rinsing in isopropanol takes it off pretty well.

    The water-soluble Kester stuff that I’ve used works okay, but it’s nasty stuff if you leave it on. I’ve seen it turn copper traces into a green, corroded mess in just days of being left on a board. So it’s easier to clean off, but it’s important that it be cleaned off quickly.

    1. “The water-soluble Kester stuff that I’ve used works okay, but it’s nasty stuff if you leave it on. I’ve seen it turn copper traces into a green, corroded mess in just days of being left on a board. So it’s easier to clean off, but it’s important that it be cleaned off quickly.”

      That is a property of water-soluble fluxes in general.

  11. @ibedazzled
    It is most likely not the water soluble flux that is giving you headaches but the rosin core solder. I use the same water soluble flux as you, with flux less solder or sometimes a lead-free solder with flux. If I use the lead free stuff it can be a bit hard to get the flux off, but it comes off easy enough with warm water.

  12. flux (rosin) removal isn’t just for looks. Flux can corrode metals (RMA and RA type flux have an activating agent, typically an acid). Cleaning flux from a solder joint also allows for proper inspection (no copper exposed). Using a cotton swab or acid brush (cut, as Pete stated) soaked in alcohol helps to break down and remove the rosin flux. If you want your projects to last longer, you really should know what kind of rosin core your solder contains. has an informal section if anyone wants to read it. I have found the solder at Radio Shack to be sub-par, order a spool (kester, Chemtronics, etc.) from a reputable source like mouser or digi-key, etc.

  13. I’ve never bothered cleaning off any of the boards I’ve soldered except for one. I had a prototype board I had soldered a circuit to. The circuit was behaving oddly so I ran a blade in between the solder joints and cleaned everything with some isopropyl alcohol, a Q-tip, and some paper towels.

  14. Polyclense? sounds like another word for acitone. i’m not sure weather acitone would be the best thing to clean a pcb with but i also can’t think of why it would hurt.

  15. Never use water soluble unless you are using a dishwasher to clean the boards. Found that out the hard way at a job years ago. Hot water is the only thing that will clean the flux off, not even alcohol. If you don’t clean it off the flux will eat at the solder connections turning them black and even chew through small leads. No clean is the way to go with a simple flux remover.

  16. I discovered lighter fluid can clean flux residue off a circuitboard, it’s also great at getting glue residue from stickytape off metal/acrylic etc.

    I only really use the lighter fluid trick occasionally for removing glue residue, however it’s useful when you had to desolder something by doing the add-more-solder technique and there’s an excess of flux left on the board. But mostly I don’t care about flux on circuitboards.

  17. Here at Sparkfun all of the flux we use here at is water soluble, to clean we use crock-pots filled with de-ionized water turned on high. We let the boards soak for a minute then blow them off with an air compressor. We recently got a expensive washing machine that uses de-ionized water and pressure jets to clean the boards really well. On water sensitive parts we use alcohol and brushes to clean off any excess flux. If the flux dries on the board it can be a pain to get off with water or alcohol so we just use the flux to help loosen up the dried flux making it easy to clean up.

  18. Yup, acetone also works.

    I found that the best way to get it without queries is to go to a boat supplies place and explain that you are using it to clean circuit boards.

    They usually don’t mind, after all they sell it by the gallon for cleaning fibreglass hulls prior to repair among other things.

    BTW nail polish remover made now tends to be “acetone free” thanks to irresponsible teenagers sniffing it to get high.
    This does work somewhat but the results can be unpredictable with it even damaging some component markings as I discovered.

  19. I’ve been using Kester 63/37 organic core for a while, ever since being introduced to it at work. It uses an organic flux that is /highly/ water-soluble…just “dabbing” the bottom of the board in a shallow dish of water is sufficient. We used it at work because our boards contained a lot of trimpots and unsealed relays, but had to be cleaned due to sensitivity to stray capacitance. Clean the back of the board by dabbing, then blast with the heat gun on low heat.

    A good liquid water-soluble flux is Alpha 850. It’s very aggressive and makes soldering even really dirty component leads (like 50-year-old carbon comp resistors) really easy. Also washes off with dabbing in water.

    Both of these fluxes are pretty corrosive, though not track-dissolving corrosive. They’ll create stray resistance and capacitance problems in analog circuits if not cleaned, but leaving the board uncleaned with digital electronics won’t have an immediate effect. A quick dip in the ultrasonic bath or scrub under cold running water with a toothbrush will take either flux right off.

  20. “a method he found online” link ( claims “Pure isopropyl alcohol (99%) is only available as a lab or industrial solvent, so most amateurs would not be able to acquire it.” But I got 99% iso at the local drugstore, and it’s available on Amazon for $2.67 a pint (Cumberland Swan brand).

  21. in my case i get 99% iso from corner drugstore (Jean-Coutu Pharmacy) and a toothbrush.

    80%, 70% etc will not work.

    if you bake the board slightly, the flux will come off easier.

    Finish with ammonia cleaner and rinse.

  22. Isopropanol and a brush is good for small projects, but the ultrasonic bath is excellent for larger qualtities.
    Compressed air is usually fine for drying after a water based clean, but if it’s a board with small stuff like ball grid arrays (BGA) where water can become trapped and corrode over time, it best to give it a quick bath in alcohol to make sure there’s no water left.

    I noticed in Mikes pictures that he had not soldered the ends of the component legs. I’ve seen older equipment where the component legs have corroded right through the board because of this – so freaking solder the ends guys!

  23. No clean flux will work just fine and leave no visible residue as long as you don’t overheat the joint.
    Try Kester 951 (leaded) or 959 (LF) flux pens. Leaves NO residue.

  24. My first job at an electronics company also used a dishwasher to clean the boards. Then they ran them through a conveyor dryer system. I always thought the dishwasher was kind of strange. But it gets the job done.

    It was like, “this is where we wash the boards…” and im like, oookay?

  25. by the way, if you want to check how clean your board really is, use a UV light in a dark room. Flux (at least my standard rosin-based type) will fluoresce green under UV.

  26. Flux goes liquid if you heat it up a little. I just use a hot air station on the lowest heat setting and just wipe off the flux with tissue. Better doing that first so you dont have use so much alchohol to clean it up

  27. Chalk up another Alcohol/Windex guy right here. I also keep a can of contact cleaner close by for the hardcore “need current here now” moments. I can’t tell you how many ‘dead’ synths I have fixed with these tools. Also keep a lil spray bottle of distilled water to get extra stuff off the boards in extreme cases. Otherwise I try to give love taps of compressed air and a 6 year old toothbrush. Also keep a kabuki style brush for keyboard keys themselves. The real champ is the eyebrow brush. Ya can pick these up at the dollar tree and they are great for getting in extra tight. Glad to see I’m not such an outcast using alcohol and windex.

  28. @Simon
    The safety datasheet says this cleaner contains Xylene, Petroleum etc. so it is really effective stuff. And toxic too. Maybe a bit overkill for cleaning a flux residue.

  29. I use no-clean solder all the time, smells kinda fruity (might just be my odd sense of smell). You still need to clean them if you are going to coat the board with conformal coating though. I use a hot distilled water soak. I would use DI water if I were doing it in production though.

  30. When I worked manufacturing RF tubes for MRI machines, we used lead solder (leadless melts at a lower temp, isnt resistant structurally to stress or vibration, the list goes on…) and waited for it to dry, then scratched off with a spudger (compressed fiber stick, better than plastic), and finally doused with alcohol and rags. There werent any high points though and the caps were sunk-mount, flush with the tube. It was a total pain, so I’d learned to use as little flux as possible and still get good joints. They later used a clear type liquid flux, just dissolved without scrubbing if there was any remaining.

    I’d still like to get this, I’ll test certain thinners available here & see if they are safe on circ boards.

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