Whip Up Some Homemade Artisanal Flux

You don’t think much about the power company until you flip the lights on and they don’t come on. The same can be said of soldering flux. You don’t think much about it, usually, until you try soldering without it. Flux has a cleaning action on metal surfaces that allows for a proper solder joint. The problem is, do you have any idea what’s in the flux you are using? We don’t either. [Catsndogs] has a recipe to make your own flux and then you’ll know.

At the heart of rosin flux is basically tree sap. If you live near pine trees, you can source it naturally. If not, you can find it at music instrument stores. Stringed instruments use rosin, so it is readily available. If you do source it yourself, [Catsndogs] reports that it doesn’t matter if it is old or clean.  You do want to pick out as much tree bark and dead ants as you can, though. You essentially dissolve it in alcohol (at least 80% isopropyl or ethanol). Then filter it through filter paper or a coffee filter.

You can adjust the viscosity by allowing the alcohol to evaporate to make the mixture thicker or by adding more alcohol to make it thinner. Thicker flux is good for tacking down SMD parts. As you might expect, this isn’t “no clean” flux. Also, the flux is very flammable, so be careful.

This isn’t the first time we’ve heard of this recipe. Or even the second time. But it is a good reminder that you can make your own free of whatever wacky chemicals are in the commercial preparations.

32 thoughts on “Whip Up Some Homemade Artisanal Flux

  1. This is cool from a do it from scratch DIY perspective. My experience doing something similar, for restoring 1890’s technology that used lead solder to connect steel components, is that the resulting rosin flux in usage is kinda smoky: you get condensed rosin on nearby objects, like your microscope lens, and you end up inhaling the stuff and coughing a lot if you don’t have a fume extractor of some sort.

  2. Didn’t your parents teach you this? My father and uncles grew up behind the iron curtain in the 60s and 70s and when soldering they often dipped the iron in a little can of rosin.

  3. Living in a place with plenty of pin trees I have long wondered if I should just go and fetch my free soldering flux from the forest. Guess I better start before winter arrives….

    Though, have also contemplated. If rosin that is effectively pine syrup is good as a soldering flux. What about other tree sourced syrups? Perhaps that dreaded maple stuff finally has an actual valuable use.

    Then one can also ask, what about just syrup? As in sugar in a bit of water? Or just sugar?

    1. At soldering temperatures, rosin has a lot of chemicals that behave as acids and etch the surface. Sugar acts more acid at elevated temperatures, but it also caramelizes, turning into something tar-like, and burns/chars. Tree sap has a bit of sugar, but all the other stuff does a lot of the work that’s valuable for a flux.

      1. Partly joking, but it is interesting to know what regular stuff would be a half decent solder flux in a pinch.

        Having something that acts as a reducing agent is one worthwhile feature.
        A bit of acidity is indeed useful for removing some surface oxides.
        Not to mention being a liquid at appropriate temperatures.

        At least one has the big advantage of having a hot reaction mix to work with.

  4. >”alcohol (at least ethanol) does not make any toxic gases.”

    Acetaldehyde. “produced by the partial oxidation of ethanol”, causes skin, eye, throat and lung irritation from between 0.07 and 0.25 ppm and upwards. Is carcinogenic.

    1. Acetaldehyde is what alcohol turns into in your liver. That smell on someone’s breath who’s been drinking? I’m pretty sure it’s this.

      Coffee has a decent amount of the stuff in it too.

  5. I once read about a guy making flux from citric acid, water, and some substance to control the viscosity such as honey.

    I think you have to clean it after soldering as you probably don’t want to leave citric acid on your PCB, but it dissolves easily in water. I’m not sure though what happens with the sugar in the honey at soldering temperatures.

    I never tried it myself, but it seems plausible and it’s easy enough to try at home if you’re interested.

    I do wash my PCB’s after soldering with just water and soap. Rindins it with isopropanol gets rid of most of the water (and it evaporates pretty quick) I also have success with tying the PCB to a shoe lace or other string and then whirl it around. the combination of both the centrifugal force and the high wind speed dry the PCB pretty quickly.

    1. Compressed air is also pretty decent at drying off boards. But rapid moving dry air is a bit of an electrostatic hazard on insulated parts, but an assembled board shouldn’t accumulate charge, since all the resistors and ESD protection diodes will keep any built up charge at acceptable levels.

      I personally wouldn’t whirl around a board, too high risk accidentally whacking it into something or sending it flying. Two things that can destroy a lot of prior efforts spent on the board thus far.

      But even leaving a board to air dry is oftentimes good enough as is.
      Rust takes time to form, and water alone doesn’t do much in a couple of hours.
      Spraying the board in a coating of IPA can however aid with evaporation and speed up air drying a bit.

    2. After a bit of reading…
      I was probably wrong about the honey, but using citric acid (even lemon juice) is apparently quite effective. Viscosity can be controlled with glycerin or petroleum jelly, although the jelly does not dissolve in water. But there is plenty more to read and experiment if you’re interested in this.

  6. Skimming through relevant patents shows that itaconic acid is a good additive to make ‘mildly activated’ flux. It’s moderately soluble in alcohol, and decomposes into citric acid at soldering temperatures.

    An interesting alternative based on completely different chemistry is glyciline: a deep eutectic solvent made from 1mol of choline chloride (140g) and 2mol of glycerin (180g). It’s an organic salt that’s liquid at room temperature, and remains stable through soldering temperatures. Polyethylene glycol at molecular weight 400 (PEG400) works as a thickener.

  7. I knew flux is typically made from rosin. I had never considered making it myself, but I would almost certainly have considered if it I ever got in a pinch where it was the only option. And it probably wouldn’t have worked very well. Knowing that it is alcohol soluble bridges the final gap! Now if I get in a pinch where I need it, I know how to do it successfully!

    On a side note, did you know that the chemical that makes turmeric yellow is alcohol soluble? I once made ink from it. This chemical is also base sensitive, turning red in an alkaline environment, so I used the ink to make base indicator paper. I still have some of the paper somewhere. (I also put some of the ink in an old Canon printer cartridge and printed some base indicator paper, but the ink was so thin that it slowly leaked out of the cartridge even when not printing. I wonder if I could have fixed that by evaporating out some of the alcohol… I might have to revisit this…)

    1. This is the second time in my life I have ever heard this and it’s on the same day, from two very different sources. How bizarre is that?? I was actually pretty skeptical until reading it here.

  8. Actually got a baggie of pine resin sitting around that I collected some months ago and haven’t got round to “processing” in some form. I had the idea that I had to heat it for a while to drive out water and let the inclusions scum up and be skimmed.

    1. I think it’s the same reason they add glycerin: it keeps the end product from becoming too sticky. I’ll be experimenting with both, and perhaps even some waxes, at some point this winter.

  9. I have found that filtering RA flux can be quite a difficult task. The last time I made RA flux it was from cheap rosin I bought at a music store and it took several filterings under pressure using a syringe and a cotton plug to get it reasonably clear.

  10. Please don’t use ethanol. The water in it either causes the rosin to precipitate when not in use or leaves wet rosin on your PCBs which may become slightly conductive (bad for RF, high-speed designs, large-impedance designs etc.). Use isopropanol or other solvent not containing water. Also, denatured ethanol is one of the worst smells ever…

  11. Battery acid from my car works on those tough solder joints in pipe fitting copper. If you just can’t get it to suck solder a good source of sulfuric acid is a battery lol. Also the old lead solder runs better so keep some around.

  12. Someone already mentioned that colophony is the very rosin they warn us against in the solder wire rolls… not a big deal if there is ventilation. And is there anything nowadays they do NOT warn against?

    One point to remember, though, is that the lack of ‘wacky chemicals’ does not make something safe. The most poisonous (one of the botulins) and the most carcinogenic (one of the aflatoxins) substances known are both natural products.

  13. Some tips on cleaning raw pine resin: you can melt it in boiling water, esin isn’t water soluble, it will float to the top, dirt and junk sinks. you can then pick out any big chunks, let small stuff sink, then skim off and filter the top layer of water+resin through a few layers of cheesecloth to remove any small particles. Let it cool to solidify, and pull the resin puck off of the water. Then you want to heat it up above 100C for a bit to drive out any retained water – be careful, use a thermometer, don’t over heat or use a flame heat source as it is flammable. You should probably do this outside on an electric hotplate, away from any flammable structures. This will give you pure, clean, dry resin. You can break the puck up pretty easily if you stick it in the freezer, it will become quite brittle and shatter.

  14. I’ve been using homemade flux for a while now. I used a brick of cheap rosin, crumbled into a tiny bottle and filled it with isopropanol.
    I have a thick solution of it in a syringe. If for some reason you’re taking too long with soldering (perhaps you’re managing multiple different wires and although you’re using a 3rd hand, things just keep shifting out of position because of the heat from the soldering) and the joint you’re making doesn’t flow anymore, touch it with the droplet of flux hanging off the syringe and everything will flow into place.

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