More DIY solder flux

[GuShH] wrote a guide for making your own rosin-based solder flux. According to [Stephen] — who sent in the tip and tried this method himself — is works well, it’s cheap, but you will need to clean up a bit after using it on a PCB.

Only two ingredients are necessary to make your own liquid or paste flux: rosin and a solvent. The rosin being weighed in the image above, can be found from several sources. We looked in on the same method quite recently where flux was sourced from a music store. But [GuShH] suggests that if you can find some from a hardware store it is better because the music store variety tends to be ‘molten’ and doesn’t work quite as well.

Proportions are listed on his guide for light, medium, and heavy concoctions. He recommends isopropyl alcohol as the solvent, and has stored the flux in a clear dropper bottle. We’re fans of needle bottles and asked about sourcing them in a previous post (linked in the paragraph above) so check that comments section if you don’t know where to get one.

15 thoughts on “More DIY solder flux

  1. I don’t see why people are obsessed with cleaning rosin flux. It doesn’t damage the board, in fact with home etched bare copper boards it actually protects them from oxidation. Unless you do really sensitive analog circuitry i just don’t see the point apart from aesthetic reasons.

    I usually coat newly etched boards with a thin layer of pure rosin flux (not RMA flux) and this makes them easier to solder and protects them from oxidizing.

    The recipe i use is something like 10g of rosin to 100-200ml of ethanol. I use standard mildly denaturated red ethanol with no ill effects. This flux beats most professional flux i have tried since they are usually meant for soldering newly produced clean components while i often solder really old components that have a layer of oxide on the pins.

    1. I’ve seen many cases where the flux left behind caused corrosion. In high voltage circuits, the flux left behind can also pull moisture out of the air which causes arcing not always visible until the lights are out.

  2. Remember kids, rosin based flux contains colophony, which is a pulmonary sensitiser and can lead to hypersensitivity and asthma in predisposed individuals – “colophony lung”.

    Rosin can also cause colophony induced skin sensitisation and contact dermatitis.

    If you’re asthmatic, or just careful, you’ll try to keep your nose out the fumes while soldering +/- use some local air extraction.

    Same goes for burning insulation off wires – it can produce isocyanates which can annoy asthmatics.

    Far better to get a darwin award for something spactacular, like a home made x-ray machine, than boring old asthma!!

      1. there have been isolated case reports of airway symptoms from colophony free solders, and lead free soldering at higher temperatures also has the potential to produce noxious thermal degradation products from fluxes.

        Nothing like a bit of danger to inspire interest in the younger generation!!

  3. If you want to clean your board from rosin based flux just soak it in ammonia. Rosin reacts with it creating compound similiar to soap. After that all that is left to do is a pass with toothbrush under running water.

  4. This trick is very old, in fact I use it to coat it boards before soldering. Keeps them fresh forever and soldering is a treat. Nonetheless, for apply-and-solder jobs it’s suboptimal. The Propanol doesn’t evaporate quick enough and residues of it in the colophony cause it to sort of mini-explode. Does anybody have a better solvent suggestion? Something you tried?

    1. Acetone seems to be better solvent for the rosin. It evaporates faster and penetrates better than ethanol. Note: Acetone will damage many types of clear plastics. It is better to avoid using acetone near anything other than PCB and components soldered on to the PCB.

  5. I keep looking at the rosin chunks and my brain says “Crystalized Pineapple candy!” and then I want to eat them.

    Hopefully the smell would cancel this thought out, because if I can ever get enough free time to go back to pretending I’m a mad scientist, I’d like to try this.

  6. There’s a lot of Loblolly pines around my area. I may have to experiment with hard rosin I get from trees myself and see if it stacks up. You know, for next time I get stranded in the woods and need to prepare flux…

    1. I pried some chunks of dried resin off some of the pine trees around here with an old flathead screwdriver, being careful not to damage the trees. I put these chunks in a glass mushroom bottle and covered them with 91% isopropyl alcohol from my medicine cabinet. Then I shook the bottle and swished it around until the chunks were dissolved (several hours). There were lots of pieces of leftover trash in the solution, so I made a funnel out of a coffee filter to strain it into an old pill bottle. It was still too thin for my taste, so I let it sit uncovered for several days to let some of the alcohol evaporate off. Now I keep it tightly capped, and it is a beautiful, clear, honey colored rosin of just my preferred consistency. I love it! Also, I clean rosin residue off PCBs with isopropyl alcohol, a closely-trimmed horsehair acid brush, and a little elbow grease. Maybe this is not necessary, but I am a neat freak, and I love to see my shiny, well-executed solder joints!

  7. Has anyone herd of motor oil or a motor oil like substances being added to flux? I found this really old round tin of flux inside an old Weller Solder Gun box. I had avoided using it for as long as I remember until one day when i got desperate. Turns out that flux was and still is THE BEST stuff ever!!! It has the standard consistency found in those puck shaped flux paste containers found online but isnt golden in color. The color looks just like used motor oil black and smells right out of an old Ford work truck. After using it for a while now I am positive this isnt just some fluke contamination with motor oil but an intentional “mix”… probably from some old timey grease monkey from the 1950’s. who knows!.. but just a dab makes standard solder flow so GOOD; wicks right up with de-solder brade; and even makes crapy ebay solder actually workable (alloy conductivity aside.. that stuff is not purely 60/40 yuck). Long story short its the secret weapon in my soldering kit and I have never seen anything commercially that looks like or performs like it before. (and nope.. mixing old motor oil into standard flux does not yield the same consistency nor results.. =/) I would love to talk shop with the old guy who made this misery flux. haha

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