What The Flux: Buy It Or Brew It Yourself

Flux generally makes our lives easier. It’s the best bet when trying to prevent solder bridges with fine-pitch components like you see here. But it is also indispensable when it comes to desoldering components from a board (we’re talking just one component without disturbing all of the others). But have you ever looked at what it costs to pick up a syringe of liquid flux from an online retailer? In addition to the cost of the product itself there’s usually a hazardous material handling fee that is rolled into the shipping cost. So we were happy that [Christopher] sent in a link to the DIY flux page over at Dangerous Prototypes.

The concept is simple enough. Mix some rosin with some solvent. Turns out these items are really easy to source. The solvent can be acetone (which you may have on hand for removing toner transfer from freshly etched PCBs) or plain old rubbing alcohol. And an easy source for rosin is your local music store. They sell it to use on bow hair for String players. Grind it up, throw it in a bottle and you’re good to go. Now does anyone know where we can source needle-tipped bottles locally?

For those that still just want to buy flux we highly recommend watching part one and part two of [Ian’s] flux review series.

38 thoughts on “What The Flux: Buy It Or Brew It Yourself

  1. You can buy syringes at Home of Economy or Tractor supply or any farm supply shop. they usually are good and sturdy and have alot of capacity. you can just use the syringe or take off the needle and glue it in a bottle.

  2. awesome idea!

    i’ve gotten my bottles off eBay for ~$3 shipped, never had a problem. I’d say they are 90% as good as the ones we have at the soldering stations at work, and probably 20% of the price

  3. I use a normal fine-tipped painting brush to apply. (art store?) If you rinse it with solvent carefully after each usage, the hairs won’t break.

    You can even use pigment colors to colorize the resin. That gives you a nice “poor-mans-PCB-coating” to protect against corrosion. Drying can be accelerated with a hot air blower.

    1. flux brushes are still available at plumbing or welding supply store, cheap enough to be considered disposable. Not exactly fine tipped, but not that much wider than the brush used in this video, although the bristles might be stiffer. I bring it up because many rural areas that may not have art supplies, probably have a place that stocks the flux brushes.

  4. I am using ethanol (97%) and it’s been great so far, sometimes leaves white stains, but otherwise very effective. Isopropanol is best though, if you wanna waste money on it.

  5. I use a cotton swab stick that I keep in one of those funny tiny liquor bottles, it’s always submerged but even if I leave it to dry, it gets soft again very quickly once in alcohol. The flux is rosin dissolved in ethanol, basically the same colour as the original content of the bottle. I tried isopropanol but I don’t understand why people praise it, it doesn’t dissolve the rosin all that well and it smells ugly. Acetone is just too volatile and makes you hear funny things if you breathe it too much.

    I guess nail polish flasks could be useful too.

  6. You can buy flux for a couple of quid from almost any plumbing / DIY store. It’s a paste rather than a fluid. But when heated to solder temperature it melts and flows.

    1. Plumbing solder is type RA and it’s too corrosive for electrical soldering. What’s needed is RMA type – mildly active rosin flux.

      I don’t know if they still carry it but I used to buy liquid RMA flux from Sears in the tool section.

  7. A note from a greybeard. There is a fungus that eats the rosin, and when the rosin is all gone, the fungus residue is conductive. A nuisance with old 5volt logic, a genuine problem with analog circuits. I don’t know how it would affect today’s low voltage, high speed stuff.

  8. My favorite part of the flux bottle is the flux goo that collects at the base of the needle. I pick a little bit and smear a tiny amount right on the smt solder joint. Easy to clean when it doesn’t flow around the board. The broken midpoint of a toothpick, soaked in alcohol, is my tool of choice.

  9. I’ve made a small batch of homemade flux of rosin that I collected from trees.
    I was looking for good quality rosin flux online, found it, noticed that the shipping overseas is over $90 (they can’t put it in planes), ragequit my browser and ran to the hills with my knife.

    I put the lumps of rosin in to a glass jar, melted it in a water bath and took all the garbage out. I also filtered it iirc. Then I mixed it with ethanol and it does work.
    That is not the best way of doing it, though. You might want to get rid of the turpentine in the rosin by evaporating it. Also, they don’t write “low solids” on flux bottles for no reason.

    1. Is there really turpentine in that? I’ll have to try it on the next batch.

      I don’t see what good water does with the rosin, might as well go straight to alcohol.

      I’ve been making my own batches for about four months now. I have a nice old pine in the backyard that just sheds solid fist-sized bricks of sap.

      Drop a chunk into a glass jar, cover it with some denatured alcohol, leave for a few days (capped of course) until the whole thing dissolves. Filter it a couple of times through a coffee filter into a fresh jar. I keep the “dense” stuff in the jar and dole out smaller amounts thinned appropriately with more denatured alcohol in cleaned “5 Hour Energy Drink” containers.

      I was using rosin-core solder and my joints sucked. Since I started using this, my joints are 10x better and cleaner.

      Just one problem, I tend to have “rich” batches which seem to leave a white film. A quick bath in the alcohol gets rid of that.

      1. “I have a nice old pine in the backyard that just sheds solid fist-sized bricks of sap.”

        :D Having your own flux tree is a nice bonus. I would give it a name, “Old Kester” maybe :P

        “Is there really turpentine in that?”
        That’s what wikipedia says.

        “Rosin is the resinous constituent of the oleo-resin exuded by various species of pine, known in commerce as crude turpentine. The separation of the oleo-resin into the essential oil-spirit of turpentine and common rosin is effected by distillation in large copper stills.”

        I don’t know how much of turpentine is left in when you make it or how much it really affects the soldering or amount of fumes. I just figured that it’s one substance you don’t need in the flux so getting rid of it is better.

        I’d also like to use ethanol that’s not denatured to keep the total amount of impurities out.

        I didn’t add water in the rosin, I boiled water in a pot and put the rosin in the glass jar in that. I did it like that for 3 reasons: to get the turpentine and largest garbage out, control the rate of heating to prevent burning and keep my pot clean. I just think that’s not enough heat and the process is very slow.

    2. just made a tiny batch of pine tree sap rosin with iso, inverted soda can and tea candle. The resulting work looks like it came out of china, but does have a nice campy aroma.

  10. For needle-tipped bottles, take any syringe needle and drill a hole in the lid of a 3oz airplane bottle slightly smaller than the needle. I prefer a smaller needle, for more control. Don’t have any needles around? The tip of a wood splint picks up quite a bit, and makes it easy to accurately place the flux.

  11. I guess you could use powdered rosin that is used to remove hair in pig slaughter. It is being sold in large quantities and cheep. Wonder if it’s available outside Europe.

    Anyways I’ve never used anything else. It’s cheep, safe, durable and no cleanup flux. Violin rosins are overpriced compared to rosin filled cups sold in electronics shops. What I do is that I pour a few drops of high concentration ethanol on the hard rosin surface and rub it with a toothpick. I then apply that solution on pads or wires and (de)solder. When I’m done I just let the rosin surface dry.

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