I use liquid rosin flux for hand‑soldering and hot‑air desoldering. Making it in bulk means I never run out, I skip the branding and packaging markup, and I know exactly what’s in the bottle.

A quick warning: I’m sharing a process I’m comfortable with; it may not suit everyone. Do not attempt this unless you fully understand the risks and know how to handle flammable solvents safely.

The recipe you will see below works great, but I am too curious to keep it as a final version. I plan to update this article if any of the future tests turn into genuine upgrades. For now, the simple recipe does everything I ask of it.

The recipe

My base recipe was done in two stages. Don’t worry about being exact at the start, it is the process that gives you a lot of leeway, as I’ll explain in a moment.

The solvent, by volume:

• ¼ acetone
• ¾ alcohol (IPA or ethanol)

The flux, by weight:

• 70 % solvent
• 30 % crushed rosin (colophony)

For example, 30 g rosin to 70 g solvent.

This makes a saturated solution, which is fast‑acting for hand‑soldering. In cold weather some rosin can recrystallise, but the liquid stays well‑saturated and works fine. If you want a thinner flux for fine SMD work or for refilling a flux pen, just dilute a portion with a little more alcohol. This recipe is a concentrate; low‑odour commercial pen fluxes often sit closer to 10–20 % rosin, so you’ve got room to adjust.

The acetone helps the rosin dissolve faster, and it’s how I’ve always made flux. I’m curious whether the warm water bath alone would do the job without it; but for now the four‑part recipe is what I trust. Keep the acetone share modest; I believe the excess can make hot flux droplets jump around at the iron tip because acetone evaporates easily.

A note on stickiness

This flux, like many liquid rosin fluxes, is fairly sticky. You’ll get some on your fingers and maybe on the workbench. It’s a minor nuisance, nothing that a wipe with alcohol won’t fix.

Where to get the ingredients

• Rosin: I buy it by the kilogram on AliExpress. Small, branded boxes are expensive, and bulk means I avoid packaging markup and have more control of my supply.
• IPA: 3D‑printing shops, vape shops, and sometimes pharmacies. 99 % IPA is dry enough out of the bottle.
• Ethanol works great. Look for 95 %+ rectified spirit, not ordinary vodka (which contains too much water). High‑proof spirits at 96% do the job, though availability varies.
• Acetone: hardware or paint stores, pure grade; pure nail-polish remover marked as acetone and sold in 1L bottles from cosmetic shops also works in a pinch.

Crushing the rosin

I use a cheap plastic mortar from AliExpress. Rosin needs to be crushed to a coarse powder, because it dissolves much slower without that step.

Dissolving: the water‑bath method

Safety first. Alcohol and acetone vapours are extremely flammable. Work outdoors or in a well‑ventilated space, far from any open flame, hotplate, or spark. Wear eye protection.

1. Put the crushed rosin and solvent into a glass container. A clean canister that once held IPA works well, but make sure it’s thoroughly rinsed and dried, not idling with old solvent at the bottom.
2. Do not seal it too tight, leave the cap a bit loose so pressure can escape. Make sure not to breathe in the evaporated acetone.
3. Heat water to 50-70 °C (hot but not boiling) in a kettle, then pour it into a larger bowl or pot.
4. Place the glass container in the water bath. The warm water alone dissolves the rosin in a few minutes.
5. Swirl occasionally until the solution is clear. Once cooled, filter to catch any undissolved rosin fragments and stray debris, then bottle.

Optional: drying the solvents

99 % IPA is dry enough. But if you want to remove every trace of water, soak the alcohol (or acetone) with zeolite drying beads (look for them on AliExpress) and then filter through at least a coffee filter. They work by adsorbing water, so make sure to heat them in the oven before using them and let them cool in a dry place. This is a belt‑and‑suspenders step; the flux works perfectly without it.

Containers

I use 30 ml or 50 ml soft plastic drip bottles with self‑sealing caps, sold in bulk on AliExpress (often 100–200 pcs). They stay leak‑free for years. I’ve never seen mould grow in a sealed flask, so I’ve never needed additives.

What makes rosin stick

As far as I understand, rosin is mostly abietic acid, a weak carboxylic acid. At soldering temperatures it melts and the ‑COOH groups react with copper oxide on the pads and leads, dissolving the oxide and letting the solder wet the metal. That’s the whole trick.

This also explains why some modifications help and others ruin it:

• Halide activators (the things in RA‑grade fluxes) work much faster, but they give you a very active flux, so they corrode the board if not cleaned thoroughly.
• Hydrogenated and disproportionated rosins keep the acid groups intact while stabilising the double bonds that cause oxidation, and they create enough molecular diversity to prevent crystallisation. That’s why I’m curious about them.
• Esterified rosins (like glycerol esters) have their acid groups chemically blocked – they’re great tackifiers for adhesives and chewing gum, but nearly useless as flux.

Rosin only gets aggressive when it melts; below its softening point it’s chemically inert, which is why residue left on a cooled board is essentially non‑corrosive. And while it’s liquid, rosin also forms a barrier that keeps atmospheric oxygen away from the joint and lowers surface tension to help solder flow. It’s not just an oxide remover, but also a temporary shield.

Things on my wishlist

A few things I’d like to test, when this batch runs out:

• Dropping the acetone. With a warm water bath, the alcohol may dissolve rosin fast enough on its own. Worth a side‑by‑side test with my current four‑part recipe.
• Disproportionated rosin (like Foral AX) and hydrogenated rosin. Both keep the acid groups while resisting oxidation and crystallisation much better than raw gum rosin. I’d try them at the same 30 % concentration and see whether the cold‑weather crystals vanish.
• Polymerized rosin (Sylvalite, Pamite, etc.). Higher melting point, even less tendency to crystallise, but with a lower acid value, so it would be a milder flux overall. Worth a look if crystal‑free storage is the priority.
• Maleic‑modified rosin (maleopimaric acid). The opposite tradeoff: higher acid value than raw rosin, so more aggressive fluxing and less crystallisation. This would push close to RA‑grade activity, and I’d expect it to require thorough cleaning, which fits my workflow. I’ve only seen references to it; I haven’t found a small‑quantity source yet.
• A mild organic acid activator. Adding 1-3 % (by weight of solids) of adipic acid or succinic acid would shift the flux into RMA territory, helpful for stubborn leads. Most of it decomposes at soldering temperatures and the rest washes off. I’d also consider a tiny test with oleic acid. I wash PCBs with alcohol in an ultrasonic bath, so an oily residue isn’t a deal‑breaker for me, but I’d keep that batch small.
• A cleaner starting rosin. Trying a water‑white (WW) gum rosin like BW‑1200, just to see whether the paler material leaves noticeably cleaner‑looking residue. I don’t expect a dramatic difference, but it’s worth a comparison purely for the curiosity reasons.

Skipped on purpose

A few things have come up while I was reading about the topic, and I’m steering clear of them while using the current batch:

• Glycerol. It’s hygroscopic, doesn’t fully decompose at soldering temperatures, and leaves a sticky, potentially conductive residue. I tried it a bit, realized it’s used in tacky SMD reflow gels, but doesn’t fit in the kind of liquid flux I make.
• HCl or chloride‑based activators. Even at low concentrations, they risk corroding pads and tools, and they don’t belong in an electronics flux you store and reuse. I’ve seen no corrosion with my plain rosin flux; I intend to keep it that way. Besides, their availability for personal use is unreliable in many places.
• Esterified rosins. As explained above, the acid groups that do the fluxing are blocked. They’re excellent for other things, but not my goal.

A note on rosin sources

Gum rosin (from live tree resin) tends to have a milder odour and crystallises less from solution than wood rosin, so it’s the preferred raw material for fluxes.

Tall oil rosin (a paper‑mill by‑product) sees use too, praised for higher thermal stability and less insoluble residue after heating. Most “rosin” on AliExpress doesn’t specify the source; I assume it’s gum rosin unless it smells like a pine stump.

If you’re shopping specifically, gum rosin is the safe default.

Worth the trouble

The process feels primitive, but it gives me a flux that costs next to nothing, scales to a year’s supply, and is as clean and predictable as anything I’ve bought¹. I’ve left residue on test boards for weeks with no visible corrosion, which I can’t say for every commercial flux I’ve tried. The stuff works.