What flux types are there?
Flux removes oxides from metal surfaces and improves wetting during soldering. The three main types are rosin-based (R, RMA, RA), no-clean with low residue and water-soluble flux. IPC J-STD-004 classifies them by composition and activity. Whether residues must be cleaned depends on the type and its activity level.
To soldering guideWhat does flux do when soldering?
Flux removes oxide layers from the joint and the component and improves wetting, so the molten solder flows cleanly and forms a solid joint. Without flux, oxides prevent wetting and the joint stays cold or incomplete.
When heated, metal quickly re-oxidises. The flux binds these oxides chemically and protects the surface from renewed oxidation during soldering. It also lowers the surface tension of the solder so it spreads evenly across the contact areas.
What flux types are there?
There are three main types: rosin-based flux (R, RMA, RA) from natural resin, no-clean flux with very low, uncritical residue, and water-soluble flux with high activity. Rosin and no-clean are mild; water-soluble flux wets more strongly but must be washed off completely.
| Flux type | Cleaning | Activity | Use |
|---|---|---|---|
| Rosin (R, RMA, RA) | depends on activator | low to high | electronics, rework |
| No-clean | not required | low | SMT, hand soldering |
| Water-soluble | mandatory | high | where cleanliness matters |
- R (rosin): pure rosin without activator, low activity, barely corrosive residue.
- RMA (rosin mildly activated): mildly activated, a good balance of wetting and mild residue.
- RA (rosin activated): strongly activated, high wetting, residues should be cleaned.
The flux core in the wire co-determines activity and cleaning needs.
Choose solder wireWhen must residues be cleaned?
Cleaning is required whenever the residues are corrosive or ionically conductive. Water-soluble flux must therefore always be washed off. No-clean residues may remain on the assembly. With activated rosin (RA) it depends on the activator content and the application.
The IEC and IPC standard J-STD-004 classifies flux with a three-part code of base, activity and halide content. The base is RO (rosin), OR (organic) or IN (inorganic). The activity is L (low), M (moderate) or H (high). The final digit gives the halide content: 0 below 0.05 percent, 1 above.
- ROL0: rosin, low activity, halide-free - usually no cleaning.
- ORM0: organic, moderate activity, halide-free - typical for many no-clean products.
- ORH1: organic, high activity, with halide - strong wetting, cleaning required.
Frequently asked questions
What is the difference between no-clean and water-soluble flux?
No-clean leaves low, uncritical residues that may remain on the assembly. Water-soluble flux is more active and wets better, but leaves corrosive residues that must be washed off completely with water.
What do ROL0 or ORH1 mean under J-STD-004?
The code describes base, activity and halide content. RO stands for rosin, OR for organic. L, M and H stand for low, moderate and high activity. The digit 0 means halide-free, 1 means halide above 0.05 percent.
Do rosin residues always have to be cleaned?
No. Pure rosin (R) and mildly activated RMA leave largely uncritical residues. Strongly activated RA and halide-containing fluxes should be cleaned, since their residues can be corrosive.
Advice on flux and soldering
From the right flux type through no-clean to cleaning, we help you select the right soldering technology.
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