What Is Soldering Flux and How to Use It?

Maybe you’ve been soldering for a while, or maybe you're just starting out — but to better understand how flux works, I create this article. I'll explain what flux is for, what types exist, and how to solder reliably, cleanly, and with ease.

Soldering Flux is a chemical or organic compound used to prepare the surface for soldering. It removes oxides from PCB pads and component leads in electronics. Flux also reduces the surface tension of molten solder, helping it flow more easily across the surface, into vias, and between pads and component leads. Without flux, solder becomes shapeless and uncontrollable. It won’t wet the surface, sticks to the iron tip, and just smears across the board — proper soldering won’t happen. This is called poor wetting.

Soldering Fluxes comes in different forms:
- Liquid soldering flux form;
- Paste Gel soldering flux form;
- Powder inside solder wires;
- Solid rosin chunks.

Most importantly: Soldering fluxes have different activity levels. For assembling or repairing modern PCBs, it's best to use gel-based no-clean fluxes of class ROL0, REL0, or ORL0. They don’t require mandatory cleaning — but it’s still good practice to clean the board after soldering.
If you need to restore heavily oxidized leads and pads, a more active flux is required — one that can effectively break through corrosion and contamination.
⚠️ Active fluxes must be cleaned after soldering.
⚠️ Active flux should never be used on wires. During soldering, it can seep under insulation and remain trapped. Even after soldering, it stays chemically active and will slowly corrode the conductor, leading to failure.

For tinning wires, liquid fluxes are better — they easily penetrate between strands and help the solder flow evenly, forming a strong connection.
⚠️ Always use only no-clean ROL0 flux for wires.

Flux helps solder wet the metal surface — without it, reliable soldering is nearly impossible. The soldering process differs slightly for PCB components and wires.

1. Heat the soldering iron to 320°C.
2. Clean the tip using brass wool or a damp sponge.
3. Apply a small amount of flux to the pad and the component lead.
4. Take a drop of solder onto the tip and transfer it to the joint, heating both pad and lead for about 2 seconds.
5. Don’t overheat — holding longer than 2 seconds can damage the pad.
6. After soldering, it’s recommended to remove leftover flux, especially if it’s active.

1. Heat the iron to 300-320°C.
2. Clean the tip.
3. Strip the wire with a wire stripper and apply flux to the exposed strands.
4. Distribute the flux evenly between the strands.
5. Bring the iron with solder to the wire and hold for 1–2 seconds until solder flows between the strands.
6. Don’t overheat — insulation may begin to melt or retract.
7. Always use no-clean ROL0 flux for wires.

There is no universal answer — it depends on the task.
If you’re assembling or repairing electronics by hand, use gel or liquid no-clean flux, class ROL0. It’s a safe and effective option for most modern PCBs and components.
If you’re dealing with oxidized or dirty pads, you can use a more active soldering flux ROM0, but be very careful - active flux always requires cleaning. If left on the board, it will cause leakage currents, copper corrosion, and damage to the solder joints. Active flux can seep under insulation and cause corrosion and failure weeks or months later.

For wires — always no-clean ROL0.

Thank you for reading!