What Is Soldering Fluid (Liquid Flux)? Types and Selection Criteria
Soldering fluid is a liquid flux without solder alloy that dissolves oxide on the metal surface so that molten solder wets it and spreads evenly.
Soldering fluid, or liquid flux, is a liquid that dissolves the oxide on the metal surface to be soldered and helps molten solder wet the surface and spread evenly. It contains no solder alloy; the joint is made by solder wire or solder cream. It is applied with a brush or bottle tip in electronic board soldering, tinning component leads, and desoldering and resoldering work.
Types of soldering fluid
| Type | Property | Use |
|---|---|---|
| Resin (rosin) based | Residue has a low corrosive effect | Electronic assembly and repair |
| No-clean | Leaves little, non-corrosive residue | Work where the residue can stay on the board |
| Organic, water soluble | High cleaning power; residue is washed off | Production with washing after soldering |
| Inorganic (acid or salt based) | Strongest type; residue is corrosive | Sheet metal and metalwork; not used in electronics |
ISO 9454-1 classifies fluxes into three types: resin, organic and inorganic. A thinner is not a flux: alcohol-based fluxes thicken as they evaporate when left open, and a thinner is used to restore the flux consistency.
Selection criteria
- Work and surface: On electronic boards, choose a resin-based or no-clean flux recommended for electronics. Active fluxes are for metalwork only.
- Residue cleaning: With no-clean types the residue can stay on the board for most work. With other types, remove it with a cleaner such as isopropyl alcohol or with the method given in the datasheet.
- Solder alloy: Lead-free solder needs a higher temperature; check in the datasheet that the flux works at that temperature.
- Safety: Alcohol-based fluxes are flammable. See the safety data sheet for flammability, storage and ventilation.
Common mistakes
- Taking flux for solder: Flux does not make the joint; it is used with solder wire.
- Using an active flux on a board without cleaning: The residue corrodes tracks and leads over time.
- Applying too much flux: Even with no-clean types a sticky, dirt-collecting residue remains.
- Confusing thinner with flux: A thinner is not for removing surface oxide; it adjusts the flux consistency.
- Leaving the bottle open: The solvent evaporates, and the flux thickens and gets contaminated.
Soldering fluids in the catalog
The Soldering Fluid and Flux category has 5 active products, all in 1 LT packs and sold individually:
| Product | Brand | Note |
|---|---|---|
| PC-26 | Feray | Soldering fluid |
| PC-33 | Feray | Soldering fluid |
| PC-70 Thinner | Feray | Thinner; not a flux |
| SR-33 | Soldex | Presented as a no-clean liquid flux |
| ARAX | Soldex | Soldering fluid; check the application area in the datasheet before use in electronics |
In some solder wires of the same name, Arax is a high-activity flux intended for metalwork. For paste-form flux, see the solder pastes.
Frequently asked questions
What does soldering fluid do?
It dissolves the oxide on the surface so that the solder adheres and flows. It helps solder spread evenly on oxidised leads, in board repair and when resoldering after desoldering.
Does no-clean flux residue need to be cleaned?
For most work it does not. If a conformal coating will be applied to the board, or the datasheet recommends cleaning, the residue is cleaned with isopropyl alcohol.
Should I use soldering fluid or soldering paste?
Soldering fluid is more practical for a thin, even coat on a board; soldering paste suits work such as tinning cable ends, where the flux has to stay in place.
How should soldering fluid be stored?
Keep it closed, away from heat and flames and out of the reach of children. For flux that has thickened, use the thinner recommended by the manufacturer.