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What is soldering flux?

Flux is a chemical cleaning agent used in the soldering process. Its uses include the soldering of electronic components onto circuit boards. The purpose of flux is to prepare surfaces for soldering by removing any oxides and impurities. Using flux improves the strength of the soldering joints. It also protects metal surfaces from re-oxidation, improves electrical contact and helps the flow of molten solder.

Flux is made up of a base material (vehicle) and an activator. The activator removes oxides from the metal. This helps wetting the solder on the board. Flux also contains solvents and additives, to facilitate the soldering process.

What is soldering flux

Types of soldering flux

There are three main categories of soldering flux, based on electronics industry standard.

  • Rosin flux: The oldest and still one of the most common fluxes. Its base material is derived from pine tree resin. Rosin flux is inert when in solid state. It flows and becomes acid when heated, removing oxides and particles from surfaces. It can be cleaned with an isopropyl alcohol solution. Flux residue should be removed as soon as possible.
  • Organic flux: Also known as water-soluble flux. It is made up of organic materials other than rosin. Compared to the previous type, this flux is a stronger cleaner. It is corrosive and requires careful cleaning after application. Flux residue can lead to poor component performance and even short circuits. Organic flux can be cleaned with water.
  • Inorganic flux: It is made with chemicals such as borates, fluorides and chlorides. Inorganic flux is used with stronger metals (brass, copper, stainless steel). It is not recommended for electrical circuits, due to its strength. Its main application is plumbing. Very corrosive, any residue might destroy a solder joint.

Most experts recommend rosin fluxes for electrical soldering. Rosin fluxes are the least corrosive and pose a small risk of damaging components.

Some organic fluxes are no-clean. This means they require little or no cleaning after application. However, residual no-clean flux can still reduce the adhesion of conformal coatings. It could also interfere with PCB testing or sensitive components. It is best to clean flux residue whenever possible.


Flux can be in solid, paste, or liquid form. It can be applied on a PCB in a number of ways, based on the soldering process being used:

  • Hand soldering: Flux is applied manually. A solder pen or a handmade applicator can be used. Certain types of soldering wires are manufactured with flux mixed in the solder. If this is the case, heating the wire onto the surface with a soldering iron will release the flux.
  • Wave soldering: Flux is sprayed on the board prior to it going through the solder wave.
  • Reflow soldering: Flux is applied as a paste composed of sticky flux and small spheres of metal solder.

ConRo Electronics, as an electronic manufacturing and soldering systems distributor, offers a wide choice of Leaded and lead-free solders, from major manufacturers such as Indium and Surclean.

We’ll show you how to improve product reliability while increasing performance and lowering costs. Our team of technical support specialists will provide your company with dependable global supply, unrivalled efficiency, and superior technical support.

Feel free to contact us on 0208 953 1211 or send us an email to

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