Soldering flux is a chemical substance used before and during the soldering process of electronic components. Soldering flux can be used for both manual and automated soldering processes.  Soldering flux is mainly used to prepare the metal surfaces before soldering by cleaning and removing any oxides and impurities. Oxides are chemical compounds that are formed when metals are exposed to air. Oxides prevent the formation of a perfect solder joint. The presence of metal oxides on circuit boards may result in electrical conductivity and inefficient flow of electricity across the electrical board or circuit board.  Other reasons for using solder flux include:

  • Prevent reoxidation of the metal surface during the soldering process
  • Help in reducing the surface tension and the viscosity of the molten solder. This helps to improve wettability.
  • The flux allows the molten solder to form a permanent electrical and mechanical joint.




How to select an appropriate soldering flux?

The mode of activity of fluxes is based on halides. A flux with a greater halide content will perform better by removing any formed metal oxide on the metal surface and enhancing the joint’s strength. Fluxes that have very high activity tend to leave behind corrosive by-products on the soldering surface. This can result in reliability problems of the product.

When doing electronic assembly, you should consider two factors.

  • The flux needs to be inactive before and after soldering.
  • Secondly, it should remain active during the entire soldering process. This helps to prevent the formation of tarnish and oxides due to the high temperatures present during soldering.

A perfect soldering iron flux should function just below the soldering temperature to ensure that the soldering surface remains ready for soldering.  Highly active flux is not recommended because of the unwanted by-products that bring about reliability issues.

The main disadvantage of such fluxes is that even at high temperatures, they do not perform well. No clean fluxes are now available to combat this problem. An excellent solder flux should strike a balance between cleanability and activity.  Most non-clean fluxes are halide-free. They are mainly made of organic acids.

How to use a soldering flux?

Solder flux can come in the form of a paste or liquid. Most solid flux is contained in a small jar or tins, while liquid changes are included in bottles or flux pens. When soldering, it is advisable to use all precautionary gears to enhance your safety. Outlined below is a breakdown of how to use a solder flux.

  1. Clean the metal surface with an appropriate solvent. This helps remove any dust particles and grime, not to forget some of the metal oxides on the soldering surface.
  2. Smear an even coat of the flux on the soldering surface. You are not allowed to heat the surface at this stage.
  3. When the soldering gun is hot enough, place it on the metal plates covered with the flux. The soldering gun should melt the flux and spread it all over the metal surface, thus removing any metal oxides.
  4. Introduce the soldering wire after the flux has evaporated. This ensures that the solder bonds before reoxidation.




Types of Solder Flux

There are three major types of soldering flux used in the electronics industry.  Outlined below is a summary of each.

  • Rosin flux– this is the oldest type of flux.  The flux is composed of natural resins extracted from the pine trees. Modern rosin flux is mixed with other different fluxes to hence their performance. This type of flux is easy to flow, especially in hot weather conditions. The flux removes both metal oxides and foreign materials. Resin flux is acidic in nature, but once it solidifies, it becomes inert hence can be left on the soldering surface without damaging the circuit. Removing the flux after soldering is advisable because the circuit might warm up and liquefy the flux.
  • Organic acid flux–  also known as water-soluble flux. This type of flux is made of organic acids such as citric, lactic, and stearic acid.  The acids are then combined with organic solvents such as alcohol or isopropyl. Organic acid flux is more effective in removing metal oxides.  Additionally, it has robust soldering activity and is easy to clean. Organic acid flux should never be left on soldering surfaces, especially circuit boards. Organic acid flux is mainly used for soft soldering.
  • Inorganic acid flux– works better with stronger metals such as brass and copper, not to mention stainless steel. Before using inorganic flux, you need to clean the soldering surface and remove any corrosive residues thoroughly. Lastly, inorganic acid flux can be used for non-electrical soldering.


It is beyond reasonable doubt that solder flux is essentially in electrical soldering.  Soldering flux helps in cleaning and removing metal oxides from the soldering surfaces. Solder flux also helps to prevent oxidation of the metal surfaces during soldering.




About The Author

Tony ZuberbuehlerTony Zuberbuehler is a Sales Manager at Versa Electronics with a focus on electronic contract manufacturing. Tony’s career in technology manufacturing spans 25+ years and has included roles as an engineering liaison, in purchasing and material management, manufacturing and planning, customer interfaces, and product fulfillment. Connect with Tony Z on LinkedIn.