Indium Blog

Why is a Soldering Flux Needed?

  • Flux

  • There are several questions that come up when learning about soldering fluxes like, what does a flux do?  How does a flux work?  Why do I need a flux?  This post explores these questions.

    A flux removes the oxides of a base metal, cleaning its surface to better enable the formation of an excellent solder joint. But, how is a metal oxide formed? 

    A metal oxide is formed because a metal atom likes to be surrounded on all sides. Metals exert a surface energy force, which attracts oxygen atoms to the surface of the metal, causing it to form a metal oxide and allowing it to be surrounded. This is ideal for the metal, but not ideal for soldering. These oxides inhibit metals from forming high quality solder joints.

    As a result, a flux needs to do several things to be effective. These three things are:

    • remove oxides
    • prevent re-oxidation
    • displace air 

    Rosin is good at preventing re-oxidation; this is why a lot of fluxes are rosin based. Fluxes don’t necessarily have to be rosin based, but the rosin does help in keeping the surface from re-oxidizing. Fluxes also keep the air from getting back down to the surface of the metal. The flux forces the air off of the air-to-metal interface and allows the solder to flow in behind it. This happens when the surface energy is greater than the surface tension of the solder. However, when the surface tension is greater than the surface energy, the solder will ball up (non-wetting). The flux allows for the surface energy to be greater than the surface tension. Therefore you need a flux to form a solder joint with air reflow. 

    If you have any additional questions please feel free to email me or