What is solder Flux? An explanation from the pros

If you’ve landed on this page you are probably scratching and racking your brain, ‘Solder flux! What is solder flux?’ Allow us to explain. Solder flux is man’s best friend. It’s what allows us to make successful solder joints despite problems that occur when soldering, such as oxidation, dirt, oils and impurities.

Why use solder flux?
We rely on flux because soldering joints need to be free of impurities before proper connections can be made. Ordinarily, this would be easy to accomplish by chemical means, but it doesn’t pan out when soldering.
The exceptionally high temperatures that are required to melt solder encourage the solder and the work piece to reoxidize, putting us back at square one: impure work pieces that won’t make good connections. This is where flux comes to the rescue. It reduces the oxidation of the metals and also acts as a wetting agent, allowing the solder to flow more easily.
What is in solder flux?

To fully answer, what is solder flux, we should explain what is made of. In the electronics industry, it is made of rosin, an element extracted from pine sap. It’s non-corrosive and non-conductive at normal temperatures, but will become reactive and corrosive with the elevated heat of the soldering iron.  Plumbing and automotive applications call for acid-based fluxes.

Types of solder flux

Two of the main types of flux are “clean” and “no-clean” fluxes. As mentioned earlier, fluxes become corrosive when they reach high temperatures and need to be cleaned when the project cools. “No-clean” fluxes eliminate this step.

Flux can come in liquid or paste form. Some solder will have the flux built right into its core. The flux that you choose for your project will require evaluation and research.


Smoke from solder flux is toxic. Always work in a well ventilated area and wash your hands after soldering.

The next time someone asks, “What is solder flux”, profess your newfound knowledge with confidence. We are always adding new information and blogs about heat tools to our site, so be sure to check back for the latest from Master Appliance.

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