Why use flux when soldering?

One question that comes up now and again is what flux is, and why it’s important to your soldering projects. In short: flux is a chemical agent which can be roughly likened to paint primer for your solder connection. It’s an inexpensive and easy way to improve your soldering joints. Applying flux accomplishes three things: reduces oxidation, improves electrical contact, and helps even solder flow.

When flux is applied to the electrical lead you’re soldering, it helps remove oxidation and prevents airflow to the lead, preventing further oxidation. Solder attaches more easily to non-oxidized metal, which will cause the solder to flow evenly on the electrical lead. Since the solder is even in the electrical lead its electrical conductivity is also improved.

There are three main types of fluxes in use today: rosin flux, organic acid flux, and inorganic acid flux.

Rosin flux is the oldest and still one of the most common fluxes used for electrical components. It is only active when heated, and generally safe for unheated electrical circuits. Rosin flux can be cleaned using an isopropyl alcohol solution.

Organic acid flux is also commonly used for soldering electrical circuits. This flux cleans oxidation on electrical leads much more quickly. It is very important to clean this flux off electrical circuits because it leaves residue which will conduct electricity, causing shorts in your circuit. Organic acid flux is water soluble and can be cleaned with water.

Inorganic flux is used on stronger metals such as copper and stainless steel. It is not commonly used in electrical circuits, instead being used mainly for plumbing applications.

There is solder is available with flux inside the wire, called flux core solder. Flux core solder has flux in the center of the solder wire. Additional flux is unnecessary for most applications, but using additional flux still may make the solder easier to work with for some tasks. When selecting flux core solder, it is important to use rosin core solder for electrical applications. Acid core solder should only be used for plumbing applications.

Overall, I recommend rosin flux because it is most commonly used for soldering electrical circuits and is intended specifically for that purpose.