How to Solder with Flux

How to use solder flux depends on which kind of flux is used in your soldering project.

Updated: June 27, 2022

Fluxes are chemicals that remove oxide layers from the metal surface. Fluxes are widely used in soldering since they improve heat transfer and carry metal oxides and debris away from the solder joint.

When heated with soldering iron fluxes promote and accelerate the soldering process. Fluxes are available in solid, liquid or gaseous form.

During the soldering process, the molten solder makes contact with the metal surface to which it is to be joined. Most of the metals involved in soldering are oxidized during exposure to high temperatures. This oxidation prevents the formation of a solder bond between two metal surfaces. Therefore, fluxes are used in soldering to remove such a layer of metal oxide. Fluxes also protect the metal surface against re-oxidation during soldering.

Fluxes are important and efficient material to remove a thin layer of metal oxide. However, fluxes do not effectively remove all other contaminants that may be present underneath metal oxide. Thick layers of metal oxide or some other heavy scale contamination cannot be removed even with the strongest fluxes. In that case, some kind of a mechanical or chemical cleaning procedure may be required before we apply flux.

Three main ingredients in flux are:

There are four major types of fluxes based on cleaning agent:

Rosin fluxes are the most popular fluxes for applications in electronics since they are very mild in their activity - they are non-corrosive fluxes. On the other hand, inorganic fluxes are very active and corrosive.

Rosin flux is available in few different packages such as syringe or pen.

Rosin-based fluxes consist of a distillation product from pine tree sap, dissolved in a suitable organic solvent. This type of flux is solid at room temperature.

When heated with soldering iron rosin flux gains sufficient activity to remove metal oxides from copper, silver, gold and a few other materials. Copper is the most used metal in electronics. Every circuit board consists of one or more layers of thin copper foil. Most of the wires used in electronics are made of copper since copper has an excellent conductivity for electrical current. Leads of several electronic components are plated with a very thin layer of gold.

Heating causes rosin flux to liquefy and become slightly chemically active.

The electronic industry prefers the least chemically active fluxes because those fluxes have minimal corrosion. Rosin flux used in electronics is mild and can remove only a thin layer of an oxide film. If any heavy oxide film is present on the circuit board then additional cleaning of the circuit board with stronger chemicals should be done prior to the application of rosin flux. Also, any grease on PCB should be removed before application of the rosin flux.

Safety and health concerns should be taken into consideration when you learn how to use solder flux.

Fluxes are flammable since alcohol is one of the most used solvents in the fluxes. Rosin flux produces smoke when it breaks down at soldering temperature. Therefore, you should solder in a well-ventilated area when using flux during the soldering operation.

In any case, you should use a benchtop smoke extractor which will absorb all smokes caused by flux.

Portables smoke extractor, also called smoke absorber, can be easily moved along the workbench. The best extraction of fumes is achieved when a portable fume extractor is placed close to the soldering area.

How to Use Solder Flux

Select appropriate size and shape of iron tip. Replace tip when the soldering iron is cold and not plugged into a power source.

Turn on soldering iron. Modern soldering irons have a short heat-up time (a minute or less).

While waiting for soldering iron reaches the setup temperature, apply flux on both surfaces of the solder joint. Both surfaces should be clean without any debris.

The tip of soldering iron should be tinned with a thin layer of solder. This will provide a better transfer of solder to solder joint.

Many kinds of solder wire come with their core already filled with rosin flux. In that case, there is no reason to apply additional flux paste since the flux is already conveniently included in solder wire. On the other hand, it doesn’t hurt to apply flux paste in addition to flux included in the solder wire.

The soldering operation may be made in a single step or two steps. It is easier if it is done in two steps.

The first step is tinning each surface with a thin layer of solder. The second step is simply a matter of joining pre-tinned surfaces together. In the single-step soldering operation, the first and second steps are done simultaneously.

In this "how to use solder flux" guide, we will continue with an easier technique - the one which is done in two steps. In our soldering project, the first surface is the lead of the electronic component and the second surface is a pad on the circuit board..

Place the iron tip to the pad on the circuit board to start tinning the first surface. The heat from the iron tip will melt flux. Flux will float over the surface of the pad preventing oxidation by forming a liquid blanket between air and metal surface. Repeat the tinning process on the other surface (in our case on the lead of the electronic component.)

When both surfaces are tinned, place soldering iron to soldering joint. Make sure that soldering iron touches both surfaces. Keep holding soldering iron at one side of the soldering joint until the heat transfers through the joint and melts the solder. Add a small amount of solder if needed to fill deeper gaps between two surfaces.

Remove solder and soldering iron away from soldering joint. Always return soldering iron in the iron stand. Never leave the soldering iron on the table – hot iron can burn the bench or start a fire.

Inspect the soldering joint for possible imperfections.

The main reason for failures to make a good soldering joint are:

lack of solder
too much solder( excess of solder can be removed with solder wick or desoldering pump
lack of heat (this may happen when using a low-wattage soldering iron)
too much heat (this may happen when the soldering iron is applied to the soldering joint longer than needed. It may also happen when using bigger sizes of iron tips. Soldering with a high-wattage soldering station will not damage the solder joint if you choose the appropriate size and shape of iron tip)
lack of flux
the surface is not clean enough

After soldering, clean rosin flux residue on the solder joint. You may use an organic solvent to remove these residues. Isopropyl alcohol is the most common solvent used for removing rosin flux residues.

You can use a small brush or cotton swab to apply solvent to flux residue (even a small paintbrush may be good enough as long as it clean). Hold brush with fingers, dip it in isopropyl alcohol and wipe over flux residue.

See more details about how to remove solder flux residue.

Finally, rinse the surface with distilled water. Don’t rinse with tap water. Tap water contains minerals and may leave small pale stains on the circuit board.

After cleaning the circuit board should be completely free of flux and residual agents.