Why Is Soldering Flux Needed?
Solder will have a hard time bonding into oxidated metals. Oxidation is a chemical process wherein oxygen in the air reacts with a metal, making the latter corrode. The rusting of iron is an example of oxidation. Moreover, oxidation accelerates at high temperatures which means oxidation is very much likely during soldering. Prevention of oxidation is the primary purpose of applying soldering flux.
An oxidated metal will have an oxide layer as shown:
The application of soldering flux will effectively remove this oxide layer and allow the solder to make a strong bond with the metal. Flux can also help remove dirt, dust and other foreign particles that may affect the soldering process.
Soldering Flux for Electronics
There are several types of soldering flux depending on the type of soldering to be done. Soldering of electronic parts requires rosin flux. The term rosin comes from a type of resin derived from coniferous (pine) trees.
Rosin flux liquefies when heated at 120 °C and becomes acidic, allowing it to remove oxides and other impurities. As it solidifies at room temperature, it reverts back to being non-reactive and consequently, non-corrosive. Thus, rosin flux will not affect your printed circuit board at room temperature but when the temperature rises, it may begin eating away the copper on the board. And while most of the rosin will evaporate especially if you’re using very high temperatures for soldering, residues will always be left behind. Therefore you must clean away any trace of flux after soldering. Use isopropyl alcohol and an old toothbrush to remove the solder flux.
Rosin flux is distributed into two package types: in a tin can or plastic container or as a pen. If you’re having issues with cleaning flux residue, pen type flux will make your life easier.
There are other types of soldering flux but we will limit ourselves to those used in building electronic projects. If you’re interested about other types of flux, read about organic and inorganic soldering flux.
How to Apply Soldering Flux
Applying soldering flux prior to soldering is straight-forward. For soldering electronic components into a printed circuit board, apply flux to both the component’s pin and the hole track on the board. Then apply solder as described on my how to solder tutorial.
For soldering wires, dip both the wires to the soldering flux prior to tinning. Then apply solder just like how I described it on my soldering wires tutorial.
Rosin Core Solder
There are soldering leads with rosin core which makes fluxing and soldering a one-step process. Such type of solder is best to use when soldering through-hole components as they don’t necessary require a lot of soldering flux.
You don’t need to apply additional flux if using a rosin core solder for through-hole components. However, more soldering flux may be needed when soldering surface-mount components which contain hundreds of very small leads. The small size of these leads makes it very hard to add more solder without shorting them out. The flux will make the leads bond easier to the copper trace on the board without adding more solder.