Flux is a chemical that we use to control oxidation during soldering. It’s a necessary part of the soldering process. However, at the end of the soldering you generally are left a residue that covers your circuit board, and with enough time the solder flux may actually begin to corrode the parts of your circuit. It’s a process that may take a long time to occur, but it’s something that you may want to control.
In order to do so, you have to use the standard rosin flux or a particularly toxic solvent known as trichloroethylene which is a solvent that’s effective at dissolving the flux.
To avoid the use of solvents you can either leave the flux residue on the board undisturbed, or you may look for an alternative flux formulation. There’s water soluble flux which can be cleaned with water and detergent. There are mild fluxes that can be left on the board. And there are no clean fluxes that leave virtually no residue at all. These can be selected instead of the rosin flux and be just as effective.