One of the more common fuxes used is water soluble organic acid (OA) flux. Rosin flux is the most aggressive flux and in today’s sensitive electronics it is most likely to harm over time if heated to its activity point, meaning it becomes acid and will start to corrode areas where residues have not been removed after use. Ideally flux will flow easily, especially when hot, removes oxides quickly, and helps to remove foreign particles from the surface of the metal being soldered. Rosin flux is still used today, but typically is a blend of fluxes to optimize the flux, its performance, and characteristics. Some of the oldest types of flux used is based off pine sap (refined and purified) called rosin. In all cases, the best flux to use is the least acidic (least aggressive) flux that will work on the oxide on the components and result in a good solder bond. However, there are several applications where additional flux is extremely beneficial, such as wire tinning surface mount soldering and de-soldering.
For many applications, the flux included in the core of the solder wire is sufficient.
