Rosin fluxes are named for their primary ingredient, rosin, which is extracted from the oleoresin, or sap, of pine trees. Rosin has an active ingredient called abietic acid, as well as some other types of natural acid that may be present in the rosin. Three main types of rosin flux are used: rosin (R) flux, rosin mildly activated (RMA) flux and rosin activated (RA) flux. Each of these have a different level of activators, which are the agents that allow the flux to deoxidize and clean.
R flux is only used to clean surfaces that are already extremely clean. Its main advantage is that it leaves no residue behind.
RMA flux is used on slightly dirtier surfaces, but it also leaves a little more residue than R flux. After use, the surfaces must be cleaned with a flux cleaner.
RA fluxes have the highest level of cleaning ability but also leave behind the most residue and are therefore rarely used.
Water Soluble Fluxes
Water soluble fluxes are also called Organic Acid, or OA, fluxes. They are usually made from glycol bases. The biggest shortcoming of OA fluxes is that they tend to bond with either the circuit board or other metal surface and therefore require cleaning. They can also be more aggressive deoxidizers than necessary. However, organic fluxes are more active than RA rosin and are therefore more potent cleaners overall.
Inorganic water soluble fluxes are also available; they are usually even more powerful than OA fluxes.
No-clean fluxes are made from a mixture of organic resins (other than rosin) mixed with inorganic agents. The properties and behaviors of these kinds of fluxes vary considerably based upon their specific chemical composition. For instance, some will leave an enormous amount of residue that, while posing no threat to the surface, is unattractive. No-clean fluxes that leave no residue are also available. The biggest advantage of no-clean fluxes is that they do not need to be cleaned off the surface after soldering, as they will not affect electrical conductivity.