Fluxes are chemical compositions used in soldering, brazing and welding. They clean the metal’s surface, assist with heat transfer, and remove unwanted oxide films. According to J-STD-004, a popular flux classification system from the IPC and EIA, there are four broad categories of products: rosin (RO), resin (RE), organic (OR) and inorganic (IN) fluxes. Rosin fluxes contain abietic acid, a chemical extracted from pine-tree sap. They may also contain activators such as halides. There are two subcategories of RO fluxes: high-solids and low-solids. High-solids resin fluxes contain greater amounts of halide activators which, if left in place, will convert metal surfaces into metal salts. Low-solids RO fluxes contain lower levels of halide activators, but may still corrode solder wires. Resin fluxes are based on synthetic or modified organic resins. RE fluxes provide better thermal stability than RO fluxes and are often easier to clean. In addition, resin fluxes can be modified to flash-off from the heat of a solder tip.