Brazing Fluxes

The main function of flux is to clean the surface of base metals and check any further oxidation so that the filler metals can wet the surface of joints efficiently and the joints should be strong. The fluxes should be able to deoxidize the metals at high temperature without being decomposed itself.

Rosin Fluxes
Rosin is a natural substance, which is extracted from the bark of pine trees. It was earlier known as colophony. The composition of rosin varies but its general formula is C19H29COOH. Its primary constituents are abietic acid (70 to 85%) and pimaric acids(10 to 15 %). It also contain some percentage of unsaponifiable hydrocarbons. However, saponifiers must be added for rosin flux removal.

The rosin flux has solid state and glasasy structure. It isn't effective until it melts. After melting it has enough mobility to enter into reactions with the surfaces to be joined. The rosin flux starts softening at 50-70°C and entirely melts at 120°C. The joint materials are oxidised for a long time inorder to increase the flux action and give strong joints in lesser time. For this purpose, rosin is usually accompanied by a chemical system called an activator.

Rosin flux is mainly made of natural resin extracted from the oleoresin of pine trees. Some of their important properties are:

Rosin flux is readily used in the electronic industry due to the combination of its favorable properties:

Inorganic Fluxes
Inorganic fluxes are composed of inorganic acids and salts, like hydrofluoric acid, hydrochloric acid, sodium or potassium fluoride, zinc chloride, and stannous chloride. They are usually very corrosive. These fluxes are stronger and are capable of removing oxide layers of ferrous as well as nonferrous metals, like stainless steel, Kovar and nickel irons.

The inorganic fluxes are primarily used for non-electronics applications, for example the brazing of copper pipes. However sometimes they are used for lead-tinning applications in the electronics industry. Their main drawback is that the residues they leave behind are chemically active and they can cause corrosion.

Organic Acid (OA) Fluxes
Organic acid fluxes are milder than inorganic fluxes but stronger than rosin fluxes. They offer a fine balance between cleanness and flux activity. They contain polar ions that can be easily removed by a polar solvent like water. As they are soluble in water so they are environmentally desirable. OA fluxes come with or without the use of halides as activators.

Fluxes based on alkalies, inorganic acids and their salts are highly corrosive. Some inorganic fluxes are hydrochloric acid, orthophosphoric acid, ammonium chloride, zinc chloride, etc. Due to the high corrosiveness of these fluxes, they aren't used for electronic applications.