The role of the activators is primarily disruption and removal of the oxide layer on the metal surface (and also the molten solder), to facilitate direct contact between the molten solder and metal. The reaction product is usually soluble or at least dispersible in the molten vehicle. The activators are usually either acids, or compounds that release acids at elevated temperature.
The general reaction of oxide removal is:
- Metal oxide + Acid → Salt + Water
Salts are ionic in nature and can cause problems from metallic leaching or dendrite growth, with possible product failure. In some cases, particularly in high-reliability applications, flux residues must be removed.
The activity of the activator generally increases with temperature, up to a certain value where activity ceases, either due to thermal decomposition or excessive volatilization. However the oxidation rate of the metals also increases with temperature.
At high temperatures, copper oxide reacts e.g. with hydrogen chloride to water-soluble and mechanically weak copper chloride, and with rosin to salts of copper and abietic acid which is soluble in molten rosin.
Some activators may also contain metal ions, capable of exchange reaction with the underlying metal; such fluxes aid soldering by chemically depositing a thin layer of easier solderable metal on the exposed base metal. An example is the group of fluxes containing zinc, tin or cadmium compounds, usually chlorides, sometimes fluorides or fluoroborates.
Common high-activity activators are mineral acids, often together with halides, amines, water and/or alcohols:
- hydrochloric acid, most common
- phosphoric acid, less common, use limited by its polymerization at higher temperatures
Inorganic acids are highly corrosive to metals even at room temperature, which causes issues during storage, handling and applications. As soldering involves high temperatures, compounds that decompose or react with acids as products are frequently used:
- zinc chloride, which at high temperatures reacts with moisture, forming oxychloride and hydrochloric acid
- ammonium chloride, thermally decomposing to ammonia and hydrochloric acid
- amine hydrochlorides, decomposing to the amine and hydrochloric acid
Information above from http://en.wikipedia.org/wiki/Flux_(metallurgy) under the Creative Commons Attribution-ShareAlike License
