How Solder Flux Works
Soldering two pieces of metal together, whether it’s structural soldering like plumbing or stained glass or connective soldering like soldering a resistor to a circuit board is more than a simple mechanical bond. Solder forms a metallurgical bond with metals if it is properly applied, and this adds both physical strength and superior conductivity. Check out our top solder flux for electronics.
One thing that can prevent such a bond is an oxidized surface on the metal, and metals oxidize readily and constantly, unless they are coated to prevent this. Iron oxide is commonly known as rust, but all except the noble metals will oxidize. Tin, aluminum, copper, and silver oxidize easily, as will almost every metal used in electronics besides gold. They oxidize more quickly when they are heated, such as when they are being soldered.
Flux Prevents Oxidation
Flux reacts with oxides and removes them from the joint, and a coating of flux will prevent more oxide from forming, even at high temperatures. In effect, flux does for a solder joint what primer does when painting; it makes a very good metallurgical bond possible.
Three Common Types of Flux
There are several materials that work as flux, and they are chosen for different applications.
Inorganic Acid Flux gives the best strength to a solder joint, and is mostly used for structural purposes. It is made of stronger acids, such as hydrocloric acid or ammonium chloride, and it needs to be cleaned off thoroughly after use, because it remains caustic, and will erode and weaken the joint if left in place. This flux should never be used for electric or electronic work.
Organic Acid Flux is milder than inorganic acid flux, though it is still mildly corrosive, and it clears oxides away quickly to give a strong joint. It is made of milder acids, such as citric acid. It has the advantage of being soluble in water, so it can be cleaned off of PCBs, though care should be used to keep the water away from components that should not get wet. It must be removed from all electronic work, because it is conductive and will interfere with proper operation of the circuitry if it remains. Operating the circuit before the flux is removed may damage the circuit.
Rosin Flux is one of the earliest fluxes to be used. It is made from pine sap that has been refined for purity. Rosin flux is corrosive when liquid, but almost inert at room temperatures when it solidifies. Depending on the activity of the Rosin Flux, Rosin flux could be put in three categories, R (plain rosin), RMA (mildly active rosin), and RA (active rosin). If a PCB gets warm enough to liquefy the flux, corrosion will result, so it’s good policy to remove it after the soldering is finished. It can be removed with alcohol.
Smoke During Soldering
Soldering rarely gets above the boiling point of lead, so the smoke is not a lead hazard, though it is a mix of gasses from the chemical reaction of flux with oxides, and it is best to avoid breathing it.