Several main functions of electric soldering iron flux
1. Chemical Activity
To achieve a good solder joint, the object to be soldered must have a completely oxide free surface. However, once the metal is exposed to air, it generates an oxide layer, which cannot be cleaned with traditional solvents. In this case, it must rely on the chemical interaction between the flux and the oxide layer. After the flux removes the oxide layer, a clean surface of the object to be soldered can be combined with the solder.
There are several chemical projections of flux and oxide:
1. The formation of a third substance through chemical interactions;
2. The oxide is directly peeled off by the flux;
3. The above two reactions coexist.
Rosin flux removes the oxide layer, which is the first reaction. The main components of rosin are AbieticAcid and Isomeric diterpene acids. When the flux is heated, it reacts with copper oxide to form Copperabiet, which is a green transparent substance that easily dissolves into unreacted rosin and is removed together with rosin. Even if there is residue, it will not corrode the metal surface.
The reaction of oxides exposed to hydrogen gas is a typical second reaction, where hydrogen reacts with oxygen at high temperatures to form water, reducing oxides. This method is commonly used in the welding of semiconductor parts.
Almost all organic or inorganic acids have the ability to remove oxides, but most cannot be used for soldering. In addition to the function of removing oxides, flux has other functions that must be considered during soldering operations.
2. Thermal Stability
When the flux removes the oxide reaction, it must also form a protective film to prevent the surface of the solder from being oxidized again until it comes into contact with the solder. So the flux must be able to withstand high temperatures and will not decompose or evaporate at the temperature of the soldering operation. If it decomposes, it will form solvent insoluble substances that are difficult to clean with solvents. Pure rosin of W/W grade will decompose at around 280 ℃, which should be paid special attention to.
3. Activity of flux at different temperatures
Good flux not only requires thermal stability, but also its activity at different temperatures. The function of flux is to remove oxides, which usually works best at a certain temperature, such as RA flux. Unless the temperature reaches a certain level, chloride ions will not resolve to clean the oxides. Of course, this temperature must be within the temperature range of the soldering operation.
When the temperature is too high, its activity may also be reduced. For example, when rosin exceeds 600 ℉ (315 ℃), there is almost no reaction. This characteristic can also be used to purify the flux activity to prevent corrosion. However, special attention should be paid to the heating time and temperature in application to ensure activity purification.
