Introduction to the main functions of electric soldering iron flux
1. Chemical Activity
To achieve good solder joints, the object to be welded 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 and must rely on the chemical interaction between the flux and the oxide layer.
After the oxide layer is removed by the flux, a clean surface of the soldered object can be soldered together.
Several chemical reactions between 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.
The rosin flux removes the oxide layer, which is the first reaction. The main components of rosin are abietic acid and isomeric diterpene acids. When the flux is heated, it reacts with Copper(II) oxide to form copper rosin, which is a green transparent substance, easy to dissolve into unreacted rosin and be 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 acids or Mineral acid can remove oxides, but most of them cannot be used for soldering.
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.
Therefore, the soldering 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 requires special attention.
3. Activity of flux at different temperatures
Good flux not only has good thermal stability, but also needs to maintain good activity at different temperatures.
The function of flux is to remove oxides, which generally 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 be reduced. For example, when rosin exceeds 600 ℉ (315 ℃), there is almost no reaction. Using this characteristic, the flux activity is purified to prevent corrosion, but attention should be paid to heating time and temperature in application to ensure activity purification.
