The word FLUX comes from the Latin word "Flow in Soldering". The main functions of flux are:
1. Chemical activity
To achieve a good solder joint, the object to be welded must have a completely oxide-free surface, but once the metal is exposed to the air, an oxide layer will be formed. This oxide layer cannot be cleaned with traditional solvents. At this time, flux must be relied on It plays a chemical role with the oxide layer. After the flux removes the oxide layer, the clean surface of the solder can be combined with the solder.
There are several chemical reactions between flux and oxide:
a. The third substance is formed by mutual chemical interaction;
b. The oxide is stripped directly by the flux;
c. The above two reactions coexist.
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 oxide to form Copper abiet, which is green and transparent. The substance is easily dissolved in the unreacted rosin and removed together with the rosin. Even if there are residues, it will not corrode the metal surface.
The reaction of oxides exposed to hydrogen is a typical second reaction. At high temperatures, hydrogen reacts with oxygen to form water to reduce oxides. This method is commonly used in the welding of semiconductor parts.
Almost all organic acids or inorganic acids have the ability to remove oxides, but most of them cannot be used for soldering operations. In addition to removing oxides, fluxes have other functions. These functions must be considered when soldering operations. of.
2. Thermal stability
When the flux removes oxides, it must also form a protective film to prevent the surface of the solder from being oxidized again until it contacts the solder. Therefore, the flux must be able to withstand high temperatures, and will not decompose or evaporate under the temperature of the soldering operation. If it decomposes, it will form solvent-insoluble substances, which are difficult to clean with solvents. W/W grade pure rosin will decompose at about 280°C. pay attention.
3. The activity of flux at different temperatures
A good flux not only requires thermal stability, but also its activity at different temperatures. The function of flux is to remove oxides, and it is usually better at a certain temperature. For example, for RA flux, unless the temperature reaches a certain level, chloride ions will not be resolved to clean up the oxides. Of course, this temperature must be done during soldering. Within the temperature range.
When the temperature is too high, its activity may be reduced. For example, when the rosin exceeds 600°F (315°C), there is almost no reaction. This feature can also be used to purify the flux activity to prevent corrosion. However, in application Pay special attention to the heating time and temperature to ensure pure activity.