At present, the soldering process can be divided into lead soldering process and lead-free soldering process. Although lead-free alloy has been widely used, compared with Sn63/Pb37 eutectic solder with lead alloy, there are still some problems, such as high melting point, poor wettability, high price and reliability to be verified. At present, most of our country is in the period of mixed use of lead and lead-free, which requires more reasonable selection of flux.Soldering flux is a kind of mixture required by reflow soldering process. It is a medium for soldering between PCB and components. It is a paste with certain viscosity and good thixotropy, which is made of alloy solder, powder paste flux and some additives. At normal temperature, the flux can initially stick the electronic components at a given position, and when the electronic components are heated to a certain temperature, with the volatilization of the solvent and part of the additive and the melting of the alloy powder, the soldered components and the bonding pad are connected together and cooled to form a connected solder joint.There are many alloy powder particles in the composition of the flux. These metal powder particles are easily oxidized and poorly wetted, resulting in poor soldering. Therefore, the use and management of soldering tin should be carried out in strict accordance with the measures of soldering tin management.Generally, the flux should be refrigerated in a refrigerator at 0-10 ℃ to prevent the solder resist from chemical reaction deterioration and volatilization. Before use, the flux should be taken out and returned to normal temperature for 4-24h. Because the flux is composed of solder resist and alloy powder, the density of solder resist and alloy powder is different in the process of refrigeration and temperature return, and it is easy to delaminate. During the use period, the mixture shall be stirred evenly for 3-10 min.For different soldering, the selection of flux should be based on the assembly process, PCB, and the specific conditions of components to select the alloy composition. For example, 63Sn/37Pb is generally used for tin-lead plated PCBs, and 62Sn/36Pb/2Ag is used for components with poor solderability and PCBs requiring high solder joint quality. In addition, when selecting the flux, the chemical activity of the components in the flux and the PCB pad components, such as PSI flux, should be considered to avoid some chemical reactions that are not conducive to soldering in the reflow soldering process, thus reducing the reliability of the solder joint.