How to select the flux required for welding BGA devices

Now the welding process can be divided into lead welding process and lead-free welding process, although lead-free alloy has been widely used, but compared with the lead alloy Sn63/Pb37 eutectic filler metal there are still high melting point, poor wettability, expensive, reliability to be verified and other problems. At present our country is mostly in the lead and no lead mixed period, so it needs reasonable choice of flux.

Flux is a mixture required by reflow welding process. It is the medium for welding between PCB board and components. It is a paste with certain viscosity and good thixotropic name that is mixed by alloy filler metal, powder paste flux and some additives. At room temperature, the flux can initially stick the electronic components to the fixed position. When it is heated to a certain temperature, with the volatilization of solvent and some additives, the alloy powder melts, so that the components to be welded and the welding pad are connected together, and the welding spot is cooled to form a connection.

There are a lot of alloy powder particles in the composition of flux, these metal powder particles are easy to oxidize, poor wetting, resulting in welding welding, therefore, the use and management of solder should be strictly in accordance with the measures of solder management. Usually, the flux should be refrigerated in the refrigerator at 0-10 degrees Celsius to prevent the deterioration and volatilization of the flux resistance chemical reaction. Before use, take out the return temperature 4h-24h to restore to the normal temperature state. Because the flux is composed of the flux resistance and alloy powder, the density of the flux resistance and alloy powder is not the same in the refrigeration and return temperature process, easy to stratify, so, The use period should be 3-10min of uniform stirring.

For different welding, flux selection needs to be based on the assembly process, PCB, components of the specific situation of the selection of alloy components. For example, generally, 63Sn/37Pb is used for lead-plated tin PCB, and 62Sn/36Pb/2Ag is used for components with poor weldability and PCB with high solder joint quality. In addition, when selecting the flux, we should consider the chemical activity of the ingredients in the flux and the PCB solder pad, such as the flux of PSI, so as to avoid some chemical reactions that are not conducive to welding in the reflow welding process, thus reducing the reliability of the solder joint.

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