There are many types and specifications of solder paste. Even if it is the same manufacturer, there are differences in alloy composition, particle size, viscosity, cleaning methods, etc., and the price difference is also very large. How to choose the right solder paste has a great impact on product quality and cost. Generally, the selection should be based on the specific production environment, referring to the activity, viscosity, powder shape, particle size, and melting point of the solder paste.
(1) First determine the alloy composition. Alloy is the material that forms the solder joint. It forms an alloy layer with the metal to be welded. At the same time, the alloy composition also determines the welding temperature, so the alloy composition should be determined first. The alloy composition is mainly selected according to the electronic product and process, and the alloy composition that is compatible with the solder end of the component should be selected as much as possible, and process factors such as soldering temperature should also be considered. Generally, tin-plated lead printed circuit boards use Sn63Pb37; clamp gold and button silver thick film ends, components with poor pin solderability, and printed circuit boards that require high solder joint quality use Sn62Pb38.
SMT processing plant on SMT-QAS design
(2) Select the flux in the solder paste. The printability and solderability of solder paste mainly depend on the flux in the solder paste. Therefore, after determining the alloy composition in the solder paste, the flux suitable for the production process should be selected. When selecting, choose its activity according to the storage time of PCB and components and the degree of surface oxidation: general products use RMA type; high-reliability products choose R type; PCB and components have a long storage time, and when the surface is severely oxidized, use RA type, and It should be cleaned after welding.
(3) Determine the ratio of alloy composition and flux in the solder paste. The ratio of alloy composition and flux directly affects the viscosity and printability of the solder paste.
(4) Select the viscosity of the solder paste according to the process of applying the solder paste and the assembly density. There are many ways to apply solder paste, and different application methods have different requirements for the viscosity of the solder paste, see Table 2-17. At the same time, when choosing solder paste, you should choose more solder pastes from several companies for experiments to compare and evaluate printability, mold release, thixotropy, viscosity, wettability, solder joint defects, residues, etc., and then Choose the right solder paste.
