The selection criteria of leaded
solder paste mainly start from the following factors:
1. Alloy composition: In general, Sn63/Pb37 solder alloy composition can meet most welding requirements; for welding and QFN process of Ag or Pd coated devices, generally choose alloy composition Parts are Sn63/Pb35/Ag2 solder paste; for pcb soldering of devices that are not resistant to thermal shock, choose Bi-containing solder powder.
2. The viscosity of the solder paste: In the SMT workflow, there is a process of moving, placing or handling the PCB from the time the solder paste is printed (or dotted) and the components are attached to the reflow soldering heating process; In this process, in order to ensure that the printed (or spot) solder paste is not deformed and the components attached to the PCB solder paste are not displaced, it is required that the solder paste should have good viscosity before the PCB enters the reflow soldering heating and holding time; this requires the solder paste manufacturer to adjust the viscosity of the solder paste according to the production workshop of each factory.
3. The particle size of solder paste: Generally, the solder paste particles used by manufacturers are 25-45 μm, and the products with high requirements will choose solder paste particles with 20-38 μm. Most manufacturers now use these two types of solder paste.