The causes of welding cracks in SMT processing mainly include the following aspects:
1. The pad of the circuit board and the welding surface of the component are not wetted to meet the requirements of production and processing.
2. The solder paste did not meet the production standards as required.
3. The coefficient of thermal expansion of various component materials of welding and electric level is asymmetric, and spot welding is unstable during condensation.
4. The standard setting of the temperature curve of reflow soldering can't make the organic chemicals and water in the solder paste evaporate before entering the reflow area. 5. The difficulties of lead-free materials in SMT factories are high temperature, high tension at the interface and high viscosity. The increase of interfacial tension will definitely make it more difficult for the vapor to remain in the refrigeration process, and the vapor is not easy to be discharged, which will increase the proportion of cracks, because there will be many air holes and cracks in lead-free welding during chip processing.
6. In addition, the lead-free welding temperature will be much higher than that with lead, especially in some large-sized multilayer boards and electronic components with large thermal conductivity, the high-value temperature is generally about 260 degrees, and the temperature difference between refrigeration and condensation indoors will be relatively large, so the ground stress of lead-free welding is also relatively large.
