(1) Fatigue problem of thermal cycle stress caused by different thermal expansion coefficient of materials in welding site.
(2) Thermal migration, that is, metal atoms move against the temperature gradient. Causes a certain degree of component segregation of the solder.
(3) Electromigration, cavity may be formed at the cathode and hillock may be accumulated at the anode.
(4) At room temperature, the intermetallic compounds generated by welding reaction continue to grow, and even deplete the reactants of the metallized layer. Intermetallic compounds then peel off into the solder, forming counter-wetting.