Study on fatigue strength characteristics for solder joints

Recently, preventing environmental pollutions, Pb-free solders are about to replace Sn-Pb eutectic solders. Sn-Ag-Cu alloys are leading candidates for Pb-free solders. However, Sn-Ag-Cu alloys were not enough to meet severe board level reliability requirements. In particular, thermal fatigue failure still has the problem. The interfacial strength decreases with intermetallic compounds (IMC) formation between solders and electric pads. Because IMC is brittle, the growth of IMC strongly affects the reliability of solder joints. However, for lead-free solder joints, the effects of IMC layer development on fatigue strength have not been studied well. In this study, in order to inhibit the IMC formation, 6 kinds of nano particles of metal (Cu, Ni, Co, Pt, In, Sb) were added to the solders based on Sn-Ag. We carried out board level fatigue tests and observed and conducted elemental analysis of cross-section surface of solders. And then we considered the relation of the fatigue life of several lead free solder joints and the change of the IMC layer by thermal aging. We aged the specimens in the isothermal chamber under 150degC for 0 to 20 days. As a result of the fatigue tests, about the solder including nickel, the fatigue life after aging is longer and fatigue strength reduction after aging is more lowered than the other solders. And the results of observation showed that IMC layers grow grained and the irregularity of interface between the IMC layers and the solder layers is smaller about the solder including nickel. So we consider that that improves resistance to crack advance, and then the solder showed the higher fatigue strength.