Relationship between reliability and effect of solid solution hardening at solder joints at the high temperature

The Sn-3Ag-0.5Cu alloy has been already put to practical use in the world. However the crack easily propagate in the solder fillet at the big chip resister (6.4 × 3.2mm) after thermal fatigue of 2000 cycles between −55 and +125 degree centigrade at 30 minutes holding. It is the reason why the fine-needled Ag3Sn intermetallic compounds become coarse under the high stress and high temperature. The coarse Ag3Sn cannot play the role of hardening Sn matrix. Therefore some elements of solid solution in Sn matrix such as Sb, Bi and In are needed in order to restrict to propagate cracks in the solder joints after Ag3Sn became coarse. In this research, the relationship between the reliability and the effect of solid solution hardening at Sn-Ag-Cu-Sb and Sn-Ag-Cu-Bi alloys is investigated. Sn-3.9Ag-0.6Cu-3Sb and Sn-3Ag-0.8Cu-3Bi-Ni alloys have about twice life time as Sn-3Ag-0.5Cu alloys at the solder joints with big chip resister on FR4 substrate with six Cu layers at thermal fatigue test between −55 and 125C. Furthermore Sn-3.9Ag-0.6Cu-3Sb and Sn-3Ag-0.8Cu-3Bi-Ni alloys have longer life time than Sn-3Ag-0.5Cu alloys at the solder joints with LED on Al substrate at thermal fatigue tests between −55 and 125C, 150C.