Influence of Cooling Rates on Reliability of Solder Joints Using Sn-13wt.% Sb Binary Alloy for Power Semiconductor Modules

Power semiconductor devices for electric power conversion are operated at high temperatures and must exhibit high reliability. Therefore, high heat resistance and long fatigue lifetimes are necessary for the solder joints in these devices. In this paper, we discuss how the thermal cycling lifetimes of solder joints using Sn-13wt.% Sb binary alloys with supersaturated antimony are affected by the cooling rates. These alloys exhibit good high temperature properties and mechanical properties. We conducted tensile tests and low-cycle fatigue tests to investigate the influence of the cooling rates on the reliability of solder joints using Sn-13wt.% Sb alloy. Our study clarified that the tensile strength of the Sn-13wt.% Sb alloy deteriorates and the low-cycle fatigue lifetime increases as the cooling rate becomes slow. Furthermore, the size of the SbSn intermetallic compounds (SbSn IMCs) precipitated in the β-Sn matrix tends to become coarse with the slowing cooling rate. From these results, we infer that the mechanism of extending the fatigue lifetime by slowing the cooling rate is that coarsened hard SbSn IMCs act as obstructions to solder crack propagation and the path of the crack propagation is lengthened. It became clear that the lifetime of Sn-13wt.% Sb binary alloy can be prolonged by controlling the cooling rate. We conclude that the strengthening mechanism of Sn-13wt.% Sb binary alloys is described by the strengthening model of a particle-dispersed composite material.