Failure mode analysis of lead-free solder joints under high speed impact testing

Using an Instron micro-impact system, this study investigates the failure characteristics of 96.5Sn-3Ag-0.5Cu lead-free solder joints aged at either room temperature or 125 °C, respectively, and then impacted at shear rates of up to 1 m/s. Four types of failure mode are identified, namely M1: interfacial fracture with no residual solder left on the pad; M2: interfacial fracture with residual solder left on the pad; M3: solder ball fracture; and M4: substrate fracture. The experimental results reveal that the solder specimens fail in different failure modes at the same impact speed. The transition from ductile to brittle failure occurs at an impact speed of around 0.5 m/s. At an impact speed of 0.7 (±0.05) m/s or more, over 70% of the specimens fail in the M1 or M2 modes under all of the testing conditions. The isothermal aging process is found to reduce the interfacial strength, and hence the percentage of M3 and M4 mode failures reduces significantly. Overall, the experimental results suggest that the failure mode distribution obtained in high speed impact tests performed at 0.5 m/s provides a feasible component-level quality assurance index.