Study on fatigue ductility coefficient and life prediction for mixed solder joints under thermal cycle loads

Lead-free solder ball of PBGA with SnPb solder paste in the weight ratio of 1:0.5 was mounted for mixed soldering experimental sample. Temperature cycling experiment was carried out at -55 °C ~ +125 °C conditions. The event detector was employed to monitor the solder joint failure in realtime in terms of IPC-9701 standard, and the Weibull curve of mixed solder joint failure was established. Thermal fatigue ductility coefficient of mixed solder joints was deduced based on the failure data of solder joints and IPC-SM-785 standard. Moreover, the microstructure and the failure mode of mixed solder joints were analyzed during different thermal cycles using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results showed that the thermal fatigue ductility coefficient of mixed solder joints was 0.8306, which is greater than 0.325 of Sn63Pb37 solder joints because of the larger elastic modulus of mixed solder increasing its brittleness. The mixed solder joints cracking in temperature cycling test begin from the lead-rich region in corners. Crack interface presents grain boundary slip characteristics, which suggest the grain boundary dislocations caused by shear stress is still the main reason of solder joints cracking. The growth of IMC layer in mixed solder joints is not sensitive to temperature cycling condition, and there are lead-rich phase, Cu₆Sn₅ alloys and Ag₃Sn alloys coexistence inside the mixed solder joints after thermal cycles. Further research for long-term reliability regular data is still needed.