Slow Cycle Fatigue Creep Performance of Pb-Free (LF) Solders

Electronics industry is successfully transitioning into lead-free (LF) solders to comply with government regulations. There are many challenges associated with reliability life prediction for LF solders such as the selection of temperature cycle test methods, material compatibility issues, and reliability models. This paper focuses on investigating the effect of extended dwell time on LF reliability under thermal cycling for both flip chip BGA packages and sockets. Extensive experimental temperature cycle results for different pitch FC BGAs, and sockets have shown that LF (Sn-4.0Ag-0.5Cu) solder has a better fatigue performance than Sn/Pb over a wide range of extended dwell times. Long-dwell time temperature cycle results revealed that LF will not cross over Sn/Pb performance in the extended dwell time testing up to 8-hour cycle time for the temperature range between -25 and +100deg C. It is also noted that the margin in fatigue life between LF and Sn/Pb becomes smaller with the increased dwell time. This is consistent with the creep properties of LF solder. Finite element modeling results corroborated the experimental data. Detailed failure analysis for both socket and flip chip BGA units demonstrated that LF has much less crack propagation rate than Sn/Pb even with very long dwell time showing better fatigue performance compared to Sn/Pb.