Observation of Fatigue and Creep Ratcheting Failure in Solder Joints under Pulsed Direct Current Electromigration Testing

Abstract

Recent electromigration (EM) testing of wafer-level chip scale packages (WCSPs) under various duty factors (DFs) or pulse "on"/"off" ratio, of low-frequency pulsed-direct current (pulsed-DC) conditions have uncovered the in-tandem failure mechanisms which occur during pulsed-DC EM testing: 1) classical EM failure by voiding; 2) thermal fatigue; and 3) creep ratcheting. Cross-sectional scanning electron microscope (SEM) failure analysis of samples tested under a high DF pulsed-DC testing condition revealed crack propagation in addition to EM voiding near the interface between the device under test (DUT) solder bump and the Cu under bump metallization (UBM) layer. The crack and void suggest that significant plastic deformation by, dislocation gliding and thus dislocation multiplication, and strain hardening occur with fluctuating temperature and stress during pulse "on" and "off" cycles. Under the right conditions, material which undergoes a significant amount of strain hardening becomes susceptible to fatigue failure even with a small amount of cyclic stress. The stress fluctuation combined with these microscopic mechanisms lead to thermal fatigue, which, combined with the classical EM failure mechanism, enhances the EM failure kinetics. The EM acceleration causes these samples to have shorter mean-time-to-failure (MTTF) than samples tested under DC. Meanwhile, samples tested under low DF pulsed-DC conditions showed failure features by creep ratcheting and had far longer MTTF than the predicted based on the classic cumulative damage model. Cross-sectional SEM failure analysis of samples tested under a low DF pulsed-DC condition uncovered features of creep ratcheting failure, evidenced by squeezed out or displaced solder material from the solder bump. It is hypothesized that until failure by creep ratcheting occurs, the relatively extended relaxation time allowed in low DF pulsed-DC conditions allow dynamic recrystallization, which releases the driving force for the fatigue by nucleating deformation free grains, to occur. The predicted reason why this mechanism is observed in low DF pulsed-DC conditions instead of fatigue, as compared to high DF pulsed-DC conditions, is that dynamic recrystallization mechanism requires more time to dominate than the fatigue crack propagation mechanism. The two mechanisms are essentially believed to compete with one another, and the primary condition which allows one failure mechanism to dominate over the other is the DF, which dictates the "on" vs. "off" time. The finite element method (FEM) of the stress associated with thermal fatigue and creep mechanisms is implemented to investigate these three phenomena. Findings of this research are discussed and presented in this study.