Challenges in Predicting the Solder Interconnect Lifetime of High Power Electronics

To predict the degradation of the lead-free solder interconnect under critical applications is very challenging. Physics of failure-based approach, which is widely used in predicting the lifecycles of solder interconnect in the field service, involves finite element modelling and certain accelerated tests leading to a coffin-Manson type of fatigue model. However, this prediction highly depends on the availability of well-developed constitutive models for describing the solder creep behavior under various temperature and stress conditions. Since the variety in lead free solder alloys and the inconsistency in making lead free test samples in characterizing such a model, a relatively large variety does exist even for the same type of lead free solders. In addition, the microstructure of the solder will change due to recrystallization during aging which will affect the crack propagation. We see the doping elements in SAC+ solder indeed have effect on the recrystallization and on the crack propagation speed which makes the lifecycle prediction even more challenging.In this study, finite element modelling was done with typical SMD components using various constitutive models. The results are correlated with the accelerating test results. The effect of the difference of various constitutive models is demonstrated in the predicted lifecycles. In addition, crack propagation speed of the two solder alloys (SAC305 and SAC+ which is SAC based solder plus certain doping elements) is monitored in the accelerating tests and the microstructure of the test samples after different life cycles is analyzed. The dedicated test results showed that the SAC305 cracks propagate faster than SAC+ in the early test cycles. However, in the later phase of the test, the SAC+ cracks speed up due to more severe recrystallization along the zone of the crack tip, which decreases the strength of the grains so that the cracks can propagate more easily. This type of shifting microstructure gives additional challenge in predicting the lifetime.