Powercycling Reliability, Failure Analysis and Acceleration Factors of Pb-Free Solder Joints

At the present time, life acceleration factors for solder joints made of SnAgCu (SAC) ternary alloys between accelerated tests and field use conditions are not well understood. Further, the cycle time for accelerated testing of Pb-free solders is currently an industry concern since the rate of damage accumulation due to thermomechanical loading in Pb-free solders is different from the eutectic Sn-Pb solders. In this study, we develop an understanding of the acceleration factors through a combination of temperature cycling tests, powercycling tests and validated damage modeling. We begin by describing a recently developed simulated power cycling (SPC) tester that is a simplified powercycling test procedure. The tester uses an external thermoelectric device to heat the package surface. Through localized heating, the thermoelectric device enables rapid cycling of packages unlike in thermal chambers. We report temperature and powercycling reliability data on Pb-free, Sn₄Ag_0.5Cu₃₆ I/O wafer level-chip scale packages (WL-CSP). We compare the results of powercycling against -40deg to 125deg C temperature cycling test results. In both, fatigue failure by near interfacial crack growth was discovered to progress from the outside edge of the joints towards the center of the package. Creep and shear dominated microscopic failure modes were found on the solder joint fracture surfaces. Further, we observe in both cases that there are no diagonal cracks, secondary cracks are scarce and the effect of voids on crack growth is insignificant compared to the location of the joint on the package, which determines the stress on the joint. The fracture morphology due to fatigue crack growth was nearly identical close to and far from the crack initiation site, revealing similar damage mechanisms during all stages of life. Finally, we propose a procedure for extracting acceleration factors based on a recently developed (and experimentally validated for Sn-Pb- - solder joints) damage accumulation theory inspired by the cohesive zone models of modern fracture mechanics. The proposed model is non-empirical unlike the Coffin-Manson rule and simulates the actual progression of crack through the joint