Comparison of thermal fatigue reliability of SnPb and SAC solders under various stress range conditions

Most accelerated temperature cycling (ATC) tests reported the observations that SnAgCu (SAC) solders have better thermal fatigue reliability than SnPb solders due to their lower creep strain rate. But material studies revealed that this is true only when stress level is below a certain level. When the stress level in the solder increases, eventually the creep strain rate of SAC solder may outgrow that of the SnPb solder. Therefore, lead-free soldering under thermal cyclic loading may lead to more reliability concern than people expected. Further investigations on ATC tests with various stress ranges are still in demand. In the present study, by making use of distance-from-the-neutral-point (DNP) effect, a simple but effective custom-designed dummy package with various solder joint spacing is designed and manufactured to evaluate the thermal fatigue reliability of SnPb and SAC solders under different stress levels. It was found that SnPb solder could outperform SAC solder under a higher stress level condition. For the lower stress level condition, although the ATC test is still in progress, based on the present preliminary result, it is reasonable to predict that SAC solder would be better than SnPb solder in this case. The finite element analysis using the same package configuration was also conducted. The computational creep strains in the solders agreed with the ATC test results. Further discussion on the thermal fatigue life prediction is also presented in this paper.