Application of Phase Growth Approach to Prediction of Thermal Fatigue Crack Initiation Lifetime in Sn-3.0Ag-0.5Cu Solder Joints

Thermal fatigue damage of solder joints is believed to be closely related to microstructure evolution, that is phase growth. In the previous researches, the authors have obtained the following results. The phase growth is characterized by phase growth parameter S, which is defined as average phase size to the 4th power, and proceeds such that S increases proportional to the number of thermal cycles. And a simple power low relation was found between average number of cycles to thermal fatigue crack initiation lifetime and average increase in the phase growth parameter per cycle ΔS. In this paper, in order to apply the relation to the evaluation of the crack initiation lifetime in actual solder joints, thermal cycle tests were carried out by using fabricated printed circuit board (PCB) on which chip resistors were mounted by Sn-3.0Ag-0.5Cu solder. Periodically, the specimen was taken out from thermal shock chamber and the phase growth of the β-Sn phase in the solder joints was observed by scanning electron microscope (SEM). The following results were obtained. The microstructure observation showed that the lifetime prediction based on the phase growth parameter measured by the data in low cycles of 20 to 100, was within the range of 90% confidence limits of the experimentally determined lifetime. Therefore, the estimation method by the phase growth parameter can be applied to the lifetime prediction of solder joint with sufficient accuracy.