Phase Growth Process of Sn-3.0Ag-0.5Cu Solder Joints Under Mechanical Cyclic Loading at the Same Temperature

In order to improve the reliability of electronic devices, it is important to evaluate the thermal fatigue crack initiation lifetime of Sn-3.0Ag-0.5Cu solder joints. The phase growth parameter has been shown to be useful in estimating the thermal fatigue crack initiation lifetime of Sn-3.0Ag-0.5Cu solder joints through thermal cyclic tests. The phase growth parameter is affected by both temperature and the strain induced by the mismatch between the thermal expansion of a chip and substrate. A study of the influence of this strain on the phase growth process is important in estimating lifetime. In a previous paper, mechanical cyclic loading tests using a mini-lap joint type shear specimen at a temperature of 125°C were performed in order to evaluate the influence of the strain in Sn-3.0Ag-0.5Cu solder joints on the phase growth parameter. Moreover, an elastic-plastic-creep analysis of the solder joints under mechanical cyclic loading was carried out using a finite element method. Consequently, it was confirmed that the phase growth parameter of Sn-3.0Ag-0.5Cu solder joints was in good agreement with the creep strain. While the strain distribution in the solder joint of a mini-lap joint type specimen is nearly uniform, strain concentration exists in the actual solder joint. Therefore, in this paper, in order to clarify the phase growth process in the solder joint, which exhibits strain concentration at the joint end, a detailed finite element analysis of a mini-lap joint type specimen was performed, and the strain in the neighborhood of the joint corner was examined. The corrected increment for the phase growth parameter at the strain concentration occurrence zone is in good agreement with the master curve obtained by the thermal cycle tests of the PCB specimen. It is clear that the crack initiation life time can be evaluated using the increment of the phase growth parameter at the strain concentration occurrence zone, as same as for a PCB specimen. The phase growth parameter is considered to be the evaluation parameter corresponding to creep strain.