Influence of temperature and strain on phase growth process in Sn-3.0Ag-0.5Cu solder joints

This paper describes the phase growth of Sn-3.0Ag- 0.5Cu solder joints by mechanical cyclic loading. The objective of this paper is to clear the influence of temperature and strain to the phase growth. It is very important to evaluate the life time of crack initiation in Sn-3.0Ag-0.5Cu solder joints under thermal cycles. A mini-lap joint type shear specimen was used to evaluate the influence of temperature and strain on the phase growth of solder joints. The mechanical cyclic loading was carried out under a constant temperature condition. The phase growth of beta-Sn in the solder joint were observed by the scanning electron microscope (SEM). Moreover, a finite element analysis of Sn-3.0Ag-0.5Cu solder joints under mechanical cyclic loading was performed. The elastic-plastic- creep properties of Sn-3.0Ag-0.5Cu solder was considered in the analysis. The phase size of beta-Sn increases until some period but becomes constant after the period. The total phase size within the influence of both temperature and strain increase as number of loading cycles increasing. It is difficult to separate the phase size caused by strain from the total phase size, but it is found that the increment of the total phase size in solder joints is useful parameter to consider the phase growth process. In fatigue tests, the increment of the phase growth is affected by the loaded displacement amplitude and rate. The increment becomes larger as the displacement amplitude and rate become larger. In finite element analyses, the inelastic strain amplitude per one cycle, which occurred by displacement loading, depends on loaded displacement amplitude but doesn´t depend on displacement rate. The ratio of the plastic strain to the creep strain is changed by loaded displacement rate. Comparing the test results and the analysis results, the phase growth parameter of Sn-3.0Ag-0.5Cu solder joints is good agreement with the creep strain. Therefore the phase growth parameter is the evaluation parameter cor- responding to a creep strain.