Investigation of Thermal Cycling Maximum Temperature Effect on Fatigue Life of WLCSP

The accelerated thermal cycling test is a standard method which is currently used to characterize the reliability performance of electronic packaging. The test vehicles are placed in a condition which is harsher than the actual usage condition in order to reduce the testing and development time. Recently, in order to further reduce the testing time, the ramping rate of thermal cycling test is increased, and the acceleration factor empirical model could be used to predict the fatigue life under different loading conditions. The original acceleration factor model was proposed by Norris and Landzberg which includes temperature range, temperature-cycling frequency and the maximum temperature. In this research, the term maximum temperature effect will be studied, discussed and compared with simulation results. Because the temperature under accelerated thermal cycling test always exceeds one third of the melting point ( in Kelvin) of the solders, the creep effect becomes obvious and needs to be considered. To reveal this mechanics behavior, a simulation model of wafer level chip scaling package (WLCSP) is constructed in this research, which is with 14mm x 14mm die size, 0.4mm ball pitch, and 0.32mm ball size in 2D model and the Garofalo Hyperbolic Sine Model is used to simulate the creep behavior of the lead-free SnAg solder alloy. To investigate the maximum temperature effect of reliability of WLCSP on thermal cycling test, two life prediction models are used. One is Coffin-Manson strain based model and another one is Darveaux energy based model. The simulation results show that the increment of creep strain energy density has changed insignificantly, so the fatigue life does not coincide with the experimental results when Darveaux model is applied. However, the increment of creep strain increases with the increase of maximum temperature which is same as the experimental result.