Empirical Solutions and Reliability Assessment of Thermal Induced Creep Failure for Wafer Level Packaging

The accelerated thermal cycling test (ATCT) is a method of testing the electronic packaging reliability characteristic. A component must pass this test before being launched into the market. According to many studies, under thermal loading, the excessive thermal strain and stress occur between the package and substrate because of the coefficient of thermal expansion mismatch, which damages the solder bump of electrical packaging. In the ATCT, -40° C to 125° C is the regular thermal range of packages tested. Furthermore, the homologous temperature exceeds one-third of the melting point (K) of solder, and one should consider the accumulation of creep strain and estimate the creep behavior during the loading procedure; however, the creep strain is very sensitive to mesh size in finite element simulation. Obtaining a stable and reliable creep stain for different packaging structures becomes a must for solder joint reliability assessment. In this paper, we study a feasible and fixed element size, which should be chosen carefully for the same type of electronic packaging structure, for example, wafer level packaging (WLP), as it can affect simulation results and cause the prediction of the packaging life cycle to deviate. In ATCT simulation, there is a big difference in reliability estimation for different element mesh size selection, even if all material properties and structures remain the same. It is essential to find an appropriate and fixed element size for packaging reliability prediction to yield precise and reliable simulation results. This paper aims to find an appropriate element mesh size that can consistently predict the reliability life of various WLPs in an accurate range, and the reliability results of different WLPs under the thermal cycling loading have been validated by experiments. Moreover, the simulation results are summarized by using the Anand and hyperbolic sine creep models with suitable empirical reliability assessment equations, and the results show both creep models can predict the reliability life of WLPs in an accurate range with appropriate mesh control.