Characterization of Crack Fronts in a WLCSP Package: Experiments and Models for Application of a Multiscale Fracture Theory

Extensive failure analysis was performed on identical, isothermally cycled wafer-level chip scale packages with Sn3.8wt%0.7wt%Ag (SAC387) solder joints. Packages were periodically removed during the cycling process to observe crack front progression. The packages were dipped in liquid nitrogen to reduce solder ductility and then pried apart. The crack areas were observed under an optical microscope. The average crack areas on the corner and mid-edge joints of the package were measured using an image processing software. 50 packages were characterized from 240 to 7200 cycles at every 240 cycles. An average of 20 solder joints per cycle was observed to estimate the crack length and area. A finite element model of this package was constructed in ABAQUS. The solder interconnects of the model were given full plastic and creep properties. Using the failure analysis data and the finite element model, the material constant value in a previously developed hierarchal fracture model was calibrated for SAC387. The ability of the model to predict non-intuitive failure initiation sites due to the under bump metallurgy (UBM) geometry is demonstrated. The hierarchal fracture process model was inspired by information theory and continuum thermodynamics. It was earlier proposed to capture the length-scale and temporal hierarchy inherent in quasi-static fracture processes. This single parameter model allows for a non-empirical, geometry independent approach to predicting crack growth by relating equivalent inelastic dissipations and a material constant to the probability of fracture at a material point.