Reliability Assessment of Fan-Out Packages Using High Resolution Moiré Interferometry and Synchrotron X-Ray Microdiffraction

Measurement and simulation of the stress state in the silicon die of a wafer-level fan out package can be difficult due to the complexity of high density packaging structures and the uncertainties in material properties at the nanoscale, yet these stresses directly affect the electrical behavior of the embedded active devices due to the piezoresistive effect in silicon. In this study, the residual stresses and thermal stresses in die-on-board, fan-out packages are measured using high resolution moiré interferometry and synchrotron x-ray microdiffraction. The results are then used to verify the materials database used in the finite element analysis (FEA) with the purpose of developing a quantitative method to characterize the stress behavior and its effect on charge carrier mobilities. The underfill effect is investigated by comparing two underfills, one with a moderate glass transition temperature (Tg) and one with a low Tg, to a control with no underfill. High resolution moiré interferometry is used to map the actual deformation of the package cross-section subjected to a negative thermal load, while synchrotron x-ray microdiffraction produces Laue peaks, which are analyzed to extract stress, strain, and lattice deformation data at room temperature. These experiments, in which the separate underfill conditions demonstrated distinct deformation patterns, are used to validate the material and load parameters of the FEA model, which in turn is used to calculate the mobility change.