Deformation measurement on cross sections of Fan-Out Wafer Level Package by Digital Image Correlation (DIC)

Digital image correlation (DIC) is an established method for contactless, non-destructive full-field deformation measurements based on images acquired during an experiment. In this work, DIC technique is applied to non-embedded cross sections of fan-out WLP packages (Wafer Level Package, eWLB embedded Wafer Level Ball Grid Array) mounted on printed circuit boards (PCBs), in order to characterize the deformation and strain fields caused by a temperature load, due to mismatch in the coefficients of thermal expansion (CTE) of the different materials of the assembly. Deformation and strain fields of two PCB stacks are measured, which are known to result into different levels of solder joint lifetime. Finite element analysis (FEA) is used for obtaining a better understanding of the experimental findings. Both measured and simulated deformation and strain fields are compared and a virtual sensitivity study is used in order to identify the essential metric, which can be correlated with the later solder joint lifetime of the eWLB package on the respective PCB stack. A very good correlation of the identified metric from the DIC method to the solder joint lifetime is obtained. Consequently, applying DIC on cross-section is demonstrated as a fast and easy experimental method not only to validate the FEA simulation models but, in combination with FEA, also to assess the solder joint reliability of the future generations of products of eWLB packages. Therefore, the method has the potential to speed up the reliability assessment and the development of the eWLB package technology.