Sensitivity Investigation of Substrate Thickness and Reflow Profile on Wafer Level Film Failures in 3D Chip Scale Packages by Finite Element Modeling

3D chip scale package (CSP) is one of the major trends in IC packaging with the application of wafer level films (WLF) for die-to-die or die-to-substrate attachment. However, the WLF failures (voiding/cracking) are often observed in moisture sensitivity test. Substrate thickness and reflow profile were found very sensitive to the WLF failure rate. To investigate the sensitivity of the substrate thickness and the reflow profile, a novel direct concentration approach (DCA) is developed in this study, which allows updating the interfacial continuous condition as the function of the temperature and humidity. The paper also develops a simplified micromechanics-based vapor pressure model to visualize the whole-field vapor pressure corresponding to the instantaneous moisture distribution. With the applications of the DCA and the simplified vapor pressure model to a 3D ultra-thin stacked-die CSP, it was found that the moisture transport and escape during the reflow is the root cause of the WLF failures. A small reduction of the substrate thickness and an in-situ baking during the reflow can reduce greatly the moisture concentration and the vapor pressure at the bottom film, and therefore significantly decrease the WLF failure rate.