Direct Concentration Approach of Moisture Diffusion and Whole-Field Vapor Pressure Modeling for Reflow Process—Part II: Application to 3D Ultrathin Stacked-Die Chip Scale Packages

In the present study, the direct concentration approach (DCA) and the whole-field vapor pressure model developed in Part I of this work (Xie et al., 2009 “Direct Concentration Approach of Moisture Diffusion and Whole Field Vapor Pressure Modeling for Reflow Process: Part I–Theory and Numerical Implementation,” ASME J. Electron. Packag., 131, p. 031010) is applied to 3D ultrathin stacked-die chip scale packages to investigate wafer-level die-attach film cohesive failures during the reflow process. Oversaturation, which refers to the film that absorbs more moisture when reflow process begins, is observed using the DCA. The modeling results suggest that the moisture transport and escape through the substrate during the reflow is responsible for the film rupture. A small reduction in substrate thickness could result in a significant decrease in moisture concentration and vapor pressure in bottom layer film and therefore reduce failure rate greatly. A slight improvement in reflow profile while still meeting specification allows a significant amount of moisture loss during the reflow; hence failure rate could also be reduced greatly. The mechanism of soft film rupture at reflow due to moisture is discussed in detail. The simulation results are consistent with the published experimental data.