Effect of the Mass Transfer Biot Number on Moisture Desorption and Hygrothermal Stress in QFN Packages

Package-level hygrothermal simulations commonly represent drying at epoxy molding compound (EMC) surfaces using idealized boundary conditions, which may not fully capture the coupled effects of bulk moisture diffusion and surface evaporation during reflow soldering and thermal cycling. This study developed a diffusion- and evaporation-based hygrothermal mechanical model for quad-flat no-lead (QFN) packages by incorporating an evaporation boundary formulation with moisture transport parameters obtained from independent moisture absorption and desorption experiments. The mass transfer Biot number S was introduced to quantify the relative roles of bulk moisture diffusion and surface evaporation in package desorption. Comparative simulations demonstrated that S influenced surface moisture removal kinetics and moisture retention during thermal loading, resulting in variations in predicted hygrothermal stress evolution. The proposed approach provides an experimentally calibrated and physically representative treatment of desorption boundaries for reliability analysis of plastic encapsulated packages.