The objective of this study is to empirically investigate the dependence of solder voids and related interconnect defects in QFN packages, on such design variables as stencil aperture pattern and size, as well as quantity and location of thermal vias. A non-traditional fractional-factorial matrix was used for this study. The QFN packages produced by implementing this experiment were analyzed using destructive and non-destructive failure analysis techniques. The effect of design variables on different types of defects such as voids, and taper were evaluated. This study showed that stencil aperture size influences void area percentage the most in comparison with other design variables. An optimal stencil configuration was proposed based on void area fraction calculations. A 2-D steady-state finite element analysis was then conducted to parametrically evaluate the effect of solder voids on the performance of thermal vias under QFN thermal pads. The model confirmed that the performance of thermal vias is not very sensitive to void area fraction.