A molded wafer-level package simulation model was successfully developed for calculating solder ball fatigue life during a temperature cycle test, revealing that the most crucial factor affecting fatigue life, rather than being the maximal stress in solder balls, is the equivalent strain range in solder balls during the creep effect. Accordingly, in a temperature cycle test, the fatigue life of the solder balls, which are located from the outer edge to the center, is negatively correlated with the equivalent strain range; the earliest solder balls to rupture are those located at the outer edges, which have the highest equivalent strain range but not the highest stress. Regarding the fatigue life distribution, the simulation results differed from the experiment results by only 6.4%. Additionally, the effects of mold compound protection type and thickness on fatigue life were investigated. When the thickness was changed from 85 to 25 µm, the solder ball fatigue life increased to approximately 1230 cycles, which satisfies the production standard of 500 cycles and is 1.86 times longer than the fatigue life in the existing production line. Reduction of mold compound thickness reduced the amount of material required to 29% of that in the current production line. The model established in this study is expected to be applied in future integrated circuit package design for product reliability.