Power modules development is becoming more and more important for switching applications, to improve electric and reliability performances. This paper presents an experimental-numerical method to characterize the reliability behavior of ACEPACK(Trade Mark) SMIT package, a top side-cooled module designed with a half bridge topology, which employs high-voltage silicon MOSFETs. The experimental test involves power cycling to study the reliability and thermal behavior. Then, a finite-element based model is developed to simulate test conditions, to calculate temperature behavior inside the package.