The electronic packages are subject to various constraints of thermal and mechanical order both during the manufacturing process and at the state of their operation causing thus their failure. It is this aspect which was the subject of our study of the thermal characterization of an electronic package QFN. For this purpose a thermal study was carried out via development of a finite element model 3D integrating JEDEC recommendations, in order to calculate the thermal resistance accompanied by a reliability study through application of methods FORM/SORM and HBRM (Heuristic Based Reliability Method) based on a response surface approach proved fast and efficient and this in order to design more reliable electronic systems.