Abstract: With the increasing integration of electronic packaging, electronic chips power capacity and heat power is also growing, encapsulation body internal temperature distribution and thermal stress have the influence on the reliability of the chip. The heat sink is one of the major cooling methodfor electronic equipment. By using ANSYS finite element software, 3D finite element model was founded for chip-eutectic layer-heat sink tosimulate package thermal stressin -55~125℃ in the thermal cycling, and the eutectic welding stresswas studied at different heat sink materials. The results showed that the maximum stressin the encapsulation structure appears in the heat sink on the surface of the four corner. The heat sink materials with the thermal stress from high to low in order are tungsten copper, AlSi50 and AlN. The results of the study provide a design basis fordesignment of X-band power module package.