Abstract: The finite element modelling（FEM） simulation software ANSYS was used to investigate the influence of the package thickness,die thickness,leadframe（LF） thickness,exposed paddle size and die attach paddle size on the thermal stress and warpage of small size dual flat non-leaded（DFN） device packaging,which went through the reflow condition.The results show that the position of maximum von mises stress is located at the joint area which is encapsulated by the LF,die and molding compound（i.e.the die attach epoxy fillet height area）,and the value is decreased upon the package thickness being decreased.The whole thermal stress distribution will be increased along the joint line of die,epoxy and LF from the package center position to the epoxy fillet height area.For the thin DFN package,the die thickness,exposed paddle size and die attach paddle size have little effect on the package warpage,while the LF thickness makes big effect on the von mises stress and warpage.Therefore,the package thermal stress can be decreased effectively by reducing the package thickness and the LF thickness,moreover,the warpage can also be controlled within 1μm.