Effect of shell-tube dimensions on the reliability of power VDMOS by temperature cycling

Abstract

With the improvement of packaging technology, small packaging type is gradually becoming mainstream. Packaging dimension has a significant influence on the performance and reliability of power Vertical Double diffused MOSFET (VDMOS) device. In this paper, the thermal stress of different shell-tube dimensions in power VDMOS device is simulated by finite element methods using ANSYS software. The simulation results show that the solder joint between the power VDMOS chip and the shell was under the maximum thermal stress, and an inverse relationship was found between shell-tube dimensions and the thermal stress of the solder joint. In order to validate the simulation conclusion for the inverse relationship, this paper selected TO-257 and TO-254 package type for the same VDMOS chip. The shell-tube dimensions of TO-254 is larger than TO-257. The temperature cycling experiment was carried out with five samples drawn in each package type. The author found that the on-state resistance of the TO-257 package increased significantly after 500 cycles, but the TO-254 package was almostly unchanged. The evolution of the on-state resistance would be explained by the thermo-mechanical effects. Furthermore, some voids were observed in the solder joint of the TO-257 package after the temperature cycling experiment according to the results of scanning acoustic microscope (SAM), which is mean that the solder of the TO-257 package severely degrade after temperature cycling. Due to the smaller shell-tube dimensions of the TO-257 package, the larger thermal stress of the solder joint is generated in temperature cycling. It has been demonstrated that the ANSYS software simulation results were in accord with the experimental results which indicate that the shell-tube dimensions can seriously impact on the solder joint of power VDMOS device.