VDMOS Degradation Experiment And Research On The Thermal Contact Resistance

For high power VDMOS device with high switch speed, high input impedance, easy to drive and no secondary breakdown phenomenon, the high power VDMOS is widely used in automatic control, hybrid system, military industry, aerospace and some other fields. For this reason, the operational reliability of VDMOS is focused. Then a high temperature accelerated life test is taken to explore the working reliability of VDMOS under normal condition in this paper. During the experiment a phenomenon is found that the device is burnt out in a sudden. From the analysis, we found the reason of the phenomenon is the thermal contact resistance.Because of the gap of the contact interface and the thermal coefficient mismatch, the thermal contact resistance(TCR) will be introduced between device and cooling fin, the heat dissipating way will be blocked. Consequently, the heat couldn’t be dissipated, the chip temperature will be higher and the reliability and lifetime of the device will be reduced.For this two problems, some researches is taken:1. A high temperature(150℃、180℃、200℃、230℃) accelerate life test is taken for high power VDMOS device, According to obtaining the degradation rules of the sensitive parameters(Ron, VGS, IGS, IDRS, IDSS), the failure time and distribution, then the life of the high power VDMOS under normal condition is acquired.2. The TCR can’t be distinguished directly from the differential structure function curve. In order to obtain the TCR between the power device and cooling fin, the relationship between the total thermal resistance and the contact force imposed to the high power diode is derived. The total thermal resistance from the chip to heat sink is measured under different contact force, and the TCR can be extracted nondestructively from the derived relationship. Finally, the experimental results are compared with the simulation results.3. In order to obtain the thermal characteristics of TCR, an experiment is taken in the paper. A TCR sample is put on the constant temperature platform. Changing the temperature of the platform, the TCR is measured under the different temperatures. Through the results obtained in this experiment, an analysis is discussed through the microscopic combining the elastic-plastic deformation. When the power device is attached to a cooling fin, the TCR won’t keep a constant value. The TCR is changing with the time. In order to obtain the change, three samples are used to do the high temperature accelerated life test. The temperatures of 130℃、140℃、150℃ is chosen in the life test. At the beginning of the test, the initial TCR values of three samples is measured, the failure criteriais set at 20%. The failure times of the three simples at high temperature is measured, and Arrhenius model is used to acquire the failure time of simple at the room temperature. A high and low temperature cycling test is taken, 120℃ is set to the high limit temperature and-30℃ is set to the low limit temperature. The cycle is set in 2 hours and holding time is set in 40 minutes. And the changing of the TCR is analyzed under the condition.