Contact Pressure Distribution Measurement for the Press Pack IGBT by Ultrasonic Measurement Method

Even contact pressure distribution among submodules in a press-pack insulated gate bipolar transistor (PP-IGBT) is an important factor in reliability screening tests before engineering application. However, the current stress-sensitive method for contact pressure measurement is an invasive method where each measurement would change the contact pressure distribution of submodules. It is necessary to study a noninvasive measurement method for contact pressure distribution in PP-IGBT. In this article, a noninvasive measurement method based on the ultrasonic reflection coefficient is proposed to measure the contact pressure distribution within PP-IGBTs. First, the characteristic of ultrasonic wave propagation at the contact interface of two different materials is analyzed. The ultrasonic measuring platform for the contact pressure distribution in PP-IGBTs is designed, and the new assembly measured part has little effects on the pressure distribution within the device by different clamping force experiment. Second, an efficient contact pressure measurement system for PP-IGBTs is designed, and a calibration method of ultrasonic reflection coefficient for contact pressure measurement is proposed. Finally, the accuracy of the proposed ultrasonic measurement method for contact pressure distribution is verified by comparing the measurement results with those of the stress-sensitive film method, where the relative error of contact pressure within PP-IGBTs in three different conditions is less than 10%.