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Spot IGBT degradation through power cycling

Posted: 17 Sep 2015 ?? ?Print Version ?Bookmark and Share

Keywords:Dissipated heat? IGBT? 3D? power cycler? thermal interface material?

The base-line model, which we created based on the measured geometry and the best guess of the material parameters, showed a significantly different thermal transient behaviour than the real device. Such discrepancies can be eliminated by calibrating the model, successively refining the model data. We did this by fitting the structure functions obtained from the thermal transient simulation of the model (red curve in figure 4), to the structure functions generated from measurements of the real device (blue curve).

Next, we began calibrating the device by fitting the internal features of the package. We then went outward from the package in the direction of the heat flow path, fitting successively the thermal capacitance and thermal resistance values of the different regions. To adjust the die capacitance properly, we made sure the physical dimensions of the die were correct and the area of the heat sources was set properly. In this case, we needed to increase the heated area until the capacitance values at the die region overlapped each other on the structure function.

Figure 4: Simulation result of the baseline model shows an offset between simulated devices (red) and physical devices (blue).

Then, we needed to make sure that the thermal resistance of the ceramics layer was set to the appropriate level. With the increase of the thermal conductivity of the ceramics, the length of the corresponding thermal resistance section on the structure function could be reduced to achieve a new fitting section. After that, we set the bottom copper layer and the thermal interface material (TIM) between the device and the cold-plate to the appropriate thermal conductivity level for a proper match of the curves figure 5.

Figure 5: Structure function of the simulated (blue) and measured (red) transients after model calibration shows a tight fit.

Running the device in the power tester
As soon as we recorded the initial status of the IGBT's thermal structure, we exposed the device to reliability tests so we could assess its long-term behaviour. We fixed the selected IGBT module to a water-cooled cold plate with a thermal pad. The thermal pad has poor conductivity compared to most of the thermal pastes and gels, but it showed great thermal stability in earlier experiments. Thus, it didn't affect the measured results. The cold-plate temperature was set to 25C.

The module under test contained two half-bridge modules, that is, four IGBTs. The gates of the devices were connected to the drains, and the half-bridge modules were powered using separated driver circuits (figure 6). All IGBTs were connected to separate channels of the thermal transient tester equipment.

Figure 6: We used these electrical connections of the IGBTs for power cycling and thermal transient measurements.

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