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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?

We decided to apply a 100C temperature swing on the device under test to accelerate the power-cycling process. This value was selected so that the maximum junction temperature would be 125C, which is device's allowed maximum. We maximised the power applied to the module to reduce the cycle time and selected the appropriate timing to achieve the target 100C temperature swing. The IGBT module can handle currents up to 80 A, but because of the high voltage drop on the devices, the power rating became the limiting factor. Based on previous trial measurements, 25 A was selected as heating current.

We needed to use 200 W of heating power for 3 s to heat the chips to 125C. The cooling time was set to ensure that the chips would have sufficient time to cool, and the average temperature wouldn't change during the tests. The timing diagram and the temperature profile are shown in figure 7.

Figure 5: Switching diagram of heating power and junction temperature during power cycling shows the expected thermal delay.

The applied heating current and timing remained constant during the whole testing process regardless of the changing voltage drop or increasing thermal resistance. The cooling transient of the devices was recorded in every cycle that was made possible to monitor the junction temperature change continuously. After every 200 cycles, a full-length transient measurement was performed using a 10-A heating current to check the structural integrity of the heat flow path.

Failed gate oxide, not bond wires
In our experiment, we continued the power cycling until the device totally malfunctioned (short circuit or open circuit), which was our failure criteria. Out of the four IGBT devices tested, one device failed significantly earlier than the others, after just 10,158 power cycles (figure 8). This premature failure was probably caused by incorrect mounting in the cold-plate or some random error. The other three devices, samples 0, 1, and 2, showed similar behaviour and failed after 40660, 41476, and 43489, respectively.

Figure 8: One IGBT device failed significantly earlier than others.

After all the IGBTs failed, we disassembled the modules and examined the condition of the chips and bond wires. Figure 9 shows an image of one of the chips, illustrating that several bond wires broke during the tests and an area on the chip surface was burned, which was probably caused by the arc when a wire detached while high current was applied.

Figure 9: Photo of the device shows broken bond wires and burnt chip surface.

Failure from overheating
Despite the obvious defects of the bond wires, the broken bond wires did not cause failure of the devices. All of the chips failed because of overheating and damage to the gate oxide. These effects could later be examined and tracked using electrical tests: the cracking of the bond-wires was indicated by the increase of the VCE (collector-emitter) voltage, and the damage of the gate oxide resulted in the increase of the IG, gate leakage current. When designing IGBT power cycler equipment, these parameters ought to be measured.

The joints between the substrate and the baseplate and the die-attach also need to be investigated for us to understand the source of the overheating, which is why we needed the calibrated simulation model. Figure 10 shows the temperature distribution of two adjacent IGBTs at the end of the heating period simulated using the calibrated detailed model. The thermal coupling between the adjacent chips was negligible; thus, each chip could be investigated individually.

Figure 10: Simulated temperature distribution of one half-bridge module after 3 seconds shows little thermal coupling.

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