Spot IGBT degradation through power cycling

This article describes an experiment in which we conducted thermal transient tests from one steady-state to another to determine cause of failure for a small sample of IGBTs. These types of tests may support proper re-design of the physical structure of the module, and if needed, it can also serve as an input for thermo-mechanical stress simulations.

Our aim was to investigate the common failure modes emerging in current IGBT modules using a replicable process. Although the tests weren't done in a high-enough enough volume to predict lifetime, they let us examine the degradation process. We started by performing conducted thermal-transient tests on the samples. Trial measurement showed that the thermal transient of the device from steady state to steady state was 180 s. The hot steady state was achieved using 10 A of driving current on the device, which was switched to 100 mA sensor current when we started to acquire data.

Figure 1 shows the thermal transient function that describes the initial "healthy" status of the sample. This curve and the corresponding structure function were used as a basis for calibration of a detailed numerical representation of the package. Structure functions are direct models of one-dimensional, longitudinal heat-flow. In many frequently used 3D geometries, structure functions are direct models of the "essentially" 1D heat flow, such as radial spreading in a disc (1D flow in polar coordinate system), spherical spreading, conical spreading, etc. As such, structure functions can be used to approximately identify geometry/material parameters. The structure functions are obtained by direct mathematical transformations from the heating or cooling curves. These curves may be obtained either from measurements or from the simulations of the detailed structural model of the heat-flow path.

Figure 1: Thermal transient response of the studied IGBT prior to subjecting it to power and thermal stress.

Creating a thermal simulation model
We then built and verified a detailed 3D model of the module so that we could analyse the temperature distribution inside the structure. The geometric parameters were measured after all devices failed and the module has been disassembled. The model layout is shown in figure 2 (the cross-section of the structure is shown in figure 3).

So that we could be assured that the simulation model would behave the same way as the real device, we adjusted the material parameters until the structure functions obtained from the simulated transient results fitted the experimentally derived structure function. This process required a number of iterations.

Figure 2: Layout of the simulation model.

Figure 3: Cross-section of an IGBT module shows the bond wires attached to the device.

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