Find IGBT degradation through power cycling

September 07, 2015 //By Andras Vass-Varnai
Find IGBT degradation through power cycling
Andras Vass-Varnai outlines how to pinpoint IGBT degradation using power cycling methods.

Dissipated heat in a junction is one of the major effects that can influence the reliability of die-attach materials used in an IGBT’s chip. Power cycling tests are ideal to mimic the lifecycle of a module because the number of switching cycles corresponding to an IGBT module can be predicted based on the target application.

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

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