Post mortem analysis is a process that identifies the failure modes involved with electronic component failure through mechanical, electrical and chemical tools. Systematic identification is useful in determining the underlying physical failure mechanisms, which can ultimately lead to pinpointing the failure's root cause.
The techniques involved with post mortem analysis span a broad application of tools and processes which can typically be accomplished within two failure analysis categories: non-destructive and destructive.
The non-destructive post-mortem technique involves non-altering means for analyzing the sample. No part of the sample undergoes chemical, electrical or mechanical related stress. Typically, non-destructive techniques involve quick external visual inspections, which can range from sophisticated microscopy methods to x-ray radiography analyses. Examples of non-destructive methods include the following:
Destructive techniques cause permanent and usually irreversible changes to the sample. This can be done by electrical, chemical or mechanical means, or a combination thereof. Typically, destructive techniques should proceed after non-destructive methods have been exhausted, because the outcome of this test is intended to cause physical alteration to the sample. The various applications of destructive failure analysis methods include:
IGBT electronic components are important to investigate since they are quickly becoming the device of choice in electronic power engineering. Their hybrid structure, consisting of a BJT and MOSFET, makes them an attractive choice for designs which call for fast switching speed with minimal power consumption. Unfortunately, these components are not without their faults, and investigating these faults involves a broad range of tools and applications. Two non-destructive methods are explored below.
Several IGBTs that had undergone stress tests were examined. Based on the responses during tests, the parts were considered to have experienced only mild to medium damage. Electrical parameters of the two parts were measured and compared with the datasheet limits. Then, the parts were examined via X-Ray inspection using facilities in collaboration with CALCE (UMD).
The X-ray inspection revealed damage that may have occurred at the die-attach of the parts. The X-ray image of an un-stressed part is shown for comparison. The die attach at the stressed part shows possible voids. Such voids are likely to increase die temperature of the parts.
Figure 1: X-Rays of multistate degradation in IGBTs; a) unstressed component; b) component with mild operational damage; c) component with severe operational damage