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INDUCTIVE LOADS Onmany PCBs, inductive loads are either hidden (e.g., voltage converters and LC filters) or they are controlled by the circuitry. Sudden switching or changes in load current leads to generation of inductivity-caused high voltage pulses. These pulses should be minimized by snubber circuitry. Otherwise early life field returns in semiconduc- torsmay appear, showing typical EOS failure signatures. In principle, the failure signature is similar to those causedby ESDwhen the device is powered. If the inductive load is an electric motor, high voltage pulses from the collector can be easily buffered at the source by small capacitors from each carbon commutator contact to GND or between the carbon contact pieces. Similar to ESD, not every inductive pulse will immediately kill the device. Usually, it will take a certain amount of time until latent damage becomes This article highlights some of themost frequent types of early life failures in automotive applications. Most of these failures can be prevented by careful precautions in the design phase; however, they sometimes require a bit more investment for additional protection of circuitry or improved packaging design. The most common failures are summarized in Table 1 (page 22). Looking at the root causes, it is obvious that these findings usually cannot be obtained from device failure analysis alone. A sound failure investigation at the system level, towhich a device failure signature is an important input, is the only proper way toward root cause findings. Moreover, for a success- ful approach, the involvement of all partners within the supply chain is mandatory and will require a culture change away from 8D thinking toward new, top-down interdisciplinary approaches. REFERENCES 1. https://nationalmaglab.org/education/magnet-academy/ watch-play/interactive/kelvin-water-dropper. 2. T. Knapen: “Analyzing and Optimally Controlling the Kelvin Water Dropper,” Master Thesis of Applied Mathematics, August 2015, University of Twente, the Netherlands, http://essay.utwente. nl/68015/1/Knapen_MA_EEMCS.pdf. 3. G. Vogel: “Avoiding Flex Cracks in Ceramic Capacitors: Analytical Tool for a Reliable Failure Analysis and Guideline for Positioning Cercaps on PCBs,” Microelectron. Reliab., October 2015, 55 (9), p. 2159-2164. 4. P. Jacob, A. Kunz, and G. Nicoletti: “Reliability and Wear- out Characterisation of LEDs,” Microelectron. Reliab., Sep- tember-November 2006, 46 (9-1), p. 1711-1714. 5. J. Tharian: “Degradation- and Failure Mode Analysis of III-V Nitride LEDs,” Master Thesis, May 2007, Vorarlberg University of Applied Sciences, Bregenz, Austria. an evident failure. CONCLUSION

ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 20 NO. 4

Fig. 9 Degradation of the forward I-V characteristics of two LEDs, switched in parallel directly. At the beginning (left curve), the red LED drawsmore current than the blueoneand thus suffersmoredegradation, resulting in flatter characteristics (right image). Now the red LED receives only a small part of the current and provides only dim light emission, while the blue one suffers current overload. are one of the most ESD-sensitive components. Suffering ESD strikes, LEDswill not fail or change their light emission immediately, but ESD shots can activate intrinsic defects, thus decreasing their reverse bias characteristics. Another frequent issue is LED circuitry. Direct paral- lel circuiting of LEDs will end up creating an asymmetric current distribution, which intensifies over time. As a result, the LED with the lowest starting voltage suffers current overload, while the others are not under opera- tional current (Fig. 9) and become dark. Regarding LED packaging-related failures, the sealing often cannot withstand the harsh environmental chal- lenges of automotive applications. Delamination of the primary lens from the housing can occur, resulting in pulling bond wires or the whole chip from the die-attach. This mechanism usually starts slowly, appearing at the first intermittent or temperature-dependent contact behavior. In automotive applications, LEDs are frequently not switched off completelywhen not in use, but a voltage of roughly 0.5-1 V is always applied. Electric potential, elevated temperature, and the presence of humidity are ideal dendrite growth conditions. The dendrite growth usually starts fromthe die-attach, where silver epoxy glue is used for fixing the dies. Dendritesmay either climb over the chip border or along the housing bottom to neighbor- ing pads. In the end, they cause more or less low-ohmic shorts, so that the LED reduces its light emission and is finally bypassed completely by dendrites.

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