November_EDFA_Digital

ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 20 NO. 4 20 stitchbond of the bondwire will shear and crack, which frequently shows electrically as an intermit- ting open or a temperature-dependent contact. • Printed circuit board (PCB) bending should be avoided, both during assembly and in the applica- tion. (See Fig. 7 for an example.) This requires laser cutting instead of stamping and enough backside

EARLY LIFE AUTOMOTIVE ELECTRONICS FAILURES AND THEIR ROOT CAUSES (continued from page 18)

Fig. 6 Typical stitch bond crack. The delamination between pin and potting is completed by the crack through the bond wire.

reversebias leakage. Inmany cases,manufacturers specify their LEDs as rather tolerant in the reverse bias direction. For instance, − 30 uA at − 3 V. LEDswith such characteristics will pass all tests, but their lifetime is significantly shorter than those with reverse bias characteristics, similar to a Zener diode, [4] Fig. 8. The reverse bias leakage canbe easily localized by using emission microscopy and/or optical beam induced resistance change (OBIRCH). At the localiza- tion spot, a dark spot will later grow in regular operation mode, thus weakening overall light emission. Therefore, binning criteria should not only focus on the LED color, but reverse bias characteristics should also be included. A study based on a few thousand blue LEDs [5] has shown that devices with a low reverse bias voltage suffered sig- nificant lifetime loss. It is also not widely known that LEDs

support points for the PCB, especially if it would suffer bending stress from push button switches or connectors with screws or plug pins. Beyond this, placement rules for crack-sensitive components such as ceramic capacitors must be considered. [3] LEDS IN CAR APPLICATIONS When discussing LEDs, it is important to distinguish between semiconductor-related and packaging-related failures. For both types, failures typically do not arise immediately. In fact, they fail slowly in most cases. When LED semiconductors are the source of the malfunction, small failures in the crystal may result in a

Fig. 8 Reverse bias leakage of LEDs. All LEDs are still working. Magenta: LED shortly before end of lifetime with high RBL leakage. Red: medium RBL, showing that the LED has at least one intrinsic failure. Dark blue: perfect LED, noRBL, RBL characteristics similar to a Zener diode.

Fig. 7 A PCB has been mounted and fixed into a car door grip. Just bymounting, a certain bending of the PCB applies. If the door is banged,mechanical shock adds on. Ceramic capacitors are at high risk of cracking in such applications.

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