This is an optical image (top view) of an LED that failed open circuited.

The next image is a side view of the microsectioned LED. This fracture shows striations (red arrow) propagating away from the bond wire. There is also a discontinuity in the bond wire (white arrow) that appears to be the break that explains the open circuit condition.

How can this damage be explained? The CTE of cast epoxy (the body/lens) is ~55 ppm/C and the CTE of gold (the bond wire) is ~14 ppm/C. So the epoxy thermally expands ~4X more than the gold for a given temperature excursion. It seems likely that the break in the gold bond wire occurred due to an increase in temperature, which would put the gold wire in tension. On the other hand, it seems likely that the fracture in the lens material occurred due to cold temperatures (or alternatively due to pulse heating of the bond wire) where the epoxy just of the gold/epoxy interface would be in tension. In any case, our experience shows that LEDs tend to fail if forward current and ambient temperatures approach the maximum operating limits in the LED specification. Since lighting applications are often pushing for maximum lumens (maximum forward current and power dissipation), many failures could be avoided by applying derating principles at the design stage.

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