How Do LEDs Fail?
Q. In my mistreatment of the LEDs, I ended up with one which puts out no light at 10mA, but will light up OK at 100mA! A. You have probably ‘cratered’ the ball-bond. Another poster mentioned ‘dark line defects’. This is, indeed, one mechanism for declining output. However, many of the more modern LED processes are not prone to DLDs. Here are some other defect types: 1) Cratering: A crack develops under the ball bond metallization zone. If you pull on the bond wire (not possible in encapsulated LEDs), a chip pulls out, leaving a ‘crater’, hence the name. Symptoms: If you apply pressure to the ball (sometimes just by pressing on the top of the LED, light output momentarily increases (or perhaps, is restored, in the case of total failure). Decapsulate and test bond/chip integrity. If you check the VI characteristic with a curve tracer, you may see ‘breakover’ characteristic if you apply sufficiently high bias Voltage. Causes (singly or in combination): a) incorrect ball bonding parameters
