Thermally activated degradation and package instabilities of low flux LEDS

The results achieved in an accelerated life-time test on Phosphor-Converted Light Emitting Diodes (PC-LEDs) have been reported. Two different families of commercially available low-flux devices have been widely characterized and a comparative analysis on performances has been carried out. A wide set of devices has been submitted to a combined electrothermal accelerated stress under different aging conditions. The stress induced a luminous flux decay on LEDs from both series. In particular, the lumen decay was found to be thermally activated for one set of devices. The aged devices showed also a degradation of chromatic properties, in terms of a blue or yellow shift for the two different families. The failure modes found have been detected also in devices aged at constant temperature and no bias. The degradation mechanism responsible for lumen decay and chromatic shift was ascribed to the thermally activated package instabilities. A failure analysis has been carried out on failed devices, detecting different failure modes related to the package (chip detachment) and to the chip (generation of low impedance paths that shorted the junction).