Extensive analysis of the degradation of phosphor-converted LEDs

This paper reports an extensive analysis of the degradation of Phosphor-Converted Light-Emitting Diodes (pc-LEDs) submitted to accelerated stress tests. The aim of the study has been to achieve a description of the degradation modes and to extrapolate the dependence of the Time-To-Failure on the adopted stress conditions. The analysis has been carried out on two different sets of commercially available low-power LEDs, that have been stressed under different current (in the range 30-50 mA) and temperature levels (in the range 80-140 °C). 80 devices have been considered for the investigation, and stressed under different current/temperature levels for 1600 hours. A detailed analysis of the thermal properties of the LEDs allowed to extrapolate the junction temperature of the devices under the different applied stress conditions. The results of this work indicate that stress can induce both a gradual and a catastrophic degradation of the LEDs. The failure modes detected during gradual degradation consist in a decrease in the luminous flux and in the worsening of the chromatic properties of the LEDs. The analysis of the degradation kinetics under different stress conditions indicated that the degradation kinetics can be strongly influenced by the stress temperature level. On the basis of the failure analysis results presented within this paper, gradual degradation is ascribed to the worsening of the optical properties of the package and phosphors-encapsulant of the LEDs. Furthermore, we show that high stress conditions can also determine the increase in the thermal resistance of the samples: by means of X-Ray analysis, we demonstrate that this effect is due to the partial detachment of the LED-phosphors system from the device package. Finally, we show that catastrophic degradation is mostly correlated to the shortening of the junction. This effect is due to the generation of parasitic conductive paths that are possibly localized at native defect sites, and can be identified by means of emission microscopy.