Long-term degradation mechanisms of mid-power LEDs for lighting applications

This paper reports the results of a reliability investigation performed on four different groups of commercially available mid-power white LEDs. In order to determine the robustness of this continuously growing class of lighting devices we arranged an experiment of accelerated aging: the four groups of LEDs (from different manufacturers) were submitted to a series of stress tests in environmental chambers with set-point temperatures ranging from 45 °C to 105 °C, in accordance to the IES LM-80-08 lumen maintenance measurement standard. The experimental data gathered all along the 4000 h of stress accumulated up to now suggest the presence of multiple degradation mechanisms that may limit the useful lifespan of the light-emitting diodes under test. In particular we observed the following phenomena: i) a decay of the luminous flux; ii) an increase in the reverse and forward leakage current; iii) the worsening of the chromatic properties of the emitted light; and iv) the presence of a thermally activated degradation mechanism. The results provide a first insight into the reliability of those widely used LEDs; the results on the temperature-dependence of the degradation kinetics can be used as a guideline for the thermal design of modern distributed-light lamps.