Thermal stability analysis of power LED during aging

The aim of this work is therefore to analyze the thermal stability by lifetime accelerated tests for three different constructions of LED lighting fixtures. In result of studied we believe to provide information on the role of temperature in determining devices degradation, since in most of the cases high current and high temperatures acted as degradation driving forces. In order to high light the influence of the ageing tests on the LEDs, we firstly measured their characteristics at the initial state and than the measurements were carried out in a cyclical way after every ageing step. The accelerated current test implied a faster degradation than the constant thermal stress: even if the degradation modes were similar for both stress and appeared to be thermally activated, the high carrier flow acted as an acceleration factor on the failure of devices. These results underline that package design in terms of heat dissipation is significant for devices with longer lifetime, and essential requirement for solid state lightning market. Thermal modeling of the LED packages was done in FLOTHERM. Using an IR camera, we measured the die surface temperature of the high-power single dies in LED packages, which are heat sunk at the case.