Innovative methodology for testing the reliability of LED based systems

With this work we propose an innovative method for the analysis of the reliability of LED and Laser devices and systems. The basic idea of the proposed method is the separation of the different degradation forces that lead to the decrease of LED performances during ageing. By using a specific reliability analysis procedure it is possible to separately evaluate the effects of the single accelerating factors: temperature, current intensity, applied signal waveform, voltage overstress, optical and mechanical solicitation. To individually determine the degradation kinetics it is fundamental to separate the effects of temperature and current. For these reasons we carried out iso-currents reliability tests, where several devices have been stressed with the same current at different junction temperatures, and iso-thermal stresses, where junction temperature is instead constant for different applied currents. The result of the analysis will be a multivariable law that relates the several degradation parameters in the form of degradation kinetics. This will allow the estimation of the devices lifetime for a very wide operating conditions region. During degradation an extensive set of measurements have been carried out at fixed steps in the form of photometric, optical, electrical, capacitive, mechanical and thermal characterization. The combination of these results allows the understanding of what degradation mechanisms are taking place and therefore it is a fundamental tool to improve system reliability. Degradation has also been studied by analyzing catastrophic damages by means of failure analysis; the failure investigation is useful for the catastrophic damage: melting of bonding wire, contacts evaporation, facet melting (for laser diodes).