Thermal stability analysis of High Brightness LED during high temperature and electrical aging

In this paper we report the analysis of thermal stability of High Brightness Light Emitting Diode subjected to thermal and bias ageing. The degradation mechanisms of several families of commercial available devices were investigated. In the first part of the work we estimated thermal resistance and thermal behaviour under dc bias condition. After this thermal characterisation two different ageing tests were carried out on devices: thermal aging at high temperature levels without biasing the devices and accelerated dc stress at nominal current value (400mA). At each step a complete electrical and optical characterisation of aged devices was performed, in order to find a correlation between different aging and a better understanding of degradation mechanism. This characterisation included I-V measurements, optical power vs current characteristics and spectral analysis. During thermal stress we observed the increase of forward voltage at nominal current and the degradation of optical power with nearly exponential kinetics. We found that lifetimes were well correlated with stress temperature: therefore it was possible to find an activation energy of degradation mechanism of about 1.5eV. Moreover, modifications of spectral properties during electrical and thermal stress were found. Thus, a package level analysis was carried out in order to clarify the role of modification in optical properties of reflector cup and the efficiency of phosphors. Finally, evaluation of differential structure functions indicated that stress induces also the worsening of the properties of the chip-to-package thermal path: this phenomenon has been attributed to the partial detachment and degradation of the ohmic contacts.