Environmental Acceleration Factors of Nonhermetic Packaged AlGaAs LEDs

摘要

Light output degradation of nonhermetic packaged AlGaAs LEDs has been characterized, following application of accelerated stress testing of the LED component. Our data have been applied to a previously reported "Black Box" acceleration model, to determine the model coefficients for temperature, ambient humidity and current biase of the LED device. The stress testmparameters included 3 levels of moisture, between 55% and 100%, 3 elevated temperature levels between 35℃ and 121℃, and 3 current bias conditions between 0mA and 40mA. Where device failure has not occurred during the test period, the time to failure (TTF) for an individual part has been projected to the time at which 50% reduction in the light output power will occur. The TTF data was then fit to a lognormal distribution to obtain the mean time to failure (MTTF) for each stress level of the study. The dependence of MTTF on temperature and relative humidity is assumed to be, C_0@exp[Ea/KT]@exp[-Arh(RH)~2]. The activation energy (Ea), and relative humidity coefficient (Arh) were determined from our accelerated test data. Our results show different behavior when testing with no in-test current bias, indicating a different physical mechanism for degradation under these conditions. Finally, physical analysis of aged devices point to a moisture driven corrosion mechanism, leading to eventual failure of the LED.