Abstract
By solving the problem of very long test time on reliability qualification for Light-emitting Diode (LED) products, the accelerated degradation test with a thermal overstress at a proper range is regarded as a promising and effective approach. For a comprehensive survey of the application of step-stress accelerated degradation test (SSADT) in LEDs, the thermal, photometric, and colorimetric properties of two types of LED chip scale packages (CSPs), i.e., 4000 °K and 5000 °K samples each of which was driven by two different levels of currents (i.e., 120 mA and 350 mA, respectively), were investigated under an increasing temperature from 55 °C to 150 °C and a systemic study of driving current effect on the SSADT results were also reported in this paper. During SSADT, junction temperatures of the test samples have a positive relationship with their driving currents. However, the temperature-voltage curve, which represents the thermal resistance property of the test samples, does not show significant variance as long as the driving current is no more than the sample’s rated current. But when the test sample is tested under an overdrive current, its temperature-voltage curve is observed as obviously shifted to the left when compared to that before SSADT. Similar overdrive current affected the degradation scenario is also found in the attenuation of Spectral Power Distributions (SPDs) of the test samples. As used in the reliability qualification, SSADT provides explicit scenes on color shift and correlated color temperature (CCT) depreciation of the test samples, but not on lumen maintenance depreciation. It is also proved that the varying rates of the color shift and CCT depreciation failures can be effectively accelerated with an increase of the driving current, for instance, from 120 mA to 350 mA. For these reasons, SSADT is considered as a suitable accelerated test method for qualifying these two failure modes of LED CSPs.
Highlights
With a wide range of applications of Light-emitting Diode (LED) products in the lighting market, the reliability of the phosphor converted white LED has become a global spread hot research [1,2,3].A white LED with high reliability is expected to have a lower luminous flux degradation and color shift under a long-term operation time
Materials 2017, 10, 1181 developed to assist people to understand more about the essence of reliability of pc-white LEDs, the traditional reliability evaluation test methods still require a test time of at least
In order to shorten the test duration, the constant stress accelerated degradation tests (CSADTs) under manifold uni/multi-environmental overstress conditions are developed for pc-white LEDs and the relevant products [2,16]
Summary
With a wide range of applications of LED products in the lighting market, the reliability of the phosphor converted white LED (pc-white LED) has become a global spread hot research [1,2,3]. When combined with stochastic degradation models such as Gamma process and Wiener process, and statistical luminous flux distributions such as Gaussian distribution, Weibull distribution and Lognormal distribution, SSADT is capable to provide more efficient lifetime evaluation test plan with a smaller sample size and less test time for LED products, as compared to CSADT [24,25,26]. This makes the optimization of the SSADT test strategy become a very hot research topic.
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