In recent years, the light emitting diode (LED) technology has experienced an impressive progress. High power phosphor-converted white LED (high power LED) has attracted considerable interest in recent years owing to its diverse range of applications, such as outdoor lights and automobile headlamps. For the various application, the reliability of LED package should be guaranteed.
LED lifetime is conventionally measured using the Alliance for solid-state illumination systems and technologies(ASSIST), which requires at least 6,000 hours. It is strongly needed to develop a methodology of accelerated life tests(ALT) in order to decrease the research and development period.
From reviews of ALT for high power LED, it is well-known that long-term degradations and failures of commercial high power LED package are primarily associated with the package itself rather than the blue LED chip. In the conventional aging test, LED package are exposed to several temperatures and electrical current, to identify both their degradation mechanism and the defects of LED chip and package materials. Recently, humidity is also considered as an effective stress factor for accelerating the failure of LED packages.
The purpose of this paper is to suggest of an ALT based on the failure mechanisms of the LED package. In this paper, I have summarized all the experiment results.
(1) Accelerated stress test of high power LED package
It was found that the saturated water vapor pressure is the most dominant stress factor for the degradation phenomenon in the package for high-power LED. Also, it was proved that the saturated water vapor pressure is an effective acceleration stress of LED package degradation from an acceleration life test. Test conditions were 121 ℃, 100% RH(Unbiased autoclave), and maximum 168 h storage with and without 350 mA. The accelerating tests in both conditions cause optical power loss, reduction of spectrum intensity, increment of device leakage current and thermal resistance of the package. Also, the dark brown color and pore induced by hygro-mechanical stress partially contribute to the degradation of LED package. From these results, it was shown that the saturated water vapor pressure stress is adequate as an acceleration stress for shortening life test time of LED packages.
(2) Accelerated life test of high power LED package
ALT of high power LEDs were studied. Samples of LED packages were aged at 110℃/85% RH and 130℃/85% RH up to 900 hours under non-biased condition. The test conditions induced a luminous flux decay on LEDs in all the conditions. Aged devices exhibited modification of package silicon color from white to yellowish brown. The instability of the package contributes to the overall degradation of optical lens and structural degradations such as generating bubbles. The degradation mechanisms of lumen decay and reduction of spectrum intensity were ascribed to hygro-mechanical stress which results in package instabilities.
Main failure mechanisms have been identified from the different stress conditions (temperature/humidity accelerated tests). MTTF of 12,615 h have been evaluated for real working conditions by applying Arrhenius model and activation energy was found to be 0.33 eV. The extrapolated activation energy was similar to the results of previous studies. In order to compare the results of accelerated life test to normal operation condition, a typical current aging was performed. The light output decrease over time is exponential in nature and MTTFADTof 10,173 h have been estimated for real working condition. The difference between their results is small.
As a result of these experiments, it was proved that the high temperature and humidity stress are adequate for ALT of high power LED package to reduce test time.