| It has been widely accepted that high power light emitting diodes (LEDs) will be the next generation light source due to their excellent performance in terms of high efficiency, low power consumption, and long life. However, the reliability problems exist in the application of LEDs, while have hindered large-scale application of LED devices. Thus, the reliability is becoming an essential barrier for LED devices to substitute the traditional light sources, and great attentions have been paid to the reliability of high power LED devices at present.In this dissertation, the reliability issues of the high power LEDs are mainly studied by means of experiment, simulation, and theoretical analysis. The defects in material selection, structural design and manufacturing technology are the main reasons that will cause reliability problems for LEDs. In harsh environments, these defects will be developed and then lead to degradation and may let earlier failure of LEDs. Therefore, the key issues in the material properties, the effects of thermal and humidity stresses on the reliability for LEDs, and failure analysis are studied in this dissertation, and some contributions are achieved as bellow:The effective thermal conductivity of silicone/phosphor composites is studied experimentally and numerically. Thermal conductivity measurements are conducted from 30 to 150℃for the composites with phosphor volume fraction up to 40%. In numerical study, a finite element model with empirical particle size distribution and random particle position is constructed by using probability density function and Monte Carlo method, with the interfacial thermal resistance layer between phases also introduced into the model. The temperature jumps across the interfaces between phosphor and silicone are investigated, and the temperature jumps at the interface are compared between different size phosphors.High humidity and high temperature test (85℃/85%RH) and thermal aging test (85℃) are performed on silicone/phosphor composites. The luminescence properties, photoluminescent excitation spectra and emission spectra, of silicone/phosphor composites during the 85℃stress and the 85℃/85%RH stress are obtained. The mechanism of high temperature and high humidity effects on the luminescence properties of phosphor/silicone composite is investigated. The curing reaction processes of the silicone under isothermal and nonisothermal conditions are performed using a differential scanning calorimetry (DSC). The curing reaction kinetics model for silicone is derived. A nonlinear transient heat transfer finite element model based on commercial finite element software, ABAQUS, is developed to simulate the curing process of silicone. Good agreement between experimental data and numerical analysis by the curing model is obtained.Dynamic mechanical analysis is adopted to study the dynamic mechanical properties of the transparent silicone resins for LEDs packaging. The viscoelastic behavior of silicone is obtained from multi-frequencies dynamic mechanical temperature spectra. Glass transition and the activation energy of it are analyzed. A master curve of storage modulus ia generalized according to time-temperature superposition principle.The effects of the phosphor layer on the junction temperature and temperature in packaging for the white phosphor-conversion light-emitting diodes are investigated using special LED devices with varying phosphor concentrations. The relationship of phosphor concentration and junction temperature are obtained by experiments and simulation.To investigate the high temperature effects on the reliability of LEDs, the accelerated tests for LEDs with different structure are performed under high environment temperature and high current density. The failure modes and failure mechanisms for LEDs under high temperature are obtained by failure analysis. The failure analysis methods include microtopography, decapping, and thermal gravimetric analysis.To evaluate the reliability and failure modes of LEDs under rapid temperature changes, thermal shock testing and thermal cycle testing are performed, respectively. The dynamic thermo-mechanical responses of the LED under thermal cycling and thermal shock loading are investigated by thermo-mechanical finite element modeling and analysis. The failure mechanisms of LED under thermal shock are investigated by comparison and synthesizing the results of failure analysis and thermal-mechanical simulation.A series of life tests for warm white and nature white high power LEDs, respectively, under different temperature and humidity induced stresses are performed. The lifetime data about LED under different stresses are obtained. The temperature and humidity effects on the operating life of LEDs are investigated. The mechanisms of degradation for LEDs under temperature and humidity stress are obtained. An operating life model for LED under temperature and humidity induced stresses is also obtained.
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