【作者】 张芹；

【导师】 刘胜；

【作者基本信息】 华中科技大学 ， 机械制造及其自动化， 2011， 博士

【摘要】 基于大功率发光二极管(LED)的半导体照明被认为是新型的下一代光源,其具有低能耗、高效和长寿命等优点。但是,由于大功率LED光源在实际应用中出现了诸多可靠性问题,阻碍了LED光源的大规模应用。大功率LED光源要想取代传统照明光源,必须解决可靠性问题。因此,大功率LED可靠性的问题正受到日益广泛的关注和研究。本论文主要采用试验、数值模拟与理论分析相结合的方法来研究大功率白光LED的可靠性问题。引起大功率LED可靠性问题的主要原因是大功率LED技术还不是很成熟,有很多结构设计、材料选择和加工工艺上的缺陷。在实际使用中一旦遇到恶劣的环境,这些缺陷就会被放大,使LED较早失效。所以本论文主要从材料性能、温度和湿度对LED可靠性的影响、失效分析等方面进行可靠性问题的研究,具体的工作主要包括以下几个方面：用试验和模拟两种方法研究了荧光粉/硅胶复合材料的热导率。对荧光粉体积分数从0到40%的荧光粉/硅胶复合材料在30℃到150℃下的热导率进行了测试。并率先利用概率密度函数和Monte Carlo方法,依据荧光粉颗粒的实际粒径分布,得到了荧光粉/硅胶复合材料的理论建模,利用理论计算得到硅胶和荧光粉颗粒之间的界面热阻,将界面热阻引入模型,对不同大小荧光粉颗粒与硅胶界面上的温度阶跃进行了比较和研究。通过对荧光粉/硅胶复合材料进行在85℃高温和85℃/85%RH高温高湿条件下的老化试验,得到了荧光粉/硅胶复合材料的激发光谱和发射光谱随时间的变化规律,分析了温度和湿度对荧光粉/硅胶复合材料性能的影响机制。利用差示扫描量热仪对硅胶进行了等温和动态条件下的固化试验,建立了硅胶的固化动力学模型。基于有限元软件ABAQUS,建立了能够进行固化仿真模拟的非线性瞬态传热模型,该模型解决了对硅胶固化过程模拟计算的问题,和试验结果吻合得较好。对硅胶进行了多频条件下的动态力学性能测试,由硅胶在多频下的动态力学温度谱分析了硅胶的粘弹性性质,得到了玻璃化转变温度和激活能等重要参数,根据时温叠加原理构建了硅胶储能模量的主曲线。通过制备在荧光粉层中具有不同荧光粉含量的LED,采用试验和模拟两种方法研究了荧光粉层对结温的影响,得到了荧光粉含量与结温之间的对应关系。通过采用高环境温度和大电流的试验条件,对一系列不同结构的LED器件进行了加速试验,研究了温度对LED器件可靠性的影响。对失效的LED器件,使用显微形貌观察、开封检测和热重分析等分析手段进行了失效分析,揭示了LED器件在高温条件下的失效模式和失效机制。通过对大功率LED进行在热冲击和温度循环条件下的可靠性试验,研究了大功率LED在快速温度变化下的可靠性和失效模式,对LED在热冲击和温度循环下的瞬态温度场和应力应变进行了精确的建模仿真,利用失效分析和模拟结果相结合的方法,揭示了LED在快速温度变化下的失效机制。通过对暖白光和正白光大功率LED进行在一系列温度和湿度下的工作寿命试验,得到了LED的寿命数据。分析了温度和湿度对LED工作寿命的影响,揭示了温度和湿度对LED工作寿命影响的机制,并建立了LED在温度和湿度下的寿命模型。更多还原

【Abstract】 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.更多还原