Research on lumen depreciation related to LED packages by in-situ measurement method

In this paper, lumen depreciation of LED in reliability experiment was monitored by in-situ measurement method. The partial LED flux on the receiving surface of fiber cable was captured, and it was proportional to the total luminous flux of LED light source when we provided an exact distance. The high temperature operating life test was used to find the weakness elements of LED packages with a limiting maximum temperature stress of 125 °C. Four kinds of packaged samples were constituted with difference components, and the lumen depreciations were presented. Combined with the lumen depreciation data and sampling inspection, the results could be summed up as follows: (i) the luminous flux of LED chip had a steady and slow depreciation, however, that of the samples coated with the phosphor–silicone composites had an initial sharp decline and then reach the stable state. (ii) The samples of only chip encapsulated by silicone and those of commercial white LEDs were carbonized on the center surface between chip and materials of encapsulant. As a conclusion, the silicone as an LED encapsulant could induce flaws, the material properties in larger coefficient of thermal expansion (CTE) and stronger adhesion should be considered in the package design stage, and the degradation of phosphor–silicone composites led to a fast light energy loss during the initial high temperature aging test, and then reached up to steady.     

Improvement of lumen efficiency in white light-emitting diodes with air-gap embedded package

In this paper, white light-emitting diodes (LEDs) with air-gap embedded package were proposed and fabricated by a simple method including pulsed spray coating. The lumen efficiency of air-gap embedded LED was enhanced by 8.8% at driving current of 350 mA, compared to conventional remote phosphor white LED. This improvement was due to the enhanced utilization of blue and yellow rays, which were confirmed by pulse current-dependent correlated color temperature (CCT). The utilization efficiency of blue rays was enhanced by 12.4% due to the embedded air-gap layer. The simulation results performed by Monte-Carlo ray tracing method agreed with our experiments, which showed enhancement in lumen efficiency and similar CCT. Finally, the electric field intensity versus different thickness for air-gap and no air-gap embedded white LED was calculated to check the incident blue rays trapped in phosphor layer.     

Thermal/luminescence characterization and degradation mechanism analysis on phosphor-converted white LED chip scale packages

According to the requirements on minimizing the package size, guaranteeing the performance uniformity and improving the manufacturing efficiency in LEDs, a Chip Scale Packaging (CSP) technology has been developed to produce white LED chips by impressing a thin phosphor film on LED blue chips. In this paper, we prepared two types of phosphor-converted white LED CSPs with high color rendering index (CRI > 80, CCT ~ 3000 K and 5000 K) by using two mixed multicolor phosphor materials. Then, a series of testing and simulations were conducted to characterize both short- and long-term performance of prepared samples. A thermal analysis through both IR thermometry and electrical measurements and thermal simulation were conducted first to evaluate chip-on-board heat dissipation performance. Next, the luminescence mechanism of multicolor phosphor mixtures was studied with the spectral power distribution (SPD) simulation and near-field optical measurement. Finally, the extracted features of SPDs and electrical current-output power (I-P) curves measured before and after a long-term high temperature accelerated aging test were applied to analyze the degradation mechanisms. The results of this study show that: 1) The thermal management for prepared CSP samples provides a safe usage condition for packaging materials at ambient temperature; 2) The Mie theory with Monte-Carlo ray-tracing simulation can be used to simulate the SPD of Pc-white LEDs with mixed multicolor phosphors; 3) The degradation mechanisms of Pc-white LEDs can be determined by analyzing the extracted features of SPDs collected after aging.     

Reliability investigation of light-emitting diodes via low frequency noise characteristics

Investigation of changes of operation and noise characteristics during aging process of light-emitting diodes (LEDs) has been carried out. Several groups of different design (different optics) LEDs based on different materials (nitride-based blue and white LEDs, phosphide-based red LEDs) have been investigated. It is found that leakage current components appear due to LED’s defects and their affect is observed as increase of both the low frequency electrical noise intensity and non-ideality factor of current-leakage characteristic in small current region. No considerable changes of light intensity characteristics during LEDs aging have been observed. Noise modeling, spectral and correlation analysis of optical and electrical fluctuations show on partly correlated optical and electrical fluctuations caused by defects in the active region of the LED. Degradation processes of investigated LEDs foremost occur in the diode chip and lead to the leakage current that has important affect to the electrical fluctuation level, but practically has a weak influence to the light emission properties of LED. Phosphorous layer of white LEDs and additional optical elements have no significant influence to the reliability of investigated LEDs under given aging conditions.     

Study of Phosphor Thermal-Isolated Packaging Technologies for High-Power White Light-Emitting Diodes

A novel packaging configuration for high-power phosphor-converting white light-emitting diodes (LEDs) application is reported. In this packaging configuration, a thermal-isolated encapsulant layer was used to separate the phosphor coating layer from the LED chip and the submount. Experimental and finite-element method simulation results proved that this thermal management can prevent the heat of LED chip from transferring to the phosphor coating layer. The surface temperature of the phosphor coating layer is a 16.8degC lower than that of the conventional packaging at 500-mA driver current for 1-mm power GaN-based LED chip. Experimental results also show that this packaging configuration can improve the light-emitting power performance and color characteristics stability of the white LED, especially under high current operating condition.     

硅酸盐型LED荧光粉的表面处理及其性能

采用硅溶胶对Sr2–xBaxSiO4∶Eu2+LED荧光粉进行表面处理,以改善其稳定性、在树脂中的分散性及封装应用的老化效果。考察了硅溶胶用量、处理温度和时间对荧光粉性能影响,分析了表面处理对荧光粉的晶体形貌和发光性能的影响,并对其应用老化性能进行了研究。结果表明,表面处理使硅酸盐荧光粉的初始发光强度提高约2%,可靠性也有所提高,可使封装后的LED的光衰减少6%。     

提高基于粉浆法的功率型白光LEDs发光效率的研究

基于水溶性感光胶的自光LEDs平面涂层技术,在蓝光LED芯片表面上得到了可控的荧光粉层．采用降低粉浆中ADC的浓度和提高荧光粉的含量两种措施,减少Cr^3＋在433．6和620nm两处吸收对器件出光效率的影响;新配制的粉浆在暗室中静置3～5h,既可以提高器件的出光效率同时又避免了暗反应带来的影响;在蓝光LED表面上得到粉层后,再涂覆硅胶层,由于硅胶的折射率与粉层的更接近,不但使出光色调偏向蓝光区域而且有更多光子出射,光通量由未加硅胶层时的44．8～59lm提高到了79．4～84．9lm．     

大功率发光二极管的寿命试验及其失效分析

以GaN基蓝光LED芯片为基础光源制备了大功率蓝光LED,并通过荧光粉转换的方法制备了白光LED.对大功率蓝光和白光LED进行了寿命试验,并对其失效机理进行了分析.结果表明,大功率LED的光输出随时间的衰减呈指数规律,缺陷的生长和无辐射复合中心的形成,荧光粉量子效率的降低,静电的冲击,电极性能不稳定,以及封装体中各成分之间热膨胀系数失配引起的机械应力都可能导致大功率LED的失效.     

High performance of InGaN LEDs on (111) silicon substrates grown by MOCVD

We report on the high-performance of InGaN multiple-quantum well light-emitting diodes (LEDs) on Si (111) substrates using metal-organic chemical vapor deposition. A high-temperature thin AlN layer and AlN-GaN multilayers have been used for the growth of high-quality GaN-based LED structure on Si substrate. It is found that the operating voltage of the LED at 20 mA is reduced to as low as 3.8-4.1 V due to the formation of tunnel junction between the n-AlGaN layer and the n-Si substrate when the high-temperature AlN layer is reduced to 3 nm. Because Si has a better thermal conductivity than sapphire, the optical output power of the LED on Si saturates at a higher injected current density. When the injected current density is higher than 120 A/cm/sup 2/, the output power of the LED on Si is higher than that of LED on sapphire. The LED also exhibited the good reliability and the uniform emission from a large size wafer. Cross-sectional transmission electron microscopy observation indicated that the active layer of these LEDs consists of the dislocation-free pyramid-shaped (quantum-dot-like) structure.     

Reliability of AlGaInP light emitting diodes with an ITO current spreading layer

Three aging experiments were performed for AlGalnP light emitting diodes (LED) with or without indium tin oxide (ITO), which is used as a current spreading layer. It was found that the voltage of the LED with an ITO film increased at a high current stressing, while there was little change for that of the LED without the ITO. The results of the LEDs with different thicknesses of the ITO film show that the LED with a thicker ITO has a higher reliability. The main reason for the voltage increase of the LED with the ITO film might be the current crowding in the ITO film around the P-type electrode.     

ITO作为电流扩展层的AlGaInP 发光二极管可靠性研究

Three aging experiments were performed for AlGaInP light emitting diodes（LED） with or without indium tin oxide（ITO）,which is used as a current spreading layer.It was found that the voltage of the LED with an ITO film increased at a high current stressing,while there was little change for that of the LED without the ITO.The results of the LEDs with different thicknesses of the ITO film show that the LED with a thicker ITO has a higher reliability.The main reason for the voltage increase of the LED with the ITO film might be the current crowding in the ITO film around the P-type electrode.     

Phosphor Thermometry in White Light-Emitting Diodes

A method for the in situ measurement of phosphor temperature in high-power white light-emitting diodes (LEDs) was demonstrated. The method is based on the dependence of the fluorescence decay time in inorganic phosphors on temperature. The decay time was estimated using the frequency-domain technique, which relies on the measurement of the phase shift of the sinusoidal waveform of fluorescence relative to that of the photoexcitation, obtained by driving the LED at high-frequency current. LEDs containing a single phosphor ("cool-white") and a phosphor blend ("warm-white") were examined. By comparison of the phosphor temperature with that of the LED junction and metal mount, the character of the spatial profile of temperature within the LED packages was revealed     

A review on discoloration and high accelerated testing of optical materials in LED based-products

Reduction of intensity of light output is one of the most common degradation modes in light-emitting diode (LED) systems. It starts from the failure of the various components in the system, including the chip, the driver, and optical components (i.e. phosphorous layer). The kinetics of degradation in real life applications is relatively slow and in most cases it takes several years to see an obvious deterioration of optical properties. Highly Accelerated Stress Testing (HAST) set-up and a methodology to extrapolate the results to real time applications are therefore needed to test the reliability of LED packages and lens materials. Using HAST concept in LED industry is inevitable due to the necessity of assessing the reliability of new products in a short period of time. This paper aims at briefly clarifying the degradation mechanisms of optical components in LED packages and explaining how they contribute to the depreciation of light output of the LED systems. The concept of HAST and the way the reliability of LED packages can be evaluated will also be discussed.     

Preparation of GaN-on-Si based thin-film flip-chip LEDs

GaN based MQW epitaxial layers were grown on Si(111) substrate by MOCVD using AIN as the buffer layer.High light extraction LEDs were prepared by substrate transferring technology in combination with thin-film and flip-chip design.The blue and white 1.1×1.1 mm~2 LED lamps are measured.The optical powers and external quantum efficiency for silicone encapsulated blue lamp are 546 mW,and 50.3%at forward current of 350 mA, while the photometric light output for a white lamp packaged with standard YAG phosphor is 120.1 lm.     

Methods of Increasing Luminous Efficiency of Phosphor-Converted LED Realized by Conformal Phosphor Coating

White light output was realized through combining a cerium-doped yttrium aluminum garnet (YAG:${hbox{Ce}}^{3+}$ ) phosphor with a gallium nitride (GaN)-based blue light-emitting diode (LED). In order to coat a phosphor layer of high quality on a GaN chip, the slurry coating technique was applied and investigated because of the coating efficiency and uniformity. The phosphor layer of conformal structure was realized on the LED chip with the self-exposure method. The influences of ammonium dichromate (ADC) as photosensitizer of photoresist in the slurry are discussed. Two methods are brought forward to reduce the absorption of Cr ions introduced by ADC. And experimental results indicate that these two methods are feasible to increase the luminous efficiency of white LEDs.      

功率型白光LED可靠性的实验及仿真建模研究

快速推广LED产品到照明市场最主要的障碍在于长期的可靠性测试问题。由于长期的LED可靠性测试不符合快速且低成本的要求,因此本实验对白光LED进行步进式加速寿命老化测试,步进式环境温度从55oC到145oC变化。同时,对LED进行结温测试,并使用COMSOL仿真软件进行温度场分布仿真来模拟器件的发热情况,结果表明荧光胶处的温度高于LED芯片温度。在长期老化之后,对LED的光度进行测量,可以对LED的失效机理进行分析。通过研究温度对LED的光谱功率分布衰减的影响及光通量衰减的影响,发现在经过近3500 h的老化之后,LED的光衰很严重,表明电流及高温对LED的影响是极大的。     

一种利用有限元法模拟LED芯片结点温度的方法

提出了一种以有限元法估算发光二极管(LED)光源模块结点温度的方法,提出了较详细的计算步骤,最后以6只1 W大功率LED组成的光源模块为例,演示如何以实测为基础,实测与软件试算相结合来估算LED光源模块的芯片结点温度.结果证明该方法具有较好的预测性,可以用来研究LED光源模块的温度分布,从而为研究LED封装材料匹配性、系统可靠性提供一定的参照.     

白光LED色度特性研究

为了解决白光发光二极管（LED）光衰问题,采用包膜技术对YAG：Ce^3＋荧光粉进行了实验。分别用Al2O3和La2O3对YAG：Ce^3＋荧光粉包覆,然后封装成白光LED。实验结果表明：La2O3较Al2O3包覆的YAG：Ce^3＋荧光粉初始光强强;包膜后的白光LED的光衰和光色变化都非常缓慢,表现出良好的光学稳定性。     

荧光粉涂覆方式对白光LED光色指标的影响

文章采用TRACEPRO软件建立了三种白光LED的光学模型:(1)芯片直接涂覆荧光粉;(2)芯片涂覆硅胶后涂覆荧光粉层;(3)芯片涂覆荧光粉后涂覆硅胶层.通过改变荧光粉的摩尔浓度或硅胶厚度来考察白光LED的光色指标,如光通量、色温及显色指数的变化.研究结果表明:第一种和第二种涂覆方式中,光色指标随着荧光粉摩尔浓度或硅胶厚度的改变呈规律性变化;第三种涂覆方式中,色温及显色指数的变化趋势不稳定,第三种方式光通量高于前两种,最大值可达到87.31m.三种方式的显色指数在70左右.研究结论为白光LED工艺设计提供参考和依据.     

功率LED柔性封装结构的设计与热特性分析

根据功率LED的柔性封装要求,提出了基于贴片式(SMD)封装的功率型LED柔性封装结构。对各层结构进行了优化设计,采用有限元分析(FEA),模拟了柔性封装结构LED的热场分布。对比研究了柔性LED与传统封装结构LED的热特性,并对弯曲状态下柔性衬底材料对芯片的应力进行了分析。结果表明,采用金属Cu箔衬底的柔性封装结构,其散热特性较好;Cu/超薄玻璃复合衬底替代Cu箔衬底,可以减少弯曲的应力,减少幅度达到2.5倍,散热特性基本相同。SMD柔性封装的LED不仅具有较好的热稳定性,且具有柔性可挠曲特性,其应用潜力很大。