Reliability and optical properties of LED lens plates under high temperature stress

In this investigation the thermal degradation mechanisms of Bisphenol A Polycarbonate (BPA-PC) plates at the temperature range 100–140 °C are studied. The BPA-PC plates are currently used both in light conversion carriers in LED modules and optical lenses in LED-based products. In this study BPA-PC plates are aged at elevated temperature of 100–140 °C for a period up to 3000 h. Optical and chemical properties of the thermally-aged plates were studied using UV–Vis spectrophotometer, FTIR–ATR spectrometer, and integrated sphere. The results show that increasing the thermal ageing time leads to yellowing, loss of optical properties, and decrease of the light transmission and of the relative radiant power value of BPA-PC plates. The results also depict that there is not much discoloration within the first 1500 h of thermal ageing. The rate of yellowing significantly increases at the end of this induction period. Formation of oxidation products is identified as the main mechanism of yellowing. An exponential-based reliability model is also presented to calculate the rate of degradation reaction and to predict the life-time of BPA-PC plates.     

Accelerated life time testing and optical degradation of remote phosphor plates

In this investigation the thermal stability and life time of remote phosphor encapsulant plates, made from bisphenol-A polycarbonate (BPA-PC), are studied. Remote phosphor plates, combined with a blue-light LED source, could be used to produce white light with a correlated colour temperature (CCT) of 4000 K. Spectral power distribution (SPD) and photometric parameters of thermally-aged phosphor plates were measured by Integrated-Sphere. Results show that thermal ageing leads to a significant decrease in the luminous flux and chromatic properties of plates. The photometric properties of thermally-aged plates, monitored during the stress thermal ageing tests, showed a significant change both in the correlated colour temperature (CCT) and in the chromaticity coordinates (CIE x, y). It is also observed that there is a significant decay both in the phosphor yellow emission and in the blue peak intensity. The decrease in the luminous flux is strongly correlated to the deterioration of the chromatic properties of the phosphor plates. The results also show a significant decay of CCT, postulating that the degradation of the remote phosphor plates affects the efficiency of light and the colour of emitted light as well. The decrease of CCT takes place with almost the same kinetics as the lumen depreciation.     

Reliability and diffusion-controlled through thickness oxidation of optical materials in LED-based products

In this study the temperature-induced oxidation reaction in bisphenol A polycarbonate (BPA-PC), leading to discoloration of the polymer used in solid state lighting applications, is studied. BPA-PC plates used as optical material in LED-based products suffer from yellowing and discoloration during service, resulting in a gradual deterioration of optical performance of the product. Discoloration reaction is greatly influenced by the temperature-induced oxidation of the polymer. In this study BPA-PC plates are aged at elevated temperature of 100–140 °C for a period up to 3000 h, aiming for accelerated oxidation of the plates. Diffusion of oxygen through the aged sample is studied up to 40 μm. A significant oxygen uptake is observed up to 20 μm. Activation energy of oxygen diffusion is measured to be 0.25 eV, which is close to activation energy of degradation reaction, reported in our previous works.     

Lifetime predictions of LED-based light bars by accelerated degradation test

The accelerated degradation of light bars was tested under different stresses called junction temperatures, which result from the combination of current and ambient temperature. Light bars are used as a light source in laptops. A general procedure for an accelerated degradation test was used to analyze the useful lifetime of light bars under operating conditions. The degradation behavior for each light bar was fitted by an exponential function. The impact of parameter variations and measurement errors were also considered. The failure criterion was defined as the 50% decrease of the emitted optical power, when compared with the initial level. The failure time is, accordingly, the time required to achieve that failure criterion. A response model based on an inverse power (exponential) law for the failure time under different stresses was then calculated to predict the lifetime under operating conditions. The results show that the failure time of a light bar under operating conditions is about 11,571 h.     

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.     

Degradation of silicone in white LEDs during device operation: a finite element approach to product reliability prediction

Silicone, which is a very common material for Light Emitting Diode (LED) packaging components like lens, casting and housing, undergoes degradation during high temperature and current operation. Indeed, electrical and optical losses cause material shrinkage and hardening, inducing mechanical stress within the LED assembly, which can end up into crack formation in silicone. In order to evaluate the reliability of LED package regarding the silicone crack, a degradation material model is developed, which is based on the experimental investigation of the mechanical properties of silicone during degradation. A thermo-optical model is used for the calculation of the temperature distribution in the device during steady-state operation. The crack reliability model, which is build combining the stress simulation results based on finite element approach and the visual inspection of the corresponding LED package during steady-state operation, is used to estimate the package lifetime depending on the operation conditions.     

Color shift acceleration on mid-power LED packages

Mid-power LED packages are now widely used in many indoor illumination applications due to several advantages. Temperature stress, humidity stress and current stress were experimentally designed and performed to accelerate the color shift of mid-power LED packages and color shift mechanisms have been discussed based on the color shift results obtained from measurements. Conclusions could be drawn:

• -Linear function fitting demonstrates a good linear relationship between color shift (Δu′ Δv′) and aging time almost for all the aging conditions. We can extrapolate the color shift Δu′ and Δv′ based on the fitted regression equations and then make the prediction for the total color shift Δu′v′.
• -Current stress can induce a different failure mode. Peak intensity reduction analysis reveals that the current stress accelerates the degradation of LED die.
• -Humidity test induced a substantial color shift both in u′ and v′. The u′ has an increased degradation rate after aging of 3000 h at 85%RH & 85 °C, there should be different degradation mechanisms during the whole humidity test. The molecular structure decomposition of silicone plates and then follows the silicone carbonization due to the long-term (3000 h) accumulated high localized temperature aging.

     

Failure and degradation mechanisms of high-power white light emitting diodes

The investigation explores the factors that influence the long-term performance of high-power 1 W white light emitting diodes (LEDs). LEDs underwent an aging test in which they were exposed to various temperatures and electrical currents, to identify both their degradation mechanisms and the limitations of the LED chip and package materials. The degradation rates of luminous flux increased with electrical and thermal stresses. High electric stress induced surface and bulk defects in the LED chip during short-term aging, which rapidly increased the leakage current. Yellowing and cracking of the encapsulating lens were also important in package degradation at 0.7 A/85 °C and 0.7 A/55 °C. This degradation reduced the light extraction efficiency to an extent that is strongly related to junction temperature and the period of aging. Junction temperatures were measured at various stresses to determine the thermal contribution and the degradation mechanisms. The results provided a complete understanding of the degradation mechanisms of both chip and package, which is useful in designing highly reliable and long-lifetime LEDs.     

Analysis of electrical parameters of InGaN-based LED packages with aging

As the light-emitting diode (LED) becomes a mature technology in the general illumination space, there is a tendency to operate LEDs at high current densities and temperatures in order to gain higher light output at lower cost. Further, there is interest among intelligent-lighting platform developers to offer predictive maintenance capabilities to users. The existing useful life prediction model defines LED lifetime based on parametric failure; however, there is a need for a useful life prediction model based on catastrophic failure, which can occur with the degradation of components in an LED package. Electrical parameters, especially package series resistance, are good indicators of LED package health (i.e., remaining useful life) and could potentially be sensed real-time in an application. In this study, the series resistance variation pattern until catastrophic failure was measured at different current and temperature stress conditions. The degradation mechanisms at each phase of variation were explained and, using available models, activation energies and exponents were extracted. The experimental data suggest electromigration-induced metal migration from the contact metallization layer to the semiconductor is the cause of short circuit catastrophic failure of LED packages. The variation patterns of ideality factor and reverse leakage current support this hypothesis. The information presented can be used to develop a catastrophic life estimation model for LED packages under current and temperature stress.     

荧光层形状对大功率白光LED光学性能的影响

为确定荧光层形状对大功率白光LED光学性能的影响,对蓝光LED发光晶片激发黄色荧光粉产生白光的荧光涂布工艺进行了研究。分别通过大面积点胶、晶片表面点胶和保形荧光胶涂布工艺制得白光LED样品,利用积分球和角度测试机对白光LED的光学性能进行测试,结果表明,保形荧光层白光LED的色温、光强分布和发光角度等光学性能优于大面积点胶和晶片表面点胶白光LED的光学性能。     

A Review on the Physical Mechanisms That Limit the Reliability of GaN-Based LEDs

We review the failure modes and mechanisms of gallium nitride (GaN)-based light-emitting diodes (LEDs). A number of reliability tests are presented, and specific degradation mechanisms of state-of-the-art LED structures are analyzed. In particular, we report recent results concerning the following issues: 1) the degradation of the active layer induced by direct current stress due to the increase in nonradiative recombination; 2) the degradation of LEDs submitted to reverse-bias stress tests; 3) the catastrophic failure of advanced LED structures related to electrostatic discharge events; 4) the degradation of the ohmic contacts of GaN-based LEDs; and 5) the degradation of the optical properties of the package/phosphors system of white LEDs. The presented results provide important information on the weaknesses of LED technology and on the design of procedures for reliability evaluation. Results are compared with literature data throughout the text.     

Reliability test and failure analysis of high power LED packages

A new type application specific light emitting diode(LED) package(ASLP) with freeform polycarbonate lens for street lighting is developed,whose manufacturing processes are compatible with a typical LED packaging process.The reliability test methods and failure criterions from different vendors are reviewed and compared.It is found that test methods and failure criterions are quite different.The rapid reliability assessment standards are urgently needed for the LED industry.85℃/85 RH with 700 mA is used to test our LED modules with three other vendors for 1000 h,showing no visible degradation in optical performance for our modules,with two other vendors showing significant degradation.Some failure analysis methods such as C-SAM,Nano X-ray CT and optical microscope are used for LED packages.Some failure mechanisms such as delaminations and cracks are detected in the LED packages after the accelerated reliability testing.The finite element simulation method is helpful for the failure analysis and design of the reliability of the LED packaging.One example is used to show one currently used module in industry is vulnerable and may not easily pass the harsh thermal cycle testing.     

GaN基发光二极管寿命测试及失效分析

将来自相同外延片和相同制作工艺的30只GaN基绿色发光二极管管芯分成3组,分别加30、40和60 mA电流进行不同时间的老化试验.在老化之前和老化期间测量了器件的光输出功率和I-V特性.将测得的光输出功率数据对时间进行指数函数拟合,得到了每一组器件的退化率及寿命,并推出器件在正常使用条件下的寿命.实验数据分析表明在电流应力作用下,GaN基绿色发光二极管的正向电压随着老化时间的增加单调上升,同时光功率下降.在60 mA下老化的管芯的串联电阻退化严重.对失效器件进行了失效机理分析.     

Life-time estimation of high-power blue light-emitting diode chips

We have proposed a new concept of metal package by which we can estimate the lifetime of blue light-emitting diode (LED) chips with high accuracy. Components in conventional LED package which may obscure the degradation behavior of LED chip itself were removed or replaced by other materials or components. Three kinds of chips from different manufacturers were analyzed in this study using proposed metal packages. In this paper, the optical and electrical characteristics such as light-output degradation and reverse leakage current of high-power blue LED chip were investigated and analyzed. Also, the relationship between light-output degradation and electrical characteristics of LED chip was described. With aging time of 5000 h, only one kind of blue LED chip shows enough light-output degradation to estimate life-time.     

GaP表面粗化对AlGaInP发光二极管光电特性的影响

采用湿法溶液粗化AlGaInP基红光LED表面GaP层 ,并在粗化后的GaP表面沉积ITO,研究了粗化时间对GaP表面形貌的影响,并利用SEM、半导 体 芯片测试机、X射线衍射仪、X射线光电子能谱对LED器件表面形貌、光电特性曲 线、界面晶向、元素特性进行表征,比较了粗化前后的LED亮度和光电特性变 化。测试结果表明:采用HIO₄、I₂、HNO₃系列粗化液在室温、粗化时间为30 S 时,有效增加了光在通过GaP面与ITO界面时的出光角度,使AlGaInP发光二极管 的发光效率提高21.4%,同时引起界面处的缺陷密度升高,费米能级 远离价带,主波长蓝移0.36 nm,正向电压上升0.04 V。     

A design and qualification of LED flip Chip-on-Board module with tunable color temperatures

with the increasing requirements on guaranteeing the color uniformity and improving the luminous efficacy and manufacturing efficiency, a wafer level chip scale packaging (WLCSP) technology has been developed by thermally impressing a thin multiple phosphor film on a LED wafer, then being segmented into individual LED chips. In this paper, a high power white LED Chip-on-Board (COB) module with high color rendering index (CRI, Ra > 93) and tunable correlated color temperatures (CCTs) is prepared by the flip chip technology. In this COB module, the tunable color temperatures are achieved by using two types of white LED CSPs with different target CCTs of 3000 K and 5000 K. The thermal and photochromatic properties and the photochromatic stability of the COB module are studied through both experiments and simulations. The results show that: 1) The measured spectral power distribution (SPD) intensity of the prepared COB module is smaller than the arithmetic sum of SPD intensities of its each series connected CSPs, which may be attribute to the light absorption happened among CSPs; 2) The junction temperature and driven current have the different effects on the photochromatic properties of COB module (i.e. luminous flux, CCT and CRI); 3) The nonlinearity of luminous flux as a function of driven current and junction temperature should be considered in its modeling.     

Kinetics of red AlGaAs light-emitting diodes degradation

The kinetics of the gradual degradation of red AlGaAs light-emitting diodes (LED) has been studied. This degradation has been shown to be due to diffusion of Zn atoms from the GaAs substrate to the p-n⁺ junction. The gradual degradation kinetics has been described on the basis of a diffusion model. It has been ascertained that the Zn atom diffusion process is not recombination-enhanced. At the same time the process of elastic stress relaxation, occuring at the initial stage of the LED degradation, is enhanced by the flowing current.     

Half‐duplex visible light communication using an LED as both a transmitter and a receiver

In typical visible light communication (VLC) systems, light‐emitting diodes (LEDs) are used as optical transmitters and photodiodes are used as optical receivers. Currently, many communication devices such as smart phones have a built‐in LED lamp whereas they usually do not have a built‐in photodiode. If we find a way to receive VLC signals without the need to add an additional photodiode on the communication devices, it will contribute to the spread of VLC. Therefore, we propose and demonstrate a VLC scheme without the need for a photodiode. As the first step, we investigate the characteristics of an LED as a VLC receiver and find out that an LED can also be used as a VLC receiver in certain conditions. Then, we demonstrate a half‐duplex VLC system using an LED as both an optical transmitter and an optical receiver, without the need for a photodiode. This technique could be used in various applications such as the VLC between smart phones with a built‐in LED lamp and the VLC between LED traffic lights. Copyright © 2015 John Wiley & Sons, Ltd.     

Kelvin probe force microscopy – An appropriate tool for the electrical characterisation of LED heterostructures

Light Emitting Diodes (LEDs) are commercially important devices in opto-semiconductor industry. The light emitting properties of LEDs degrade with time of operation and may lead to device failure. Even though the stability and reliability of LEDs are important topics, they are not well researched with AFM to date. This work demonstrates that Kelvin Probe Force Microscopy (KPFM) is an appropriate method to identify specific sites of increased degradation in a semiconductor heterostructure. Furthermore, the study shows that KPFM provides the metrological basis for further investigations with respect to the progress of degradation and its physical background. In this study, KPFM has been used to measure the potential gradient over cross-sectioned LED heterostructure in operation at different states of degradation. The results show significant differences between new and aged LEDs, markedly at specific layers of the LED heterostructure.     

A comprehensive modeling framework for gate stack process dependence of DC and AC NBTI in SiON and HKMG p-MOSFETs

A comprehensive modeling framework involving mutually uncorrelated contribution from interface trap generation and hole trapping in pre-existing, process related gate insulator traps is used to study NBTI degradation in SiON and HKMG p-MOSFETs. The model can predict time evolution of degradation during DC and AC stress, time evolution of recovery after stress, impact of stress and recovery bias and temperature, and impact of several AC stress parameters such as pulse frequency, duty cycle, duration of last pulse cycle (half or full) and pulse low bias. The model can successfully explain experimental data measured using fast and ultra-fast methods in SiON and HKMG devices having different gate insulator processes. The trap generation and trapping sub components of the composite model have been verified by independent experiments. Data published by different groups are reconciled and explained. The model can successfully predict long time DC and AC stress data and has been used to determine device degradation at end of life as EOT is scaled for different HKMG devices.