Overdriving reliability of chip scale packaged LEDs: Quantitatively analyzing the impact of component

The objective of this study is to quantitatively evaluate the impacts of LED components on the overdriving reliability of high power white LED chip scale packages (CSPs). The reliability tests under room temperature are conducted over 1000 h in this study on CSP LEDs with overdriving currents. A novel method is proposed to investigate the impact of various components, including blue die, phosphor layer, and substrate, on the lumen depreciation of CSP LEDs after aging test. The electro-optical measurement results show that the overdriving current can lead to both massive light output degradation and significant color shift of CSP LEDs. The quantitative analysis results show that the phosphor layer is the major contributor to the failure in early period aging test. For the long-term reliability, the degradations of phosphor and reflectivity of substrate contribute significantly on lumen depreciation. The proposed reliability assessment method with overdriving loadings can be usefully implemented for LED manufacturers to make a cost- and effective-decision before mass production.     

Light emitting diodes reliability review

The increasing demand for light emitting diodes (LEDs) has been driven by a number of application categories, including display backlighting, communications, medical services, signage, and general illumination. The construction of LEDs is somewhat similar to microelectronics, but there are functional requirements, materials, and interfaces in LEDs that make their failure modes and mechanisms unique. This paper presents a comprehensive review for industry and academic research on LED failure mechanisms and reliability to help LED developers and end-product manufacturers focus resources in an effective manner. The focus is on the reliability of LEDs at the die and package levels. The reliability information provided by the LED manufacturers is not at a mature enough stage to be useful to most consumers and end-product manufacturers. This paper provides the groundwork for an understanding of the reliability issues of LEDs across the supply chain. We provide an introduction to LEDs and present the key industries that use LEDs and LED applications. The construction details and fabrication steps of LEDs as they relate to failure mechanisms and reliability are discussed next. We then categorize LED failures into thirteen different groups related to semiconductor, interconnect, and package reliability issues. We then identify the relationships between failure causes and their associated mechanisms, issues in thermal standardization, and critical areas of investigation and development in LED technology and reliability.     

Reliability improvement of InGaN LED backlight module by accelerated life test (ALT) and screen policy of potential leakage LED

In this paper, to improve the reliability of InGaN white LED Backlight module, we have analyzed the module level accelerated life test (ALT) for LED which is used for the front display in the refrigerator. In addition, we have suggested a screen method that enables to screen out LEDs which have potential leakage problems. The stress factors and levels were decided based on the end user environment in the field. The acceleration factor (AF) was logically calculated according to the stress factor. After 12 h of ALT, we have found one brightness degradation in 6480 LEDs (98 modules). It was found from the failure analysis that the leakage was from the P-pad electrode failure and caused the brightness degradation. We have suggested a method that enables to screen the potential field failure LEDs in mass production which the failure can be occurred by external noise factor. We have added “turn on current level screen” on “operating current level screen”. As a result, we could make possible to screen leakage LEDs effectively which may fail in the end user side. This method was applied by 20 LED makers.     

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.     

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.     

Application of multi-output Gaussian process regression for remaining useful life prediction of light emitting diodes

Light-emitting diodes (LEDs) are the preferred technology today when it comes to lighting both for indoor and outdoor applications, predominantly due to their high efficiency, environmental resilience and prolonged lifetime. Given their widespread use, there is a need to quickly qualify them and accurately predict the reliability of these devices. Due to their inherently long operational life, most LED reliability studies involve the use of degradation tests and application of filter-based prognostic techniques for dynamic update of degradation model parameters and estimation of the remaining useful life (RUL). Although they are in general very effective, the main drawback is the need for a specific state-space model that describes the degradation. In many cases, LED degradation trends are affected by a multitude of unknown factors such as unidentified failure modes, varying operational conditions, process and measurement variance, and environmental fluctuations. These variable factors that are hard to control tend to complicate the selection of a suitable state-space model and in some cases; there may not be a single model that could be used for the entire lifespan of the device. If the degradation patterns of LEDs under test deviate from the state space models, the resulting predictions will be inaccurate. This paper introduces a prognostics-based qualification method using a multi-output Gaussian process regression (MO-GPR) and applies it to RUL prediction of high-power LED devices. The main idea here is to use MO-GPR to learn the correlation between similar degradation patterns from multiple similar components under test and thereby, bypass the need for a specific state space model using available data of past units tested to failure.     

High-Temperature Degradation of GaN LEDs Related to Passivation

This paper describes the thermally activated failure mechanisms of GaN light-emitting diode (LED)-test structures related with the presence of a hydrogen-rich SiN passivation layer. It is shown that the properties of the passivation layer can remarkably affect devices' stability during high-temperature stress: Degradation mechanisms identified consist of radiative efficiency loss, emission crowding, and forward-current decrease. The radiative efficiency degradation was found to be thermally activated, with activation energy equal to 1.3 eV. This failure mechanism of LEDs is attributed to the thermally activated indiffusion of hydrogen from the passivation layer to p-type region of the diodes, with subsequent p-doping compensation and/or worsening of the transport properties of the p-side ohmic contact and p-type semiconductor     

A novel multiple-stress-based predictive model of LEDs for rapid lifetime estimation

This paper proposes a novel multiple-stress-based predictive model (MSBPM) to rapidly assess the lifetime of light-emitting diodes (LEDs). The MSBPM addresses the lifetime estimation of LEDs with respect to temperature, humidity, and current; these three stresses are rarely considered simultaneously in the assessment of reliability. Using several degradation data sets from accelerated life tests (ALTs) without using extrapolation method, a designed adaptive genetic algorithm is employed to identify five unknown parameters of the MSBPM. A simulation of the proposed MSBPM is presented as validation. By applying the degradation data from the ALTs under high stresses, an MSBPM for the LEDs is established. Under the nominal conditions of 25 °C/22.5% RH and a current of 0.35 A, the lifetime of the LED is estimated using the established MSBPM. The effectiveness of the proposed MSBPM is further verified through the estimated results.     

A review on the humidity reliability of high power white light LEDs

High power white LEDs are replacing current lighting sources, not only for indoor usage, but also for outdoor and harsher environmental applications. This calls for higher reliability with respect to electrical, thermal as well as humidity. In this work, a comprehensive review on the study of humidity reliability of high power LEDs is provided, and the humidity induced degradation mechanisms in packaged high power white LEDs and their failure sites are described. The failure degradation mechanisms are divided into three groups, namely the package level, chip level and interconnect level degradations. Modeling of the moisture degradation is also described, and new test designed for the humidity study is also introduced. The inability of current acceleration model to extrapolate accelerated test results to normal operating conditions for high power LEDs is shown, and this provides a new challenge for the estimation of the lifetime of high power LEDs under normal applications, along with other challenges that need to be addressed.     

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.     

Chip and package-related degradation of high power white LEDs

With this paper we present an analysis of the degradation of state-of-the-art high power LEDs. Three different kinds of commercially available samples, from leading manufacturers, were submitted to stress under various current and temperature levels. Based on an accurate estimation of the thermal resistance of the devices, iso-thermal and iso-current stress tests have been carried out, with the aim of separately evaluating the role of current and temperature in determining the degradation of the LEDs. Results indicate that state-of-the-art LEDs can show a significant degradation of their electrical and optical characteristics, when they are operated close to their current/temperature limits. In particular, data reveal the presence of two different degradation mechanisms: (i) the degradation of the blue semiconductor chip, due to the increase in non-radiative recombination, or to the decrease in the acceptor dopant concentration at the p-side of the diodes; (ii) the chemical degradation of the package, with subsequent worsening of its optical properties. Results suggest that even high-performance LEDs can suffer from limited lifetime: thermal management and driving conditions must be carefully optimized with the aim of achieving high reliability for LEDs to be adopted in high efficiency lighting systems.     

基于一维漂移布朗运动的SLD退化失效建模

超辐射发光二极管(SLD)集LD大输出功率和LED宽光谱优点于一体,是光纤陀螺仪中的关键元件与薄弱环节,其可靠性在很大程度上决定了光纤陀螺仪的可靠性。针对SLD寿命长、失效数据难于获取的特点,研究了基于性能退化数据的可靠性评估方法。在对SLD进行失效机理分析的基础上,提出用一维漂移布朗运动模型对产品在环境应力作用下的退化特性进行建模,基于所得模型,由SLD的性能退化信息估计模型中的参数进而评估得到SLD的可靠性指标。这克服了传统可靠性分析方法依赖寿命数据的缺点,能够在没有寿命数据的情况下评估得到SLD的可靠性指标,从而可节约大量的试验经费和试验时间,在工程应用上具有重要的价值。     

Analysis of thermal characteristics and mechanism of degradation of flip-chip high power LEDs

The purpose of this study is to investigate the thermal behavior at the die-attached interfaces of flip-chip GaN high-power light emitting diodes (LEDs) using a combination of theoretical and experimental analyses. The results indicate that contact thermal resistance increased dramatically at the die-attached interfaces with aging time and stress, degrading the luminous flux. The junction temperature and thermal uniformity of the flip-chip structure both strongly depend on the arrangement of gold bumps. Local hot spots effectively reduce light output under high electric and thermal stress, influencing the long-term performance of the LED device. The results were validated using finite element analysis and in experiments using an infrared and an emission microscope. A two-step thermal transient degradation mode was identified under various aging stresses. A simulation further optimized the bump configuration that was associated to yield a low junction temperature and high temperature uniformity of the LED chip. Accordingly, the results are helpful in enhancing the performance and reliability of high-power LEDs.     

GaN基发光二极管的可靠性研究进展

高效高亮度GaN基发光二极管(LED)在图像显示、信号指示、照明以及基础研究等方面有着极为广阔的应用前景,器件的可靠性是实现其广泛应用的保证.本文从封装材料退化、金属的电迁移、p型欧姆接触退化、深能级与非辐射复合中心增加等方面介绍了GaN基LED的退化机理以及提高器件可靠性的措施,并对GaN基LED的应用前景进行了展望.     

On the crack and delamination risk optimization of a Si-interposer for LED packaging

3D-integration becomes more and more an important issue for advanced LED packaging solutions as it is a great challenge for the thermo-mechanical reliability to remove heat from LEDs to the environment by heat spreading or specialized cooling technologies. Thermal copper-TSVs provide an elegant solution to effectively transfer heat from LED to the heat spreading structures on the backside of a substrate. But, the use of copper-TSVs generates also novel challenges for reliability as well as also for reliability analysis and prediction, i.e. to manage multiple failure modes acting combined – interface delamination, cracking and fatigue, in particular. In this case, the thermal expansion mismatch between copper and silicon yields to risky stress situations.To overcome cracking and delamination risks in the vicinity of thermal copper-TSVs the authors performed extensive simulative work by means of fracture mechanics approaches – an interaction integral approach within a simulative DoE and the X-FEM methodology to help clarifying crack propagation paths in silicon. The results provided a good insight into the role of model parameters for further optimizations of the intended thermal TSV-approaches in LED packaging applications.     

两种高温老化方式对功率白光LED光热参数的影响

测试和比较了大功率白光LED在高温耐电(HTCD)和高温存储(HTS)两种老化条件的光热性能变化。结果表明,在HTCD老化下,光通量衰减达到40～60%;而HTS下的衰减只有10～14%。这说明,电流应力对LED的寿命影响比较大。利用热阻瞬态响应法测试和结构函数理论分析两种高温老化条件下LED的热特性,结果表明,在HTCD老化下LED热阻的变化较HTS更为明显,并且热阻变化大多体现在导热Ag胶层。这主要是由于高温条件下电流应力引起Ag颗粒的空间分布不均,使粘结界面产生空隙导致热阻发生不同程度的改变。     

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.     

Applications of finite element methods for reliability study of ULSI interconnections

Three main failure mechanisms of ULSI interconnects are the electromigration (EM), stress induced voiding (SIV) and low-k dielectric breakdown. Reliability tests for these mechanisms are too long to meet the development time requirement, and the underlying dominant mechanisms cannot be identified, rendering difficulty in design-in reliability for integrated circuit. Facing the challenges in the reliability study of the interconnect system, physics based simulation and modeling is found to be essential, and finite element method (FEM) is a suitable tool. A few examples on the application of FEM to study the degradation processes and identification of potential failure sites in interconnects due to EM and SIV are given here. The study of the process induced degradation of the effective k value of low-k dielectric in ULSI interconnect system using FEM is also presented.     

Life prediction of LED-based recess downlight cooled by synthetic jet

This paper details the adaptation and implementation of a proposed hierarchical model to the reliability assessment of LED-based luminaires. An Edison base ? 6 in., compatible can, downlight ? LED replacement bulb, cooled by active synthetic jets, is used as the test vehicle. Based on the identified degradation mechanisms and the experimentally obtained degradation rate of the cooling device, the reduction in the heat sink enhancement factor, and thus the increase in the LED junction temperature, is determined as a function of time. The degradation mechanisms of the dual-function power electronics – providing constant power to the LEDs and to the drivers of a series of synthetic jets – are also analyzed and serve as the basis for a hybrid model which combines these two effects on the luminaire lifetime. The lifetime of a prototypical luminaire is predicted from LED lifetime data using the degradation analyses of the synthetic jet and power electronics.     

Degradation mechanisms of high-power white LEDs activated by current and temperature

This paper presents a study of the degradation mechanisms that limit the reliability of commercially-available white LEDs. Purely thermal stress and biased iso-thermal stress were carried out for several thousands hours on 1W-power LEDs, produced by a leading manufacturer. Results reveal that temperature and operating current have different roles in determining the optical degradation of these devices: (i) pure thermal stress induces a short-term optical power decay, strictly correlated to the decrease in the reflectivity of the package/reflector system and with no effects on the electrical characteristics of the blue chip; the activation energy of thermally-induced degradation is equal to 1.8 eV; (ii) constant current stress induces a long-term degradation process, with a degradation rate which is strongly dependent on the stress current level. In this latter case, optical degradation is ascribed to the degradation of the blue semiconductor chip: details are provided through the analysis of forward voltage and wavelength shift during stress time.