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High power light-emitting diodes (LED) are widely applied in many fields for their special advantages compared with other light sources. Many researches on LED reliability have been conducted in order to build the better LED design. However, seldom detailed reasons for the LED performance degradation are illustrated in the previous research when LE...
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... blue chips were packaged with the same package structure and material under the same procedural conditions. The Y AG phosphor layer for yellow conversion in the package structure, as shown in Fig 1, is the mixture of yellow Y AG phosphor and silicon glue. The samples were divided into four groups depending on the chips, and were labelled group 1, group 2, group 3 and group 4. Every group contained five samples which exhibited the similar behaviour. ...
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... the experiment, we encounter some samples in which cracks happen at the center of the packaged LED chip surface. The luminous tlux is sharply descent by 13%, as shown in Fig. 10. Thus, the moisture or critical stress in the modules, especial the interface between chip and phosphor layer, is very important. Through the accelerated life test under 85 'C /85%RH, we find the descent slopes of the four sample groups are not the same. We detect the surfaces of LED chips under AFM. The AFM micrographs show that the ...
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... It was found that during the absorption cycles, the moisture concentration was highest near the die-attach region and during the desorption cycle, the moisture concentration was high in the silicone than ambient. FEM results also indicated that moisture can reach the die attach under 24 hours of test [31,32]. ...
Applications of LEDs have increased significantly, and increasing outdoor applications are observed. Some outdoor applications require high reliability as their failure can lead to hazardous consequences. Examples are their applications in automotive, street lamp lighting etc. To ensure the reliability of LEDs in outdoor applications, reliability test that include humidity on the LEDs must be done. However, it is found that accelerated life test of LEDs at high humidity level cannot be extrapolated to standard condition of lower humidity as the mechanism of degradation depends critically on humidity level. In fact, the degradation of LEDs in outdoor applications is mainly due to the degradation of their encapsulation and housing materials (or called packaging material as a whole) instead of the semiconductor chip itself. The decrease in lumen is the results of crack and discoloration of the LED packaging material. Detail understanding of the failure physics of the packaging material for LED under humidity is needed for extrapolation performed at accelerated stress condition so that LED luminary reliability can be predicted. This chapter reviews the different types of degradation physics of the packaging material using ab-initio simulation with excellent verification from experiments. The method of extrapolation is therefore derived from the physics-based model after detailed understanding of the degradation physics of LEDs. The model also provides strategy for industry to prolong the usage of LEDs in outdoor applications, either through materials or operating conditions selection.
... Tan et al. [65] showed that in humid conditions, if there is any defect such as a crack existing at the interface of LED chip or the die attach, it is possible for the crack to develop into a delamination which is disadvantageous to thermal management. Wu et al. [66] used FEM modeling and observed that the stress induced by the coefficient of moisture expansion difference is large at the interfaces between polycarbonate (PC) lens, molding compound and heat sink. Their results indicate that more attention to the moisture spreading along the interface of the different material should be given. ...
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.
... Thermal management is an important issue [10], because thermal resistance and characteristic of LED are affected negatively from the temperature. In addition to this, thermal conditions have an effect on the LED thermal [11] and reliability [12] performance. ...
... The maximum stress and strain are found in the low ebb and peak points on the surface of the phosphor layer. These points may become the potential defect in the LED packaging modules, and the potential defects existing in LED packaging modules can increase with the increase of LED chip's surface roughness [4]. Fig. 11 Contour plots of stress and strain in the model. ...
The heteroepitaxial growth of GaN-based epitaxial layers on sapphire and SiC substrates increases process complexity during light-emitting diodes (LEDs) manufacturing process. The large lattice mismatch and coefficient of thermal expansion (CTE) mismatch between substrate materials and GaN materials, resulting in wafer curvature issue which can impact wavelength and intensity uniformity. Obtaining tighter control over the wavelength and intensity uniformity of the active MQW region will be critical. With the increasing size of wafers from 2in, 4in, 6in, to even 8in, the curvature and co-planarity will be an even challenging issue. Current MOCVD is from from being perfect in terms of wafer to wafer, run to run variations, which could be resulted from temperature and flow control of the reactor, heater, and gases. Additionally, the manufacturing process parameters of LED chip has significant effect on the LED device performance. One of the challenges for the application of wafer-level packaging in LED is the poor wavelength uniformity and reproducibility in the LED wafer. Several co-design issues for LEDs manufacturing using DfX is given in this paper.
Yttrium Aluminum Garnet transparent ceramics with different Gd³⁺ concentrations ((Ce0.001GdxY0.999-x)3Al5O12) ceramics were prepared by the conventional solid state reaction technology and vacuum sintering method, and the ceramic-based LEDs with blue LED were prepared by flip chips and Chip on board package. The extreme circumstance of −40 °C and 150 °C alternate environments, high humidity and high temperature as well as xenon lamp aging test for a long time are mentioned to explore the reliability of phosphor ceramics, respectively. And the thermostability and the effect of Gd³⁺ doping on the PL properties of ceramics were also investigated. What is more, the effects on the optical performance of the ceramic-based LEDs have been also investigated. As Gd³⁺ concentration added from 0 to 0.5, the PL spectra peaks of phosphor shifted from 542 nm to 575 nm under the excitation of 460 nm. It can be summarized that the increase of Gd³⁺ concentration could not improve the thermostability, but enhance the reliability when the x value is x is not greater than 0.4. What is more, the ceramics-based LED of x = 0.3 instead of x = 0.4 has the best reliability after the xenon lamp aging and thermal cycling test. And the ceramics-based LED of x = 0.45 has the best reliability after the high humidity and high temperature test. Compared with Ce:YAG and Ce:LuAG ceramics, although the increase of Gd³⁺ concentration decreases the thermostability and luminous efficiency, the Gd substitution can enhance the reliability and solve the lack of red light in the spectrum of Ce:YAG and Ce:LuAG ceramics, which has improved the performance of Ce:GdYAG ceramics for high-power warm white LEDs.
With the recent improvements in LED luminous efficacy that have been achieved by the lighting industry, greater attention is being devoted to understanding the long-term performance of LEDs. Specifically, there is a need to understand and predict the ability of LEDs to maintain luminous flux and chromaticity over time. The issue of chromaticity maintenance (aka, chromaticity stability and color shift) is especially important for LED devices since they may be used for 10 years or more without replacement of the light source. The characteristics of LEDs have a significant impact on chromaticity maintenance for LED-based lamps and luminaires. However, other components of an LED device, including lenses, reflectors, and device design, can also have an impact on chromaticity. Understanding the aging characteristics of LEDs, lenses, and reflectors is critical to designing and building LED-based products for long-term use.
Serious silicone carbonization has been reported in mid-power white-light LEDs during highly accelerated temperature and humidity stress test, although the junction temperature of the LED chip is much lower than the critical temperature of silicone carbonization (300 °C). This paper presents a comprehensive investigation, through which the effects of Joule heating and phosphor self-heating have been eliminated as main failure mechanisms. As a result, the over-absorption of blue lights by silicone bulk is considered as the root cause for silicone carbonization. This is mainly due to the scattering effect of water particles inside the silicone materials, which can increase the silicone temperature significantly. Furthermore, finite-element thermal simulation has also been performed, confirming the validity of the failure mechanism.
Light-emitting diode (LED) and SSL products, including packages, arrays, and modules are in the initial adoption stage, and there are many reliability and design challenges facing the industry. This chapter discusses several key aspects focusing on the reliability and the life time prediction for LED SSL products. Upfront product design for reliability activities to enable reliable SSL products are studied from both the product construction, manufacturing and application point of view. © Springer Science+Business Media, LLC 2013. All rights reserved.
This research adopts comprehensive evaluation to evaluate lifetime of LED light source for lighting system. By the method, under the condition of LED light source working outside, the situations of LED light source lifetime are analyzed with attenuation rate of illumination got from illumination data of LED light source in practical work. The real lifetime characteristics and real life curve of the LED light source are obtained eventually.
