Lifetime assessment of Bisphenol-A Polycarbonate (BPA-PC) plastic lens,used in LED-based products

In this investigation, the accelerated optical degradation of two different commercial Bisphenol-A Polycarbonate (BPA-PC) grades under elevated temperature stress is studied. The BPA-PC plates are used both in light conversion carriers in LED modules and encapsulants in LED packages. BPA-PC plates are exposed to temperatures in the range of 100–140 °C. Optical properties of the thermally-aged plates were studied using an integrated sphere. The results show that increasing the exposure time leads to degradation of BPA-PC optical properties, i.e. decrease of light transmission and increase in the yellowing index (YI). An exponential luminous decay model and Arrhenius equation are used to predict the lumen depreciation over different time and temperatures. Accelerated thermal stress tests together with the applied reliability model are used to predict the lifetime of plastic lens in LED lamps in real life conditions.     

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.     

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.     

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.     

A statistical study to identify the effects of packaging structures on lumen reliability of LEDs

The development of high-power light-emitting diode (LED) devices has been bedeviled by the reliability problems. And most reliability issues are caused by the packaging materials rather than the chips. However, which packaging material is the most influential remains unrevealed. To answer this question, a statistical method was introduced in this paper. Optical simulations were conducted to calculate the optical output power of LED package according to the orthogonal experimental design. Range and variance analyses were carried out to determine the significance of the relevant factors on the LED's light output. The results showed that the dome lens among the non-luminescent packaging materials had the most significance in affecting the light output. It is concluded that this method is useful in detecting the most significant part of LED packaging materials during the development of new packaging structures and is beneficial for enhancing the whole reliability of LED package effectively.     

元素掺杂对LED器件空间辐射性能的影响

发光二极管(LED)是卫星光通信系统中重要的光源部件,它的工作性能直接影响着光通信系统的可靠性。对粒子辐照条件下LED器件的少数载流子寿命和输出光功率变化进行了深入的理论分析,依据理论分析展开数值模拟计算。结果表明,提高LED有源区Zn元素掺杂浓度可有效降低少数载流子的辐射寿命,在提高LED输出功率的同时有效提升器件的空间抗辐射性能。     

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.     

Degradation of adhesion between Cu and epoxy-based dielectric during exposure to hot humid environments

Degradation of adhesion between Cu and epoxy-based dielectric build-up film (EDF), which is used for fan-out wafer level packaging applications, was investigated by the reliability test. The samples were composed of four layers: epoxy molding compound substrate, laminated EDF, electroless Cu, and electrolytic Cu. Adhesion was evaluated by the 90° peel test before and after the temperature and humidity (T&H) test and the highly accelerated temperature and humidity stress test (HAST). The effects of the peel strip width and humidity exposure of the peeled edges on adhesion were determined. It was found that adhesion gradually decreased during the reliability tests. The adhesion reduction mechanism was explained by the combinatorial effect of the loss in chemical bonding and the degradation of mechanical interlocking due to moisture. A black strip was observed along the edges of the peeled Cu strips (herein referred to as the black band, BB). AES revealed the BB as the superficial oxidation of the Cu strip. HAST and the T&H test showed the effect of the BB in an additional adhesion loss. SEM revealed a local cohesive fracture of the polymer, which led to an additional loss due to an early failure of the degraded polymer. Furthermore, BB growth showed a linear correlation with the square root of time and was accompanied with adhesion reduction. Since the modern packages have micro-sized Cu lines, the formation of BB should be avoided for better reliability of devices.     

Enhancement of light-emitting diode reliability using silicone microsphere in encapsulant

The aim of this study was to prevent the delamination of encapsulant for light-emitting diode (LED) without sacrificing other optical performance. Silicone microsphere was employed to control the properties of shrinkage and hardness of encapsulant during curing process. The effects of microsphere in encapsulant were investigated by using the different concentration of microsphere. After curing reaction, the shrinkage percentage and hardness of both shore A and D encapsulants were reduced and reinforced, as increasing the concentration of microsphere. Despite the addition of microsphere, the LED packages fabricated without/with microsphere showed the similar initial optical efficiencies. It is because the light scattering effect compensated their small difference of refractive indexes between encapsulants and microsphere. Using the silicone microsphere, the improvement of reliability of LED packages could be achieved after 1000 h under condition of temperature (85 °C)/humidity (RH 85%). It is attributed to the synergy effects of shrinkage suppression and hardness improvement.     

Thermal analysis of remote phosphor in LED modules

Phosphor plays an important role in LED packages by converting the wavelength of light and achieving specific color.The property and degradation of phosphor are strongly affected by the temperature.Some structural factors have been investigated in this paper and their effects are evaluated.Remote phosphor is an effective approach to improve the performance and reliability of LED modules and products.It is a trade-off that the final product design depends on both the thermal performance and the cost.     

Increasing the reliability of solid state lighting systems via self-healing approaches: A review

Reliability issues in solid state lighting (SSL) devices based on light emitting diodes (LED) is of major concern as it is a limiting factor to promote these optoelectronic devices for general lighting purposes. This postulate is even truer for high power devices in which high current and thus high thermal load are involved. In order to increase reliability and lighting efficacy, LED designs related to thermal management are evolving parallel to LED research and development. However there are still some issues mainly related to the degradation of LED’s constituents with time involving a faster decay of the lightning efficacy. In order to increase reliability of SSL devices, components presenting self-repairing properties could be implemented. In this review we will first briefly expose the state of the art on inorganic semiconductor based LED research and development, trends and challenges that lead to an increase of lighting efficiency. In a second part the different failure mode occurring for SSL devices have been compiled highlighting what are the main mechanism influencing and limiting LED reliability. Strong from this knowledge, in the last part, self-healing concepts will be proposed to further improve LED’s reliability.     

Chip package interaction for LED packages

Solid-state lightings (SSL) rapidly penetrate the global illumination market because of the energy efficiency and the reliability. The energy efficiency can be easily evaluated but the reliability is not convenient to be estimated. Among several reliability issues, a LED chip level's reliability could be a difficult problem because chip failures related to electromigration phenomenon are hard to be detected in the early stages. In order to remove potential leakage LEDs in modules, additional screening method is necessary to be performed occasionally. In this study, chip package interaction (CPI) for LED packages was investigated in order to estimate stresses of the LED chip in the module level. This methodology would help LED manufacturers to perform a robust design of LED packages in terms of the LED chip reliability. The electromigration is related to metal diffusion, which belongs to a creep phenomenon. As the creep strain is a function of temperature, stress and time, quantifying stresses in the metal layers of the LED die can be useful information for LED manufacturers to make an engineering decision in the early stages of manufacturing.     

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.     

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.     

LED光源的应用探索

LED是英文Light Emitting Diode的简称,中文名称叫发光二极管。LED具有发光效率高、使用寿命长、不易破损、开关速度高、高可靠性诸多优点。LED光源在可见光范围的应用主要包括显示屏、交通信号、汽车工业、LED背光源、情景照明、情调照明。LED光源在红外光区域的应用主要体现在红外成像、遥感、遥测以及光纤通信方面。     

Accelerated life test of high power white light emitting diodes based on package failure mechanisms

Accelerated life tests of high-power white light emitting diodes (LEDs) were conducted under an unbiased highly accelerated temperature and humidity test (HAST) and a normal aging test. The conditions in the unbiased HAST were 110 °C-85% RH, 130 °C-85% RH without bias. During the aging, the degradation mechanisms of optical power reduction and degradation of 455 mm blue wavelengths that were better than phosphors related yellow emission bands were observed. The microscopy analysis showed that this effect could be ascribed to the bubbling and discoloration of the silicone encapsulating material of the package. It is thought that these features are also responsible for the optical power reduction and thermal resistance increase.     

Improvement of Reliability of GaN-Based Light-Emitting Diodes by Selective Wet Etching With p-GaN

In an attempt to enhance the reliability of GaN-based light-emitting diodes (LEDs), the selective wet chemical etching of p-GaN surface in the GaN-based LEDs using KOH+NaOH in an ethylene glycol solution was investigated. The leakage currents of the etched LED under forward and reverse bias voltages were much lower, compared to those of a nonetched LED. The etched LED also showed improved light extraction efficiency and the degradation rate of light output power at a high injection current of 300 mA was slower than that for a nonetched LED. These results can be attributed to a decrease in the surface defects, an increase in hole concentration, and the increased surface roughness of the etched p-GaN     

Corrosion testing of anisotropic conductive adhesive interconnections on FR4, liquid crystal polymer and polyimide substrates

Anisotropic conductive adhesive films (ACF) have been widely studied for numerous applications. However, their resistance to corrosion in highly corrosive environments has been studied only very little. This study investigated the reliability and behaviour of ACFs in corrosive salt spray environment. ACF was used to attach flip chip (FC) components on FR4, liquid crystal polymer (LCP) and polyimide (PI) substrates and the FC packages were subjected to a salt spray test lasting 3000 h. The FC packages had daisy chain structures which were measured continuously in real time during testing. After testing cross sections of the tested packages were examined using an optical microscope and a scanning electron microscope (SEM). Most components failed during the test and the results showed significant differences between the various substrate materials. The LCP substrate performed considerably better than the other substrates and the PI substrate proved to have the poorest reliability. Corrosion of the pads on the substrates as well as open joints was seen in all substrate materials. The corrosion behaviour as well as the differences between the substrates showed that the substrate structure and material are critical factors in corrosive environments and should be carefully considered. The reliability of the ACF FC package with the LCP substrate was found to be good, as the test was very severe and no failures occurred during the first 625 h of testing and only 20% failed during the first 1000 h.     

High power LED package with vertical structure

A LED package structure with vertical geometry of W5II is proposed for high power optoelectronic semiconductor devices. To provide an efficient and easy method of assembling high-power LEDs to sockets of the fixtures, a vertical design referenced from the typical Edison screw base found in US-based lamp systems is used in the structure. The thermal simulation of the structure, fabrication processes and thermal characteristic are evaluated. Thermal resistance measurements are performed to characterize the thermal performance of W5II, and the optical and electrical characteristics of W5II are also verified. The good heat dissipation of W5II could be utilized to enhance the reliability and thermal fatigue capability of high-power LED packages.     

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。