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.     

Thermal cycling reliability of SnAgCu and SnPb solder joints: A comparison for several IC-packages

This paper deals with a comparison study between SnPb and SnAgCu solder joint reliability. The comparison is based on non-linear finite element modelling. Three packages have been selected: silicon CSP, underfilled flip chip and QFN package. Also the effect of thermal cycling conditions has been investigated. Comparing the induced inelastic strains in the solder joint, the lead-free SnAgCu generally scores better thanks to the lower creep strain rate. On the other hand for the CSP and flip chip package, SnAgCu scores worse for the more extreme loading conditions when the inelastic dissipated energy density is selected as damage parameter. The main reason is that due to the lower creep strain rate, the stresses become higher for SnAgCu resulting in higher hysteresis loops with more dissipated energy per cycle. For the QFN package, SnAgCu scores much better.     

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.     

铜互连电迁移失效的研究与进展

Cu/低k互连的电迁移失效与互连材料、工艺、结构和测试条件都有着密切的联系。论述了近年来铜互连电迁移可靠性的研究进展,讨论了电迁移的基本原理、失效现象及其相关机制和微效应以及主导失效的机制——界面扩散等,同时探讨了改善铜互连电迁移性能的各种方法,主要有铜合金、增加金属覆盖层及等离子体表面处理等方法,并指出了Cu互连电迁移可靠性研究有待解决的问题。     

LED器件的质量评定方法

本文提出了一种LED器件质量的评定方法,将电光转换效率和光功率保持率引入表征LED器件质量的关键参数;以芯片尺寸和电流密度来规范评定LED器件质量的测试条件;为了剔除早期失效的器件,给出了筛选方法;最后,介绍了与可靠性相关的LED器件耐久性试验。     

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.     

大功率LED的驱动电路设计

LED(Light Emitting Diode)作为新一代绿色光源,具有节能、环保和光转换效率高等特点,在照明应用方面已广泛展开.尤其是大功率LED光源更是备受喜爱,由于LED光源不能直接用市电220v电压直接供电,需特殊电压供电,因此,需要专门的驱动电路来点亮LED.本论文主要介绍一种LED恒流驱动电路,其采用恒流芯片PT4115来实现对大功率LED的高效恒流驱动.此电路具有效率高、成本低,可靠,安全等优点,适合当今大功率LED驱动电路的市场发展前景.     

LED芯片脏污与几种芯片工艺现象的辩识

文章介绍了几种常见的LED芯片表面脏污类别以及预防对策,包括芯片制造商生产过程中产生和封装厂生产中带入的脏污,另外列举了几种与脏污容易混淆的芯片工艺异常规象。     

Interfacial thermal stresses in ACF bonding assembly

The interfacial stresses and chip cracking stress produced because of thermal and mechanical mismatch in layered electronic assembly are one of main reasons for the failure of electronic packages. The analytical model considering the nonlocal deformation of assembly was developed and applied to predict the interfacial stresses produced due to temperature variation for the short and long anisotropic conductive adhesive film (ACF) bonding assembly. The conditions of zero shear stress at the free ends and self-equilibrated peeling stresses were satisfied. Simultaneously the interfacial stresses of ACF assembly were also predicted by the corrected Suhir’s model, Wang’s model, Ghorbani’s model, local model and finite element model (FEM), which were compared with the results by the present model. In addition, the analytical expression of chip cracking stress was also obtained for layered electronic assembly. The approach is mathematically straightforward and can be extended to include the inelastic creep behavior.     

Electromigration: A review

The aggressive scaling planned for integrated circuits is placing more demands on the materials, processes and designs. The circuits must be reliable and electromigration is a key reliability issue. Because many of the factors that contribute to electromigration are not completely understood, manufacturers must proceed cautiously. In this article electromigration is reviewed from the prospective of the reliability engineer, focusing on those areas of greatest applicability to the manufacturing environment. First, the fundamental physics of electromigration are examined to provide a basis for understanding the factors that affect the lifetimes under the various test conditions. Then, empirical data concerning the impact on reliability of metal stripe geometry, structure and composition are reviewed. The care necessary to make fast, wafer-level tests an important process control tool is discussed. It is shown that pulsed-dc and ac waveforms can provide longer lifetimes than predicted previously, providing some relaxation of current density requirements for higher circuit densities. An understanding of these phenomena is necessary for the reliability engineer to assess today's and tomorrow's integrated circuits.     

高功率LED热管理方法研究最新进展

LED芯片结温的高低直接影响其出光效率、工作寿命和可靠性。在分析系统各个环节热阻的基础上,详细评述了高功率LED产品从芯片到系统级的热管理研究新动向,包括:自然对流冷却,采用压电风扇、电离方法所进行的强迫空气对流冷却,采用水、液态金属作为冷却工质的液冷方法,采用热管实现的相变冷却,采用热电片进行的固态冷却方案以及利用热电片对余热进行回收利用的热管热回收方案和液体金属冷却方法。并在上述基础上提出了发展更高功率密度LED热管理方法的关键科学问题。     

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.     

Assessment of Ni/Pd/Au–Pd and Ni/Pd/Au–Ag Preplated Leadframe Packages Subject to Electrochemical Migration and Mixed Flowing Gas Tests

Preplated leadframes packages with a new noble metal finish type were exposed to various electrochemical migration tests, and mixed flowing gas environments to assess the quality and reliability due to these environmental conditions. Packages were tested as discrete parts and also as parts mounted onto printed circuit boards to assess the effect of reflow soldering. Selected packages with SnPb plated leadframes were tested for comparison purposes. After test exposure, optical microscopy, scanning electron microscope, and energy dispersive spectroscopy analyses were conducted to evaluate the corrosion behavior. Corrosion and creep corrosion on the leads were observed on all the preplated test samples subject to the mixed flowing gas. Creep corrosion over the mold compound surface was identified on the unmounted quad flat no-lead packages. The creep corrosion products were found to be electrically conductive and could bridge adjacent lead pairs     

激光钻孔GaN基LED可靠性研究

对GaN基大功率LED工作的可靠性进行了分析,进而提出采用激光钻孔技术,利用高能激光束将蓝宝石基板打出孔洞,并在孔洞内壁蒸镀金属层薄膜。利用金属良好的导热性能,将芯片表面热能传导至基板,并利用封装技术使LED工作时产生的热量能迅速传导至环境中,降低热效应带来的不良影响。实验结果表明:在注入电流350 mA条件下,采用激光钻孔技术的LED较常规结构的LED,散热效率增加约15.0%,抗静电能力提高约500 V,连续工作1 000 h,亮度平均衰减低2.8%左右。     

Field failure due to creep corrosion on components with palladium pre-plated leadframes

Creep corrosion is a mass-transport process during which solid corrosion products migrate over a surface. It has been identified as a failure mechanism responsible for the malfunction of electrical contacts and connectors. For components with noble metal pre-plated leadframes, creep corrosion is a potential reliability risk for long-term field applications. This paper explains the source, process and possible products of creep corrosion on integrated circuit (IC) packages. The operating environment for telecom equipment is introduced as an example demonstrating the corrosive environments for electronic components. Field failure due to creep corrosion on plastic encapsulated components with palladium pre-plated leadframes is presented and analyzed. Similar phenomenon has been reproduced in an accelerated manner using mixed flowing gas (MFG) testing in the laboratory.     

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.     

LQFP和eLQFP热机械可靠性的有限元分析

封装形式的差异性对产品可靠性具有重要影响。基于有限元法,对比分析了薄型四方扁平封装(LQFP)和载体外露薄型四方扁平封装(eLQFP)在室温和回流焊温度下的翘曲、芯片和粘片胶的应力水平以及各材料界面应力分布。研究表明,LQFP的翘曲比eLQFP的大,但芯片和粘片胶上的最大应力无明显差别;eLQFP在塑封材料与芯片有源面界面的应力水平比LQFP的大;eLQFP在芯片与粘片胶界面、粘片胶与芯片载体界面的剪切应力比LQFP的大,但eLQFP在芯片与粘片胶界面、粘片胶与芯片载体界面的剥离应力比LQFP的小;eLQFP在塑封材料与芯片载体镀银区界面的应力水平高于LQFP的应力水平,由于塑封材料与镀银芯片载体的结合强度弱,eLQFP更易发生界面分层。     

Mechanical fatigue test method for chip/underfill delamination in flip-chip packages

Underfill resin between Si chips and printed circuit boards is useful for improving the reliability of flip-chip packages. Generally, thermal cycle tests (TCTs) are applied to electronic packages under development in order to prove their reliability. At the early stage of development, however, a more effective test method is desirable, because TCTs are time-consuming. A new mechanical fatigue test for the underfill resin in flip-chip packages, namely the four points support test method, is proposed in this paper. The validity of the mechanical test method could be verified from the results of stress analyses and experiments. Considering the chip/underfill delamination statistically based on the assumption of Markov process, it was shown that the delamination probability during cyclic loads could be estimated with equations of the displacement range and number of cycles.     

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.     

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.