两种热电分离式MCPCB导热性能的对比研究

利用SMT工艺将两种功率不同的LED分别与设计完全相同的热电分离式铜基板及铝基板组装成模组,然后借助结温测试系统及积分球系统对两种金属基板的散热性能进行了对比研究。结果表明,热电分离式铜基板较之热电分离式铝基板仅具备微弱的散热优势,这种优势随着LED的功率增加有所扩大。当LED功率为9 W时,铜基板及铝基板所对应的LED模组热阻分别是3.16℃/W、3.26℃/W;当LED功率为15 W时,铜基板及铝基板所对应的LED模组热阻分别是2.33℃/W、2.46℃/W。     

Heat dissipation design and analysis of high power LED array using the finite element method

High-power Light Emitting Diode (LED) technology has developed rapidly in recent years from illumination to display applications. However, the rate of heat generation increases with the LED illumination intensity. The LED chip temperature has an inverse proportion with the LED lifetime. High-power LED arrays with good thermal management can have improved lifetime. Therefore, for better optical quality and longer LED lifetime it is important to solve the LED thermal problems of all components. In particular, Metal Core Printed Circuit Board (MCPCB) substrate heat sink design and thermal interface materials are key issues for thermal management. This paper presents an integrated multi-fin heat sink design with a fan on MCPCB substrate for a high-power LED array using the finite element method (FEM). The multi-fin heat sink design and simulation results provide useful information for LED heat dissipation and chip temperature estimation.     

The optimal design of the thermal spreading on high power LEDs

New energy and energy saving are the two basic energy issues. This study proposes an innovative optimal method to design LEDs with high thermal spreading holder. The optimization uses the FEM combined with the simplified conjugated gradient method (SCGM). The minimal temperature of the MCPCB backside of the LEDs is obtained throughout this design. The simulations have been proofed by the experiment of IR and thermal couple measurement. The optimal temperature decreases at about 4 °C compared with the original temperature without any active cooling device. This design can prolong the LEDs? life to benefit the energy saving obviously. In addition, this design combined with the active cooling device will achieve better heat dissipation on cooling of the electronic component.     

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.     

Investigation of silicon-based light emitting diode sub-mounts: Enhanced performance and potential for improved reliability

Aluminum nitride (AlN) and aluminum oxide (Al₂O₃) ceramic light emitting diode (LED) sub-mounts are the most widely used package substrate for recently-developed high-brightness (HB) LED package applications because they exhibit superior thermal conductivity compared to conventional printed circuit board (PCB) package substrates. Nonetheless, the Al₂O₃ ceramic sub-mount exhibits thermal conductivity in an unacceptable range, and manufacturing the AlN ceramic sub-mount is problematic due to high material cost and difficult processing. Wafer-level packaging (WLP) technology has shown noticeable improvements in manufacturing and silicon exhibits outstanding thermal conductivity. Thus silicon might become an alternative package substrate for HB LEDs. This research studied the feasibility of replacing conventional ceramic sub-mounts with WLP LED sub-mounts. The performance features of thermal dissipation, insulation, and high temperature reliability of LED sub-mounts with variable SiO₂ thickness were analyzed and compared to the results obtained from conventional Al₂O₃ and AlN ceramic sub-mounts. Experimental results show that silicon LED sub-mounts lead to better thermal dissipation performance than do Al₂O₃ ceramic sub-mounts, and the results also reveal acceptable insulation performance and high temperature reliability for silicon sub-mounts.     

陶瓷复合FR4结构界面形貌与导热性能

基于热电分离式设计理念,将AlN陶瓷片金属化后作为微散热器嵌入FR4材料内形成了复合散热基板.采用电镜扫描、光学显微,通过冷热循环冲击试验对FR4与AlN两相界面处在高低温突变情况下的界面形貌进行了分析.利用ANSYS软件对基板进行了仿真热模拟,研究了AlN嵌入后FR4导热性能的变化规律.利用结温测试仪、功率计和半导体制冷温控台等仪器设备,通过结温测试对比研究了该复合散热结构与金属芯印刷电路板(MCPCB)对大功率LED封装散热效果的影响.结果表明,该复合散热基板在经低温-55℃,高温125℃,1 000个冷热循环后,FR4和AlN界面无剥离现象发生,在环境温度急剧变化的条件下结合力良好.同时,FR4在嵌入AlN之后,导热性能得到了明显改善,且与MCPCB相比,能更有效降低LED芯片结温.     

倒装大功率白光LED热场分析与测试

散热是影响大功率LED正常工作及器件寿命的关键因素,本文利用有限元法(FEM)对W级倒装大功率白光LED的空间温度场分布进行了模拟计算,结果与用自制的测试装置测量的温度分布相吻合。在此基础上,保持电流密度不变,研究芯片尽寸与结区温度的关系,计算出在当前的发光效率和封装结构条件下,由于散热条件的限制,芯片能承受的最大功率和能达到的最大尺寸。     

Status and prospects for phosphor-based white LED packaging

The status and prospects for high-power, phosphor-based white light-emitting diode (LED) pack-aging have been presented. A system view for packaging design is proposed to address packaging issues. Four aspects of packaging are reviewed: optical control, thermal management, reliability and cost. Phosphor materials play the most important role in light extraction and color control. The conformal coating method improves the spatial color distribution (SCD) of LEDs. High refractive index (RI) encapsulants with high transmittance and modified surface morphology can enhance light extraction. Multi-phosphor-based packaging can realize the control of correlated color temperature (CCT) with high color rendering index (CRI). Effective thermal management can dissipate heat rapidly and reduce thermal stress caused by the mismatch of the coefficient of thermal expansion (CTE). Chip-on-board (COB) technology with a multi-layer ceramic substrate is the most promising method for high-power LED packaging. Low junction temperature will improve the reliability and provide longer life. Advanced processes, precise fabrication and careful operation are essential for high reliability LEDs. Cost is one of the biggest obstacles for the penetration of white LEDs into the market for general illumination products. Mass production in terms of CoB, system in packaging (SIP), 3D packaging and wafer level packaging (WLP) can reduce the cost significantly, especially when chip cost is lowered by using a large wafer size.     

LED封装中的散热研究

文章论述了大功率LED封装中的散热问题,说明它对器件的输出功率和寿命有很大的影响,分析了小功率、大功率LED模块的封装中的散热对光效和寿命的影响。对封装及应用而言,增强它的散热能力是关键技术,指出对大功率LED和LED模块散热设计很重要,因为大功率白光LED的光效和寿命取决于其散热。目前大功率LED的重点是提高散热能力,说明封装结构和封装材料在提高大功率LED散热中的影响,LED模块的散热是未来的重点。通过选用高热导率材料可以使温度得到显著控制,重点论述了封装的关键技术,最后指出了未来LED封装技术的发展趋势。     

Light degradation test and design of thermal performance for high-power light-emitting diodes

Light-emitting diodes (LEDs), which are generally used for indicator lights, have been continuously developed for the past 50 years. With an urgent need for energy conservation and pollution reduction, the trend of replacing traditional incandescent or fluorescent lamps with high-power LEDs is growing more and more popular. Consequently, LEDs have attracted the attention of many industries that incorporate LED researches and designs into their products. However, a low electro-optical conversion efficiency of LEDs can induce a high percentage of input power that transforms into redundant heat. This leads to an increase in the junction temperature, which indicates that thermal management is an important issue in high-power LEDs. In this research, a light degradation test is implemented and a chip-in-substrate-type LED packaging structure is proposed. A finite element (FE) model of the chip-in-substrate-type structure with an effective methodology, which is validated through the forward voltage method, is established. With regard to the design concepts of LED packaging, various parameters of chip-in-substrate-type structure are investigated.     

如何确定大功率LED的工作电流

针对大功率LED应用,建立了大功率LED的热阻模型,分析了环境温度、器件的最大允许结温、热阻以及输入电流之间的关系,提出了一种保证器件结温低于容许最大结温的工作电流的确定方法,并给出了一个应用本方法绘制某大功率LED工作电流与环境温度的关系曲线的实例．本方法对大功率LED应用具有重要的指导意义。     

Thermal Management and Interfacial Properties in High-Power GaN-Based Light-Emitting Diodes Employing Diamond-Added Sn-3 wt.%Ag-0.5 wt.%Cu Solder as a Die-Attach Material

The thermal management of high-power GaN-based light-emitting diodes (LEDs) soldered with Sn-3 wt.%Ag-0.5 wt.%Cu (SAC305) solder and diamond-added SAC305 solder was evaluated. Diamond addition was found to significantly reduce the surface temperature and total thermal resistance of the LEDs, revealing that diamond-added SAC305 solder is a promising die-attach material for high-power LED packaging. Interfacial reactions in the LED solder joints were also investigated. The thin Au wetting layer in the chip’s backside metallization was rapidly consumed in the initial stage of reflow, forming an AuSn₄ phase at the interface. Subsequently, the AuSn₄ phase detached from the interface, leading to dewetting of the SAC305 solder from the LED chip. To avoid dewetting, a new backside metallization of LED chips should be developed for SAC305 solder.     

带有TSV的硅基大功率LED封装技术研究

介绍了一种带有凹槽和硅通孔(through silicon via,TSV)的硅基制备以及晶圆级白光LED的封装方法。针对硅基大功率LED的封装结构建立了热传导模型,并通过有限元软件模拟分析了这种封装形式的散热效果。模拟结果显示,硅基封装满足LED芯片p-n结的温度要求。实验结合半导体制造工艺,在硅基板上完成了凹槽和通孔的制造,实现了LED芯片的有效封装。热阻测试仪测得硅基的热阻为1.068K/W。实验结果证明,这种方法有效实现了低热阻、低成本、高密度的LED芯片封装,是大功率LED封装发展的重要方向。     

大功率LED热管散热器的模块化设计与研究

针对当前大功率LED散热器多采用肋片式散热器,散热效果不是很理想,同时当前灯具都采用一体化的集成结构,在维护和检修的过程中,只能整体返厂,给大功率LED灯具的使用带来不便等问题,提出了大功率LED热管散热器模块化思路。通过模块化设计,大功率LED的散热效果得到了提高,且能够进行及时的维护,多数部件可以循环利用,延长了灯具的生命周期,降低了维修成本。对大功率LED热管散热器模块进行了设计优化,并且通过ICEPAK散热软件进行了模拟,结果表明此热管散热器模块大大提高了大功率LED的散热性能。     

LED封装的热控制优化设计

基于某型LED模块的实验结果,从热膨胀匹配、冷却技术的选择、发光效率、焊料及硅胶选择等方面对大功率LED的热控制问题进行了优化设计。实验结果表明,优化设计后的大功率LED的转换效率和光通量等指标都有了一定程度的改善。     

Structural and electrical characteristics of lanthanum oxide gate dielectric film on GaAs pHEMT technology

This paper investigates the feasibility of using a lanthanum oxide thin film (La_2O_3) with a high dielectric constant as a gate dielectric on GaAs pHEMTs to reduce gate leakage current and improve the gate to drain breakdown voltage relative to the conventional GaAs pHEMT. An E/D mode pHEMT in a single chip was realized by selecting the appropriate La_2O_3 thickness. The thin La_2O_3 film was characterized: its chemical composition and crystalline structure were determined by X-ray photoelectron spectroscopy and X-ray diffraction, respectively.La_2O_3 exhibited good thermal stability after post-deposition annealing at 200, 400 and 600 ℃ because of its high binding-energy (835.6 eV). Experimental results clearly demonstrated that the La_2O_3 thin film was thermally stable.The DC and RF characteristics of Pt/La_2O_3/Ti/Au gate and conventional Pt/Ti/Au gate pHEMTs were examined.The measurements indicated that the transistor with the Pt/La_2O_3/Ti/Au gate had a higher breakdown voltage and lower gate leakage current. Accordingly, the La_2O_3 thin film is a potential high-k material for use as a gate dielectric to improve electrical performance and the thermal effect in high-power applications.     

溶胶-凝胶法制备硼硅玻璃掺杂BST陶瓷的研究

研究了Si-B-O系玻璃掺杂对钛酸锶钡(BST)陶瓷的相结构和介电性能的影响.实验结果表明,当x(SiO_2)＞10%时,Si-B-O系玻璃掺杂BST陶瓷易出现杂相,即Ba_2TiSi_2O_8相.x(SiO_2)≤10%,Si-B-O系玻璃掺杂BST陶瓷粉体的相结构为立方钙钛矿相结构,其合成温度大于等于600 ℃,不存在第二相. Si-B-O系玻璃掺杂BST陶瓷的烧结温度低于传统工艺.Si-B-O系玻璃掺杂BST陶瓷的显微结构呈细晶结构(晶粒尺寸＜1 μm).随玻璃含量的增加,Si-B-O系玻璃掺杂BST陶瓷介电常数ε降低,介电峰变低,平坦,峰形宽化,介电损耗降低,居里温度TC向低温移动.     

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.     

Polyhedral Oligosilsesquioxane‐Modified Boron Nitride Nanotube Based Epoxy Nanocomposites: An Ideal Dielectric Material with High Thermal Conductivity

Dielectric polymer composites with high thermal conductivity are very promising for microelectronic packaging and thermal management application in new energy systems such as solar cells and light emitting diodes (LEDs). However, a well‐known paradox is that conventional composites with high thermal conductivity usually suffer from the high dielectric constant and high dielectric loss, while on the other hand, composite materials with excellent dielectric properties usually possess low thermal conductivity. In this work, an ideal dielectric thermally conductive epoxy nanocomposite is successfully fabricated using polyhedral oligosilsesquioxane (POSS) functionalized boron nitride nanotubes (BNNTs) as fillers. The nanocomposites with 30 wt% fraction of POSS modified BNNTs exhibit much lower dielectric constant, dielectric loss tangent, and coefficient of thermal expansion in comparison with the pure epoxy resin. As an example, below 100 Hz, the dielectric loss of the nanocomposites with 20 and 30 wt% BNNTs is reduced by one order of magnitude in comparison with the pure epoxy resin. Moreover, the nanocomposites show a dramatic thermal conductivity enhancement of 1360% in comparison with the pristine epoxy resin at a BNNT loading fraction of 30 wt%. The merits of the designed composites are suggested to originate from the excellent intrinsic properties of embedded BNNTs, effective surface modification by POSS molecules, and carefully developed composite preparation methods.     

Thermal investigation of LED array with multiple packages based on the superposition method

In this paper, the superposition method is used to investigate the complete temperature field of a light-emitting diode (LED) packaging substrate, based on the results of transient temperature rise measurements and the thermal resistance coupling matrix. The feasibility of use of the superposition method in an LED array with multiple packages has been proved first by temperature comparisons with the simultaneous operation of an array (5×5) of 25 high power LEDs mounted on a metal core printed circuit board (MCPCB). Compared with existing approaches, the superposition method will measure the internal temperature of chip directly, accurately and nondestructively. According to the relatively accurate and reliable self-heating and coupling temperature rise data, optimization scheme of LED lamp with multiple packages is proposed. The results show that increasing the heat source separation distance and improving the thermal conductivity of thermal interface materials will reduce the temperature rise and thermal non-uniformity.