Experimental investigation on the performance of CPU coolers: Effect of heat pipe inclination angle and the use of nanofluids

An experimental study of heat transfer performance of a CPU cooling heat pipe, examining the effects of inclination angle and nanofluids, has been conducted. It is shown that inclination angle of the unit has a significant effect on the cooling process, since it directly influences the operation of the evaporator. The effect is mainly due to the capillary effect and boiling limits of the heat pipe. The results demonstrate that for a given CPU temperature, there is a threshold angle at which the thermal resistance of the heat pipe increases dramatically. It is observed that as the CPU temperature increases, the threshold angle decreases from 60° to 30°. Introduction of 0.5 wt% Al₂O₃ nanoparticles to the water coolant of heat pipe has led to a decrease in thermal resistance. It is shown that at 10 W, the presence of nanofluid has reduced the thermal resistance by 15%, while at 25 W, the thermal resistance has dropped by 22%.     

Phase-change immersion cooling high power light emitting diodes and heat transfer improvement

A new cooling method of ethanol direct-contact phase-change immersion cooling was proposed in the thermal management of high power light emitting diodes (LED) and the feasibility of this cooling method was investigated. The heat generated by LED was measured firstly using two types of power systems: DC power and LED driver. Then the heat dissipation performance was evaluated under different experimental conditions. The results indicate that startup process of the cooling system is quick and only 450 s is needed to reach steady-state under heat load of 42.78 W. The minimum thermal resistance of 1.233 °C/W is obtained when liquid filling ratio is 33.14%. The junction temperature of LED under different absolute pressures is much lower than the limited value of 120 °C. Baffle with total height of 140 mm, bottom space height of 20 mm and distance away from substrate surface of LED of 8 mm improves heat transfer performance best due to ethanol self-circulating in the cooling receiver. Overall, the ethanol phase-change immersion cooling is an effective way to make sure high power LED work reliably and high efficiently.     

Study on a cooling system based on thermoelectric cooler for thermal management of high-power LEDs

To improve the heat dissipation of high-power light-emitting diodes (LEDs), a cooling system with thermoelectric cooler (TEC) is investigated. In the experiment, the 6 × 3 W LEDs in two rows are used to compose the light source module and the environment temperature is 17 °C. The temperatures of heat dissipation substrate of LEDs and cooling fins of a radiator are measured by K type thermocouples to evaluate the cooling performance. Results show that the temperature of the substrate of LEDs reaches 26 °C without TEC. However, it is only 9 °C when the best refrigeration condition appears. The temperature of the substrate of LEDs decreases by 17 °C since the heat produced by LEDs is absorbed rapidly by TEC and dissipated through the radiator, and the junction temperature of LEDs reaches only 45 °C which is much lower than the absolute maximum temperature of LEDs (120 °C). The experiment demonstrates that the cooling system with TEC has good performance.     

大功率LED灯具散热器的优化设计研究

LED的散热性能对其寿命有至关重要的影响,而现阶段LED的散热器设计方法很多依靠的是经验值。本文针对这一问题,提出一种散热器的优化设计方法,该方法首先基于大量实验数据研究了散热器翅片单一因素（包括翅片高度、翅片间距、翅片厚度、底板厚度）对散热性能的影响,然后运用正交试验法,综合分析了各因素对散热性能的影响程度,最终得到了翅片参数的最优水平组合。最后本文利用此方法,设计了一款新的散热器,实验表明该LED灯源芯片温度降低,并计算了优化后翅片的效率。     

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.     

一种回路热管对大功率LED散热的研究

针对大功率LED散热能力较其它照明灯具差这一问题,研制了一种应用在多芯片大功率LED散热上的回路热管装置,并研究了热负荷、倾角等对热管的起动性、均温性和热阻等的影响。试验结果表明,所设计的热管散热器的热阻在0.48～1.47K/W之间;在蒸发器倾斜角为0°和30°时,蒸发器的均温性分别被控制在1.5℃和4.3℃以内。因此,将这种结构的热管应用在大功率LED散热系统中时,首先应该对蒸发器倾斜角度对系统散热性能的影响进行测试评估。     

Thermal analysis of high power LED package with heat pipe heat sink

The goal of this study is to improve the thermal characteristics of high power LED (light-emitting diode) package using a flat heat pipe (FHP). The heat-release characteristics of high power LED package are analyzed and a novel flat heat pipe (FHP) cooling device for high power LED is developed. The thermal capabilities, including startup performance, temperature uniformity and thermal resistance of high power LED package with flat heat pipe heat sink have been investigated experimentally. The obtained results indicate that the junction temperature of LED is about 52 °C for the input power of 3 W, and correspondingly the total thermal resistance of LED system is 8.8 K/W. The impact of the different filling rates and inclination angles of the heat pipe to the heat transfer performance of the heat pipe should be evaluated before such a structure of heat pipe cooling system is used to cool high power LED system.     

基于微通道致冷的大功率LED阵列封装热分析

采用微通道致冷技术,设计了大功率LED阵列封装的微通道致冷结构,并应用热分析软件模拟了其热性能,探讨不同鳍片结构尺寸、流速、功率等参数对LED多芯片散热效果的影响.文中提出了采用交错通道以提高LED封装的散热能力,模拟结果显示,交错微通道致冷的封装结构能很好地满足大功率LED阵列的散热需要.     

Novel Thermoelectric Modules for Cooling Powerful LEDs: Experimental Results

We present the results of an experimental study of a cooling system based on a novel thermoelectric module specifically designed for thermal management of high-power light-emitting diodes (LEDs). The Seoul Semiconductor LED W724C0 device was chosen for experimental validation of the efficiency of the proposed cooling unit. Two cooling systems with identical heat sinks were tested for comparison: a state-of-the-art one based on an insulated metal substrate-printed circuit board (IMS-PCB), and a system with thermoelectric cooling. The obtained results show that use of thermoelectrics results in a considerable reduction of the LED operating temperature, providing increased light output and greatly increased LED lifetime.     

LED筒灯散热器正交试验优化设计研究

为了达到降低大功率LED筒灯制造成本同时又保证散热能力的目的,采用正交试验法优化设计了散热器。分析了散热器基板直径、基板厚度、翅片厚度、翅片间距、翅片高度和翅片轮廓直径等6个因素对散热器重量与LED温度的影响。然后利用CFD热仿真软件对LED筒灯散热器进行建模与散热仿真,最终得出了一个较为理想的优化结果。仿真与实验测试数据结果表明,采用优化后散热器的LED温度较优化前略有下降,但散热器重量降幅超过30%,实现了较好的优化目标。     

应用于大功率LED器件的回路热管散热研究

为解决大功率LED的散热问题,提出一种应用于大功率LED散热的微型回路热管,研究了充液率和倾斜角度对热管冷却大功率LED的启动性能、结温和热阻等特性的影响.研究结果表明:热管的最佳充液率为60％,系统的总热阻为7.5 K/W,此时对应的热管的热阻为1.6 K/W;热管的启动时间约为6.5 min,LED的结点温度被控制在42℃以下,很好地满足了大功率LED的结温稳定性要求.     

Diffusional creep induced stress relaxation in thin Cu films on silicon

The results of stress measurements during annealing of thin copper films deposited on 100 μm Si substrates are presented. The stress in thin films was determined by using an optical system for curvature measurements. The annealing experiments were done during thermal cycles of heating and cooling procedures from room temperature up to 400 °C with a rate 10 °C/min. The total thickness of thin films was between 20 and 100 nm. The obtained results showed that the difference between the end and the initial values of the ratio of force to width increases with the thickness of the samples. The initial linear shape of the temperature-stress plots reaches higher temperature values with an increase in film thickness. In order to explain the observations, the dependence of stress on temperature was calculated using the rate of Coble creep. It was found that the theoretical curves reveal the same features as the experimental data. It was concluded that diffusional creep mechanism dominates for thin film of thickness below 100 nm.     

Fabrication and thermal analysis of flip-chip light-emitting diodes with different numbers of Au stub bumps

The thermal performance of flip-chip (FC) light-emitting diodes (LEDs) with different numbers of Au stub bumps has been investigated by using thermosonic bonder. The LEDs were mounted on the aluminium nitride (AlN) sub-mounts which have superior thermal conductivity (230 W/mK), and the high power Chip-on-Plate (COP) package was proposed to be used for our measurement. In order to understand the thermal performance of the high power FC-LEDs, the experimental measurement and finite-element model (FRM) numerical simulation have been used. It is found that the thermal performance of our 1 × 1 mm² FC-LEDs can only be improved when using at least 6 Au stub bumps as interconnected metals. Moreover, the surface temperature of FC-LEDs is significantly reduced while using 20 Au stub bumps.     

Solder-joint reliability of HVQFN-packages subjected to thermal cycling

In this work experimental results of thermal cycling tests on HVQFN-packages mounted on printed circuit boards are combined with finite element analyses. Validating the finite element analyses by a selected series of small, medium and large HVQFN-packages assembled on printed circuit boards, allows us to determine the performance of this family. To be able to do that, the discriminating parameters that determine the board level performance of this family need to be understood. The emphasis is on the fatigue life of the soldered interconnections as it is influenced by the thermal stress load, the board thickness, and the dimension of the package. Data from different experimental set-ups are compared. An important parameter in this respect is the inclusion of the base material of the panels. The test loads were set to cycling at −40 °C/+125 °C and −20 °C/+100 °C. The results prove that the essential physical properties governing the fatigue life are the stiffness of the complete assembly and the thermal expansion mismatch between the parts.     

Fabrication of periodic square arrays by angle-resolved nanosphere lithography

We investigate the fabrication of periodic square arrays of solid gold islands by angle-resolved nanosphere lithography (ARNSL) in conjunction with thermal evaporation and etching. By varying θ (the tilt angle between the direction of gold deposition beam and the substrate surface normal) and ? (the substrate rotation angle about the beam axis), adjacent islands on a deposited hexagonal gold array will have a constant and periodic difference in height. Upon etching, this height bias will result in the shorter structures being removed to produce an array with a different symmetry from the original hexagonal symmetry of the parent mask. By depositing at three directions of ? = 0°, 120° and −120° with a constant θ = 20°, experimental results show that deposited two-dimensional gold periodic arrays will have a measurable difference in height between adjacent islands. Etching of the resulting patterns produced periodic near-square arrays with triangular nanostructures. Thus the combination of ARNSL and etching can allow selective periodic nanostructures to be removed, increasing the diversity of array symmetries available through nanosphere lithography.     

Optimization of flexible printed circuit board electronics in the flow environment using response surface methodology

A flexible printed circuit board (FPCB) is flexible, thin and lightweight; however, FPCBs experience more deflection and stress in the flow environment because of fluid–structure interaction (FSI), which affects their performance. Therefore, the present study focuses to optimize a typical FPCB electronic in order to minimize the deflection and stress induced in the system. In this study, numeric parameters (i.e., flow velocity, component size, component thickness, misalignment angle, as well as the length and width of the FPCB) were optimized using response surface methodology (RSM) with the central composite design technique. The separate effects of the independent variables and their interactions were investigated. The optimized condition was also examined to substantiate the empirical models generated using RSM. At a flow velocity of 5 m/s, the optimum values of the component size, component thickness, misalignment angle, as well as the length and width of the FPCB were determined at 11.69 mm, 12.37 mm, −0.73°, as well as 180 mm and 180 mm, respectively. This optimized condition resulted in a maximum deflection of 0.402 mm and a maximum stress of 0.582 MPa. The findings conveyed can contribute to the development of FPCB industries.     

A Microjet Array Cooling System for Thermal Management of High-Brightness LEDs

In this paper, a microjet-based cooling system is proposed for the thermal management of high-power light-emitting diodes (LEDs). Preliminary experimental investigation and numerical simulation on such an active cooling system are conducted. In the experiment investigation, thermocouples are packaged with LED chips to measure the temperature and evaluate the cooling performance of the proposed system. The experimental results demonstrate that the microjet-based cooling system works well. For a 2 times 2 LED chip array, when the input power is 5.6 W and the environment temperature is 28degC, without any active cooling techniques, the temperature of 2 times 2 LED chip array substrate reaches 72degC within 2 min and will continue to increase sharply. However, by using the proposed cooling system, when the flow rate of micropump is 9.7 mL/s, the maximum LED substrate temperature measured by the thermocouples will remain stable at about 36.7degC. As for the numerical optimization, the comparison between the simulation and experimental results is presented to confirm the feasibility of the simulation model. By using the simulation model, the effects of microjet diameter, top cavity height, micropump flow rate, and jet device material on system performance are numerically studied. According to the preliminary test and numerical optimization, an optimized microjet cooling system is fabricated and applied in thermal management of a 220-W LED lamp. The temperature test demonstrates that the cooling system has good performance.     

Pressure Drop and Heat Transfer Correlations for Triangular Folded Fin Heat Sinks

Experiments have been performed to investigate the cooling performance of triangular folded fin heat sinks made of 6000 series aluminum in a duct flow. The dimension of the triangular folded fin heat sink is 70mm in width and 92 mm in length with an 8-mm-thick base plate. The fin height is varied from 19 to 36 mm and the fin pitch from 5.0 to 9.0 mm. The duct air velocity is in the range of 1.0 to 5.0 m/s and the corresponding Reynolds number based on the hydraulic diameter is varied from 212 to 1974. The experimental results show that the cooling performance of triangular folded fin heat sink is influenced by the fin pitch, the Reynolds number, and the fin height. It increases substantially as the fin pitch decreases and the Reynolds number and the fin height increase. By compiling the experimental data, the heat transfer and the friction factor correlations with plusmn6.5% and plusmn20% accuracy, respectively, are provided for effective design of triangular folded fin heat sinks     

Barrier layer thickness analysis for reliable copper plug process in CMOS technology

Copper (Cu) replacing conventional tungsten (W) based 1st contact plug has become necessary for high performance CMOS. For a reliable and well optimized Cu plug, process selection and integration of plug diffusion barrier is an important task. In this work, we investigate barriers for Cu plug technology for CMOS process. Single layer TaN and bi-layer Ta + TaN barrier materials were studied for their effectiveness in preventing Cu diffusion into device active regions for backend thermal stress conditions. The degradation of device characteristics was used as monitor of robustness of barrier reliability. Diffusion of Cu in multilayer plug structure is modeled to explain observed stress behavior. From the model studies, the critical barrier layer thickness needed to prevent Cu diffusion is determined. We show than that a ∼7 nm sidewall barrier is effective in preventing Cu diffusion into Si at up to 350 °C/60 min only, while a minimum thickness of ∼10 nm is needed for blocking copper diffusion at 420 °C/30 min. Using multilayer Cu diffusion model, an optimized process window for reliable, low contact resistance Cu plug technology for CMOS process can be obtained.     

Design methodologies for reliability of SSL LED boards

This work presents a comparison of various LED board technologies from thermal, mechanical and reliability point of view provided by an accurate 3-D modelling. LED boards are proposed as a possible technology replacement of FR4 LED boards used in 400 lumen retrofit SSL lamps. Presented design methodology can be used for other high power SSL lamp designs. The performance of new LED board designs were evaluated by numerical modeling. Modeling methodology was proven by measurement on reference FR4 LED board. Thermal performance was compared by extracting of LED boards thermal resistances and thermal stress has been inspected considering the widest temperature operating range according to standards (−40 to +125 °C). Thermo-mechanical and reliability analysis have been performed to study parameters of each LED board technology, using thermal boundary conditions extracted from the thermal simulation of a whole LED lamp. Elastic–plastic analysis with temperature dependent stress–strain material properties has been performed. The objective of the work is to optimize not only the thermal management by thermal simulation of LED boards, but also to find potential problems from mechanical failure point of view and to present a methodology to design SSL LED boards for reliability.