Performance of Isotropic and Anisotropic Heat Spreaders

Anisotropic heat spreaders (flexible graphite and continuous carbon fiber polymer-matrix composite) and isotropic heat spreaders (copper and aluminum) have been evaluated numerically in terms of thermal resistance. Anisotropic ones are attractive for their through-thickness thermal insulation ability. Flexible graphite is superior to carbon fiber composite in providing lower thermal resistance. Carbon fiber composite is advantageous in its superior through-thickness thermal insulation ability and its smaller critical thickness (the optimal thickness for maximizing heat spreading while minimizing thickness). The isotropic heat spreaders are superior to the anisotropic ones in providing low thermal resistance, provided that the thickness is large, but they do not have the through-thickness thermal insulation ability. A higher value of the in-plane thermal conductivity enhances the effectiveness of flexible graphite. As the heat source area decreases, the thermal resistance increases while the critical thickness decreases. For the same heat source area, a greater in-plane dimension of the heat source perpendicular to the intended heat spreading direction decreases the thermal resistance and critical thickness. Flexible graphite is comparatively more advantageous when the thickness is smaller and when the heat source area is larger. For the same thickness below 2?mm, flexible graphite with in-plane conductivity of 1500?W/(m?K) is superior to copper and that with in-plane conductivity of 600?W/(m?K) is superior to aluminum. The highest thermal conductance obtained is 6.1?×?10⁴?W/(m²?K) when the thermal interfacial resistance is neglected and 5.1?×?10⁴?W/(m²?K) when this resistance is included. The conductance increases with decreasing heat source area and with decreasing heat spreader length.     

热管等效导热系数的数值模拟

为了分析热管简化模型的准确性,采用CFD(Computational Fluid Dynamics)软件Fluent对热管的等效导热系数进行了模拟,在导热系数的三种设定方式下,将简化模型热阻的模拟值与实验值进行了对比,结果表明各向异性模型比各向同性模型具有更高的精度。蒸发段和冷凝段的长度是影响模拟精度的主要因素,当蒸发段和冷凝段长度增大时,简化模型非传热方向上的导热热阻减小,适当减小该方向上的导热系数能更准确地模拟真实热管。     

Composite Spreader for Cooling Computer Chip With Non-Uniform Heat Dissipation

The performance of a composite spreader, with a 0.4 mm thick top layer of porous graphite (PG), for enhanced cooling with nucleate boiling of FC-72 dielectric liquid, and a 1.6 mm copper (Cu) substrate, for achieving better cooling of underlying 10 X 10 mm computer chip, with a non-uniform surface heat flux, is investigated. This spreader takes an advantage of the enhanced nucleate boiling heat transfer of FC-72 dielectric liquid on PG and the good heat spreading by Cu. The dissipated thermal power by the chip has a cosine-like distribution with a peak-to-average heat flux, Phi_max, which varied up to 2.467. The spreader surface area, the total thermal power dissipated by the chip, removed from the surface of the spreader, and the total thermal resistance are calculated and compared with those of PG and Cu spreaders of same thickness, 2.0 mm. With Phi_max = 2.467, 39.48 W and 72.0 W can be removed from the surface of composite spreaders cooled with saturation and 30 K subcooled boiling, compared to 43.0 and 65.3 W for Cu spreaders. The calculated surface areas and total thermal resistances of the composite spreaders, 6.82 cm² and 4.90 cm² and 0.284 and 0.68degC/W, are smaller than for Cu spreaders, 12.26 cm² and 11.92 cm², and 0.51 and 0.83degC/W. In addition, the calculated chip maximum surface temperatures of 62.37degC and 72.2degC, are lower than with Cu spreaders (72.67degC and 76.30degC).     

一种均温板加速条件下性能试验研究

针对机载电子设备散热应用条件,研究热管均温板在机载加速环境下的传热性能。为提高热管散热稳定性,将热源布置在热管中部,热管两端作为散热端。试验中试验单元安装在离心加速机上,模拟机载加速环境,测试了不同加速度条件下热管传热性能,得到热管均温板当量导热系数变化曲线,分析了加速度对热管当量导热系数的影响。试验结果表明:在0 ~13g加速条件下,热管均温板均能正常工作。结合试验结果给出热管均温板用于机载电子设备散热的设计建议。     

Heat Removal in Silicon-on-Insulator Integrated Circuits With Graphene Lateral Heat Spreaders

Graphene was recently proposed as a material for heat removal owing to its extremely high thermal conductivity. We simulated heat propagation in silicon-on-insulator (SOI) circuits with and without graphene lateral heat spreaders. Numerical solutions of the heat-propagation equations were obtained using the finite-element method. The analysis was focused on the prototype SOI circuits with the metal–oxide–semiconductor field-effect transistors. It was found that the incorporation of graphene or few-layer graphene (FLG) layers with proper heat sinks can substantially lower the temperature of the localized hot spots. The maximum temperature in the transistor channels was studied as function of graphene's thermal conductivity and the thickness of FLG. The developed model and obtained results are important for the design of graphene heat spreaders and interconnects.      

An analysis of the thermal response of power chip packages

Since power densities in integrated circuits and power semiconductor devices are continuously increasing due to miniaturization of circuitry, the design of optimum heat spreaders and heat sinks for these applications requires rather sophisticated calculational methods. The chips and spreaders are usually rectangular in shape and although the problem is three-dimensional in nature, it is usually approximated by two-dimensional configurations. Steady-state and transient analytic solutions are presented for the axisymmetric, two-dimensional, and three-dimensional spreader geometries, which can be used to calculate the thermal resistance of the base alone. To determine the thermal resistance of the chip-base combination, the one-dimensional chip thermal resistance should be added to that of the base. These analytic solutions provide calculational means which are easier than the numerical methods. The exact analytic steady-state and transient solutions developed for the axisymmetric, two-dimensional, and three-dimensional configurations are in excellent agreement with the numerical calculations. The parametric calculations provide information on the important guidelines that a packaging engineer should bear in mind while designing and optimizing heat spreaders for power semiconductor applications. These points can be summarized as follows: 1) for a given chip area there exists an optimal base area, 2) increasing the base thickness initially decreases the thermal resistance and beyond a certain limit the latter increases with base thickness, and 3) the convective heat transfer coefficient strongly affects the thermal resistance and the usual assumption of an isothermal base is not always appropriate.     

Development of conducting adhesive materials for microelectronic applications

Electrically and/or thermally conducting adhesive materials are classified into two categories depending on their conduction modes: isotropic and anisotropic materials. Silver-particle filled epoxy is the most common example of the class of isotropic materials which are conductive in all directions. This material has been long used in the electronic applications as a die-bonding material, where its good thermal conduction rather than its electrical conduction property is utilized. The silver-filled epoxy material has several limitations for high performance electrical interconnections, such as low electrical conductivity, increase in contact resistance during thermal exposure, low joint strength, corrosion issue due to silver migration, difficulty in rework, and so forth. The anisotropic conducting material provides electrical and/or thermal conduction only in one direction. An anisotropic conducting film (ACF) is used for interconnecting TAB mounted chips to a liquid crystal display panel, where fine pitch interconnection and low temperature assembly are required. In this paper, a brief review of the state-of-art conducting adhesive technology is provided. Subsequently, development of new conducting adhesive materials is presented for several different applications, which include high temperature materials for ceramic substrates, and low temperature materials for organic substrates.     

Thermal contact resistance of cured gel polymeric thermal interface material

This paper reports the experimental results on the contact resistance of curable polymer gel thermal interface materials (TIMs) that have different mechanical properties due to difference in the rheology of the polymers. A semi-analytical model for the prediction of the thermal contact resistance of cured gel TIMs is also introduced in this paper. A novel method of finding the transition from grease type behavior to gel type behavior, which is very important for post reliability stress performance, based on G' (storage shear modulus) and G' (loss shear modulus) measurements is reported. Further, post thermal cycling thermal resistance degradation rate of gel TIMs are related to the ratio of G and G'. Finally, design guidelines for gel TIMs for use in flip-chip packages comprising heat spreaders are proposed.     

A practical investigation on nickel plated copper heat spreader with different catalytic activation processes for flip-chip ball grid array packages

This study investigates the effects of two different catalytic activation techniques on the thermal performance of the flip-chip heat spreaders. The two activation techniques studied are thin nickel-copper strike and galvanic initiation. Thermal diffusivity and surface roughness of these heat spreaders were studied using the Nano-flash Apparatus and Infinite Focus Microscopy. High temperature storage tests were carried out to investigate the extent of intermetallic diffusion between the nickel and copper layers. The results show that heat spreaders with thin nickel-copper strike catalytic activation technique have a lower thermal diffusivity due to the low thermal conductivity of nickel-copper layer. Moreover, the nickel-copper layers grew thicker from around 0.2 μm at initial time to around 0.55 μm after high temperature storage duration of 168 h. On the other hand, heat spreaders processed using the galvanic initiation technique did not form any nickel-copper intermetallic diffusion layer. As a conclusion, the galvanic initiation technique can potentially provide better thermal performance for heat spreaders used in semiconductor packages.     

Investigation of the thermal performance of micro-whisker structured silicon heat spreaders for power devices

The driving forces of developments in power electronics are the continuing miniaturization and enhancement of power densities. New packaging concepts are required allowing the dissipation of a power loss density of up to several hundred W/cm² at operation temperatures as low as possible. A promising attempt to decrease the thermal resistance to the ambient is the development of silicon substrates structured with microwhiskers perpendicular to its surface. An industrial application of this new heat spreader technology in power electronic modules makes necessary the specification of the substrate properties. In this work, a new method for determination of thermal qualities based on laser heating of the heat spreader, surface temperature measurement by thermovision, and dynamic reverse modeling is described. For numerical determination of the thermal characteristics, the measured data are evaluated with the help of a thermal model of the heat spreaders under various boundary conditions. The respective temperature distributions are calculated with a new simulation tool using an alternating-direction implicit algorithm (ADI-method). Results obtained from heat spreaders with microwhisker treatment are compared with those from reference samples with a polished surface. Based on these results a view on future applications for power electronics assemblies are derived.     

Evaluation of analytical models for thermal analysis and design of electronic packages

The objective of this study is to evaluate the use of several analytical compact heat transfer models for thermal design, optimization, and performance evaluation in electronic packaging. A model for heat spreading in orthotropic materials is developed. The developed model is used in conjunction with the other available heat transfer models in a resistance network for calculation of heat transfer rate and junction temperatures in a multi-chip module (MCM). Refrigeration cooled MCM of an IBM server is used to illustrate the methodology. Results of the analytical model and resistance network analysis are compared with a numerical solution. Capability of the analytical model in predicting the thermal field is discussed and effectiveness of using the analytical models in thermal design and optimization of electronic packages is demonstrated.     

大功率LED散热鳍片扩撒热阻研究

通过设计两组实验,利用热分析软件Flotherm模拟各模型热分布情况,分析了影响LED散热鳍片扩散热阻的各个因素。结果表明:热源与鳍片的接触面积是影响扩散热阻的主要因素,接触面积愈大扩散热阻愈小,当接触面积与鳍片基座面积相等时,扩散热阻消失;扩散热阻随着鳍片基座厚度增加而减小,冷却空气流速对扩散热阻的影响很小。     

直热管传热系统热阻的试验研究

对常温直热管传热系统的总热阻进行了试验研究,着重分析了真空和大气环境下该系统裸接并用不同的螺钉扭矩安装时热管的总热阻及热管与热端安装面、冷端安装面的热阻。研究表明,试验环境对该系统的传热能力有一定的影响,热端和冷端安装面在真空中的热阻均比在大气环境中的大。螺钉的安装扭矩会影响接触面的热阻。两种环境中,在一定的热负荷下,冷端安装面在扭矩为0.4 Nm时的热阻约是扭矩为1.2 Nm时的2.4倍。     

Limits of Heat Removal in Microelectronic Systems

Thermal management issues play an increasingly prominent role in microelectronic system design. The constraints on heat removal are a major factor limiting the performance of a microelectronic system. This work presents the thermodynamic limit of performance for a thermal solution utilizing air cooling to reject thermal energy as the inverse of its mass flow-heat capacity product. The minimum resistance to heat flow offered by a thermal solution is further refined by including the effects of thermal interface materials, substrate materials, and the impact of nonuniform device layer heating. Active cooling solutions may offer additional needed cooling for microelectronics systems, but the system thermal resistances limit its applicability. This work describes the minimum efficiency that an active cooling solution must provide to offer a thermal advantage over passive cooling. This minimum efficiency is dictated by the thermal resistances involved in drawing heat into the active cooler and expelling heat to the ambient environment. Knowledge of the fundamental limitations of thermal solutions gives system designers realistic expectations to set roadmaps, define architecture specifications, and evaluate the validity of thermal system performance claims     

半圆柱板翅片散热器传热特性的数值模拟

在板式翅片散热器的基础上,通过增加不同数量和半径的半圆柱肋片,构造了6种不同结构的半圆柱板翅片散热器(HPPFHS),并通过数值模拟的方法对这两种散热器的流动和传热特性进行了研究。结果表明,随着入口风速的增加,两种散热器的热阻减小的同时压降随之增大,但其热阻减小的趋势变小。入口风速相同时,相比板式翅片散热器,流经半圆柱板翅片散热器的空气受到半圆柱肋片的影响产生涡流,因而减少了热阻,同时增加了压降,但其综合性能远好于板式翅片散热器。     

Heat Generation and Transport in Nanometer-Scale Transistors

As transistor gate lengths are scaled towards the 10-nm range, thermal device design is becoming an important part of microprocessor engineering. Decreasing dimensions lead to nanometer-scale hot spots in the transistor drain region, which may increase the drain series and source injection electrical resistances. Such trends are accelerated by the introduction of novel materials and nontraditional transistor geometries, including ultrathin body, FinFET, or nanowire devices, which impede heat conduction. Thermal analysis is complicated by subcontinuum phenomena including ballistic electron transport, which reshapes the heat generation region compared with classical diffusion theory predictions. Ballistic phonon transport from the hot spot and between material boundaries impedes conduction cooling. The increased surface to volume ratio of novel transistor designs also leads to a larger contribution from material boundary thermal resistance. This paper surveys trends in transistor geometries and materials, from bulk silicon to carbon nanotubes, along with their implications for the thermal design of electronic systems.     

The influence of various common assumptions on the boundary-condition-independence of compact thermal models

It has been repeatedly shown that high accuracy can be obtained when comparing boundary-condition-independent compact thermal models (CTMs) to detailed model results. However, it should be realized that these results have been generated using certain assumptions (e.g., uniformly distributed boundary conditions), and the question remains about the validity of these assumptions when CTMs are used in "real-life" environments. The results show that the assumptions are justified, at least for the package studied, with the exception of boundary conditions associated with heat spreaders and heat sinks. For this type of boundary condition, we need to specify separate heat transfer coefficients for the two nodes at the surface. Finally, an important conclusion is that the "traditionally" generated CTMs perform very well in "real-life" environments.     

Modeling of Cylindrical Pin-Fin Heat Sinks for Electronic Packaging

Analytical models are developed for determining heat transfer from in-line and staggered pin-fin heat sinks used in electronic packaging applications. The heat transfer coefficient for the heat sink and the average temperature of the fluid inside the heat sink are obtained from an energy balance over a control volume. In addition, friction coefficient models for both arrangements are developed from published data. The effects of thermal conductivity on the thermal performance are also examined. All models can be applied over a wide range of heat sink parameters and are suitable for use in the design of pin-fin heat sinks. The present models are in good agreement for high Reynolds numbers with existing experimental/numerical data.      

Miniature Loop Heat Pipe With Flat Evaporator for Cooling Computer CPU

This paper presents an experimental investigation on a copper miniature loop heat pipe (mLHP) with a flat disk shaped evaporator, 30mm in diameter and 10-mm thick, designed for thermal control of computer microprocessors. Tests were conducted with water as the heat transfer fluid. The device was capable of transferring a heat load of 70W through a distance up to 150mm using 2-mm diameter transport lines. For a range of power applied to the evaporator, the system demonstrated very reliable startup and was able to achieve steady state without any symptoms of wick dry-out. Unlike cylindrical evaporators, flat evaporators are easy to attach to the heat source without need of any cylinder-to-plane reducer material at the interface and thus offer very low thermal resistance to the heat acquisition process. In the horizontal configuration, under air cooling, the minimum value for the mLHP thermal resistance is 0.17degC/W with the corresponding evaporator thermal resistance of 0.06degC/W. It is concluded from the outcomes of the current study that a mLHP with flat evaporator geometry can be effectively used for the thermal control of electronic equipment including notebooks with limited space and high heat flux chipsets. The results also confirm the superior heat transfer characteristics of the copper-water configuration in mLHPs     

基于有限元方法的光纤加速度计灵敏度仿真分析

散热是大功率LED封装的关键技术之一,散热基板的选用直接影响到LED器件的使用性能与可靠性。文章详细介绍了LED封装基板的发展现状,对比分析了树脂基板、金属基板、陶瓷基板、硅基板的结构特点与性能。最后预测了今后大功率LED基板的发展趋势及需要解决的问题。 更多还原