固态T/ R组件冷板扩展热阻分析与优化准则

电子设备的功率器件和散热冷板之间由于存在面积差异,产生扩展热阻,在高热流密度条件下(>100 W/ cm2 )温升效应尤为明显。文中通过对固态T/ R组件冷板扩展热阻的简化近似解的分析计算,研究了冷板的导热系数、厚度、半径、对流换热系数等参数扩展热阻的影响规律,发现冷板存在厚度和半径的工程最优值使得扩展热阻或总热阻最小,建立了相应的尺寸优化准则,然后通过计算流体力学(computiatonal fluid dynamics,CFD)数值试验进行了计算验证,结果表明CFD解和简化解结果一致,研究结论可用于指导冷板或热扩展板的工程设计。     

Backscatter RCS for TE and TM excitations of dielectric-filledcavity-backed apertures in two-dimensional bodies

Transverse electric (TE) and transverse magnetic (TM) scattering from dielectric-filled, cavity-backed apertures in two-dimensional bodies are treated using the method of moments technique to solve a set of combined-field integral equations for the equivalent induced electric and magnetic currents on the exterior of the scattering body and on the associated aperture. Results are presented for the backscatter radar cross section (RCS) versus the electrical size of the scatterer for two different dielectric-filled cavity-backed geometries. The first geometry is a circular cylinder of infinite length which has an infinite length slot aperture along one side. The cavity inside the cylinder is dielectric filled and is also of circular cross section. The two cylinders (external and internal) are of different radii and their respective longitudinal axes are parallel but not collocated. The second is a square cylinder of infinite length which has an infinite length slot aperture along one side. The cavity inside the square cylinder is dielectric-filled and is also of square cross section     

固态组件冷板热扩展体性能分析

固态技术和相控阵雷达是雷达领域研究的重要课题,固态末级功放组件、T／R组件内晶体管等大功耗、高热流密度器件在冷板上会形成局部热障,产生扩展热阻。文中对扩展热阻特性进行研究,探讨了影响扩展热阻的三个参数：厚度、对流换热系数和导热系数,并分析得出提高热扩展体的等效导热系数是减小扩展热阻、强化传热最有效的技术途径之一,最后在此基础上利用数值模拟方法对铜块、热管及蒸发室等三种热扩展体的散热性能进行了分析和比较。     

Thickness Corrections for Capacitive Obstacles and Strip Conductors

Capacitive thickness corrections are derived exactly for two basic geometries involving pairs of semi-infinite plates. In one arrangement the pair of plates are coplanar, while in the other they are parallel to each other. In each case the total capacitance per unit length between the pair of plates is infinite, but the incremental increase of capacitance when the thickness is increased from zero to a value t is finite. These capacitance increments are evaluated, and it is shown how they may be used as approximate thickness corrections in a great varity of more complicated geometries involving capacitive obstacles in waveguide, coaxial line, and artificial dielectric media. They may also be applied to coupled-strip-line conductors. As examples, the corrections are applied in detail to a waveguide iris, and to three useful coupled-strip-line cross sections.     

Compact Models of Spreading Resistances for Electrical/Thermal Design of Devices and ICs

Building upon our recent work (IEEE Electron Device Lett., vol. 26, pp. 909-912, Dec, 2005), we present simple and continuous closed-form models for several rectangular and circular spreading resistance geometries encountered in electrical/thermal design of devices and integrated circuits. The resistance geometries considered involve current/heat flow between parallel contacts of rectangular or circular shape and concentric or eccentric nature, between a horizontal and a vertical stripe, and between a horizontal circular contact and a surrounding vertical cylindrical contact. The modeling procedure involves normalization of the spreading resistance with respect to its value under 1D flow conditions, followed by a curve-fit of the variation of this normalized resistance with contact area in terms of physical parameters. The resistance models may also be used to estimate the reciprocal of capacitance of similar insulator-electrode geometries, by replacing the resistivity by the reciprocal of the insulator permittivity.     

Thermal issues in microsystems packaging

Miniaturization of electronic/mechanical systems is achieved by packing different functional components into tight space. The heat transfer analysis for such systems has to deal with geometrically complex heat transfer paths, and it needs to be done quickly in response to every design alteration. This paper proposes a concept that aims at reduction of analysis load on the packaging designer. The proposed scheme is composed of two major steps. First, a computer program "configuration generator" is used to generate possible geometric configurations of heat transfer paths in a systematic manner. Second, temperature solutions for these configurations are compressed into "fast estimate" formulas that free the packaging designer from the need to perform involved heat transfer analysis. This approach is illustrated using a topic of heat spreading on the planar substrate as an example. Miniaturization of systems also raises another issue for the thermal design; that is the coupling between the system configuration and the overall heat dissipation to the environment. A model situation is considered where heat diffuses from a zone on the system shell and to the environment by natural convection and radiation. In the parametric domain spanned by the thermal conductivity of shell material and the system's characteristic length there is a zone where the system-level heat transfer is sensitive to the system's configuration. Such characteristic length is around 1 cm for systems encapsulated in plastics, 3-10 cm for those in ceramic and alloy shells, and 10-40 cm in copper or aluminum clad systems.     

Heat Sinks for Miniature Thermoelectric Coolers: Selection and Characterization

The effect of heat sink (HS) thermal resistance on the performance of a thermoelectric cooler (TEC) has been studied. A method of HS characterization is proposed for the typical case of substantial mismatch in the dimensions of these components. This dimension mismatch causes significant heat spreading resistance in the HS base plate which is highly dependent on the lack of conformity of the components. In this study we investigated modern a HS whose thermal resistance R _d was determined experimentally by use of a dummy heater of fixed dimensions. An analytical method is proposed for predicting HS thermal resistance for arbitrary heater dimensions using the certified R _d value as sole reference point. The method is based on an HS thermal model with three-dimensional heat spread in its base plate. This theoretical approach was used for characterization of HS from Alpha Company, a manufacturer of HS for microelectronics. HS with low pressure drop were considered under conditions of natural and forced convection. It is shown that their thermal resistance, which depends on TEC dimensions, can differ substantially from the certified value.     

Using microchannels to cool microprocessors: a transmission-line-matrix study

As microprocessors components density and clock frequency increase, so do heat dissipation. The heat results from Joule effect due to leakage currents in the components area or active region. This region is only few microns thick and can quickly reach destructing temperatures if heat is not quickly removed. On this critical issue depends the system reliability. The active region is separated from the ventilated heat sink by a silicon substrate and a metal integrated heat spreader, both hundreds of microns thick. This interface region is the microprocessor's heat transfer plate where heat exchange is achieved by conduction. Because of the localized heat source, the thermal spreading resistance of the interface region can be high. A novel way of spreading heat in that region is the use of microchannel arrays where an appropriate thermal compound or a phase change liquid can be trapped to increase heat transfer by conduction or to create micro-heat-pipes. Traditional cooling methods, with conventional and well optimised heat sinks, can then be used with less burden.In this paper, the Transmission-Line-Matrix (TLM) technique is used to simulate the effect of microchannels on the temperature distribution in the active region. To minimize the interface heat resistance various microchannel and patterns are examined. In this part of the work, the microchannels are filled with the heat spreader material copper or aluminium. The results show an improved thermal transient behaviour and a reduced active region temperature in steady state.     

Maximum thermal spreading resistance for rectangular sources and plates with nonunity aspect ratios

An exact three-dimensional (3-D) solution is derived for the steady-state heat conduction equation with the source on an otherwise adiabatic surface and Newtonian cooling on the opposing surface. Dimensionless solutions are provided for maximum and source-averaged thermal spreading resistances. The solution for unit-source contours in a plane perpendicular to the source plane is also included. Maximum thermal spreading resistance plots are provided for several source-to-plate edge length ratios and source aspect ratios (plate maintained square). The Newtonian cooling effects are incorporated via a Biot number. Examples illustrate application of the theory.     

On the crack and delamination risk optimization of a Si-interposer for LED packaging

3D-integration becomes more and more an important issue for advanced LED packaging solutions as it is a great challenge for the thermo-mechanical reliability to remove heat from LEDs to the environment by heat spreading or specialized cooling technologies. Thermal copper-TSVs provide an elegant solution to effectively transfer heat from LED to the heat spreading structures on the backside of a substrate. But, the use of copper-TSVs generates also novel challenges for reliability as well as also for reliability analysis and prediction, i.e. to manage multiple failure modes acting combined – interface delamination, cracking and fatigue, in particular. In this case, the thermal expansion mismatch between copper and silicon yields to risky stress situations.To overcome cracking and delamination risks in the vicinity of thermal copper-TSVs the authors performed extensive simulative work by means of fracture mechanics approaches – an interaction integral approach within a simulative DoE and the X-FEM methodology to help clarifying crack propagation paths in silicon. The results provided a good insight into the role of model parameters for further optimizations of the intended thermal TSV-approaches in LED packaging applications.     

An incremental `add-on' approach to determining the effect ofmodification of the geometry of a thin plate on its electromagneticscattering properties

The problem of rapid estimation of the effect of perturbing a thin plate structure is addressed using the add-on technique that was introduced recently by R. Kastner (see ibid., vol.37, p.353-61, 1989). An important feature of this method is its ability to make efficient use of previously computed results for the current distribution on a portion of the scatterer that remains unchanged as the geometry of the plate is modified. The procedure, which has been successfully applied to large irregular strip arrays and to a class of planar geometries that introduce negligible cross-polarization effects in the scattering process, is extended in this work to apply to arbitrary plates. In order to demonstrate the versatility of the method, numerical results are given for a number of plate geometries, including one with apertures     

Thermal properties of annular and array geometry semiconductor devices on composite heat sinks

The thermal spreading resistance and temperature-profile properties of a circular semiconductor device mounted on a composite heat sink are considered theoretically. The results are applied to annular and array-geometry contacts by the principle of superposition. It is found that considerable reductions in thermal resistance can be achieved which are due both to contact geometry, and the composite heat sink. Substantial improvements in the uniformity of the temperature profile in the contact due to the composite nature of the heat sink is also demonstrated and optimum heat sink geometries are illustrated.

An annular-geometry structure (inner radius to width ratio of 8) with an area of 3·14 × 10⁻⁴ cm² has a predicted thermal resistance of 2·6°C/W and a temperature difference at the interface of only 7·5 per cent, when the main heat sink is copper, the conductivity ratio is 3, and the heat sink geometry is optimum. The results are shown to agree in the special case of a single-medium heat sink with those of Frey. The results are also applicable to the case where interface layers are present in the heat sink with conductivity lower than the main heat sink and in which heat-spreading is not negligible.     

Analysis of Convective Thermal Resistance in Ducted Fan-Heat Sinks

The thermal performance of plate fin, round pin-fin, and offset strip-fin heat sinks with a duct-flow type fan arrangement was analytically evaluated. Heat sinks of 65mm$times hbox60, hboxmm hboxplan hboxareatimes hbox50 hboxmm hboxheight$with a 4300-RPM dc fan (60mm$times$15mm) were chosen for the performance comparison. A constant temperature, 6-mm thick heat sink base plate is assumed so that thermal spreading resistance is not involved. The operating point on the fan curve is based on the flow pressure drop impedance curve through a heat sink using the friction factor correlation for the chosen heat sink. The loss coefficients at both the entrance and the exit of the heat sink are included in the flow impedance curve. The operating point is defined by the balance point of the flow impedance curve and the fan performance curve. After determining the operating air velocity, the convective thermal resistance of heat sinks is evaluated from the Nusselt number correlation for the chosen heat sink. Results obtained show that optimized round pin-fin heat sinks provide 32.8%–46.4% higher convective thermal resistance compared to an optimized plate-fin heat sink. The optimized offset strip-fin heat sink shows a slightly lower convective thermal resistance than the plate-fin heat sink. As the offset strip length decreases, however, thermal performance seriously deteriorates.     

BER Performance of Walsh Hadamard Like Codes Based on Complementary Sequence Sets in a CDMA and Cognitive Underlay System

In this paper, a method of constructing a sequence set similar to Walsh Hadamard (WH) codes is described. In general, two complementary sequence set of length 2 is used as a basic kernel to construct WH codes. In this work, four complementary sequence set of length 3 is used as a basic kernel. The proposed code set have a nearly non linear phase sequences with a more evenly spread frequency spectrum when compared with WH codes. It is shown that the proposed spreading codes perform comparable to WH in a Direct Sequence code division multiple access (DS-CDMA) system in an Additive White Gaussian Noise and Rayleigh channel at a reduced code length. In this work, bit error rate performance of the Underlay Cognitive Radio system implemented using DS-CDMA system is simulated. Primary and Secondary Users are distinguished by assigning spreading codes from different families and it is assumed that the channel information is known by both the Primary and Secondary User receivers. This new code set offers good performance in a Cognitive Radio Underlay system.     

On the asymptotic decay of coupling for infinite phased arrays

The asymptotic decay of coupling with distance from a single excited element it an infinite linear array has been analyzed for several special cases and is shown to behave according to exp (-jkr)/r^3/2where r is the distance from the excited element. This behavior is also characteristic of propagation over a lossy surface. By geometric extrapolation one would expect an asymptotic decay of coupling for planar arrays to behave according to exp (-jkr)/r². This extension is carried out for an arbitrary infinite periodic planar current sheet with the expected result. In addition, this behavior is shown to be valid for an infinite parallel plate array immersed in a magnetized cold plasma. In all cases, the asymptotic behavior is shown to depend on the form of the singularity of the derivative of the reflection coefficient with scan angle. This singularity occurs at grazing incidence of the radiated beams.     

Calculation and analysis of thermal impedance of microelectronic structures from analytical models

This article deals with thermal impedances of microelectronic components that are useful in Simulation Programs with Integrated Circuit Emphasis (SPICE). In devices like heterojunction bipolar transistors, the active regions thicknesses are often much smaller than the substrates thicknesses. The devices can then be thermally assimilated to heat densities located on top of solid media. In addition to that, when the other dimensions of the heat sources are also much smaller than the substrates dimensions, it is reasonable to consider that the substrate is semi-infinite. First, the expression of the thermal impedance Z of a circular shape heat source centered on top of a half space is presented. For this purpose, the integral transform technique has been used to solve the tri-dimensional heat conduction equation in the frequency domain. The original expression is explicit, exact and allows obtaining results very quickly. After that, the case of a circular heat source on top of a cylinder is treated. A complete analysis of the substrate dimensions influence on the thermal impedance is done. It is based on the impedance decomposition into the one-dimensional impedance and the spreading impedance. By comparing these impedances with that obtained for the heat source on top of the semi-infinite medium, the threshold pulsation at which the thermal impedance of the finite medium differs from the thermal impedance of the half space is extracted. Moreover the geometrical criteria resulting in an error of less than 2% between the spreading impedance of the finite medium and the semi-infinite one are extracted. When these criteria are observed the impedance can be calculated using two perfectly known impedances: the spreading impedance of the semi-infinite medium and the one-dimensional impedance. The results are plotted on the Nyquist diagram, providing a compact representation. Finally the assumption of a circular shape heat source to approximate the thermal impedance of a square shape heat source is validated by evaluating the associated error. The calculation times have been compared to confirm the interest of using this hypothesis.     

Heat transfer enhancement at solid-liquid and solid-gas interfacesby near-surface coolant agitation

Heat transfer enhancement from glass ceramic plate to water or air by agitation of the coolant near the heated surface was investigated. The experimental arrangement consisted of a ceramic plate of dimensions of 47×35×0.26 mm³, attached to a brass vessel with inner dimensions of 34×30×4 mm³. The ceramic plate was provided with a 5×5 mm² size printed heater on the opposite side from the vessel. The temperature field across the plate was recorded by an infra-red camera. The agitation of the coolant in the vessel (air or water) was performed by a vibrating piezoelectric beam of dimensions of 26.5×12×0.6 mm³, fixed at 1 mm distance from the heated plate, or alternatively by a magnetic rod of diameter of 2.2 mm and length of 15 mm. The vibration of the beam with amplitude of some tenths of mm peak to peak at frequencies of 200 to 400 Hz caused the sinking of the peak temperature of the heater from about 90°C to 45°C in case of water as the coolant and from about 110°C to 100°C in the air. The magnetic rod, rotating in water at speed of some rounds per second lowered the heater's peak temperature from 85°C to 50 to 60°C. The ambient temperature in all experiments was 22 to 25°C and the heating power 1-2 W. The power needed for agitation was about 50 mW in case of piezoelectric vibrator and about 1 W in case of the rotating agitator drived by a fan. Using numerical simulation by ANSYS, it was demonstrated, that the temperature distribution across the plate with heater can be satisfactorily simulated using a two-dimensional (2-D) model with appropriately enhanced heat conductivity of the plate and heat transfer coefficient from the plate. For the experimental arrangement used the equivalent heat conductivity of the ceramic plate in case of agitated liquid cooling was up to 150 W/m·K and heat transfer rate up to 300 W/m ²·K     

An investigation of new FETD/ABC methods of computation ofscattering from three-dimensional material objects

Finite-element time-domain (FETD) and absorbing boundary condition (ABC) methods for computation of scattering from three-dimensional (3-D) material objects are developed and investigated. The methods involve discrete-time FETD solution of the time-domain Helmholtz equation in a region that comprises the 3-D scatterer and its immediate vicinity. Coupling of the solution to the surrounding infinite space is achieved through the ABC. This FETD/ABC formulation is examined for a number of various geometries: sphere, plate, and ogive     

一种改进型百叶窗翅片的性能数值模拟研究

传统导航大功率设备大多采用平板翅片换热，导致实际空气侧换热系数较低，换热效果不理想。传统板式百叶窗翅片虽然能克服上述缺点，但其压降比平板翅片高。本文提出一种改进型 S 型百叶窗翅片，具有较大的扰流及导流作用，从而增加换热力、减少压降。本文针对平板翅片、传统板式百叶窗翅片及 S 型百叶窗翅片进行数值建模，并对速度场、温度场、压力场、 换热系数，拟合传热因子、阻力系数与雷诺数的关系式进行模拟分析。仿真结果表明，在相同工况下，的综合换热性能最优。     

Thermal compact modeling of parallel plate heat sinks

Growing complexity of electronic systems has resulted in an increased computational effort in CFD modeling of electronic systems. To reduce the computational effort, one or several heat sinks can be represented by a compact "porous block" model, with an effective thermal conductivity and pressure loss coefficient. In this study of parallel plate heat sinks in laminar forced convection, a methodology is developed to rigorously determine the thermal properties of compact heat sink models that provide a high level of accuracy. The results of an extensive set of CFD simulations for a three heat sink channel covering two distinct heat sink geometries, air velocities from 0.25 m/s to 2 m/s and various spacings between the heat sinks, were used to create and evaluate the fidelity of compact models. The results of this study establish the validity and value in using the porous block compact model representation for noncritical heat sinks in an electronic assembly. The results also reveal that a location-independent porous-block representation can yield excellent agreement in the prediction of the thermal characteristics of state-of-the-art heat sinks.