In vivo measurement of swine endocardial convective heat transfer coefficient

We measured the endocardial convective heat transfer coefficient h at 22 locations in the cardiac chambers of 15 pigs in vivo. A thin-film Pt catheter tip sensor in a Wheatstone-bridge circuit, similar to a hot wire/film anemometer, measured h. Using fluoroscopy, we could precisely locate the steerable catheter sensor tip and sensor orientation in pigs' cardiac chambers. With flows, h varies from 2500 to 9500 W/m2 x K. With zero flow, h is approximately 2400 W/m2 x K. These values of h can be used for the finite element method modeling of radiofrequency cardiac catheter ablation.     

Free convective heat transfer coefficient for high powered and tilted QFN64 electronic device

Heat exchanges occurring between the electronic assemblies and their environment are an essential data in order to control their temperature, enhance their performance and improve their reliability. In this survey, the average natural convective heat transfer coefficient is determined for an assembly constituted by a Quad Flat Non-lead QFN64 generating a high power ranging from 0.1 to 1.0 W during operation. It is welded on a PCB which may be inclined with respect to the horizontal plane by an angle varying between 0° and 90°. The calculations done by means of the finite volume method show how the free convective heat transfer on every part of this electronic assembly is influenced by these physical parameters. The correlations proposed in this work allow calculating the convective heat transfer coefficient in any area of the considered assembly according to the generated power and the tilt inclination. These original and unpublished tools allow increasing reliability and better thermal control of this conventional device widely used in electronics for many engineering applications.     

Simultaneous estimation of heat transfer coefficient and thermal conductivity with application to microelectronic materials

An estimation of unknown properties of materials arises naturally when one considers some aspects of thermal modeling, especially carried out by widely used numerical methods, e.g. Finite Element Method (FEM).We propose a new approach of simultaneous thermal conductivity and heat transfer coefficient estimation based on thermographic measurements. A linear, steady-state distributed parameter model is used in order to describe the test sample. Thermal properties measurement is equivalent to the unknown parameter estimation of this system. The proposed method is practically applied for estimation of thermal conductivity and heat transfer coefficient of thick-film modules made on alumina (96% Al₂O₃) and DP951 ceramic substrates. In these experiments a high-resolution thermographic scanner is used.The obtained results for thermal conductivity and heat transfer factor are fully comparable with previously published ones.     

对流热损失对双曲两步热传导的影响

在激光热源加热时间远小于达到热平衡所需时间的条件下,研究了对流热损失对金属薄膜热行为的影响。指出当热源强度非常高且薄膜厚度满足L<20hc,e/G时,电子气的对流热损失对薄膜热行为的影响是不能忽略的;当L<20hc,l(θl-1)/Gθe时,固体晶格的对流热损失对薄膜行为有显著的影响。从双曲两步热传导模型出发,运用Laplace变换给出在考虑对流热损失的情况下,金属薄膜的电子气和固体晶格温度在Laplace变换域内的解析表达式,同时给出各种工作条件和材料参数对薄膜热损失的影响。     

Theoretical solutions for the thermal spreading resistance of ring-geometry diodes

Theoretical solutions are given for the thermal spreading resistance of ring-geometry diodes. Numerical results are presented that are more complete than previous published data obtained from analog electrolytic tank measurements. Analytic expressions are given for the two limiting forms of ring geometry (a small ID to OD ratio, and an ID to OD ratio approaching unity).     

微通道的流动阻力分析

针对在恶劣温度条件下工作的芯片热控制问题,提出由微泵、微通道、热电模块、智能控制单元、散热片以及液体管道等构成的微通道对流换热设计思想。数值仿真分析了换热微通道结构的速度和压力分布,计算了管道、微通道等部件的阻力损失和换热系统的流动阻力曲线,确定了系统必需的微泵工作性能参数,求解了微泵的最佳工况点。最后,制作了微通道对流换热装置,测得了系统流量和流动阻力,证实了可用经典流体理论来分析该系统的流动阻力。     

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.     

The spreading resistance of MOSFET's

A simple mathematical expression for source and drain spreading resistance near the channel end of MOSFET's has been derived by calculating the capacitance of the same geometry, using the analogy between the resistivity of conductors and the permittivity of dielectrics (R = ρε/C). Furthermore, it is shown qualitatively and experimentally that the value of the resistivity, which is the most influential parameter in the equation derived, should be determined by a doping concentration equal to the inversion channel carrier concentration, rather than by the source and drain bulk properties.     

Hybrid micro-nano structured thermal interfaces for pool boiling heat transfer enhancement

Boiling heat transfer from hybrid micro-nano structured thermal interfaces has been studied experimentally using PF5060 and deionized water as working fluids for the range of saturation temperatures T_sat between 35 °C and 60 °C. Various enhancement structures were fabricated using surface micromachining in silicon. The performance of the smooth and roughened silicon surfaces, bare and fully coated with carbon nanotubes (CNTs), the silicon-etched as well as CNTs-based pin-fin arrays, and the 3D microstructures have been studied in detail. The highest heat fluxes were obtained using the 3D microstructure without CNTs—these are 27 and 130 W/cm² for PF5060 and water, respectively. Experimental results indicate that use of the CNT-enabled, purely nano-structured interfaces appear to improve boiling heat transfer only at very low superheats, as compared to the smooth surfaces.     

Flow and heat transfer in porous micro heat sink for thermal management of high power LEDs

A novel porous micro heat sink system is presented for thermal management of high power LEDs, which has high heat transport capability. The operational principle and heat transfer characteristics of porous micro heat sink are analyzed. Numerical model for the micro heat sink is developed to describe liquid flow and heat transfer based on the local thermal equilibrium of porous media, and it is solved with SIMPLE algorithm. The numerical results show that the heated surface temperature of porous micro heat sink is low at high heat fluxes and is much less than the bearable temperature level of LED chips. The heat transfer coefficient of heat sink is very high, and increasing the liquid velocity can enhance the average heat transfer coefficient. The overall pressure loss of heat sink system increases with the increasing the inlet velocity, but the overall pressure drop is much less than the pumping pressure provided by micro pump. The micro heat sink has good performance for thermal management of high power LEDs, and it can improve the reliability and life of LEDs.     

Thermal spreading resistance of ring-geometry diodes

Mesa avalanche diodes shaped in ring structures have lower thermal spreading resistances than conventional disk mesa diodes of equal junction area. A simple analog electrolytic tank was built to measure the spreading resistance of ring geometries, A normalized curve was obtained which allows the determination of thermal spreading resistance for any ring structure.     

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.     

Using wavelength-dependent emissivity of semiconductor wafer tomodel heat transfer in rapid thermal processing station

This paper develops an approach for using a wavelength-dependent emissivity model of a semiconductor wafer in calculating heat transfer in a rapid thermal processing (RTP) station. The wafer emissivity is modeled by a generalized polynomial in wavelength where the coefficients may be functions of temperature. A comparison of experimental data with simulated results for a silicon wafer is provided     

A unified compact model of electrical and thermal 3-D spreading resistance between eccentric rectangular and circular contacts

The spreading resistance between two parallel contacts, the smaller of which is located arbitrarily over the area of the larger, is of wide interest in electrical/thermal design of devices and ICs. We propose a simple, continuous, and asymptotically correct closed-form expression to predict the accurate but involved calculations of this resistance for all contact conditions. The contacts may be rectangular or circular, and the boundary condition on these may either be equipotential/isothermal or uniform current density/uniform heat flux. The validity of the model is established by comparisons with accurate numerical calculations of spreading resistances having aspect ratios in the range zero to 1000 and with experimental observations.     

On the calculation of spreading resistance correction factors

It is shown that the calculations of spreading resistance correction factors for graded structures can be readily carried out by using a simple recurrence formula for the integration factor that occurs in Schumann and Gardner's multilayer theory. The number of layers that can be used in the practical application of this theory has hitherto been limited by the computer core size requirement, because the earlier method of calculating the integration factor requires the inversion of a 2N × 2N matrix for an N-layer approximation. The use of the recurrence formula effectively removes this constraint. In terms of computation time, the recurrence-formula method is also very efficient. The economy thus achieved both in computation time and in core size requirement makes it possible now to make spreading resistance correction a routine matter, without having to resort to such measures as Hu's interpolation and space partitioning scheme.     

Influence of thermal contact resistance on thermal impedance of microelectronic structures

The thermal impedance Z_th(jω) has been calculated numerically for a silicon chip glued on a ceramic substrate. The non perfect thermal contact is taken into account by modelling the chip-substrate interface as a thermal contact resistance r_c. If Z_th is represented as a Nyquist plot, mainly two circular arcs are observed. The high frequency arc is found to be almost independent from r_c, whereas the low frequency part is largely influenced by r_c. The thermal resistance R_th = Z_th(jω = 0) increases linearly with r_c, as known from the literature. Additionally, our simulations have shown that similar conclusions can be drawn for the real and imaginary part of Z_th at a fixed frequency ω ≠ 0.     

Out-of-plane thermal expansion coefficient of biphenyldianhydride-phenylenediamine polyimide film

The out-of-plane thermal expansion coefficient α_⊥ of biphenyldianhydride-phenylenediamine (BPDA-PDA) polyimide thin film between 20 and 400°C has been measured using a laser spot scanning interferometer. The α_⊥ varies from 100 ppm/°C at 20°C to 400 ppm/°C at 400°C. As the result of highly anisotropic microstructure, the α_⊥ is much larger than the in-plane thermal expansion coefficient α_∥ and increases dramatically when the temperature exceeds the glass transition temperature (≈320°C).     

高膨胀系数玻璃-陶瓷复合材料的性能研究

选用具有良好介电性能和膨胀性能的硼硅酸盐(SiO2-BaO-B2O3-Al2O3)和石英,采用固相法合成了一系列具有高热膨胀系数的玻璃-陶瓷复合材料,并对这些复合材料进行了XRD、SEM分析,及其热、力、电性能的测试。结果表明:所制复合材料的热膨胀系数和弯曲强度随着石英含量的增加而增大,其相对介电常数则随之减小。石英质量分数为40%的复合材料在980℃烧结时,析出了大量的方石英相,复合材料的热膨胀系数增大。最终制备的复合材料具有高的热膨胀系数[(10.3～25.5)×10-6/℃]、较高的弯曲强度(146MPa)、较低的相对介电常数(5.6～6.4)及介电损耗(0.10%～0.30%)。     

自学习热转印机控制系统设计

基于提高热转印热转印质量和效率的目的,对热转印机工作原理进行探讨,并分析影响热转印效果的因素,设计了可变参数的自学习热转印机控制系统。通过对比实验,验证该控制系统能使转印成功率达到98%,转印效率是手工转印的2.94倍,是固定参数热转印机的1.76倍。