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 These original and unpublished tools allow increasing reliability and better thermal control of this conventional device widely used in electronics for many engineering applications.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).     

Influence of heat transfer on thermal effects of the endpumped laser crystal

A thermal model of crystal is established. The temperature field differential equation of the diode-end-pumped laser crystal with circular cross-section and new boundary conditions, in which the convection heat transfer is supposed to exist between laser crystal ends and air, is established. The analytical expressions of temperature field, thermal distortion and additional optical path difference (OPD) of crystal are obtained. By numerical calculation, the influence of heat transfer on the thermal effects of laser crystal Nd:YAG is studied. Results show that crystal’s thermal effects, including temperature field, thermal distortion, OPD and thermal focal length, are all weakened as the heat transfer through ends of crystal is strengthened. This conclusion could be used to control thermal effects of laser crystal and improve laser working stability.     

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.     

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。     

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.     

Thermal performance impacts of heat spreading lids on flip chip packages: With and without heat sinks

Heat spreading lids on a flip chip package can provide many thermal and mechanical advantages. Major drawbacks are higher module costs and potentially poorer thermal performance with a heat sink. This study compares thermal performance of direct lid attach (DLA) and bare die flip chip packages and addresses the roles of interface resistance and lid thickness. The IBM SLC^TM package is tested and modeled. JEDEC standard wind tunnel tests as well as CFD models are used for analysis. The study reveals that the DLA design without additional heat sinking can provide significantly better thermal performance compared to the bare die package, depending on package size and airflow rate. With a heat sink the performance of the lidded package can be superior or inferior depending on interface resistance, lid design and the standard used for comparison.     

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.     

Spectroscopic analysis of multi-walled carbon nanotube–alumina composite films: Optimization of temperature coefficient of resistance and thermal hysteresis for thermal sensor applications

In this paper, we report a novel approach to study the potential use of multi-walled carbon naotubes (MWCNTs)–alumina (Al₂O₃) composite for heat sensing applications. This is achieved by optimizing the temperature coefficient of resistance (TCR) and thermal hysteresis of the composite. The composites were developed by uniform dispersion of MWCNTs in alumina in different concentrations following sol‐gel route. MWCNT loading in the alumina was found to be very effective to control the TCR as well as the hysteresis loss. The room temperature TCR versus MWCNTs concentration plot first shows an increasing trend with increase of MWCNTs concentration in the composite and reaches a threshold followed by drop in TCR. The maximum value of TCR that has been achieved is −0.56%/°C for 4 wt% MWCNTs content and is found to be ~1.5 times higher than the conventional metals and semiconductors. The hysteresis loss was found to decrease gradually to almost zero from 5 wt% onwards. The TCR and hysteresis variation is correlated with MWCNTs concentration dependent Raman, FESEM, EDS studies in the composite and there is a fair agreement in support of the observations.