Showing papers by "Avram Bar-Cohen published in 2005"

Thermal design and optimization of natural convection polymer pin fin heat sinks

Abstract: The design and optimization methodology of a thermally conductive polyphenylene sulphide (PPS) polymer staggered pin fin heat sink, for an advanced natural convection cooled microprocessor application, are described using existing analytical equations. The geometric dependence of heat dissipation and the relationships between the pin fin height, pin diameter, horizontal spacing, and pin fin density for a fixed base area and excess temperature are discussed. Experimental results of a pin finned thermally conductive PPS heat sink in natural convection indicate substantially high thermal performance. Numerical results substantiate analytical modeling results for heat sinks within the Aihara et al. fin density range. The cooling rates and coefficient of thermal performance, COP/sub T/, that relates cooling capability to the energy invested in the formation of the heat sink, has been determined for such heat sinks and compared with conventional aluminum heat sinks.

Thermoelectric-powered convective cooling of microprocessors

Abstract: Forced convection cooling of a personal computer microprocessor, using power generated by thermoelectric (TE) conversion, has been modeled, analyzed, and demonstrated. This study was motivated by the desire to meet the demanding cooling requirements of notebook computers without consuming valuable battery power. The modest power generated by the TE necessitated a careful match between the TE device and the fan/heat-sink sub-system. The models and methodology used to maximize the cooling capability of TE-powered convection are presented and experimentally validated using a notebook computer prototype. In the experimental study described herein, a commercial fan was successfully driven by electricity generated from the heat of the microprocessor. It was determined that, at a junction temperature of 95 /spl deg/C, thermoelectric-powered cooling could perform nearly four times better than the best natural convection design.

Thermoelectric Micro-Cooler for Hot-Spot Thermal Management

Abstract: Driven by shrinking feature sizes, microprocessor “hot-spots” — with their associated high heat flux and sharp temperature gradients — have emerged as the primary “driver” for on-chip thermal management of today’s IC technology. Solid state thermoelectric micro-coolers offer great promise for reducing the severity of on-chip “hot-spots”, but the theoretical cooling potential of these devices, fabricated on the back of the silicon die in an IC package, has yet to be determined. The results of a three-dimensional electro-thermal finite-element modeling study of such a micro-cooler are presented. Attention is focused on the hot-spot temperature reductions associated with variations in micro-cooler geometry, chip thickness, and chip doping concentration, along with the parasitic Joule heating effects from the electrical contact resistance and current flow through the silicon. The modeling results help to define the optimum solid-state cooling configuration and reveal that, for the conditions examined, nearly 80% of the hot-spot temperature rise of 2.5°C can be removed from a 70μm × 70μm, 680W/cm2 hot-spot on a 50μm thick silicon die with a single micro-cooler.Copyright © 2005 by ASME

Thermal management of systems having localized regions of elevated heat flux

Abstract: A thermal management system ( 300 ) includes a first heat transfer body ( 330 ) for providing a opposing heat flux to at least one localized region of elevated heat flux residing in adjacency to a region of lesser flux, such as on a surface ( 315 a ) of a circuit die ( 315 ) due to a integrated circuit hot-spot ( 310 ). A contact ( 320, 321 962 a, 962 b, 970 a, 970 b or 950 ) defines a thermal conduction path for the opposing flux. A second heat transfer body ( 350 ) is in a heat transport relationship with the first heat transfer boy ( 330 ) and a second heat transport relationship with the region of lesser heat flux. In such arrangement, each region of heat flux is provided a thermal solution commensurate with the level of heat flux in the region. For example, the opposing heat flux of an active first heat transfer body ( 330 ), such as a thermoelectric cooler, may be provided at the hot-spot ( 310 ), while at the same time the lesser heat flux is absorbed by a passive second heat transfer body ( 350 ), such as a heat spreader.

Superlattice microrefrigerators fusion bonded with optoelectronic devices

Abstract: A three-dimensional (3-D) electrothermal model was developed to study the InP-based thin-film In/sub 0.53/Ga/sub 0.47/As/In/sub 0.52/Al/sub 0.48/As superlattice (SL) microrefrigerators for various device sizes, ranging from 40/spl times/40 to 120/spl times/120/spl mu/m /sup 2/. We discussed both the maximum cooling and cooling power densities (CPDs) for experimental devices, analyzed their nonidealities, and proposed an optimized structure. The simulation results demonstrated that the experimental devices with an optimized structure can achieve a maximum cooling of 3/spl deg/C, or equivalently, a CPD over 300W/cm/sup 2/. Furthermore, we found it was possible to achieve a maximum cooling of over 10/spl deg/C; equivalently, a CPD over 900W/cm/sup 2/, when the figure of merit (ZT) of InGaAs/InAlAs SL was enhanced five times with nonconserved lateral momentum structures. Besides monolithic growth, we also proposed a fusion bonding scheme to simply bond the microrefrigerator chip on the back of the hot spots, defined as two-chip integration model in this paper. The cooling effect of this model was analyzed using ANSYS simulations.

Orthotropic Thermal Conductivity Effect on Cylindrical Pin Fin Heat Transfer

Abstract: There is growing interest in the use of polymer composites with enhanced thermal conductivity for high performance fin arrays and heat sinks. However, the thermal conductivity of these materials is relatively low compared to conventional fin metals, and strongly orthotropic. Therefore, the design and optimization of such polymer pin fins requires extension of the one dimensional classical fin analysis to include two-dimensional orthotropic heat conduction effects. An analytical equation for heat transfer from a cylindrical pin fin with orthotropic thermal conductivity is derived and validated using detailed finite-element results. The thermal performance of such fins was found to be dominated by the axial thermal conductivity, but to depart from the classical fin solution with increasing values of a radius- and radial conductivity-based Biot number. Using these relations, it is determined that fin orthotropy does not materially affect the behavior of typical air-cooled fins. Alternatively, for heat transfer coefficients achievable with water cooling and conductivity ratios below 0.1, the fin heat transfer rate can fall more than 25% below the “classical” heat transfer rates. Detailed orthotropic fin temperature distributions are used to explain this discrepancy. Simplified orthotropic pin fin heat transfer equations are derived and validated over a wide range of orthotropic conditions.Copyright © 2005 by ASME

Experimental Characterization of Bonded Microcoolers for Hot Spot Removal

Abstract: In this paper we describe the experimental results of Si/SiGe superlattice microcoolers, which are used to cool the target hot spot on a 65μm-thick silicon substrate. The device areas under test range from 50×50 to 150×150 μm2 . We measured the cooling temperature at the hot spot region versus the current supplied to the microcooler, as well as the thermal resistance, and the cooling power density (CPD, also defined as heat flux — the flow of heat per unit area in W/cm2 ) of these devices. The experimental results show the maximum cooling at the hot spot region approaches 1°C for device area 150×150μm2 at 80°C, and CPD up to ∼110W/cm2 for device area 50×50×2 μm2 (two 50×50μm2 device array, as illustrated in Figure 3) at 80°C. The two-chip bonded configuration will allow the integration of spot coolers and integrated circuit chips with minimum impact on the processing of microelectronic devices. Key parameters limiting the cooling performance at the hot spots are also discussed.Copyright © 2005 by ASME

Surface Effects on Confinement-Driven Pool Boiling Enhancement in Vertical Parallel-Plate Channels

Abstract: Evidence of confinement-driven boiling heat transfer enhancement in vertical channels is very well documented in the literature and much has been observed about its nature and behavior. However, the majority of the available correlations is empirically-based and they tend to be very restricted in their range of applicability and portability. In order to further elucidate the effect of this type of geometrical confinement on boiling heat transfer, an experimental study has been performed on vertical, rectangular parallel-plate channels immersed in the dielectric liquid FC-72. The enhancement of nucleate boiling performance with decreased channel spacing was found to depend on the type of heater employed but could not be explained by the surface roughness. On the other hand, degradation of the Critical Heat Flux (CHF) limit with decreasing channel spacing was found to be independent of the surface and to be well predicted by a correlation available in the literature.Copyright © 2005 by ASME

Design, fabrication, and characterization of thin film PZT membranes for high flux electronics cooling applications

Abstract: An extensive numerical and experimental study of deep reactive ion etched (DRIE) thin film Pb(Zr,Ti)O3 (PZT) membranes used to enhance heat transfer between an immersed integrated circuit and a dielectric fluid is presented Modal and harmonic analysis of the vibrating PZT membranes was performed using finite element techniques for a frequency range from 0 to 5 MHz Experimental and numerical results showed that as the size of the PZT membranes increased, their fundamental resonance frequency decreased and they exhibited an increase in the number of resonance points in a given frequency range Square and rectangular PZT membranes showed similar resonant characteristics, while elliptic membranes displayed more resonant points than circular devices For a specified frequency range and equivalent surface areas, square membranes had more resonant points than circular membranes When the thin film PZT membranes are immersed in a dielectric fluid, substantial vortices and turbulences, along with distinct particle flow paths, were observed

Thermal performance limits of polymer composite pin fin heat sinks

Abstract: The thermal performance limits of air cooled heat sinks made of a polyphenylene sulphide composite (PPS, 20W/m-K), are predicted and compared to aluminum and copper pin fin heat sinks, using a defined heat sink volume and a range of pumping powers. The thermal performance is analytically predicted across an extensive parametric space in terms of the primary thermal metrics and identifies the thermal performance limits. PPS heat sinks are seen to constitute a viable alternative material for energy efficient heat sink design and show comparable thermal performance to aluminum and copper heat sinks at low fin densities and pumping power. The analytical model used to predict the heat sink thermal performance is seen to provide good agreement with typical aluminum pin fin heat sink experimental and CFD modeling results.

Thermal Analysis and Design of Electronic Systems

Abstract: The challenges posed by high chip heat fluxes and ever more stringent performance and reliability constraints make thermal management a key enabling technology in the development of microelectronic systems for the twenty-first century. Thus, thermal packaging efforts must be performed in the context of the salient trends and parameters that characterize the IC technology and the electronic products industry. Recent road-mapping efforts, have affirmed the expectation that improvements in CMOS semiconductor technology will continue unabated into the early part of the twenty-first century. Exploiting the potential of this IC technology, with the attendant increase in chip size, switching speed, and transistor density, will necessitate significant improvements in packaging technology. Keywords: thermal management; thermal packaging; thermal modeling; thermal resistance networks; convection