Showing papers by "Avram Bar-Cohen published in 2006"

Direct Liquid Cooling of High Flux Micro and Nano Electronic Components

Abstract: The inexorable rise in chip power dissipation and emergence of on-chip hot spots with heat fluxes approaching 1 =kW/cm2 has turned renewed attention to direct cooling with dielectric liquids. Use of dielectric liquids in intimate contact with the heat dissipating surfaces eliminates the deleterious effects of solid-solid interface resistances and harnesses the highly efficient phase-change processes to the critical thermal management of advanced IC chips. In the interest of defining the state-of-the-art in direct liquid cooling, this paper begins with a discussion of the thermophysics of phase-change processes and a description of the available dielectric liquid cooling techniques and their history. It then describes the phenomenology of pool boiling, spray/jet impingement, gas-assisted evaporation, and synthetic jet impingement with dielectric liquids. Available correlations for predicting the heat transfer coefficients and limiting heat transfer rates, as well as documented empirical results for these promising techniques for on-chip hot spot cooling, are also provided and compared

Analytical modeling of silicon thermoelectric microcooler

Abstract: Due to its inherently favorable properties, doped single-crystal silicon has potential application as an on-chip thermoelectric microcooler for advanced integrated circuits. In this paper, an analytical thermal model for silicon microcooler, which couples Peltier cooling with heat conduction and heat generation in the silicon substrate, and which includes heat conduction and heat generation in the metal lead, is derived and used to study the thermal characteristics of silicon thermoelectric microcoolers. The analytical modeling results are shown to be in good agreement with the experimental data and the results from electrothermal numerical simulations. The effects of metal lead, electric contact resistance, silicon doping concentrations, and microcooler sizes on the cooling performance are investigated. The cooling potential of such thermoelectric devices, represented by peak cooling and maximum cooling heat flux on the microcooler surface, is addressed.

“Heat Shield”—An Enhancement Device for an Unshrouded, Forced Convection Heat Sink

Abstract: The inherent advantages of forced air cooling have led to the widespread use of fully and partially shrouded heat sinks for the thermal management of high power microprocessors. The superior thermal performance that is achievable in the fully shrouded configuration is accompanied by a significant pressure drop penalty. The concept introduced in the current study, employs a thin sheet-metal "heat shield," placed around a partially shrouded heat sink, to channel the flow directly into the heat sink. A combined numerical and experimental study has shown that the use of this "heat shield" can substantially enhance heat sink thermal performance, in a channel geometry and airflow range typical of commercial chip packages; making it comparable to that of a fully shrouded heat sink, with a substantially lower pressure drop (∼50%). In addition, this thermal enhancement device can be easily retrofitted into existing systems; improving performance without major channel and/or fan modifications.

Thermoelectric mini-contact cooler for hot-spot removal in high power devices

Abstract: Cooling hot-spots with high heat flux (e.g., >1000W/cm/sup 2/) is becoming one of the most important technical challenge facing today's IC industry. More aggressive thermal solutions, than would be required for uniform heating, are highly desired. Solid state thermoelectric coolers (TECs) have received recent attention for hot-spot thermal management. However, present day TECs typically have cooling flux much lower than heat flux in the hot-spots. In this work, we reported an innovative technique - TE mini-contact - to significantly increase cooling flux of TECs for the application in hot-spot cooling. A chip package featuring a TE mini-contact cooler integrated with conventional integrated heat spreader (IHS) and heat sink is designed. The cooling performance of such chip package has been investigated by using a 3-D numeric model. It is found that the cooling in the hot-spot (1250W/cm/sup 2/, 400 /spl mu/m by 400 /spl mu/m) can be about 19/spl deg/C better in the proposed package than that achieved in the conventional chip package without TEC. The effects of trench, die thickness, and TEC misalignment on the cooling of the hot-spot are also discussed.

The Impact of a Thermal Spreader on the Temperature Distribution in a Plasma Display Panel

Abstract: Plasma display panels (PDPs) are a leading technology for large size television displays. Screen inefficiencies result in considerable localized heat generation within the PDP and necessitate aggressive thermal management to reduce spatial temperature variations across the screen. In the current study, experimental and numerical techniques were used to investigate the effect of a natural graphite heat spreader on hot spots and the temperature distribution across the screen. A commercial, 42 inch high-definition PDP, retrofitted with natural graphite heat spreaders with in-plane thermal conductivity varying from 140 to 440 W/mK and thicknesses varying from 0.5 mm to 1.4 mm, served as the test vehicle for this study. Infrared thermography was used to obtain a thermal map of the entire screen for different illumination patterns for each spreader configuration. Using a luminance meter, the luminosity of a selected point on the screen has been measured. A three-dimensional numerical model of this typical PDP, including the glass and spreader layers, as well as the chassis electronics, has been developed and the numerically obtained temperature distributions were compared to the experimental data

Prediction of thermal performance degradation of air-cooled fine-pitch fin array heat sinks due to fouling

Abstract: The fouling of air-cooled fine-pitch heat sinks by air born dust particles has become a major reliability concern for desktop and notebook personal computers, where significant thermal performance degradation can result. This paper investigates for the first time heat sink fouling mechanisms by both analytical and experimental analyses. The contribution of two fouling modes, namely accumulation of a thermally insulative dust coating on the fins within the heat sink channels, and blockage of the heat sink leading edge entrance, is quantified. It is found that the former fouling mode does not significantly increase heat sink thermal resistance. Instead, heat sink thermal performance degradation is essentially attributable to leading edge entrance blockage, which reduces the airflow rate through the heat sink by increasing pressure drop

Analysis and Design of a Least Material Orthotropic Pin Fin Heat Sinks

Abstract: The thermal anisotropy inherent in pin fins fabricated of polymer matrix composites necessitates the use of two-dimensional orthotropic relations for the prediction of the temperature distribution and heat transfer rate in large Biot number fins. Such fins offer a promising alternative to conventional metallic fins and heat sinks for electronic cooling applications, providing the benefits of moldability, lower density, lower fabrication energy and reduced cost. To realize these benefits and to compete favorably with the heat rejection capability of metallic fins and heat sinks, the polymer composite components must be thermally-optimized for the least-material configuration. This paper begins with a brief review of the available literature and a previously derived rigorous analytic model for orthotropic pin fins. The parametric trends are explored and attention is focused on identifying the least-material pin fin aspect ratio for a range of volumes and operating conditions. The orthotropic least-material configuration is found to closely approximate the aspect ratio associated with an isotropic fin. Simplified equations are proposed and analyzed for the cooling rate of an orthotropic pin fin. Further it is combined together with the least-material relation in order to design a least-material natural convection heat sink. The performance of the polymer least-material heat sinks is evaluated and compared, using common metrics, to conventional aluminum heat sinks

Analytical Modeling of On-Chip Hotspot Cooling Using Silicon Thermoelectric Microcoolers

Abstract: Thermal management of microprocessors has become an increasing challenge in recent years because of localized high flux hotspots which can not be effectively removed by conventional cooling techniques. This paper describes the novel use of the silicon chip itself as thermoelectric microcooler to suppress the hotspot temperature. A three-dimensional analytical thermal model of the silicon chip, including localized silicon thermoelectric cooling, thermoelectric heating, Joule heating, hotspot heating, background heating, and conductive/convective cooling on the back of the silicon chip, is developed and used to predict the on-chip hotspot cooling performance. The effects of chip thickness, microcooler size, doping concentration and parasitic Joule heating from the electric contact resistance on hotspot cooling are investigated in details.Copyright © 2006 by ASME

Characterization and Modeling of Anisotropic Thermal Conductivity in Polymer Composites

Abstract: The thermophysical properties of a commercially available Polyphenylene Sulphide (PPS)-carbon fiber composite material are experimentally characterized and used to validate application of the Nielsen thermal conductivity model to this category of polymer matrix composites. The PPS-fiber thermal conductivity was measured in the three orthogonal directions, using the laser flash thermal diffusivity method, and found to display significant anisotropy. Thermal conductivity predictions based on the Nielsen model, and using E-SEM measured values of fiber dimensions and fiber orientation and carbon fiber mass content determined from TGA analysis, were found to be within 5% of the corresponding measurements. The close agreement between predictions and measurements permitted the determination of the morphological influence of fiber material, volume content and orientation, and resin and fiber thermal conductivities, on thermal conductivity to be parametrically investigated and optimized for a given formulation. The impact of this optimization on the thermal performance of a PPS-fiber pin fin is described.Copyright © 2006 by ASME

Optimization of doping concentration for three-dimensional bulk silicon microrefrigerators

Abstract: We designed and fabricated a three-dimensional (3D) silicon microrefrigerator, which demonstrates a cooling power density over 200W/cm with only ~1degC cooling. The high cooling power density is mainly due to the high thermal conductivity and heat spreading effects. These devices have potential application in hot-spots management to reduce the chip peak temperature and realize on chip thermal management. A finite element model is developed to study and optimize these 3D devices. The simulation results showed that the optimized doping concentration to achieve the maximum cooling for these 3D silicon microrefrigerators (5e18 cm-3) is different from the conventional ID device, where S2sigma achieves the maximum at the doping of 5e19 cm-3. At its optimized doping concentration, these silicon microrefrigerators could reach a maximum cooling of 3degC. Further studies prove that this deviation is due to the nonidea factors inherent within the device, e.g. semiconductor-metal contact resistance, Joule-heating from probe contact resistance etc. Thus to optimize the real device, it is necessary to chose a full model considering all the nonideal factors

Thermal Packaging Challenges and Opportunities at the Micro and Nano Scales

Abstract: Microsystem technology is a key driver in the rapid migration of thermal science and technology towards the micro and nano scales. The deleterious effects of heat generation and thermally-induced failures of micro/nano electronic and photonic components have necessitated the development of thermal packaging solutions at progressively smaller scales. This lecture will define the emerging micro/nano thermal packaging challenges and explore solid state refrigeration and dielectric liquid cooling techniques for addressing on-chip hot spots in high heat flux chips, as well as the use of high conductivity polymer composites for cooling of notebook and portable computers. Thermo-optic issues in the use of polymer waveguides and gratings will also be considered.