Showing papers by "Avram Bar-Cohen published in 2012"

Thermal Management of On-Chip Hot Spot

Abstract: The rapid emergence of nanoelectronics, with the consequent rise in transistor density and switching speed, has led to a steep increase in microprocessor chip heat flux and growing concern over the emergence of on-chip “hot spots”. The application of on-chip high heat flux cooling techniques is today a primary driver for innovation in the electronics industry. In this paper, the physical phenomena underpinning the most promising on-chip thermal management approaches for hot spot remediation, along with basic modeling equations and typical results are described. Attention is devoted to thermoelectric microcoolers — using mini-contcat enhancement and in-plane thermoelectric currents, orthotropic TIM’s/heat spreaders, and phase-change microgap coolers.Copyright © 2009 by ASME

Polymer Heat Exchangers—History, Opportunities, and Challenges

Abstract: For the past 40 years considerable attention has been devoted to the innovation, characterization, and implementation of polymer heat exchanger technology, driven by the corrosion resistance, low density, low cost, and ease of manufacture of many polymeric materials. Moreover, new polymer composites, with higher impact and yield strengths, higher temperature limits, and higher thermal conductivities, promise to bridge the performance gap that exists between polymers and corrosion-resistant metals. This paper begins by reviewing the history of polymer heat exchangers and the technical limitations that have motivated much of the research on this technology. The notable developments that have taken place in the last decade and primary potential applications for polymer heat exchangers are then discussed, including solar water heaters, heat recovery systems, and seawater heat exchangers, in particular, for the desalination industry. The paper closes with a review of compact polymer heat exchangers, with milli...

Advanced thermal management technologies for defense electronics

Abstract: Thermal management technology plays a key role in the continuing miniaturization, performance improvements, and higher reliability of electronic systems. For the past decade, and particularly, the past 4 years, the Defense Advanced Research Projects Agency (DARPA) has aggressively pursued the application of micro- and nano-technology to reduce or remove thermal constraints on the performance of defense electronic systems. The DARPA Thermal Management Technologies (TMT) portfolio is comprised of five technical thrust areas: Thermal Ground Plane (TGP), Microtechnologies for Air-Cooled Exchangers (MACE), NanoThermal Interfaces (NTI), Active Cooling Modules (ACM), and Near Junction Thermal Transport (NJTT). An overview of the TMT program will be presented with emphasis on the goals and status of these efforts relative to the current State-of-the-Art. The presentation will close with future challenges and opportunities in the thermal management of defense electronics.

Two-Phase Thermal Transport in Microgap Channels—Theory, Experimental Results, and Predictive Relations

Abstract: A comprehensive literature review and analysis of recent microchannel/microgap heat transfer data for two-phase flow of refrigerants and dielectric liquids is presented. The flow regime progression in such a microgap channel is shown to be predicted by the traditional flow regime maps. Moreover, Annular flow is shown to be the dominant regime for this thermal transport configuration and to grow in importance as the channel diameter decreases. The results of heat transfer studies of single miniature channels, as well as the analysis and inverse calculation of IR images of a heated microgap channel wall, are used to identify the existence of a characteristic M-shaped heat transfer coefficient variation with quality (or superficial velocity), with inflection points corresponding to transitions in the two-phase cooling modalities. For the high-quality, Annular flow conditions, the venerable Chen correlation is shown to yield predictive agreement for microgap channels that is comparable to that attained for macrochannels and to provide a mechanistic context for the thermal transport rates attained in microgap channels. Results obtained from infrared imaging, revealing previously undetected, large surface temperature variations in Annular flow, are also reviewed and related to the termination of the favorable thin-film evaporation mode in such channels.

Hybrid 3D-IC Cooling System Using Micro-fluidic Cooling and Thermal TSVs

Abstract: 3D-ICs bring about new challenges to chip thermal management due to their high heat densities. Micro-channel based liquid cooling and thermal through-silicon-vias (TSVs) have been adopted to alleviate the thermal issues in 3D-ICs. Thermal TSV (which have no electrical significance), enables higher interlayer thermal conductivity thereby achieving a more uniform thermal profile. While somewhat effective in reducing temperatures, they are limited by the nature of the heat sink. On the other hand, micro-channel based liquid cooling is significantly capable of addressing 3D IC cooling needs but consumes a lot of extra power for pumping coolant through channels. This paper proposes a hybrid 3D-IC cooling scheme which combines micro-channel liquid cooling and thermal TSV with one acting as heat removal agent while the other enabling beneficial heat conduction paths to the micro-channel structures. The experimental results show that, the proposed hybrid cooling scheme provides much better cooling capability than using only thermal TSVs, while consuming 55% less cooling power compared with pure micro-channel cooling.

Trench-assisted thermoelectric isothermalization of power switching chips

Abstract: One embodiment includes a power module. The power module includes a power switching device, at least one spot cooler and a base cooler. The at least one spot cooler and base cooler are configured to lower an average surface junction temperature and to isothermalize the surface junction temperature of the power switching device. The at least one spot cooler is embedded in at least one of a heat sink base or base cooler of the power module, and the at least one of the heat sink base or base cooler are attached onto a double side metalized substrate that is attached to the power switching device. In one embodiment, the power module further includes a trench structure cut into the double side metalized substrate.

GaN HEMT Junction Temperature Dependence on Diamond Substrate Anisotropy and Thermal Boundary Resistance

Abstract: A thermal model of Gallium Nitride High Electron Mobility Transistors (GaN HEMTs) examines the impact of diamond substrate parameters on device thermal performance. These parameters include substrate thickness, GaN-substrate thermal boundary resistance (TBR), and a simplified anisotropic substrate thermal conductivity. Diamond substrates appear to only provide thermal improvement over GaN-on-Silicon Carbide (SIC) devices when the corresponding GaN-on-diamond TBR is not substantially larger than the GaN-on-SiC range, independent of the degree of substrate anisotropy. The reduced lateral conductivity due to substrate anisotropy also proves to be of less significance to substrate thermal resistance when vertical conductivity is very high, but decreased spreading in the substrate can significantly impact cold plate temperature rise. Finally, for any degree of anisotropy, substrates thinner than 150µm are shown to significantly increase cold plate temperature rise as they restrict heat spreading and impose higher heat fluxes to downstream components.

Two-phase microgap cooling of a thermally-simulated microprocessor chip

Abstract: Forced flow of refrigerants and dielectric liquids, undergoing phase change in a microgap channel above an active chip is a promising candidate for the thermal management of advanced semiconductor devices. This paper presents two-phase heat transfer and pressure drop results for a chip-scale, uniformly heated, microgap channel using HFE-7100 and FC-87 as the working fluids. Results for two channel configurations are presented: a chip-size, short channel, representative of a product configuration and a longer channel, representing a laboratory prototype. Each microgap channel was tested with three nominal gap heights: 100, 200, and 500 micrometer.

Energy Efficient Two-Phase Micro-Cooler Design for a Concentrated Photovoltaic Triple Junction Cell

Abstract: This paper investigates the potential application of an R134a-cooled two-phase micro/mini-cooler for thermal management of a triple junction solar cell under 2000 suns concentration An analytical model for the triple-junction solar cell temperature based on prediction of two-phase flow boiling in mini/microchannel coolers is developed and exercised with empirical correlations from the open literature for the heat transfer coefficient, pressure drop, and critical heat flux The thermofluid analysis is augmented by detailed energy modeling, using two uniquely defined coefficient of performance metrics — COP relating the solar energy harvest to pumping power consumption and COPT relating the solar energy harvest to the “parasitic” work expended to provide the requisite cooling, including pumping power and the energy consumed in the formation and fabrication of the microcooler itself Three constant fin thicknesses of 100μm, 300μm and 500μm are examined for a range of R134a flow rates and geometries to determine the energy efficient design for a 10mm×10mm triple junction CPV cell The results reveal that two-phase cooling of CPV’s can achieve very high COPT values, substantially exceeding 104 for much of the design space of interest, though the energy efficiency is dependent on microcooler geometry and the number or pitch of the microcooler channelsCopyright © 2012 by ASME

Two-phase Fluid Selection for High-temperature Automotive Platforms

Abstract: : High-temperature environments and device self-heating are pushing the thermal limits of power electronics. This is particularly true in automotive applications, where there is a transition from internal combustion architectures to hybrid electric and full electric vehicles. In general, two-phase cooling has emerged as an attractive solution to meeting the high-temperature, high-power cooling needs of the aforementioned electronics. However, it is important to understand the benefits and limitations of various fluids when designing a two-phase cooling system. To aid in the selection process and add to the current knowledge base, this report briefly discusses the current state of high temperature electronics, reviews platform considerations when selecting a fluid, and introduces a two-phase Figure of Merit (FOM) to analyze over 110 different fluids for cooling high-temperature electronics up to 500 deg C.

Transient thermoelectric self-cooling of a germanium hotspot

Abstract: Thermoelectric cooling has been well documented as a solution for hotspots in electronic devices. Hotspots arise from the heterogeneous power dissipation of most computer chips and power electronics. This paper reviews the recent progress in thermoelectric self-cooling and goes on to study the transient behavior of a germanium thermoelectric self-cooler. A 3-D thermoelectric simulation is used to explore the effects of various initial conditions, current pulse durations, current pulse magnitudes, and die thicknesses. Additionally, a new cooling metric is introduced in order to more effectively characterize the advantage of transient cooling. The results suggest that transient thermoelectric cooling has the potential to improve hotspot temperature reduction by approximately 30% relative to what is achievable in steady state operation.

Quantum-Well Si/SiC Self-Cooling for Thermal Management of High Heat Flux GaN HEMT Semiconductor Devices

Abstract: Wide bandgap semiconductor technology is expected to have a dramatic impact on radar and communications systems. To take full advantage of the power capabilities and small device sizes of wide bandgap semiconductors, new and novel thermal management solutions, especially for high power density, monolithic microwave integrated circuits (MMICs) are in high demand. In this paper, a quantum-well Si/SiC self-cooling concept for hot spot thermal management at the multi-fingered GaN high electron mobility transistor (HEMTs) in the GaN-on-SiC package is proposed and investigated using a three dimensional (3-D) thermal-electric coupling simulation. The impact of electric current, cooler size, Si/SiC substrate thickness, Si/SiC thermal conductivity, and interfacial parasitic effect on the hot spot cooling is examined and discussed. The preliminary modeling results strongly suggest that self-cooling phenomenon inherent in the quantum-well Si/SiC substrate can be used to remove local high heat flux hot spot on the semiconductor devices.© 2012 ASME

A Hybrid Multi-gate Model of a Gallium Nitride (GaN) High Electron Mobility Transistor (HEMT) Device Incorporating GaN-substrate Thermal Boundary Resistance

Abstract: : This report describes a pseudo-analytical thermal model of gallium nitride (GaN) high electron mobility transistors (HEMTs), which combines analytical heat spreading models with spreading width boundary conditions derived from two-dimensional finite element thermal simulations We successfully produced an accurate GaN HEMT hybrid model capable of evaluating the impact of thermally important device parameters on junction and individual layer temperatures A parameter space investigation, covering GaN-substrate thermal boundary resistance (TBR), gate pitch, substrate thickness, substrate thermal conductivity, and GaN thickness validated the hybrid model against full finite element numerical analysis and provided insight into device thermal behavior This modeling showed near junction thermal resistance contributions from the GaN and interface TBR stay relatively constant with gate number and pitch down to 5 m Alternatively, the thermal profiles in the substrate layers and below show strong interaction between gates; the magnitude of those components scale directly with gate number and increase significantly with decreasing gate pitch Also finite substrate and GaN thicknesses produce a minimum temperature rise dependent on downstream thermal resistance Finally, increasing substrate thermal conductivity, by replacing a silicon carbide (SiC) substrate with higher thermal conductivity diamond, appears to only be advantageous if the TBR does not increase substantially beyond the SiC range

Thermal–structural modeling of polymer Bragg grating waveguides illuminated by a light emitting diode

Abstract: This study reports both analytical and numerical thermal–structural models of polymer Bragg grating (PBG) waveguides illuminated by a light emitting diode (LED). A polymethyl methacrylate (PMMA) Bragg grating (BG) waveguide is chosen as an analysis vehicle to explore parametric effects of incident optical powers and substrate materials on the thermal–structural behavior of the BG. Analytical models are verified by comparing analytically predicted average excess temperatures, and thermally induced axial strains and stresses with numerical predictions. A parametric study demonstrates that the PMMA substrate induces more adverse effects, such as higher excess temperatures, complex axial temperature profiles, and greater and more complicated thermally induced strains in the BG compared with the Si substrate.