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Characterization of Single-Phase Heat Transfer and Hydraulic Performance in Non-Uniformly Heated Microgap Channels
Keywords: liquid cooling, microchannels, conjugate heat transfer
It is an industry-wide known fact that the trend in electronics compactness, functionality and feature count and thus power dissipation is going in an upward direction. The electronics market shall, if it has not already, experience unprecedented trends in chip powers and heat fluxes that could by 2012 reach 300 watts and 150 watts/cm2 in heat dissipation and heat flux, respectively. On the other hand, changes in chip layout and packaging technologies, throughout the recent years, leading to a wide adoption of flip chip die technology and non-uniform power dissipation has had a strong impact on thermal management needs as design constraints. Recently, the objective of cooling recent flip chip dies with non-uniform power dissipation, but continuously improving heat sinks, had shifted into the need for significantly lowering the maximum chip-to-heat sink thermal resistance as this resistance became dominant and most critical resistance of the overall total system thermal resistance. As a consequence, the thermal packaging community started searching for and investigating new cooling technologies for high heat flux chips. Advanced liquid cooling has started to emerge as a feasible solution for even cost performance electronics systems. The recent increase in CPU heat fluxes coupled with the need to thermally design for the highest possible heat flux, for speed reasons, caused thermal system to become more complex had paved the way for active cooling. Microgap coolers provide direct contact between chemically inert, dielectric fluids and the back surface of an active electronic component, thus eliminating the significant interface thermal resistance associated with thermal interface materials and/or solid-solid contact between the component and a microchannel cold plate. This presentation focuses on studying the thermal performance of dielectric fluid single phase heat transfer through microgap coolers, implemented and packaged on non-uniformly powered, thermal test chip. This presentation focuses on the single-phase thermo-fluid characteristics of a dielectric liquid, HFE-7100, flowing in an asymmetrically-heated chip-scale micro-gap channel, 14 mm wide by 10.5 mm long, with channel heights varying from 100 m to 500 m. The single-phase, area-averaged heat transfer coefficients of HFE-7100 to be evaluated and assessed against literature correlations and CFD simulations. The focus of this presentation is on the modeling, analysis and empirical characterization of single-phase, high heat flux cooling of non-uniformly heated semiconductor chips, focusing on the conjugate effects in the heated surface and spatially varying heat transfer performance in a microchannel. This configuration is representative of the thermal management challenge facing next generation computers. The presentation analyses a very complex, geometrical, thermal and hydrodynamic problem. The geometric complexity involves packaging the microgap channel in a chip-scale housing that is mounted directly on the thermal chip-substrate assembly. The thermal complexity is due to the conjugate heat transfer phenomena, i.e., the direct coupling between convective flow and conduction through the substrate, along with external convection and radiation with the ambient air. The non-idealities associated with chip power non-uniformity and the highly developing flow in the very short micro-gap channel length add more complexity to the study and take it outside the realm of available single phase thermal correlations in the literature. This presentation seeks to experimentally determine the thermal performance of such complex microgap coolers and makes extensive use of CFD modeling to analyze non-uniform heating and conjugate heat transfer, i.e., conduction and convective flow effects and capture single phase heat transfer coefficients of these microgap coolers.
Ihab Andre Ali, VP of Thermal Products
Pipeline Micro, Inc.
Santa Clara, CA

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