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Prof. Avram Bar-Cohen (popularly known as Avi), the past President of IEEE Electronics Packaging Society (EPS), and the past Editor-in-Chief of the IEEE Transactions on Components and Packaging Technologies, passed away on October 10, 2020. He was a giant in the field of thermal management and made seminal research and professional contributions over a five-decade period. Prof. Bar-Cohen received his bachelor’s and master’s degrees in mechanical engineering from the Massachusetts Institute of Technology (MIT), Cambridge, MA, USA, in 1968. Subsequently, he joined Raytheon Corporation, while continuing to pursue his Ph.D. degree at MIT, and receiving it in 1971. His Ph.D. work was titled “Boiling and Condensation in a Liquid- Filled Enclosure.” Shortly afterward, he moved to Ben-Gurion University, Be’er Sheva, Israel, as a faculty member, where he helped to establish the Department of Mechanical Engineering. He served there for nearly 15 years, with stints away during visiting positions at MIT, Naval Postgraduate School, Control Data Corporation, and University of Minnesota. In 1989, he joined the faculty of the Department of Mechanical Engineering, University of Minnesota. There he served as the Director of the Thermodynamics and Heat Transfer Division, and held the James J. Renier Visiting Chair in Technological Leadership. In 2000, he was appointed as the Director of the Center for the Development of Technological Leadership, and the H. W. Sweatt Chair in Technological Leadership. In 2001, Prof. Bar-Cohen was appointed as the Chair of the Department of Mechanical Engineering, University of Maryland (UMD), College Park, MD, USA. After serving two terms as the Chair, he accepted the position of Program Manager at the Microsystems Technology Office, U.S. Defense Advanced Research Projects Agency (DARPA), on assignment from the UMD in 2010. Subsequently, in 2015, he joined Raytheon Corporation, while on leave from the UMD, and served there until his passing. [ABSTRACT FROM AUTHOR]

The Intrachip Enhanced Cooling Fundamentals (ICECool Fun) effort was launched by the Defense Advanced Research Projects Agency (DARPA) under the leadership of Dr. Avram Bar-Cohen during 2012–2015 to target an order of magnitude improvement in chip level and hot spot heat fluxes, compared to the then state-of-the-art (SOA). Evaporative cooling technologies to achieve potential targets of 1 kW/cm2 at the chip level and 5 kW/cm2 at the hot spot level were targeted. A key goal was to improve fundamental understanding of the evaporative cooling physics at the relevant scales, and a numerical modeling capability to enable the co-design of such solutions in emerging computing and communications systems. A summary of the five projects pursued under this effort is provided, including the key accomplishments and developed capabilities. [ABSTRACT FROM AUTHOR]

Manifold microchannels have shown high performance in embedded two-phase cooling of high heat flux electronics, demonstrating several examples of heat dissipation of 1 kW from 1-cm2 chips, with low pumping power requirements. The heat transfer coefficients (HTCs) in previous studies of chip-scale manifold-microchannel coolers exhibited a peak at relatively low quality (about 0.2–0.4) and steeply declined thereafter with increasing quality. However, manifold microchannels—arrays of manifolded-microgap channels—are a more complex geometry than traditional, straight, and circular channels and therefore generate more complex two-phase flow phenomena, which influences the resulting heat transfer performance. To better characterize the flow and heat transfer in such channels, in this study, the fluid flow and wall heat transfer in a visualization test section was imaged with a high-speed optical camera and thermography. R245fa served as the working fluid due to its low-pressure operation (approximately 1.8 atm at 30 °C) and excellent thermal properties, including high latent heat, high specific heat, and high thermal conductivity. The results are reported in terms of the channel outlet flow regime and HTCs under varying heat fluxes and mass fluxes with two manifold designs. The flow regimes and HTCs are compared with well-regarded predictive relations developed for more traditional channel geometries. The emergence of intermittent, and then persistent, dryout was associated with intermittent rivulet flow at the channel outlet and was observed to cause declining average wall HTCs, even while high HTCs occurred at the consistently wetted inlet region. [ABSTRACT FROM AUTHOR]

In a more electrified future, development of high power, compact, and synergistically cooled power electronics for improved performance and reliability is of critical importance to a broad range of defense and commercial applications. As power levels increase, power converters must manage higher thermal loads in smaller and lighter weight packages. This necessitates smarter packaging configurations and more aggressive cooling schemes. This work introduces a novel cooler design for thermal management of high-power switches in a dual active bridge converter: a vertically enhanced manifold microchannel system (VEMMS). As opposed to traditional flat heatsinks, this cooler shrinks in footprint but expands away from the board in the direction of its height ($z$ -direction), for increased heat transfer area while keeping the footprint on the printed circuit board (PCB) at minimum. Since most power converters’ total height is dictated by magnetic elements such as the core, expanding the cooler in vertical direction does not reduce packing density, while it significantly enhances its thermal performance. Both liquid and air-cooled versions of the cooler are introduced through case studies, and their advantages are assessed through extensive numerical modeling. Volume based power densities of 20 kW/L for the overall converter and 286 kW/L for one full bridge assembly for a 10-kW power converter were attained. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This paper presents details of an optimization process of electrode geometry for an electrohydrodynamically (EHD) driven thin-film evaporator. The operation principle of the device is based on the action of the EHD force on the molecules of a dielectric liquid in a highly convergent electric field. The force starts at the end of a pair of electrodes, where the electric field changes from zero far from the electrodes to a finite value in between the electrodes. This force drives the liquid up into the spacing between the electrodes. The electrodes in this study were deposited thinly on a SiO2 /Si wafer, so the liquid could be held within micrometers of thickness over the surface. Since the performance of the device in removing heat from the surface is a function of its pumping head and consequently its electrode geometry, the performances of different electrodes were evaluated by testing twelve sets of electrode pairs with different geometries. Then the optimum electrode design was incorporated into the design of a large size (32 × 32 mm²) EHD thin-film evaporator. The device was fabricated, and its pumping and heat transfer performances were tested. A pumping head equal to the full height of the electrodes and a heat transfer coefficient of 1.9 W/cm² .°C was achieved using HFE-7100 liquid. [ABSTRACT FROM AUTHOR]

This paper presents the design, fabrication, and testing of a novel electrohydrodynamic (EHD) ion-drag micropump. In order to maximize the electrical field gradients that are responsible for EHD pumping, we incorporated three-dimensional (3-D) triangular bumps of solder as part of the EHD electrodes. To form these bumps, Niobium was sputter-deposited onto a ceramic substrate, coated with photoresist, optically exposed and etched using a reactive ion etcher to define the electrode pattern. The substrate was then "dipped" into a molten solder pool. Since the solder adheres only to the metallic film, bumps of solder form on the electrodes, giving the electrodes a significant 3-D character. The overall dimensions of the micropump are 19 mm × 32 mm × 1.05 mm. Four different designs were fabricated and tested. Static pressure tests were performed with a 3M Thermal Fluid (HFE-7100) as the working fluid and the optimum design was identified. The results with the thermal fluid were highly promising and indicated a pumping head of up to 700 Pa at an applied voltage of 300 V. The experimental results for the four different designs show that the presence of the 3-D bump structures significantly improves the pumping performance. Also, a much better pumping performance was obtained with the micropump in which the emitter had a saw-tooth shape. [ABSTRACT FROM AUTHOR]