Avram Bar-Cohen

Bio: Avram Bar-Cohen is an academic researcher from University of Maryland, College Park. The author has contributed to research in topics: Heat transfer & Heat sink. The author has an hindex of 50, co-authored 329 publications receiving 8329 citations. Previous affiliations of Avram Bar-Cohen include Auburn University & DARPA.

Structural Reliability of Novel 3-D Integrated Thermal Packaging for Power Electronics

Abstract: The continual increase of device power and package integration levels has driven the development of advanced power electronics packaging solutions. This study will focus on a numerical modeling approach to design analysis and material selection to improve solder joint reliability in one of these advanced solutions — a thermally integrated power electronics package that aims to dissipate hot-spot heat flux (5 kW/cm2) via mini-contact based thermo-electric (TE) cooling in addition to removing background heat flux (1 kW/cm2) by manifold-microchannel cooling. The methodology used for performing the structural reliability modeling is a non-linear finite element analysis (FEA) approach. Combined thermal and mechanical analyses were run to obtain stresses and strains in the solder joint used to integrate the TE cooler with the mini-contact and the mini-contact with the Silicon Carbide (SiC) chip. To predict the Mean Time to Failure (MTTF) of SAC305 at various levels of integration, a Physics of Failure (PoF) based methodology was applied using Engelmaier’s failure model.In this paper, we will discuss the results of analyses of tapered, t-shaped, and lofted shaped mini-contacts made out of SiC, copper and diamond. Both structural design and material selection affect hot-spot heat dissipation and solder joint reliability. SiC has a good thermal conductivity at room temperature (RT), however, with increase in temperature, its thermal conductivity drops, and this can adversely affect device performance in high temperature applications. On the other hand, one can take advantage of high conductivity materials like copper, diamond or silver-diamond composite to keep the device cool and thus, improve package life time. However, for such high conductivity materials, one will need to take into account the cost of manufacturing complex shapes without any compromise in package thermal or reliability performance.It was found that a ductile mini-contact material will share the thermal mismatch strain with the solder interconnection, while a brittle mini-contact material will shift the failure site inside the TE cooler. It was determined that a mini-contact structure tapered near its top base and lofted (constant cross-sectional area) near the chip (bottom base) would provide the best reliability results. Application of high conductivity composite material (silver-diamond composite) to enhance structural reliability is discussed.Copyright © 2015 by ASME

Energy Efficient Polymers for Gas-Liquid Heat Exchangers

Abstract: The present study explores the thermofluid characteristics of a seawater-methane heat exchanger that could be used in the liquefaction of natural gas on offshore platforms. The compression process generates large amounts of heat, usually dissipated via plate heat exchangers using seawater as a convenient cooling fluid. Such an application mandates the use of a corrosion resistant material. Metals such as titanium, expensive in terms of both energy and currency, are a common choice. The “total coefficient of performance,” or COPT , which incorporates the energy required to manufacture a heat exchanger along with the pumping power expended over the lifetime of the heat exchanger, is used to compare conventional metallic materials to thermally conductive polymers. The results reveal that heat exchangers fabricated of low energy, low thermal conductivity polymers can perform as well as, or better than, those fabricated of conventional materials, over the full lifecycle of the heat exchanger. Analysis of a prototypical seawater-methane heat exchanger, built from a thermally conductive polymer, suggests that a COPT nearly double that of aluminum, and more than ten times that of titanium, could be achieved.Copyright © 2008 by ASME

The Classroom of the Future: An Internet‐Delivered National Course on Thermal Management of Electronics

Abstract: Teaching inter-disciplinary material poses special challenges due to the diversity of student backgrounds. This problem is compounded if the material being taught is intended for both undergraduate and graduate students. Mechanical Engineering faculty members from three universities have come together to address this problem using a layered, multimedia delivery mechanism via the Internet. This has resulted in the first-ever, live, full-duplex, Internet course taught at any of the three partner universities: Auburn University, the University of Maryland, and the University of Minnesota. With the addition of colleagues from industrial sites such as Philips in the Netherlands and three other universities in Japan, Singapore, and Australia, the next offering will expand to become an international course. The authors hope to illustrate that a course delivered over the Internet adds significantly to the learning process in a cost-effective manner.

Numerical modeling and simulation of laser diode diamond microcoolers

Abstract: High heat flux management schemes in laser diodes require appropriate cooling applications Micro channel coolers are now widely used in high power laser diode industry with the highest total thermal resistance reported as low as 003 cm 2 -K/W with pressure drops as low as 10~50 psi Since, the geometries, flow rates as well as high heat fluxes of current SOA LD micro-coolers differ, it is necessary to understand their thermal performance relative to conductive, convective and caloric thermal resistance To do this comparison an equivalent effective micro-cooler thermal model is developed and then iterated with scaled input parameters from SOA LD micro-coolers The objective is to identify the dominant thermal resistance - that plays the major role in decreasing the total thermal resistance This paper will then predict a micro cooler that will perhaps be able to reduce total thermal resistance lower than 001 K-cm 2 /W with minimal pressure drop for next generation high heat flux applications The current study will be restricted to only single phase liquid cooling

Light-emitting diodes

Abstract: Light-Emitting Diodes. (Monographs in Electrical and Electronic Engineering.) By A. A. Bergh and P. J. Dean. Pp. viii+591. (Clarendon: Oxford; Oxford University: London, 1976.) £22.

A review of Ga2O3 materials, processing, and devices

Abstract: Gallium oxide (Ga2O3) is emerging as a viable candidate for certain classes of power electronics, solar blind UV photodetectors, solar cells, and sensors with capabilities beyond existing technologies due to its large bandgap. It is usually reported that there are five different polymorphs of Ga2O3, namely, the monoclinic (β-Ga2O3), rhombohedral (α), defective spinel (γ), cubic (δ), or orthorhombic (e) structures. Of these, the β-polymorph is the stable form under normal conditions and has been the most widely studied and utilized. Since melt growth techniques can be used to grow bulk crystals of β-GaO3, the cost of producing larger area, uniform substrates is potentially lower compared to the vapor growth techniques used to manufacture bulk crystals of GaN and SiC. The performance of technologically important high voltage rectifiers and enhancement-mode Metal-Oxide Field Effect Transistors benefit from the larger critical electric field of β-Ga2O3 relative to either SiC or GaN. However, the absence of clear demonstrations of p-type doping in Ga2O3, which may be a fundamental issue resulting from the band structure, makes it very difficult to simultaneously achieve low turn-on voltages and ultra-high breakdown. The purpose of this review is to summarize recent advances in the growth, processing, and device performance of the most widely studied polymorph, β-Ga2O3. The role of defects and impurities on the transport and optical properties of bulk, epitaxial, and nanostructures material, the difficulty in p-type doping, and the development of processing techniques like etching, contact formation, dielectrics for gate formation, and passivation are discussed. Areas where continued development is needed to fully exploit the properties of Ga2O3 are identified.

Low loss dielectric materials for LTCC applications: a review

Abstract: Small, light weight and multifunctional electronic components are attracting much attention because of the rapid growth of the wireless communication systems and microwave products in the consumer electronic market. The component manufacturers are thus forced to search for new advanced integration, packaging and interconnection technologies. One solution is the low temperature cofired ceramic (LTCC) technology enabling fabrication of three-dimensional ceramic modules with low dielectric loss and embedded silver electrodes. During the past 15 years, a large number of new dielectric LTCCs for high frequency applications have been developed. About 1000 papers were published and ∼500 patents were filed in the area of LTCC and related technologies. However, the data of these several very useful materials are scattered. The main purpose of this review is to bring the data and science of these materials together, which will be of immense help to researchers and technologists all over the world. The comme...