Bahgat Sammakia

Thermal Challenges in Next-Generation Electronic Systems

Thermal challenges in next-generation electronic systems, as identified through panel presentations and ensuing discussions at the workshop, Thermal Challenges in Next Generation Electronic Systems, held in Santa Fe, NM, January 7-10, 2007, are summarized in this paper. Diverse topics are covered, including electrothermal and multiphysics codesign of electronics, new and nanostructured materials, high heat flux thermal management, site-specific thermal management, thermal design of next-generation data centers, thermal challenges for military, automotive, and harsh environment electronic systems, progress and challenges in software tools, and advances in measurement and characterization. Barriers to further progress in each area that require the attention of the research community are identified.

Bending Fatigue Study of Sputtered ITO on Flexible Substrate

Recently, there has been a tremendous rise in production of portable electronic devices. To produce flexible devices, flexible substrates should replace conventional glass substrates. Indium-tin-oxide (ITO) is the preferred transparent conducting layer used in the display technology. Although ITO has excellent sheet resistance and very good optical properties, ITO can crack at very low tensile strains which might cause failure in the conductive layer because of the unusual structure of a very thin film of brittle ceramic material applied to a polymer substrate. Therefore, the mechanics of ITO on flexible substrates has been thoroughly considered in the design and manufacturing of flexible devices. In a typical roll-to-roll manufacturing process, many challenges exist during the travel of the coated web over the rollers which produce bending stresses that might cause failure even if the stresses are below the yield strength of the material. Therefore, the high cycle bending fatigue of ITO thin films on flexible substrates is of a significant importance. In this work, high cycle bending fatigue experiments were conducted on ITO coated PET substrate. The effect of bending diameter, bending frequency, and sample width on the change in electrical resistance was investigated. High magnification images were obtained to observe crack initiation and propagating in the ITO layer. The goal of this work is to establish a baseline for a comprehensive reliability study of ITO thin films on flexible substrate. It was found that bending diameters as well as the number of bending cycles have a great influence on the electrical conductivity of the ITO layer.

A Review of Recent Developments in Some Practical Aspects of Air-Cooled Electronic Packages

The recent emphasis on low-cost high-end servers and desktop workstations has resulted in a renewed interest in the development of high-performance air-cooled systems. A new generation of advanced heat sink designs capable of dissipating up to 10 5 W/m 2 have been proposed and developed. Better manufacturing tolerances, lower defects, and an improved understanding of card and enclosure effects have been attained and shown to be critical to achieving the desired thermal performance, Advanced internal thermal enhancements, encompassing high thermal conductivity adhesives and greases have also been implemented. This review article covers recent developments in heat sink designs and applications intended for high-end high-power dissipation systems. A review of recent studies of card effects in the thermal enhancement of electronic packages is also presented. In certain applications the card heat-sinking effect can play a major role in the thermal management of a package, accounting for more than 50 percent of the total power dissipation of the package

Thermal challenges in next generation electronic systems - summary of panel presentations and discussions

The presentations made, as well as the discussions, in the panels at the workshop, Thermal Challenges in Next Generation Electronic Systems (THERMES), are summarized in this paper. The panels dealt with diverse topics including thermal management roadmaps, microscale cooling systems, numerical modeling from the component to system levels, hardware for future high performance and Internet computing architectures, and transport issues in the manufacturing of electronic packages. The focus of the panels was to identify barriers to further progress in each area that require the attention of the research community.

Flexible chemiresistor sensors: thin film assemblies of nanoparticles on a polyethylene terephthalate substrate

The thin film assembly of metal nanoparticles on flexible chemiresistor (CR) arrays represents an intriguing way to address the versatility of chemical sensor design. In this work, thin film assemblies of gold nanoparticles in size range of 2–8 nm diameters with high monodispersity (unlinked or linked by molecular mediators) were assembled on a CR array with a polyethylene terephthalate (PET) substrate to demonstrate the flexible chemiresistor characteristics of the nanostructured materials. The correlation between the relative change in electrical conductivity and the change in dielectric medium constant in response to flexible wrapping of the device demonstrated the viability of manipulating the electrical responses in terms of wrapping direction. The responses of the devices in response to volatile organic compounds (VOCs) were analyzed in terms of particle size, interparticle properties, and substrate–film interactions. For molecularly linked films with small particle size and large interparticle spacing, which is characterized by a high percentage of organics and linker molecules, the relatively low electrical conductivity renders the change in interparticle spacing able to play a dominant role in the sensor response to VOCs with small dielectric constants. The combination of a high percentage of linker molecules in the thin film assembly and a high dielectric constant for the VOCs was found to produce a negative response characteristic. In contrast, the response characteristic for the unlinked film via weak interparticle interactions was dominated by the change in interparticle spacing regardless of the percentage of organics in the nanostructure. The delineation between these factors and the sensing characteristics is useful in enabling a rationale design of the nanostructures on flexible chemiresistors.

Comparative Analysis of Different Data Center Airflow Management Configurations

Increase in computing power resulting from high performance microprocessors, packages, and modules and the deployment of high heat load computer rack units in high density configurations, has escalated the thermal challenges in today’s data center systems. One of the key issues is the location of hot recirculation regions in the room and the mixing of hot rack exhaust air with the cold supply air. Along with many factors such as the rack heat load and the cooling capacity of the supply air, the data center thermal management architecture plays an important role in determining the reliability of the electronic equipment and the general thermal performance of the data center. There are several candidate configurations available for the air ducting designs for data centers. The overall energy efficiency of the system is highly dependant upon the selection of the specific configuration. This paper will summarize the results of a broad numerical study carried out to assess the effectiveness of different data center configurations. The numerical modeling is performed using a commercial computational fluid dynamics (CFD) code based on finite volume approach. The configurations studied include different combinations of raised floor and ceiling supply and return vent location subject to specific constraints. The performance of the data center has been characterized on the basis of average and maximum mean region rack inlet air temperature. Among the seven different configurations compared, the raised floor/ceiling return type configuration is found to be the most effective configuration for the given set of constraints and assumptions.Copyright

Optimization of Data Center Room Layout to Minimize Rack Inlet Air Temperature

In a typical raised floor data center with alternating hot and cold aisles, air enters the front of each rack over the entire height of the rack. Since the heat loads of data processing equipment continue to increase at a rapid rate, it is a challenge to maintain the temperature of all the racks within the stated requirement. A facility manager has discretion in deciding the data center room layout, but a wrong decision will eventually lead to equipment failure. There are many complex decisions to be made early in the design as the data center evolves. Challenges occur such as optimizing the raised floor plenum, floor tile placement, minimizing the data center local hot spots, etc. These adjustments in configuration affect rack inlet air temperatures, which is one of the important keys to effective thermal management. In this paper, a raised floor data center with 12 kW racks is considered. There are four rows of racks with alternating hot and cold aisle arrangement. Each row has six racks installed. Two air-conditioning units supply chilled air to the data center through the pressurized plenum. Effect of plenum depth, floor tile placement, and ceiling height on the rack inlet air temperature is discussed. Plots will be presented over the defined range. A multivariable approach to optimize data center room layout to minimize the rack inlet air temperature is proposed. Significant improvement over the initial model is shown by using a multivariable design optimization approach.

Predictive Model for Optimized Design Parameters in Flip-Chip Packages and Assemblies

An analytical model is developed to predict the out-of- plane deformation and thermal stresses in multilayered thin stacks subjected to temperature. Coefficient of thermal expansion mismatches among the components (chip, substrate, underfill, flip-chip interconnect or C4s) are the driving force for both first and second levels interconnect reliability concerns. Die cracking and underfill delamination are the concerns for the first level interconnects while the ball grid array solder failure is the primary concern for the second level interconnects. Inadvertently, many researchers use the so-called rule of mixture in its effective moduli for the flip chip solder (C4)/underfill layer. In this study, a proper formula for effective moduli of solder (C4)/underfill layer, is presented. The classical lamination theory is used to predict the out-of-plane displacement of the chip substrate structure under temperature variation (DeltaT). The warpage and stresses resulting from the analytical formulation are compared with the 3-D finite element analysis. The study helps to design more reliable components or assemblies with the design parameters being optimized in the early stage of the development using closed form analytical solutions.

Experimental-Numerical Comparison for a High-Density Data Center: Hot Spot Heat Fluxes in Excess of 500 W/FT2

The current trend of using denser server environments is continuously increasing to satisfy the growing needs of e-commerce and other emerging technologies. The resulting high room-level heat fluxes present significant challenges with respect to maintaining acceptable computer rack inlet temperatures and minimizing total data center energy consumption. Numerical methods are widely used to model existing and new facilities. Validation of existing numerical techniques is an important step in facilitating good thermal design of data centers. This paper uses previously published experimental data to present a comparison between test results and numerical simulations. The example considered is a large 7400 ft2 data canter that houses over 130 heat-producing racks (1.2 MW) and 12 air conditioning units. Localized hot spot heat fluxes were measured to be as high as 512 W/ft2 (5.5 kW/m2) for a 400 ft (37 m) region. A numerical model based on computational fluid dynamics (CFD) was constructed using inputs from the measurements. The rack inlet air temperature was considered to be the basis for experimental vs. numerical comparison. The overall mean rack inlet temperature predicted numerically at a height of 1.75 m is within 4degC of the test data with a rack-by-rack standard deviation of 3.3 degC

UNSTEADY NATURAL CONVECTION GENERATED BY A HEATED SURFACE WITHIN AN ENCLOSURE

The unsteady buoyancy-induced flow generated by an isothermal flat vertical surface enclosed in a long rectangular cavity is numerically analyzed. The enclosed fluid is assumed to be Newtonian and the walls are adiabatic. The transient process is initiated by suddenly and uniformly raising the temperature of the surface to a steady value higher than that of the fluid in the cavity. The full two-dimensional conservation equations, representing mass, momentum, and energy balance, are solved using a modified finite-difference scheme originally called the simple arbitrary La-grangian-Eulerian (SALE) technique. The resulting transport is found to be composed of several distinct regimes. At very short times a quasi-one-dimensional conduction regime occurs adjacent to the surface. The temperature distribution and heat transfer coefficient are found to closely follow the known one-dimensional conduction solution. At intermediate times the flow adjacent to the surface is observed to resemble the boundary-layer reg...