Dereje Agonafer

Steady-state heat conduction in multi-layer bodies

Abstract The mathematical formulation of the steady-state temperature field in multi-dimensional and multi-layer bodies is presented. The numerical examples are for two-layer bodies and they include boundary conditions of the first, second, and third kind. This study includes tables to assist the selection of eigenfunctions and computation of the eigenvalues. The computations include the contribution of contact resistance to the temperature solution. An efficient computational scheme for calculating the eigenvalues is used. For multi-dimensional, multi-layer bodies, the eigenfunctions are real if each layer is homogeneous; they may become imaginary if layers are orthotropic.

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.

A simulation-based multi-objective design optimization of electronic packages under thermal cycling and bending

In this study, a simulation-based multi-objective design optimization methodology was developed for improving electronic packaging reliability. It was demonstrated using a generic model of an electronic package on a printed wiring board. The objective for the optimization was to improve the reliability of solder joints under both thermal cycling and bending by optimizing a group of design parameters. A parametric finite element model was developed using ANSYS for both load conditions. To improve the numerical efficiency of the optimization, a multi-quadric response surface method was implemented to approximate the response of finite element simulations for each loading condition. Subsequently, the multi-objective optimization of solder joint reliability was implemented using a Minmax principle on all response surfaces and a differential evolution algorithm as optimal search engine, which is capable of finding global minimum when local minima exist. Our study demonstrated that the reliability of the solder joints is significantly improved for this given generic model of electronic package. The proposed methodology can be effectively used in improving the reliability of electronic packages.

Method for Determining a Dynamical State–Space Model for Control of Thermal MEMS Devices

A method is presented for determining lumped dynamical models of thermal microelectromechanical systems (MEMS) devices for purposes of feedback control. As a case study, an electrothermal actuator is used. The physical properties and a set of assumptions are used to determine the basic structure of the dynamical model, which requires the development of the electrical, thermal, and mechanical dynamics. The importance of temperature-dependent parameters is emphasized for dynamical modeling for purposes of feedback control. To confront temperature dependence in a practical yet effective manner, an average temperature is introduced to preserve the energy balance inside the structure. This allows the development of a practical method that combines structure of the model, through the average body temperature, with finite element analysis (FEA) in novel way to perform system identification and identify the unknown parameters. The result is a lumped dynamical model of a MEMS device that can be used for the design of feedback control systems. We compare computer simulated results using the dynamical model with experimental behavior of the actual device to show that our procedure indeed generates an accurate model. This dynamical model is intended for further synthesis of driving signal and control system but also gives a qualitative insight into the relationship between devices geometry and its behavior. The method enables fast development of the model by conducting relatively few static FEA and is verifiable with dynamic experimental results even when temperature measurements are not available.

Design optimization and reliability of PWB level electronic package

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Effect of Under Floor Blockages on Data Center Performance

As the Electronic Packaging industry develops technologies for fabrication of smaller, faster, economical and reliable products; thermal management and design play an important role. The major part of the failures of the electronic components is temperature related. During thermal cycling, fatigue failures are caused due to mismatch of coefficient of thermal expansion (CTE) of different materials present in the components. Increased power dissipation and density in modern electronics system require efficient and intelligent design and thermal management strategies to ensure the reliability of electronic products. This paper discusses the reliability and design optimization of a generic Printed Wiring Board (PWB) level electronic package under thermal cycle loading. Finite element tool ANSYS is used to estimate the cycles to fatigue failure of solder joint of the package coupled with optimization module present in ANSYS for providing the details on determining optimal design parameters which affect the product reliability. Combining finite element analysis with optimization would significantly reduce the design time and increases the product reliability. Four model characteristics: PWB core in-plane Youngs Modulus, PWB core in-plane coefficient of thermal expansion, PWB core thickness and the stand-off solder joint height are chosen as the optimization inputs (design variables) that ensure higher reliability and improved performance of the assembled product. The objective of the optimization is to improve the fatigue life of solder joints of the package. Sub approximation, Design of Experiment (DoE) and Central Composite Design based Response Surface Modeling Methodology are used to study the effects of each design variables on the fatigue life.

Analytical Thermal Solution to a Nonuniformly Powered Stack Package With Contact Resistance

The convergence of computing and communications dictates building up rather than out. As consumers demand more functionality in their hand-held devices, the need for more memory in a limited space is increasing, and integrating various functions into the same package is becoming more crucial. Over the past few years, die stacking has emerged as a powerful tool for satisfying these challenging integrated circuit (IC) packaging requirements. In this paper, a review of thermo-mechanical challenges for stacked die packaging is discussed.

Design Optimization and Reliability of PWB Level Electronic Package

In a typical raised floor data center, under floor plenum supplies cold air to the computer room via perforated floor tiles. Thermal management of any data center primarily depends on air flow rates through the tiles. These tile flow rates are a function of several factors that govern the flow pattern under the raised floor. CFD analyses of an ideal raised floor can often be misleading as they may not incorporate the effect of under floor blockages such as cables, ducts, pipes and other random blockages. In this paper, the effect of under floor blockages on data center performance is discussed. A raised floor data center with 8 kW racks is considered. There are four rows of racks with six racks per row. Two air conditioning units supply chilled air to the data center through a 2 ft. deep plenum. Blockages are included in the model and are shown to have significant impact on tile flow rates and eventually the rack inlet temperatures. Parameters like blockage size and locations are investigated to come up with a set of broad guidelines which will help the facility designer to rearrange the blockages and improve the thermal performance without altering the layout or cold air supply

Multi-Objective Optimization to Improve Both Thermal and Device Performance of a Nonuniformly Powered Micro-Architecture

The semiconductor industry, following Moores law, has consistently followed a trajectory of miniaturization that enables design engineers to achieve greater levels of innovation in the same or smaller die footprints. According to Samsung technologists, the next generation of semiconductor technology will cost about