bluuebunny/crossref_metadata_2025 huggingface dataset:
Flow Analysis and Modeling of Field-Controllable, Electro- and Magneto-Rheological Fluid Dampers
Authors: Wang Xiaojie , Gordaninejad Faramarz
Published: December 2006

This study combines a fluid mechanics-based approach and the Herschel-Bulkley constitutive equation to develop a theoretical model for predicting the behavior of field-controllable, magneto-rheological (MR), and electro-rheological (ER) fluid dampers. The goal is to provide an accurate theoretical model for analysis, design, and development of control algorithms of MR/ER dampers. Simplified explicit expressions for closed-form solution of the pressure drop across a MR fluid valve are developed. The Herschel-Bulkley quasi-steady flow analysis is extended to include the effect of fluid compressibility to account for the nonlinear dynamic behavior of MR/ER fluid dampers. The advantage of this model is that it only depends on geometric and material properties of the MR/ER material and the device. The theoretical results are validated by an experimental study. It is demonstrated that the proposed model can effectively predict the nonlinear behavior of field-controllable fluid dampers.
Three-Dimensional Sharp Corner Displacement Functions for Bodies of Revolution
Authors: Huang C. S. , Leissa A. W.
Published: December 2006

Sharp corner displacement functions have been well used in the past to accelerate the numerical solutions of two-dimensional free vibration problems, such as plates, to obtain accurate frequencies and mode shapes. The present analysis derives such functions for three-dimensional (3D) bodies of revolution where a sharp boundary discontinuity is present (e.g., a stepped shaft, or a circumferential V notch), undergoing arbitrary modes of deformation. The 3D equations of equilibrium in terms of displacement components, expressed in cylindrical coordinates, are transformed to a new coordinate system having its origin at the vertex of the corner. An asymptotic analysis in the vicinity of the sharp corner reduces the equations to a set of coupled, ordinary differential equations with variable coefficients. By a suitable transformation of variables the equations are simplified to a set of equations with constant coefficients. These are solved, the boundary conditions along the intersecting corner faces are applied, and the resulting eigenvalue problems are solved for the characteristic equations and corner functions.
Rigid Body Dynamics, Constraints, and Inverses
Authors: Hemami Hooshang , Wyman Bostwick F.
Published: December 2006

Rigid body dynamics are traditionally formulated by Lagrangian or Newton-Euler methods. A particular state space form using Euler angles and angular velocities expressed in the body coordinate system is employed here to address constrained rigid body dynamics. We study gliding and rolling, and we develop inverse systems for estimation of internal and contact forces of constraint. A primitive approximation of biped locomotion serves as a motivation for this work. A class of constraints is formulated in this state space. Rolling and gliding are common in contact sports, in interaction of humans and robots with their environment where one surface makes contact with another surface, and at skeletal joints in living systems. This formulation of constraints is important for control purposes. The estimation of applied and constraint forces and torques at the joints of natural and robotic systems is a challenge. Direct and indirect measurement methods involving a combination of kinematic data and computation are discussed. The basic methodology is developed for one single rigid body for simplicity, brevity, and precision. Computer simulations are presented to demonstrate the feasibility and effectiveness of the approaches presented. The methodology can be applied to a multilink model of bipedal systems where natural and/or artificial connectors and actuators are modeled. Estimation of the forces is accomplished by the inverse of the nonlinear plant designed by using a robust high gain feedback system. The inverse is shown to be stable, and bounds on the tracking error are developed. Lyapunov stability methods are used to establish global stability of the inverse system.
A Magnetohydrodynamic Power Panel for Space Reentry Vehicles
Authors: Steeves Craig A. , Wadley Haydn N. G. , Miles Richard B. , Evans Anthony G.
Published: December 2006

During reentry from space, a layer of high temperature air (>3000K) is formed extending tens of centimeters from the surface of the vehicle, well out into the high speed flow regime. Magnetohydrodynamics (MHD) can then be used to generate power by projecting magnetic fields outside the vehicle into the conducting air stream and collecting the resulting current. Here, we analyze a multifunctional MHD panel which generates the requisite magnetic fields, protects the vehicle from high temperatures, and is structurally stiff and strong. The analysis shows that a magnetic system weighing approximately 110kg can generate 0.6MW of power for 1000s.
Buckling and
Sensitivity to Imperfection of Conical Shells Under Dynamic
Step-Loading
Authors: Jabareen Mahmood , Sheinman Izhak
Published: December 2006

A general nonlinear dynamic analysis, based on Donnell’s shell-type theory, is developed for an arbitrary imperfect isotropic conical shell. It is used for studying dynamic stability and imperfection sensitivity under dynamic step loading. The nonlinear dynamic time history and the sensitivity behavior are examined in parametric terms over a wide range of aspect ratios. A general symbolic code (using the MAPLE compiler) was programmed to create the differential operators. By this means the Newmark discretization, Galerkin procedure, Newton-Raphson iteration, and finite difference scheme are applied for automatic development of an efficient FORTRAN code for the parametric study, and for examining the correlation of the sensitivity behavior between two different dynamic stability criteria. An extensive parametric study of the effect of the cone semi-vertex angle on the stability and sensitivity to imperfection under dynamic step loading was carried out. It was found that the dynamic buckling can indeed be derived from the nonlinear static solution.
The Response of Metallic Sandwich Panels to Water Blast
Authors: Liang Yueming , Spuskanyuk Alexander V. , Flores Shane E. , Hayhurst David R. , Hutchinson John W. , McMeeking Robert M. , Evans Anthony G.
Published: December 2006

Metallic sandwich panels subject to underwater blast respond in a manner dependent on the relative time scales for core crushing and water cavitation. This article examines the response at impulses representative of an (especially relevant) domain: wherein the water cavitates before the core crushes. Three core topologies (square honeycomb, I-core, and corrugated) have been used to address fundamental issues affecting panel design. Their ranking is based on three performance metrics: the back-face deflection, the tearing susceptibility of the faces, and the loads transmitted to the supports. The results are interpreted by comparing with analytic solutions based on a three-stage response model. In stage I, the wet face acquires its maximum velocity with some water attached. In stage II, the core crushes and all of the constituents (wet and dry face and core) converge onto a common velocity. In stage III, the panel deflects and deforms, dissipating its kinetic energy by plastic bending, stretching, shearing, and indentation. The results provide insight about three aspects of the response. (i) Two inherently different regimes have been elucidated, designated strong (STC) and soft (SOC), differentiated by a stage II/III time scale parameter. The best overall performance has been found for soft-core designs. (ii) The foregoing analytic models are found to underestimate the kinetic energy and, consequently, exaggerate the performance benefits. The discrepancy has been resolved by a more complete model for the fluid/structure interaction. (iii) The kinetic energy acquired at the end of the second stage accounts fully for the plastic dissipation occurring in the third stage, indicating that the additional momentum acquired after the end of the second stage does not affect panel performance.
A Mathematical Model for Frictional Elastic-Plastic Sphere-on-Flat Contacts at Sliding Incipient
Authors: Chang L. , Zhang H.
Published: December 2006

This paper presents a mathematical model for frictional elastic-plastic sphere-on-flat contacts at sliding incipient. The model is developed based on theoretical work on contact mechanics in conjunction with finite-element results. It incorporates the effects of friction loading on the contact pressure, the mode of deformation, and the area of contact. The shear strength of the contact interface is, in this paper, assumed to be proportional to the contact pressure with a limiting value that is below the bulk shear strength of the sphere. Other plausible interfacial-shear-strength characteristics may also be implemented into the contact model in a similar manner. The model is used to analyze the frictional behavior of a sphere-on-flat contact where the experimental data suggest that the interfacial shear strength is similar in nature to the one implemented in the model. The theoretical results are consistent with the experimental data in all key aspects. This sphere-on-flat contact model may be used as a building block to develop an asperity-based contact model of rough surfaces with friction loading. It may also serve in the modeling of boundary-lubricated sliding contacts where the interfacial shear strength in each micro-contact is coupled with its flash temperature and related to the lubricant/surface physical-chemical behavior.
Analysis of Wave Propagation in Beams With Transverse and Lateral Cracks Using a Weakly Formulated Spectral Method
Authors: Hu N. , Fukunaga H. , Kameyama M. , Mahapatra D. Roy , Gopalakrishnan S.
Published: December 2006

In this paper, a novel numerical technique based on the global-local hybrid spectral element (HSE) method is proposed to study wave propagation in beams containing damages in the form of transverse and lateral cracks. The ordinary spectral element method is employed to model the exterior or far field regions, while a new type of element (HSE) is constructed to model the interior region containing damages. To develop this efficient new element for the damaged area, first, the flexural and the shear wave numbers are explicitly determined using the first-order shear deformation theory. These wave modes, in one of the two mutually orthogonal directions for two-dimensional transient elastodynamics, are then used to enrich the Lagrangian interpolation functions in context of displacement-based finite element. The equilibrium equation is then derived through the weak form in the frequency domain. Frequency-dependent stiffness and mass matrices can be accurately formed in this manner with a coarse discretization. The proposed method takes the advantage of using (i) a strong form for one-dimensional wave propagation and also (ii) a weak form by which a complex geometry can be discretized. Numerical verification is carried out to illustrate the effectiveness of the method. Finally, this method is employed to investigate the behaviors of wave propagation in beams containing various types of damages, such as multiple transverse cracks and lateral cracks.
Plane Analysis of Finite Multilayered Media With Multiple Aligned Cracks—Part II: Numerical Results
Authors: Chen Linfeng , Pindera Marek-Jerzy
Published: December 2006

In Part I of this paper, elasticity solutions were developed for finite multilayered domains, weakened by aligned cracks, that are in a state of generalized plane deformation under two types of end constraints. In Part II we address computational aspects of the developed solution methodology that must be implemented numerically, and present new fundamental results that are relevant to modern technologically important applications involving defect criticality of multilayers. The computational aspects include discussion of the various parameters that influence the accuracy with which numerical results are generated and subsequent verification by a comparison with previously reported results in the limit, as the in-plane dimensions become very large and layer anisotropy vanishes. The present solution quantifies the thus far undocumented effects of finite dimensions, crack location, and material anisotropy due to a unidirectional fiber-reinforced layer’s orientation on Mode I, II, and III stress intensity factors in composite multilayers with single and multiple interacting cracks under different loading and boundary conditions. These effects may have significant impact on defect criticality of advanced multilayered structures when cracks are in close proximity to vertical and horizontal boundaries.
Predicting the Liquid Formation for the Solder Joints in Flip Chip Technology
Authors: Chen Wen-Hwa , Lin Shu-Ru , Chiang Kuo-Ning
Published: December 2006

An accurate and efficient analytical geometric method is presented for predicting the geometric parameters of the controlled collapse chip connection type solder joint using direct chip attach technology after a reflow process. By this method, the meridian of the solder joint is first discretized as a series of sufficiently fine fragmental arcs. After calculating the internal pressure inside the molten eutectic solder from the forces balance, the meridional and circumferential radii of curvature of each arc are then obtained from the Laplace-Young equation. As a result, the coordinates of each node of the arc and the solder joint geometry can be determined in turn. The factors that affect the final shape of the molten eutectic solder joints, including the solder volumes, external loading, pad size, surface tension of molten eutectic solder, and interfacial surface tension between the molten eutectic solder and the solid high-lead bump are considered herein. The results computed by the analytical geometric method are also compared with those obtained using the Surface Evolver program, the extended Heinrich’s model, and the experimental results. The results of the various approaches are mutually consistent.
A Numerical Analysis of the Thermal Performance of Single Sided and Back-to-Back Tape Ball Grid Array Packages
Authors: Tonapi Sandeep S. , Sathe Sanjeev B. , Sammakia Bahgat G. , Srihari K.
Published: December 2006

This paper presents the results of a comprehensive numerical study of the thermal performance of Tape Ball Grid Array package mounted on one side of a printed circuit board as well as packages mounted in back-to-back and offset configurations. A cover plate is attached to the back side of the chips to enhance heat transfer from the module. The assembled organic carrier is placed in a vertical channel. A conjugate heat transfer model is used which accounts for conduction in the packages and the card and convection in the surrounding air. The effect of location of the modules on a card with zero, one and two power planes is evaluated for thermal performance. Heat dissipation is studied for forced convection (2, 1, and 0.5m∕s). Comparison is made for single sided and back-to-back cases.
Boron Nitride Particle Filled Paraffin Wax as a Phase-Change Thermal Interface Material
Authors: Liu Zongrong , Chung D. D. L.
Published: December 2006

Wax (predominantly tricosane paraffin wax, with a melting temperature of 48°C) filled with hexagonal boron nitride (BN) particles (5-11μm) was found to be an effective phase-change thermal interface material. The thermal contact conductance, as measured with the interface material between copper surfaces, decreased with increasing temperature from 22to48°C, but increased with increasing temperature from 48to55°C. The melting of the wax enhanced the conductance, due to increased conformability to the mating surfaces. For a given BN volume fraction and a given temperature, the thermal contact conductance increased with increasing contact pressure. However, a pressure above 0.30MPa resulted in no significant increase in the conductance. The conductance increased with BN content up to 6.2vol.%, but decreased upon further increase to 8.6vol.%. The highest conductance above the melting temperature was 18×104W∕m2.°C, as attained for a BN content of 4.0vol.% at 55°C and 0.30MPa. Below the melting temperature, the highest conductance was 19×104W∕m2.°C, as attained for a BN content of 6.2vol.% at 22°C and 0.30MPa.
The Role of Fin
Geometry in Heat Sink Performance
Authors: Khan W.
A. , Culham J.
R. , Yovanovich M.
M.
Published: December 2006

The following study will examine the effect on overall thermal/fluid performance associated with different fin geometries, including, rectangular plate fins as well as square, circular, and elliptical pin fins. The use of entropy generation minimization, EGM, allows the combined effect of thermal resistance and pressure drop to be assessed through the simultaneous interaction with the heat sink. A general dimensionless expression for the entropy generation rate is obtained by considering a control volume around the pin fin including base plate and applying the conservations equations for mass and energy with the entropy balance. The formulation for the dimensionless entropy generation rate is developed in terms of dimensionless variables, including the aspect ratio, Reynolds number, Nusselt number, and the drag coefficient. Selected fin geometries are examined for the heat transfer, fluid friction, and the minimum entropy generation rate corresponding to different parameters including axis ratio, aspect ratio, and Reynolds number. The results clearly indicate that the preferred fin profile is very dependent on these parameters.
The Exergy Cost of Information Processing: A Comparison of Computer-Based Technologies and Biological Systems
Authors: Carey V. P. , Shah A. J.
Published: December 2006

Processing information (analysis, storing, retrieving, sharing) is the primary function of modern computer-based information systems. Systems of this type generally require an input flow of exergy (available energy) to function. Information processing systems now are evolving in two directions. One direction is toward bigger and more sophisticated systems. The other is toward systems that are more compact and portable. In both cases, the energy efficiency is becoming an increasingly important design issue. This paper summarizes an exploration of the exergy cost of processing information at the component and system levels in state-of-the-art information processing systems. The energy efficiency characteristics of computer-based information technologies are also compared to estimates of the energy efficiency of biological information processing in brains of mammals. Energy efficiencies of processors and systems are quantified in terms of the ratio of processing capacity to the exergy input rate. Available data suggest that for recent generations of processors, the ratio of processing capacity to exergy input rate has been increasing proportional to the square root of processor speed. Despite this increase, the energy efficiency of computer-based systems is currently substantially below the estimated efficiency of biological systems. Unless processor energy efficiencies are greatly increased, the development of information processing systems that match human brain performance will be hindered by the need for large power supplies and high-capacity heat rejection systems.
Optimization of Synthetic Jet Fluidic Structures in Printed Wiring Boards
Authors: Wang Yong , Yuan Guang , Yoon Yong-Kyu , Allen Mark G. , Bidstrup Sue Ann
Published: December 2006

The active cooling substrate in this study is a microelectromechanical system device that implements the synthetic jet concept into a printed wiring board (PWB) to enhance thermal management. Synthetic jets are oscillatory jets synthesized from the surrounding fluid using electromagnetic actuators. The jet fluid mechanics and heat transfer applications have been investigated by a variety of on-board (PWB) fluidic structures. A testbed comprising six different fluidic channels has been fabricated and characterized with a standard silicon based platinum heater. Based on the fluid mechanics measurements and cooling performance tests, an empirical correlation of synthetic jet Nusselt number with its Reynolds number, fluidic channel dimensions, and jet locations has been derived. Through a magnitude analysis, jet actuator diaphragm, fluidic channel dimension, and cooling location optimizations have been investigated.
An Exergy-Based Figure-of-Merit for Electronic Packages
Authors: Shah Amip J. , Carey Van P. , Bash Cullen E. , Patel Chandrakant D.
Published: December 2006

Chip power consumption and heat dissipation have become important design issues because of increased energy costs and thermal management limitations. As a global compute utility evolves, seamless connectivity from the chip to the data center will become increasingly important. The optimization of such an infrastructure will require performance metrics that can adequately capture the thermodynamic and compute behavior at multiple physical length scales. In this paper, an exergy-based figure-of-merit (FoM), defined as the ratio of computing performance (in MIPS) to the thermodynamic performance (in exergy loss), is proposed for the evaluation of computational performance. The paper presents the framework to apply this metric at the chip level. Formulations for the exergy loss in simple air-cooled heat sink packages are developed, and application of the proposed approach is illustrated through two examples. The first comparatively assesses the loss in performance resulting from different cooling solutions, while the second examines the impact of non-uniformity in junction power in terms of the FoM. Modeling results on a 16mm×24mm chip indicate that uniform power and temperature profiles lead to minimal package irreversibility (and therefore the best thermodynamic performance). As the nonuniformity of power is increased, the performance rapidly degrades, particularly at higher power levels. Additionally, the competing needs of minimization of junction temperature and minimization of cooling power were highlighted using the exergy-based approach. It was shown that for a given power dissipation and a specific cooling architecture (such as an air-cooled heat sink solution), an optimal thermal resistance value exists beyond which the costs of increased cooling may outweigh any potential benefits in performance. Thus, the proposed FoM provides insight into thermofluidic inefficiencies that would be difficult to gain from a traditional first-law analysis. At a minimum, the framework presented in this paper enables quantitative evaluation of package performance for different nonuniform power inputs and different choices of cooling parameters. At best, since the FoM is scalable, the proposed metric has the potential to enable a chip-to-data-center strategy for optimal resource allocation.
Thermal Behavior of Nominally Flat Silicon-Based Heat Spreaders
Authors: Lin Ta-Wei , Wu Ming-Chang , Peng Cheng-Hsien , Chen Po-Li , Hung Ying-Huei
Published: December 2006

Thermal characteristics for a horizontal heated chip mounted with three types of nominally flat silicon-based heat spreaders have been systematically investigated. They include the natural convective and radiative heat transfer from the top surface of the heat spreaders to the external ambient, external thermal resistance, and maximum overall thermal resistance. In the aspect of natural convection, an axisymmetric bowl-shaped profile of local Nusselt number is achieved, and the highest convective heat transfer performance occurs at the location near the rim of the heat spreader. The effect of surface roughness on both local and average natural convective heat transfer behaviors from nominally flat silicon-based spreader surfaces to the external ambient is not significant. Two new generalized correlations of local and average Nusselt numbers for various heat spreader surfaces are presented. The contributions of convection and radiation on the total heat dissipated from the top surface of the heat spreader to the ambient are about 72% and 28%, respectively. The effect of surface roughness on external thermal resistance for nominally flat silicon-based surfaces is not significant. The influence of the conductive thermal resistance within the silicon-based heat spreader on the maximum thermal resistance is not significant. The maximum thermal resistance is mainly dominated by external thermal resistance for flat nominally silicon-based heat spreaders.
Cooling of Power Electronics by Embedded Solids
Authors: Dirker J. , van Wyk J. D. , Meyer J. P.
Published: December 2006

Thermal issues have become a major consideration in the design and development of electronic components. In power electronics, thermal limitations have been identified as a barrier to future developments such as three-dimensional integration. This paper proposes internal embedded cooling of high-density integrated power electronic modules that consist of materials with low thermal conductivity and evaluates it in terms of dimensional, material property, and thermal interfacial resistance ranges. Enhanced component conductivity was identified as a possible economically viable internal cooling option. Thermal performance calculations were performed numerically for conductive cooling of internal component/module regions via parallel-running embedded solids. Thermal advantage per volume usage by the embedded solids was furthermore optimized in terms of a wide range of geometric, material, and thermal parameters. In the dimensional and material property range commonly found in passive power electronic modules, parallel-running cooling layers were identified as an efficient cooling configuration. Numerically based thermal performance models were subsequently developed for parallel-running cooling inserts. A multifunctional experimental setup was constructed to study the cooling of ferrite (operated as a magnetic core) by means of embedded aluminium nitride layers and to verify the thermal model. Results corresponded well with theoretically anticipated performance increases. However, interfacial thermal resistance constituted a major limitation to the cooling performance and future power density increases. With the thermal model developed, functional optimization in terms of magnetic flux density for parallel-running cooling layer configurations was performed for a wide range of material and geometric conditions.
Effects of Dissolved Air on Subcooled Flow Boiling of a Dielectric Coolant in a Microchannel Heat Sink
Authors: Chen Tailian , Garimella Suresh V.
Published: December 2006

The effects of dissolved air in the dielectric liquid FC-77 on flow boiling in a microchannel heat sink containing ten parallel channels, each 500μm wide and 2.5mm deep, were experimentally investigated. Experiments were conducted before and after degassing, at three flow rates in the range of 30-50ml∕min. The dissolved air resulted in a significant reduction in wall temperature at which bubbles were first observed in the microchannels. Analysis of the results suggests that the bubbles observed initially in the undegassed liquid were most likely air bubbles. Once the boiling process is initiated, the wall temperature continues to increase for the undegassed liquid, whereas it remains relatively unchanged in the case of the degassed liquid. Prior to the inception of boiling in the degassed liquid, the heat transfer coefficients with the undegassed liquid were 300-500% higher than for degassed liquid, depending on the flow rate. The heat transfer coefficients for both cases reach similar values at high heat fluxes (>120kW∕m2) once the boiling process with the degassed liquid was well established. The boiling process induced a significant increase in pressure drop relative to single-phase flow; the pressure drop for undegassed liquid was measured to be higher than for degassed liquid once the boiling process became well established in both cases. Flow instabilities were induced by the boiling process, and the magnitude of the instability was quantified using the standard deviation of the measured pressure drop at a given heat flux. It was found that the magnitude of flow instability increased with increasing heat flux in both the undegassed and degassed liquids, with greater flow instability noted in the undegassed liquid.
Experimental Investigation of Heat Transfer in Impingement Air Cooled Plate Fin Heat Sinks
Authors: Duan Zhipeng , Muzychka Y. S.
Published: December 2006

Impingement cooling of plate fin heat sinks is examined. Experimental measurements of thermal performance were performed with four heat sinks of various impingement inlet widths, fin spacings, fin heights, and airflow velocities. The percent uncertainty in the measured thermal resistance was a maximum of 2.6% in the validation tests. Using a simple thermal resistance model based on developing laminar flow in rectangular channels, the actual mean heat transfer coefficients are obtained in order to develop a simple heat transfer model for the impingement plate fin heat sink system. The experimental results are combined into a dimensionless correlation for channel average Nusselt number Nu∼f(L*,Pr). We use a dimensionless thermal developing flow length, L*=(L∕2)∕(DhRePr), as the independent parameter. Results show that Nu∼1∕L*, similar to developing flow in parallel channels. The heat transfer model covers the practical operating range of most heat sinks, 0.01<L*<0.18. The accuracy of the heat transfer model was found to be within 11% of the experimental data taken on four heat sinks and other experimental data from the published literature at channel Reynolds numbers less than 1200. The proposed heat transfer model may be used to predict the thermal performance of impingement air cooled plate fin heat sinks for design purposes.
Test Methods for Silicon Die Strength
Authors: Tsai M. Y. , Chen C. H. , Lin C. S.
Published: December 2006

Recently, the 3D or stacked-die packages become increasingly popular for packaging ICs into a system or subsystem to satisfy the needs of low cost, small form factor, and high performance. For the applications of these packages, IC silicon wafers have to be ground to be relatively thin through the wafer-thinning processes (such as grinding, polishing, and plasma etching). The strength of dies has to be determined for the design requirement and reliability assurance of the packages. From the published data, there still exist some issues, including a large scatter existed in die strength data and difficulties in differentiating the causes of the low strength between from the wafer grinding and from wafer sawing by either the three-point bending or four-point bending test. The purposes of this study are to develop new, reliable, and simple test methods for determination of die strength, in order to improve the data scatter, and to provide a solution for differentiating the factors that affect the variability of die strength for finding out the causes of the weakness of the die strength. In this study, two new test methods, point-loaded circular plate with simple supports test (PLT-I) and point-loaded plate on elastic foundation test (PLT-II), are proposed and then evaluated by testing two groups of silicon dies with different surface conditions. The surface conditions (roughness) of the specimens are determined by atomic force microscopy and correlated to failure strength. The failure forces from both tests have to be modified by using maximum stress obtained from theories or finite element analyses to obtain the failure strength. The test results are compared to each other and further with a widely used four-point bending test. The results suggest that, unlike the four-point bending test suffering the chipping effect, both methods provide very consistent data with a small scatter for each group of specimens and can be used for identifying the effect of surface grinding (roughness) on the die strength. It is also shown that the die strength is highly dependent on the surface roughness. Accordingly, these two methods can provide not only a (biaxial) stress field similar to temperature-loaded die in the packages, but also simple, feasible, reliable, and chipping-free tests for silicon dies of dummy or real IC chips, without strict geometrical limitation, such as beam-type geometry for the three-point or four-point bending test.
A Graphite Foams Based Vapor Chamber for Chip Heat Spreading
Authors: Lu Minhua , Mok Larry , Bezama R. J.
Published: December 2006

A vapor chamber using high thermal conductivity and permeability graphite foam as a wick has been designed, built, and tested. With ethanol as the working fluid, the vapor chamber has been demonstrated at a heat flux of 80W∕cm2. The effects of the capillary limit, the boiling limit, and the thermal resistance in restricting the overall performance of a vapor chamber have been analyzed. Because of the high thermal conductivity of the graphite foams, the modeling results show that the performance of a vapor chamber using a graphite foam is about twice that of one using a copper wick structure. Furthermore, if water is used as the working fluid instead of ethanol, the performance of the vapor chamber will be increased further. Graphite foam vapor chambers with water as the working fluid can be made by treating the graphite foam with an oxygen plasma to improve the wetting of the graphite by the water.
Constructal Peripheral Cooling of a Rectangular Heat-Generating Area
Authors: da Silva Alexandre K. , Gosselin Louis
Published: December 2006

The present paper determines numerically the optimal geometric parameters for the maximal peripheral cooling of a two-dimensional rectangular solid body with internal heat generation. The objective is to maximize the thermal global conductance (i.e., minimize the hot spot temperature on the solid body) by using the minimal cooling space. The flow is conducted around the heated solid body by a sequence of channels of independent width Di, where 1⩽i⩽4. Each configuration is free to morph itself in two directions: (a) the number of cooling channels, and (b) the aspect ratio of the heated body λ. The numerical results show that a number of cooling channels greater than one (i.e., n>1) is profitable in terms of thermal performance when the heated body resembles a square (i.e., λ∼1). However when λ is free to vary, the thermal performance does not necessarily increase with the number of cooling channels. The paper also discusses the importance to allow each configuration to morph itself in multiple directions by comparing the thermal performance of similar configurations with different number of degrees of freedom. Scale analysis is used to verify the results obtained numerically for all the degrees of freedom considered. The numerical results agree with the scaling trends.
Experimental and Numerical Investigation of the Reliability of Double-Sided Area Array Assemblies
Authors: Chaparala S. , Pitarresi J. M. , Parupalli S. , Mandepudi S. , Meilunas M.
Published: December 2006

One of the primary advantages of surface mount technology (SMT) over through-hole technology is that SMT allows the assembly of components on both sides of the printed circuit board (PCB). Currently, area array components such as ball grid array (BGA) and chip-scale package (CSP) assemblies are being used in double-sided configurations for network and memory device applications as they reduce the routing space and improve electrical performance (Shiah, A. C., and Zhou, X., 2002, “A Low Cost Reliability Assessment for Double-Sided Mirror-Imaged Flip Chip BGA Assemblies,” Proceedings of the Seventh Annual Pan Pacific Microelectronics Symposium, Maui, Hawaii, pp. 7–15, and Xie, D., and Yi, S., 2001, “Reliability Design and Experimental work for Mirror Image CSP Assembly”, Proceedings of the International Symposium on Microelectronics, Baltimore, October, pp. 417–422). These assemblies typically use a “mirror image” configuration wherein the components are placed on either side of the PCB directly over each other; however, other configurations are possible. Double-sided assemblies pose challenges for thermal dissipation, inspection, rework, and thermal cycling reliability. The scope of this paper is the study of the reliability of double-sided assemblies both experimentally and through numerical simulation. The assemblies studied include single-sided, mirror-imaged, 50% offset CSP assemblies, CSPs with capacitors on the backside, single-sided, mirror-imaged plastic ball grid arrays (PBGAs), quad flat pack (QFP)/BGA mixed assemblies. The effect of assembly stiffness on thermal cycling reliability was investigated. To assess the assembly flexural stiffness and its effect on the thermal cycling reliability, a three-point bending measurement was performed. Accelerated thermal cycling cycles to failure were documented for all assemblies and the data were used to calculate the characteristic life. In general, a 2X to 3X decrease in reliability was observed for mirror-image assemblies when compared to single-sided assemblies for both BGAs and CSPs on 62mil test boards. The reliability of mirror-image assemblies when one component was an area array device and the other was a QFP was comparable to the reliability of the single-sided area array assemblies alone, that is, the QFP had almost no influence on the double-sided reliability when used with an area array component. Moiré interferometry was used to study the displacement distribution in the solder joints at specific locations in the packages. Data from the reliability and moiré measurements were correlated with predictions generated from three-dimensional finite element models of the assemblies. The models incorporated nonlinear and time-temperature dependent solder material properties and they were used to estimate the fatigue life of the solder joints and to obtain an estimate of the overall package reliability using Darveaux’s crack propagation method.
Optimal Support Locations for a Printed Circuit Board Loaded With Heavy Components
Authors: Chen Kun-Nan
Published: December 2006

In the design of printed circuit boards (PCBs), it is preferable to increase their fundamental frequency so as to reduce the effects of the dynamic loading on them. The dynamic characteristics of a PCB carrying various electronic components and modules are most significantly affected by the geometrical and material properties of the bare board and by the boundary conditions supporting the loaded PCB. In this research, a PCB carrying a heavy CPU cooling fan and supported by six fastening screws is investigated by the modal testing and analyzed by the finite element method. After the finite element model of the PCB is verified by the experimental results, the locations of the six supporting screws are optimized to achieve a maximum fundamental frequency for the loaded PCB. The position of each fastening screw can be determined by two design variables, i.e., x and y coordinates. Two cases are studied: the symmetric case (six design parameters) with the symmetric constraint on the support locations imposed, and the asymmetric case (12 design parameters) without the constraint imposed. Finally, verification experiments are performed on the two PCBs supported by screws located at the optimal positions. Although relatively large differences between the calculated, optimized fundamental frequencies and the experimental values are observed, the experiments confirm a very significant improvement in frequency for both cases.
Graphite Foam Thermal Management of a High Packing Density Array of Power Amplifiers
Authors: Williams Z. A. , Roux J. A.
Published: December 2006

Much focus has been placed on the thermal management of electronics in recent years. An overall reduction in size of electronic components as well as advances in chip technology, leading to ever higher power dissipation, have increased the necessity for innovative cooling designs. While computational fluid dynamics (CFD) software packages have been instrumental in the design of cooling systems, it remains important to validate these CFD predictions through experimentation. The present work focuses on the experimental evaluation of several variations of an air cooled base plate channel design for an array of generic power amplifier modules. In the current study two materials, graphite foam and a microfibrous material, are investigated as mini-heat exchangers to be implemented in the cooling channel of the base plate. Computational simulations have been conducted on some of the proposed designs in order to evaluate certain parameters. Experiments were conducted measuring chip temperatures and the pressure drop across the cooling channel. Effective heat transfer coefficients were also reverse engineered.
Singulation of Electronic Packages With Abrasive Waterjets
Authors: Hashish Mohamed
Published: December 2006

Abrasive waterjets (AWJ) were used for the first time to commercially singulate electronic chips such as those used for flash memory cards found in digital cameras, cell phones, and universal serial bus (USB) storage devices. Cutting these components requires high cutting speed, high edge quality, accuracy, and precision. For example, a minimal accuracy needed is ∼0.1mm and a minimum Cpk of 1.33. A relatively small AWJ (∼0.38mm) was successfully used to accurately cut chips at speeds of 20-60mm∕s. It was determined that the use of machine vision is critical to meeting the accuracy requirements. The cutting process consisted of piercing starting holes and then cutting shaped pattern cuts to contour the chip components. Drilling holes was performed without delamination, and the cutting speed was optimized to meet the intricate chip geometry. Because of the relatively high volume of components to be cut, requiring around-the-clock duty, process and machine reliability are of critical importance. This paper discusses the results and observation of the cutting process as well as the performance of the system.
Experimental Characterization of Monotonic and Fatigue Delamination of Novel Underfill Materials
Authors: Mahalingam Saketh , Goray Kunal , Tonapi Sandeep , Sitaraman Suresh K.
Published: December 2006

No-flow underfill materials reduce assembly processing steps and can potentially be used in fine-pitch flip chip on organic board assemblies. Such no-flow underfills, when filled with nano-scale fillers, can significantly enhance the solder bump reliability, if the underfills do not prematurely delaminate or crack. Therefore, it is necessary to understand the risk of underfill delamination during assembly and during further thermal excursions. In this paper, the interface between silicon nitride (SiN) passivation and a nano-filled underfill (NFU) material is characterized under monotonic as well as thermo-mechanical fatigue loading, and fracture parameters have been obtained from such experimental characterization. The passivation-underfill interfacial delamination propagation under monotonic loading has been studied through a fixtureless residual stress induced decohesion (RSID) test. The propagation of interfacial delamination under thermo-mechanical fatigue loading has been studied using sandwiched assemblies and a model for delamination propagation has been developed. The characterization results obtained from this work can be used to assess the delamination propagation in flip-chip assemblies. Though the methods presented in this paper have been applied to nano-filled, no-flow underfill materials, their application is not limited to such materials or material interfaces.
Studies on the Effect of Cyclic Loading on Grain Boundary Rupture Time
Authors: Oh J. , Brust F. W. , Katsube N.
Published: December 2006

This paper studies intergranular creep failure of high-temperature service material under a stress-controlled unbalanced cyclic loading condition. The experimentally verified Murakami–Ohno strain-hardening creep law and Norton’s creep law are incorporated into the Tvegaard’s axis-symmetric model for the constrained grain boundary rupture analysis. Based on the physically realistic Murakami–Ohno creep law, it is shown that the cavity growth becomes unconstrained upon the stress reversal from compression to tension. This leads to the prediction that the material life under a cyclic loading condition is shorter than that under a constant loading. Based on the classical Norton’s law, the predicted material life under a cyclic loading condition remains the same as that under a constant loading. The obtained numerical results qualitatively match recent experimental results by Arai, where the life under a cyclic loading can be much shorter than that under a constant loading. There are many cases where engineers use a simple Norton’s creep law because of its simplicity. The present work suggests that more physically realistic creep laws should be used when cyclic loading must be considered.
Mixed Finite Element Analysis of Elastomeric Butt-Joints
Authors: Kakavas P. A. , Giannopoulos G. I. , Anifantis N. K.
Published: December 2006

This paper presents a mixed finite element formulation approximating large deformations observed in the analysis of elastomeric butt-joints. The rubber has been considered as nearly incompressible continuum obeying the Mooney/Rivlin (M/R) strain energy density function. The parameters of the model were determined by fitting the available from the literature uniaxial tension experimental data with the constitutive equation derived from the M/R model. The optimum value of the Poisson ratio is adjusted by comparing the experimentally observed diametral contraction of the model with that numerically obtained using the finite element method. The solution of the problem has been obtained utilizing the mixed finite element procedure on the basis of displacement/pressure mixed interpolation and enhanced strain energy mixed formulation. For comparison purposes, an axisymmetric with two-parameter M/R model and a three-dimensional (3D) with nine-parameters M/R model of the butt-joint are formulated and numerical results are illustrated concerning axisymmetric or general loading. For small strains the stress and/or strain distribution in the 2D axisymmetric butt-joint problem was compared with derived analytical solutions. Stress distributions along critical paths are evaluated and discussed.
Elasto-viscoplastic constitutive equations for the description of glassy polymers behavior at constant strain rate
Authors: Zaïri Fahmi , Naït-Abdelaziz Moussa , Woznica Krzysztof , Gloaguen Jean-Michel
Published: December 2006

In this study, a modelization of the viscoplastic behavior of amorphous polymers is proposed, from an approach originally developed for metal behavior at high temperature, in which state variable constitutive equations have been modified. A procedure for the identification of model parameters is developed through the use of experimental data from both uniaxial compressive tests extracted from the literature and uniaxial tensile tests performed in this study across a variety of strain rates. The numerical algorithm shows that the predictions of this model well describe qualitatively and quantitatively the intrinsic softening immediately after yielding and the subsequent progressive orientational hardening corresponding to the response of two polymers, amorphous polyethylene terephthalate and rubber toughened polymethyl methacrylate.