bluuebunny/crossref_metadata_2025 huggingface dataset:
Heat Transfer in a Porous Cathode of Fuel Cells
Authors: Hwang J. J.
Published: April 2009

This paper has provided an innovative aspect in the heat transfer of fuel-cell related studies. A heat/mass coupled modeling approach is presented to predict the transport phenomena inside the porous electrode of a fuel cell. The energy equations based on the local thermal non-equilibrium (LTNE) is derived to resolve the temperature difference between the solid and fluid phases inside the porous electrode. The surface heat transfer is coupled with the species transports via a macroscopic electrochemical model on the reaction boundary. A general criterion for the local thermal non-equilibrium in porous electrodes is first proposed in terms of non-dimensional parameters of engineering importance. Then, the significance of local thermal non-equilibrium in a typical porous electrode is assessed. Furthermore, detailed distributions of the local temperature, local Nusselt number, species concentration, and electric current density inside the porous electrode of fuel cells are presented. Finally, the effect of LTNE parameters on the thermal-fluid behaviors in the porous electrode is investigated.
Thermal Design and Performance of Two-Phase Meso-Scale Heat Exchangers
Authors: Hannemann Robert , Marsala Joseph , Pitasi Martin
Published: April 2009

Dramatically increased power dissipation in electronic and electro-optic devices has prompted the development of advanced thermal management approaches to replace conventional air cooling using extended surfaces. One such approach is Pumped Liquid Multiphase Cooling (PLMC), in which a refrigerant is evaporated in a cold plate in contact with the devices to be cooled. Heat is then rejected in an air or water-cooled condenser and the working fluid is returned to the cold plate. Reliable, highly efficient, small-scale components are required for the commercial application of this technology. This paper presents experimental results for two-phase meso-scale heat exchangers (cold plates) for use in electronics cooling. The configurations studied include single and multi-pass designs using R134a as the working fluid. With relatively low flow rates, low effective thermal resistances were achieved at power levels as high as 376 W. The results confirm the efficacy of PLMC technology for cooling the most powerful integrated circuits planned for the next decade.
Novel Process Techniques to Reduce Voids in Solder Thermal Interface Materials Used for Flip-Chip Package Applications
Authors: Montano M. , Garcia J. , Shi W. , Reiter M. T. , Vadakkan U. , Phillippe K. L. , Clark B. , Valles M. , Deppisch C. , Ferrara-Brown J. D. , Jadhav S. G. , Bernal E. , Kuan M. K.
Published: April 2009

In the present study, the thermal performance of flip chip electronics packages was evaluated by characterizing the amount of voiding present in the Solder Thermal Interface Material (STIM) which is placed between the die and Integrated Heat Spreader (IHS). The study found that the thermal resistance, Rjc (resistance between the Si die and IHS), is dependent upon the amount of voiding present as well as the location of the voiding in the STIM. The study also described the techniques to reduce the STIM voids in flip chip packages and identified the key process parameters to improve the thermal performance. The process parameters varied in this study consisted of STIM thickness, dwell time and temperature, flux weight, and many others. A detailed DOE and statistical analysis were carried out to determine the impact of the parameters mentioned above toward reducing the quantity of voids in the STIM. The analysis showed that for the packages under consideration, the primary process parameters that affect the STIM voiding are cure time, flux weight and TIM thickness.   This paper was also originally published as part of the Proceedings of the ASME 2005 Pacific Rim Technical Conference and Exhibition on Integration and Packaging of MEMS, NEMS, and Electronic Systems.
Reduction of Gaseous Emission From Turbine Combustor
Authors: Amano R. S. , Xie J. , Singh Shyam , Peck R. E.
Published: April 2009

A study of spray combustion with porous inserts was performed using an on-axis fuel used in a concentric Jet-A. Combustion performance was evaluated by measuring exhaust emissions and gaseous temperatures for different operating conditions with and without ceramic foam inserts. The results indicated that the enhanced heat transfer in the flame zone could reduce nitrogen oxides and unburned hydrocarbon emissions. Placing a second porous layer downstream could yield further reductions in both emissions. The results for different firing rates and equivalence ratios revealed the residence time in the porous layer is an important factor in controlling the combustor performance.
Design and Calibration of a Novel High Temperature Heat Flux Gage
Authors: Raphael-Mabel Sujay , Huxtable Scott , Gifford Andrew , Diller Thomas E.
Published: April 2009

A new type of heat flux sensor (HTHFS) has been designed and constructed for applications at high temperature and high heat flux. It is constructed by connecting solid metal plates to form brass/steel thermocouple junctions in a series circuit. The thermal resistance layer of the HTHFS consists of the thermocouple materials themselves, thus improving temperature limits and lowering the temperature disruption of the sensor. The sensor can even withstand considerable erosion of the surface with little effect on the operation. A new type of convection calibration apparatus was designed and built specifically to supply a large convection heat flux. The heat flux was supplied simultaneously to both a test and standard gage by using two heated jets of air that impinged perpendicularly on the surface of each gage. The sensitivity for the HTHFS was measured to have an average value of 20 μV/(W/cm2). The uncertainty in this result was determined to be ±10% over the entire range tested. The sensitivity agrees with the theoretically calculated sensitivity for the materials and geometry used. Recommendations for future improvements in the construction and use of the sensors are discussed.
The Wake Behavior Behind a Heated Cylinder in Forced and Mixed Convection Regimes
Authors: Hu H. , Koochesfahani M. M.
Published: April 2009

The effect of buoyancy on the wake instability of a heated circular cylinder in a contra flow arrangement is investigated experimentally in the present study. A novel optical diagnostic technique, Molecular Tagging Velocimetry and Thermometry (MTV&T), was used for qualitative flow visualization and quantitative simultaneous measurements of velocity and temperature fields in the wake of the heated cylinder. The experiment was conducted in a water channel with the temperature and Reynolds number of the approaching flow being held constant at T∞ = 24°C and Re = ρ∞U∞D/μ∞ = 130. The temperature of the heated cylinder varied between 24°C (unheated cylinder) and 85°C, corresponding to a Richardson number (Ri) varying between zero (unheated) and unity. The heat transfer process around the heated cylinder changes from the forced convection regime to the mixed convection regime. With increasing Richardson number, significant modifications of the wake instability were revealed from both qualitative flow visualization images and quantitative simultaneous velocity and temperature fields. The effect of buoyancy on the wake instability of the heated cylinder was discussed in the terms of vortex shedding pattern, vortex shedding frequency, turbulent heat flux distribution, the wake closure length and averaged Nusselt number of the heated cylinder.
Optimum Number Density of Blocks Released in Fluidic Self-Assembly of Microelectronics
Authors: Brasoava Adriana , Danescu Radu , Wells David , Reddy Nayan
Published: April 2009

An experimental study was conducted to demonstrate the existence and estimate the value of an optimum number density of silicon plates (blocks) in a fluidic self-assembly process used for parallel assembling of microelectronics. Blocks ranging in size from 350 to 1050 microns were released under water over an inclined substrate and allowed to move gravitationally to fill indentations (receptors) of matching shape. The performance of the process was evaluated in terms of the time necessary to achieve filling. Results indicate that there is an optimum block density for which the filling time is minimum. If the density is too high, blocks in contact with each other form agglomerations that reduce the mobility of individual blocks and prevent them from aligning properly with the receptors to fill them. The optimum area fraction covered by blocks was relatively similar for the three sizes of blocks considered. The findings amend a common belief that the FSA performance increases indefinitely with the density of blocks released. Economic advantages of a limited block density at industrial scale are discussed.
Characterization of Interdigital Capacitive Strain Gauges by Direct Write Technology
Authors: Li Jinggao , Longtin Jon P. , Tankiewicz Szymon , Gouldstone Andrew , Sampath Sanjay
Published: April 2009

Interdigitated capacitive strain gauges have several distinct advantages over resistive-based strain gauges, particularly for applications in harsh environments, such as high-temperature environments. In this work capacitive strain gauges have been fabricated using thermal spray technology. Gauges are fabricated using both a direct-write approach where the gauge is fabricated using a computer-controlled deposition system and by ultrafast laser micromachining in which blanket coatings sprayed onto a substrate are subsequently laser micrornachined. Silver coatings were sprayed onto plastic, polymer, composites, fiberglass and alumina to form the strain gauges. An ultrafast laser machining technique was used to fabricate capacitive strain gauges on copper coated printed circuit boards as well as NiCr coatings on alumina substrate. The typical capacitance of strain gauge was in the range of 5∼25 pF. Mechanical tests included gauge factor, linearity and zero shift. Temperature-based measurements include the temperature coefficient of capacitance (TCC) measurements and thermal cycling tests. The devices show promise for use in wireless strain monitoring applications.
Confinement of Freezing Front by Laser Irradiation During Cryosurgery
Authors: Romero-Me´ndez Ricardo , Chu Kevin , Vu Henry , Franco Walfre , Aguilar Guillermo
Published: April 2009

A new methodology to control the freezing front propagation during cryosurgical procedures is studied through the use of numerical techniques. Laser irradiation of a target tissue is explored as a new methodology for localizing heat generation and, thus, confining more accurately the desired cryoinjury region and to protect a thicker superficial layer of tissue. In addition to the irradiation of laser energy, the use of dyes is proposed as a means of localizing heat absorption and increasing the thickness of the protected region. A 2D finite volume numerical code based on the enthalpy method was developed to model the freezing process during cryoprobe cooling of a volume of tissue, while heating was applied to the external boundary protecting the superficial layer of tissue. Laser irradiation was modeled with Beer’s Law, and the energy absorption, which is proportional to the intensity, was taken as a source term in the energy equation. The thermophysical properties of the tissue are modeled as temperature dependent properties of water. Temperature contours resulting from a) constant temperature heating b) and regulated laser irradiation heating of tissue indicate that the latter methodology may be more effective in limiting cryoinjury to a predefined region. Additionally, if dyes are used, the protected area increases in thickness. The most dramatic differences between the two methodologies occur when the cryoprobe is placed near the surface, the effective attenuation coefficient of the material is low, and dyes are injected into the tissue to promote localized absorption of laser irradiated energy.
Computational and Experimental Study of the Flow in Evaporative Crystallizers
Authors: Echeverri Luiz F. , Acharya Sumanta , Rein Peter W.
Published: April 2009

Various evaporative-crystallization systems rely on the natural circulation generated by boiling as the only driving force for the fluid flow. The circulation resulting from the balance between the buoyancy forces of the vapor bubbles and the frictional resistance plays an important role in the convective-boiling heat transfer, and it is desired that this circulation be as high as practically possible to maximize the capacity of the equipment and to lead to high-quality product yield. Although the basic mechanisms that govern the individual processes of boiling, buoyancy, and two-phase interactions have been extensively studied in simpler geometries, their combined behavior in the complex geometry of evaporative-crystallizers and the interaction of numerous physical and chemical variables make it difficult to understand and optimize the key parameters leading to improved product yield. In the present study measurements and computations have been reported both in a lab-scale test rig and in a full-scale crystallizer in order to obtain a better understanding of the physical processes. It is observed that one of the key physical parameters that influence the circulation rate is the drag coefficient, and that, existing correlations have to be corrected for flow contamination and high void fractions to obtain reasonable agreement with measurements.
Verification and Validation in Computational Fluid Dynamics and Heat Transfer Using Experimental Uncertainty Analysis Concepts
Authors: Coleman Hugh W.
Published: April 2009

An approach to verification and validation (V&V) using experimental uncertainty analysis concepts to quantify the result of a validation effort is discussed. This is the approach to V&V being drafted by the American Society of Mechanical Engineers (ASME) Performance Test Code Committee, PTC 61: Verification and Validation in Computational Fluid Dynamics and Heat Transfer. The charter of the committee is “Provides procedures for quantifying the accuracy of modeling and simulation in computational fluid dynamics and heat transfer.” The committee is initially focusing its efforts on drafting a standard for V&V in computational fluid dynamics and heat transfer based on the concepts and methods of experimental uncertainty analysis. This will leverage the decades of effort in the community of experimentalists that resulted in the ASME Standard PTC 19.1 “Test Uncertainty” and the ISO international standard “Guide to the Expression of Uncertainty in Measurement.”
Parametric Sensitivity of Predicted Fuel Fire in a Building
Authors: Brown Alexander L.
Published: April 2009

The ability of current modeling and simulation tools to accurately predict a building fire of practical size and duration is at issue. Modeling is challenged by computational cost, fidelity of assumed physics, and correct knowledge of initial and boundary conditions. A series of simulations has been conducted to compare with experiments for a fuel fire in a facility. The purpose of the study was to understand the importance of simulation parameters. The test geometry is sufficiently large and the fire of long enough duration to present a challenge to model in detail. Several computational parameters have been varied at magnitudes consistent with the uncertainty in the parameter to determine the parametric sensitivities. The predicted heat flux inside the facility was sensitive to varying degrees to the parameters selected for the study, with those related to the fuel source being the most important physical parameters.
Thermal Deformation Modeling of Space Optical Systems With Variable Coefficient of Thermal Expansion
Authors: Batson Jennifer , Hashemi Ab
Published: April 2009

In modeling space optical systems, an important property affecting the wave front error is the coefficient of thermal expansion (CTE) of the materials. The change of deformation that an optical element experiences due to thermal loads is proportional to both the CTE and the change in temperature gradient. This deformation affects the performance of the optical system by introducing error in the wave front. The deformation can be reduced in part by using materials with low CTE. Alternatively, using high conductivity materials to minimize temperature gradients through the mirror can also reduce deformation. Usually, a combination of these approaches is used to optimize the performance and meet the requirements of the system. Even with the utmost attention to thermal control, often the temperature gradients cannot be completely avoided. Low CTE materials have been developed to reduce thermal deformation, including ULE (Ultra-low Expansion), Zerodur, and Silicon Carbide. However, the manufacturing process can result in non-uniformities throughout the optics. For optical systems requiring highly precise performance, modeling these non-uniformities becomes important. The non-uniformity in the CTE of a material in effect compounds the deformation in the same manner as introducing additional temperature gradient through the optics. This paper describes the methodology for integrated thermal/mechanical modeling to predict the deformation response of an optical element with assumed CTE variations and thermal disturbances. A mirror with an assumed CTE variation was modeled in a changing thermal environment and using IDEAS/TMG analysis tools, thermal deformations were predicted. Results show excellent agreement with engineering predictions. Clearly knowing the CTE variation of the material is a critical step for modeling. However, measuring and specifying the material CTE is out of the scope of this paper.
Precision Thermal Modeling and Measurement
Authors: Pytel Diane , Hashemi Ab
Published: April 2009

This paper describes a study of milli-Kelvin (mK) level temperature measurement and modeling. It describes the instrumentation for mK temperature measurement and an analysis and quantification of the measurement uncertainty. It also explains the development of an analytical model and simulation of a significant portion of the laboratory equipment configuration. Finally, it provides a comparison and correlation of the laboratory and predicted results for change in temperature. Results show very good agreement and demonstrate the capability for precision temperature measurement technique described in this paper.
Experimental Study of Liquid Flow and Heat Transfer in Microchannels With In-Line Array of Cubes
Authors: Mo H. L. , Prattipati R. , Lin C. X.
Published: April 2009

De-ionized (DI) water flow in microchannels with periodically mounted in-line array of cubes is investigated experimentally. The array of 505 micron cubes are fabricated within silicon channels to generate so-called self-sustained oscillating flow for the purpose of improving the heating wall temperature distribution, enhancing the cooling capacity and reducing the pumping power. An electrical heater is attached on the top of the test channel to simulate the real cooling situation. With periodically mounted in-line array of cubes, more heat is transferred through the cubes to the liquid, and the liquid boundary development is interrupted periodically by the cube array, resulting in heat transfer enhancement. The strong fluid mixing between the cubes will cause more uniform distribution of the heating wall temperature and thus reduce the wall thermal stresses resulted from the temperature rise along the flow direction. Within a certain range of Reynolds number, self-sustained oscillating flow is activated and expected to reduce the pressure difference in the channels. Experimental data are presented and discussed to evaluate the performance of the cube arrays periodically mounted in microchannels.   This paper was also originally published as part of the Proceedings of the ASME 2005 Pacific Rim Technical Conference and Exhibition on Integration and Packaging of MEMS, NEMS, and Electronic Systems.
Analysis of Effect of Turbine Characteristics on Performance of Solar Chimney Plants
Authors: Serag-Eldin M. A.
Published: April 2009

The present work employs a computational model for predicting the detailed internal flow inside a solar chimney plant, for various turbine characteristics. The model features governing partial differential equations expressing mass, momentum and energy conservation, as well as the transport equations for the kinetic energy of turbulence and its rate of dissipation. The equations are all expressed in an axi-symmetric boundary-fitted co-ordinate system. The turbine characteristics are embedded implicitly within the model. The paper focuses on predicting the effect of turbine characteristics on the flow field within the solar plant as well as its effect on overall plant performance. It reveals strong dependence of plant performance on the selected turbine characteristics.
Attempt at Hydraulic Analogy of Insulation on a Finite Length Cylinder
Authors: Sparrow Ephraim M. , Rohsenow Warren
Published: April 2009

In December 1947, Eph Sparrow, then working on an MS, came to me asking if there wasn’t some small project he could do instead of going home for Christmas. Just before this time Dusenberie’s book on finite differences was published. There he solved a problem of conduction through insulation on a finite L/D cylinder. I suggested that we try to build a hydraulic analogy for the insulation. Eph made up five different diameter open-ended cans. With three setscrews near the top of the cans, one can could be placed inside the other on centers with different end gaps. Water added to the space between the cans provided different magnitudes of L/2. This provided a variety of magnitudes, Ri, R0, L/2, T0 = R0 − Ri.
Single Bubble Dynamics During Flow Boiling on a Horizontal Surface at Different Gravity Levels
Authors: Li Ding , Dhir Vijay K.
Published: April 2009

Complete numerical simulations of bubble dynamics including bubble growth, sliding motion, and bubble lift-off from a horizontal surface have been carried out for different gravity levels and flow velocities. In the model, the region of interest is divided into micro and macro regions. The micro region is the ultra thin liquid film that forms between the solid and evolving vapor-liquid interface. The region occupied by vapor and liquid—excluding the micro layer—is designated as the macro region. Complete conservation equations of mass, momentum, and energy for both phases are solved in this region. The interface shape is obtained by solving for the Level-Set function. The advancing and receding contact angles obtained from experiments are used as input to the model. The predictions are compared with data from experiments. The functional dependence of bubble diameter at departure and growth period on gravity is found to weaken with the increase in flow velocity.
Two-Phase Concurrent Separated Flow Model for Boiling Heat Transfer in Narrow Vertical Rectangular Space
Authors: Pan Liang-Ming , Liang Xiangfei , Xin Ming-Dao , Jen Tien-Chien , Chen Qinghua
Published: April 2009

Compared with conventional channels, narrow and micro channels have significant characteristic of heat transfer enhancement. With smooth internal surface, such channels can efficiently avoid encrustation at the washing effect of the high-speed liquid. Moreover, heat transfer elements can be easily assembled. This type of channels have been adopted extensively in many engineering applications, e.g. microelectronic cooling, Advanced Nuclear Reactor, cryogenic, aviation and space technology and thermal engineering. In recent years, much work was focused upon flow patterns, heat transfer and pressure drop. Almost everyone thought the heat transfer enhancement mechanism of narrow and micro channels to be bubbles’ deformation and disturbance, which is insufficient to explain the heat transfer enhancement. In present work, an innovative model of quasi-one-dimensional vapor liquid two-phase concurrent separated flow was proposed for boiling heat transfer in vertical narrow rectangular space. Numerical results such as boiling heat transfer coefficient and liquid film thickness were obtained. Comparison of model results with reported experimental correlation indicates that the proposed model can predict heat transfer in narrow channels correctly, with the relative deviation less than 14%. Numerical simulating result confirms that heat conduction through liquid film is the predominant mechanism of boiling heat transfer in vapor liquid separated flow region in a vertical narrow rectangular space.
Bubbles Coalescence and Condensation of Subcooled Flow Boiling in Vertical Narrow Channels
Authors: Pan Liang-Ming , He Chuan , Xin Ming-Dao , Jen Tien-Chien , Chen Qinghua
Published: April 2009

Compared with conventional channels, narrow and micro channels have significant heat transfer enhancement characteristic. With smooth internal surface, such channels can efficiently avoid encrustation at the washing of the high-speed liquid. Moreover, heat transfer elements can be easily assembled. This type of channels have been adopted extensively in many engineering applications, e.g. microelectronic cooling, Advanced Nuclear Reactor, cryogenic, aviation and space technology and thermal engineering. In recent years, many efforts have been done which focused upon flow patterns, heat transfer and pressure drop. Almost every researcher thought the heat transfer enhancement mechanism of narrow and micro channels to be bubble’s deformation and disturbance, but the proposed mechanism is insufficient to explain the heat transfer enhancement of narrow channel. In present paper, an experimental flow visualization study has been performed under pressured conditions. Microscopic high-speed video visualization revealed that initial bubbles growth accompanied with slipping on heating wall in flow direction with slip velocity less than 0.1m/s. Several important parameters, such as heat flux and subcooling of cross-section, have important effects on the behavior of bubbles. At higher heat flux, it resulted shifting of the ONB point to the upstream and thus generated larger population of bubbles. It was observed that small bubbles (d = 0.01∼0.07mm) slipped on heating wall at a speed of 0.1 to 0.2m/s, and the velocity of larger bubbles (d = 0.1∼0.3mm) was increased to 0.25 ∼ 0.7 m/s. From flow visualization, it is showed that the process of large bubbles coalescing with small ones is the dominating mechanism of bubble growing. In some situation, the speed-increasing bubble’s size even formed vapor layer near the heating wall.
A CFD Analysis of Telecommunication Racks, Including Effects of Air Filter Locations
Authors: Yeh L. T. , Chung B. T. F. , Yeh Joseph
Published: April 2009

A CFD thermal and air flow analysis is performed for a given telecommunication rack. The commercial software, Flotherm, is employed to perform the system level analysis. The analysis is made to examine the effects of the air filter locations at the upstream and downstream of the fans on the flow distribution within the system. The analysis is then extended to the case with one-fan failure condition.
Printed Circuit Board Thermal Modeling Without the Use of an Effective Thermal Conductivity
Authors: Sampson Richard L.
Published: April 2009

The complexity of the circuit traces on the layers of a typical printed circuit board (PCB) poses a serious problem when preparing a thermal model of the board. Thermal analysts have resorted to the use of an average or so called, “effective thermal conductivity”, Keff, treating the board as a homogeneous medium in their PCB thermal models. This approach carries with it the possibility of significant error in the prediction of board temperatures. A typical PCB will have large variations in the density and pattern of the circuit traces, and a single value of Keff cannot accurately represent all board locations. An alternative approach to this long standing problem is presented in this paper. In the new procedure the thermal conductance between pairs of nodes is computed using all of the details of the circuit traces in the internodal region. The trace information is obtained from bitmap files of each circuit layer, files which may be generated from the board CAD files. The conductances are utilized in a general purpose thermal analyzer for computation of system temperatures. Using the details of the local circuit traces in the computation of internodal conductances results in a more accurate thermal model.
Optimization of Data Center Room Layout to Minimize Rack Inlet Air Temperature
Authors: Bhopte Siddharth , Agonafer Dereje , Schmidt Roger , Sammakia Bahgat
Published: April 2009

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 continues to increase at a rapid rate, it is a challenge to maintain the temperature within the requirements as stated for all the racks within the data center. 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 affects rack inlet air temperatures which is one of the important key to effective thermal management. In this paper, a raised floor data center with 4.5 kW racks is considered. There are four rows of racks with alternating hot and cold aisle arrangement. Each row has six racks installed. Two CRAC 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. Now a multi-variable 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 multi-variable design optimization approach. The results of multi-variable design optimization are used to present guidelines for optimal data center performance.
Characterization of Maximum Current Capacity for Microprocessor Sockets
Authors: Song David , Gupta Ashish , Chiu Chia-Pin
Published: April 2009

This paper presents the current-carrying-capacity (CCC) characterization of a land-grid-array type microprocessor socket. This CCC study has been performed using both computational modeling and experiments using infrared camera. A subsequent risk assessment was performed against the maximum allowed temperature at the point of pressure contact of socket pin for the use-condition socket pin current and motherboard temperature. The results from the modeling and the experimental results are compared.
Fracture and Fatigue Testing of Micro-Sized Materials for MEMS Applications
Authors: Takashima Kazuki , Halford Timothy P. , Higo Yakichi
Published: April 2009

We have developed a new type of mechanical testing machine for micro-sized specimens, which can apply a small static or cyclic load, and have investigated fracture and fatigue crack growth behavior of micro-sized specimens. Cantilever beam type specimens (10 μm × 10 μm × 50 μm), with notches were prepared from thin films of a Ni-P amorphous alloy by focused ion beam machining. Fatigue and fracture toughness tests were carried out in air at room temperature using the mechanical testing machine. Fatigue and fracture testing was completed successfully for micro-sized cantilever specimens. Once fatigue crack growth occurs, rapid sample failure was observed in these micro-sized specimens. This indicates that the fatigue life of micro-sized specimens is mainly dominated by crack initiation. This also suggests that even a micro-sized surface flaw can be a fatigue crack initiation site which will shorten the fatigue life of micro-sized specimens. As a result of fracture toughness tests, plane strain criteria for small scale yielding were not achieved for this amorphous alloy. Plane stress and plane strain dominated regions were clearly observed on the fracture surfaces and their sizes were consistent with those estimated by fracture mechanics calculations. This suggests that fracture mechanics is still valid for such micro-sized specimens.
Flow Boiling Heat Transfer in Microchannel Heat Sinks of Different Flow Orientations
Authors: Zhang H. Y. , Pinjala D.
Published: April 2009

A study of FC-72 flow boiling in a microchannel heat sink with different orientations is presented. The heat sink consisted of 21 microchannels in parallel, with the channel dimensions of 0.2mm (W) × 2mm (H) × 15mm (L). The heat sink was attached onto a flip chip BGA package with a thermal grease at the interface to minimize the contact resistance. The experiments were conducted based on a subcooling around 31 °C and inlet thermodynamic equilibrium quality x = −0.4 with varying flowrates and power inputs for both the vertical upflow (VU) and the vertical downflow (VD) orientations. Boiling with bubbles was observed at the heat sink outlet even if the exit quality x<0. It was found that the flow boiling regime can be represented by isolated bubble flow, slug flow in the present microchannels. Both the junction to inlet thermal resistances and pressure drops are presented as functions of the power input. The experimentally determined thermal resistances were also compared with an analytical analysis in the presence of both single phase convection and two phase saturated boiling heat transfer mechanisms. Good agreement is achieved at large power inputs.   This paper was also originally published as part of the Proceedings of the ASME 2005 Heat Transfer Summer Conference.
Molecular Dynamic Simulation of Crack Propagation Behavior in Nano Size HCP Crystals
Authors: Ando Shinji , Bu Leping , Tsushida Masayuki , Tonda Hideki
Published: April 2009

A crack propagation behavior of nano size hcp crystal (13nm × 24nm × 4.6nm) has been simulated by molecular dynamics method using Lennard-Jones type potential. In a case of model crystal with initial notch plane and direction were (1010), [1210], the crack propagated parallel to notch plane and two sets of prismatic slips were occurred at the crack. Therefore, the crack in this crystal is deduced to extend by alternating shear on two intersecting {1010}<1210> prismatic slip systems. In a model crystal with (0001), [1010] initial crack, {1011} first order pyramidal slip occurred at crack tip and following {1012} twin was also observed. In the model crystal with (1010)[0001] initial crack, the crack propagated parallel to initial crack plane with two prismatic slips in front of the crack. To understand strength of nano size titanium crystals as MEMS materials, stress intensity factor, K of each model crystals were also estimated from stress-strain curves obtained from these simulations.
Early-Stage Analysis for MEMS Structural Optimization II: Its Application to Microrelay Reliability
Authors: Ishikawa Koji , Miki Takahiro , Mamiya Hiroki , Yu Q.
Published: April 2009

This paper discusses a new structural optimization methodology for MEMS and its application to reliability evaluation of micro relays. Clarifying the relationship between system characteristics and design factors, our new design optimization method (called MESA) enables numerical evaluation of MEMS structures at the concept design stage. The relation is defined as sensitivity, which is calculated based on the system governing equations with an experimental method technique and a FEM analysis. The sensitivities show not only the effect of design parameters for the system performances but also the system tradeoffs. The MESA allows designers to obtain “rough” total system performance and create a new concept. The MESA is successfully applied to evaluate an electrostatic microrelay for DC/RF signal switching. With the aid of the MESA, we define existing problems of current cantilever-shape MEMS switches and propose new mechanical approaches in order to enhance the mechanical reliability. The MESA clearly shows us that there are tradeoffs in the switching phenomenon of cantilever microrelay. Based on the MESA information, a new switching concept, which has tri-state multi-finger lateral contacts, is established and the MEMS structure is designed and fabricated. The tri-state switching concept reduces the number of contacts and also disperses the impact energy, which aggravates adhesion. In addition, bi-electrostatic actuators increase the adverse force to prevent stiction without the increase of restoring force, which causes degradation or cracks of the contact surfaces. Furthermore, a new push-pull switching structure is designed as a second generation by means of the MESA. The MESA shows that the second concept will provide superior mechanical performance with keeping the high RF isolation.
Early-Stage Analysis for MEMS Structural Optimization I: Concept
Authors: Miki Takahiro , Ishikawa Koji , Mamiya Hiroki , Yu Qiang
Published: April 2009

We report on the development of a new micro-electro-mechanical systems (MEMS) optimal design method called MEMS Early-Stage Analysis (MESA), which supports the total system evaluation of MEMS devices before the design stage. Recently total system simulation and design using Computer Aided Engineering (CAE) analyses have become important in MEMS device development due to their fabrication and design complexity. Although a lot of CAE methods that can be applied to MEMS have been demonstrated, time-consuming trial-and-error processes are inevitable at the design stage in order to obtain an optimal structure. In our design method, we can clarify and simplify the relation between design parameters and the system characteristics using a MESA weighted orthogonal array. In the MESA array, the sensitivity of each design factor for the system performance shows numerically how the design parameter influences the system characteristics. The existent trade-offs between design parameters can be minimized by both modifying the design concept and adjusting the sensitivities. Therefore MEMS designers are able to optimize the total system based on the information from the MESA array. Moreover, particular system characteristics can be enhanced in order to meet the system requirement through the adjustment of weight values for the sensitivities. The MESA makes the evaluation of system validity possible at the concept design stage. To conduct the informative optimal design method at the beginning of development leads the reduction of the total MEMS design time and cost.
High Precision Compact Thermal Models
Authors: Sabry Mohamed-Nabil
Published: April 2009

This paper is a step towards building a more complete theory of compact thermal models, in which their common structure is highlighted and hence the sources of errors inevitably present in any compact model clearly revealed. This approach is further extended in order to propose an original approach to attain any desired level of precision. Different methods already proposed by other authors can all benefit from the approach proposed here in order to raise accuracy level. Traditionally, increasing precision was attained by increasing the number of nodes. This strategy faces many problems, like the difficulty to select node size and position, as well as the inability to provide an a priori estimate of the number of nodes needed. In particular, this strategy fails in the case of multiple heat sources (MCM). The actual approach relies on a “flexible” pattern model that allows us to avoid such problems. Moreover, the new approach gives access to the tangential temperature gradient. This valuable information for designers in order to assess reliability can not be predicted by classical compact model approaches.
Correlation Between Creep and Uniaxial Ratchetting of Sn/37Pb and Sn/3Ag/0.5Cu Solder Alloys
Authors: Sasaki Katsuhiko , Kobayashi Takuji , Ohguchi Ken-Ichi
Published: April 2009

Ratchetting deformations of solder alloys are significant deformations for the safety and reliability of solder joints of electronic packaging. This paper discusses a correlation between creep and uniaxial ratchetting deformations to clarify the difference in the time-dependent deformations between lead-free and lead-containing solder alloys. Uniaxial ratchetting tests are conducted by cyclic tension-compression and cyclic tension-unloading with the several ratios of maximum to minimum stresses. Additional creep tests are also conducted after the uniaxial ratchetting tests to clarify the effect of the uniaxial ratchetting on the creep deformation. A method to evaluate the uniaxial ratchetting deformation is discussed using the creep curves. The results show that the uniaxial ratchetting deformation correlates to the creep deformation and that the correlation is different between the lead-free and lead-containing solder alloys.
Grain Formation and Intergrain Stresses in a Sn-Ag-Cu Solder Ball
Authors: Park Seungbae , Dhakal Ramji , Lehman Lawrence , Cotts Eric
Published: April 2009

The thermo-mechanical behavior of near eutectic lead-free SnAgCu (SAC) solder joints under Deep Thermal Cycling (DTC) and in-situ thermal loading was examined. Crossed polarizer, optical microscopy revealed that in ball grid array (BGA) solder joints, these Sn rich, Pb-free solders exhibit large grained Sn structures. After imaging, these SnAgCu solder joints were subjected to repeated thermal stresses under an inert atmosphere. Subsequent to this thermal loading, the samples were again examined with optical microscopy. Using both data sets, the intergrain strains and deformations were quantified by Digital image correlation, a full field optical measurement technique. The relations between the positions of grains as well as intermetallics compounds, their boundaries and Sn deformation fields were examined.
Thermal Performance of Natural Graphite Heat Spreaders
Authors: Smalc Martin , Shives Gary , Chen Gary , Guggari Shrishail , Norley Julian , Reynolds R. Andy
Published: April 2009

Heat spreaders can be made from natural graphite sheet materials. These spreaders take advantage of the anisotropic thermal properties of natural graphite. Natural graphite exhibits a high thermal conductivity in the plane of the sheet combined with a much lower thermal conductivity through the thickness of the sheet. As a result, a natural graphite sheet can function as both a heat spreader and an insulator and can be used to eliminate localized hot spots in electronic components. In some cases, a natural graphite heat spreader can replace a conventional thermal management system consisting of a heat sink and cooling fan. This paper examines the properties of natural graphite heat spreaders and the application of these spreaders to thermal management problems in laptop computers. The thermal and mechanical properties of natural graphite heat spreaders are presented along with a discussion of how those properties are measured. The use of a natural graphite heat spreader to reduce the touch temperature in a laptop computer is presented. Both experimental techniques and numerical models are used to examine performance of the heat spreader in this application.
Novel Cyanide Free Electroless Au Plating for the Reliable Joint Between Ni-P/Au and Sn-Ag-Cu Based Lead Free Solder
Authors: Aiba Akihiro , Kawamura Kazumi , Kumagai Masashi
Published: April 2009

Novel cyanide free electroless Au plating with an excellent bath stability and the reliable joint with solder was developed. Although the Au plating solution was based on sulfurous acid, the solution’s stability turned out to be excellent by the effective complexing force of the novel additives. The bath stood working without any precipitations for more than 3 metal turnovers. Since the displacement reaction between Ni-P and Au took place in the mild and the uniform way by the aid of the additives involved, resulting in the uniform and the fine grain structure of the plated film, almost no pinholes on the Au plating layer and no pitting corrosion in the ground Ni-P layer could be observed. The joint strength between the BGA pad plated with this novel Ni-P/Au plating and Sn-Ag-Cu based lead free solder ball was found to be much higher than that obtained by the conventional cyanide process. The excellent joint reliability of this process was probably due to the fact that the uniform Au layer with almost no pinholes effectively suppressed the oxidation of the Ni-P surface and eventually almost no pitting corrosion was produced in the Ni-P layer without forming any fatal voids.
Nondestructive Evaluation of Thermal Phase Growth in Solder Ball Micro-Joints by Synchrotron Radiation X-Ray Micro-Tomography
Authors: Sayama Toshihiko , Tsuritani Hiroyuki , Uesugi Kentaro , Tsuchiyama Akira , Nakano Tsukasa , Yasuda Hideyuki , Takayanagi Takeshi , Mori Takao
Published: April 2009

In high-density packaging technology, one of the most important problems is reliability of the micro-joints connecting LSI (Large Scale Integrated Circuit) chips to PCBs (Printed Circuit Boards) electrically and mechanically. Development of nondestructive testing methods with high spatial resolution is expected to enhance the reliability. Our research group has developed an X-ray micro-tomography system called SP-μCT at the beamline BL47XU in SPring-8, the largest synchrotron radiation facility in Japan. In this work, SP-μCT was applied to three-dimensional evaluation of microstructure evolution; that is, phase growth due to thermal cyclic loading in solder ball micro-joints. Simulating solder micro-joints used in a flip chip, specimens were fabricated by joining a Sn-Pb eutectic solder ball 100 μm in diameter to a steel pin in the usual reflow soldering process. The phase growth process was determined by observation of the CT images obtained consecutively at the fixed point of the target joining. In the reconstructed CT images, the distribution of the constituent phases in Sn-Pb eutectic solder was identified based on the estimation value of the X-ray linear attenuation coefficient. The following results were obtained. First, each phase involves not dispersing particles but a three-dimensionally monolithic structure just like a sponge. Second, the phase growth proceeds in such a way that the average phase size to the 4th power increases proportionally to the number of cycles. Finally, in the vicinity of the joining interface, more rapid phase growth occurs in comparison to the other regions because local thermal strain due to the mismatch of thermal expansion leads to remarkable phase growth. Consequently, the microstructure images obtained by SP-μCT bring us useful information to evaluate the reliability of micro-joints.
A Thermo-Viscoelastic Analysis and Reliability Evaluation for a Single-Sided CSP
Authors: Koguchi Hideo , Ishida Mirai , Nishida Kazuto , Kuroishi Tomoaki
Published: April 2009

In the present paper, a reliability of a single-sided chip-size package (CSP) manufactured using a non-conductive adhesive stud bump direct interconnection method is investigated. The reliability of the CSP is closely related with normal stress between an IC chip and a gold bump. Total normal stress can be decomposed into two parts, deflection related and thermal expansion related. The deflection for a three-layered plate, which is taken into account viscoelastic properties for the resin-sealed sheet and the substrate, respectively, is calculated and compared with experimental results on the deflection of the single-sided CSP. A relationship between the normal stress and the curvature derived from deflection is deduced. Through the use of this relationship, the variation of normal stress with the heat cycle is obtained considering the viscoelastic properties of materials. Furthermore, a relaxation behavior for the thermal stress in the resin-sealed sheet between two rigid walls considering its viscoelastic property is investigated. Summing up normal stresses for each calculation yields the normal stress between the IC and the bump. A relationship between the normal stress and the life of single-sided CSP is investigated for heat cycle. The life in experiment can be explained by the relaxation in the normal stress and the amplitude of the normal stress.