bluuebunny/crossref_metadata_2025 huggingface dataset:
Vascular function in health, hypertension, and diabetes: effect of physical activity on skeletal muscle microcirculation
Authors: Nyberg M. , Gliemann L. , Hellsten Y.
Published: November 2015

Regulation of skeletal muscle blood flow is a complex process, which involves an integration of multiple mechanisms and a number of vasoactive compounds. Overall, muscle blood flow is regulated through a balance between vasoconstrictor and vasodilator signals. In a healthy cardiovascular system, the increase in muscle blood flow required for oxygen supply during exercise is achieved through a substantial increase in vasodilators locally formed in the active muscle tissue that overcome the vasoconstrictor signals. Most of the vasodilator signals are mediated via endothelial cells, which lead to the formation of vasodilators such as nitric oxide (NO) and prostacyclin. In essential hypertension and type II diabetes, the endothelial function and regulation of vascular tone is impaired with consequent increases in peripheral vascular resistance and inadequate regulation of oxygen supply to the skeletal muscle, which can affect muscle function. Central aspects in the vascular impairments are alterations in the formation of prostacyclin, the bioavailability of NO and an increased formation of vasoconstrictors and reactive oxygen species (ROS). Regular physical activity effectively improves vascular function by enhancing vasodilator formation and reducing the levels of vasoconstrictors and ROS.
MEMS Die Warpage During Curing of Die Attach Material
Authors: Lee Hohyung , Liu Ruiyang , Park Seungbae , Xue Xiaojie
Published: November 2015

Microelectromechanical system (MEMS) packages are vulnerable to stresses due to its functional structure. During the assembly process of the package, stresses stemming out of CTE mismatches of the structural elements and curing of the die attach material can cause warpage of the MEMS die [1]. Even though die attach material takes relatively small volumetric portion of the package, it plays a critical role in warpage of the die due to its location and sensitivity of a MEMS sensor.

Most of virgin die attach adhesives are in a state of viscous liquid and, as it is cured the material properties such as modulus and CTE change. Accordingly, residual strain is cumulated on MEMS die after curing process and signal trim process is required. Therefore, the material properties changes depending on the curing profile is valuable information for assembly process of the MEMS package.

To monitor the material properties changes and shrinkage during curing process, strain and modulus of a die attach material are measured in each curing step. Also, to investigate the material property change depending on the curing profile, two different curing profiles are used.

Experimental data show that die attach materials are gradually cured after each thermal cycling, which cause the increment of the modulus and glass transition temperature (Tg) with shrinkage at elevated temperature. Using the measurement data, FEA model is built to predict the warpage of the MEMS die. In the FEA model, residual strain on MEMS die is calculated by inputting material properties of die attach in each curing step. Also, die warpage of the package during the curing process is monitored using an optical profiler for the validation of the simulation results.
Hot Spot Elimination by Thin and Smart Heat Spreader
Authors: Ahamed Mohammad Shahed , Saito Yuji , Mochizuki Masataka , Mashiko Koichi
Published: November 2015

Heat pipes are recognized as an excellent heat transport devices and extensively investigated for applications in electronic cooling. Different types of heat pipes have been developed such as micro/miniature heat pipes, loop heat pipes and so on, and these heat pipes have been widely applied in the field of electronics cooling such as notebook, desktop, data center; as well as aerospace, industrial cooling field. However, in recent years the application of heat pipe is widening to the filed of hand held mobile electronic devices such as smart phone, tablet pc, digital camera etc. With the development in technology these devices have different user friendly functions and capabilities, which requires the highest processor clock speed. In general, high clock speed of processor generates lot of heat which need to be spread or removed to eliminate the hot spot. It becomes a challenging task to cool such electronic devices as mentioned above with a very confined space and concentrated heat sources. Regarding to this challenge, ultra thin flat heat pipe is developed; this newly developed heat pipe consists of a special fiber wick structure which can ensure vapor spaces on the two sides of the wick structure.

In this paper a novel thin spreader is proposed to eliminate the hot spot; generally the proposed heat spreader consists of 0.20mm thick metal plate and ultra thin heat pipe of 0.40mm thickness soldered in its body. Maximum thickness of this spreader is 0.63mm. Metal plate is 60mm × 110mm in size; and the ultra thin heat pipe can be fabricated from different original diameter ranges from 2.0mm to 3.0mm Cu tube. Theoretical and experimental analysis have been done to evaluate this thin spreader. In addition, some real application of this spreader will be introduced in this paper.
Use of Two-Phase CFD Simulations to Develop a Boiling Heat Transfer Prediction Method for Slug Flow Within Microchannels
Authors: Magnini Mirco , Thome John R.
Published: November 2015

This work presents a new boiling heat transfer prediction method for slug flow within microchannels, which is developed and benchmarked against the results of two-phase CFD simulations. The proposed method adopts a two-zone decomposition of the flow for the sequential passage of a liquid slug and an evaporating elongated bubble. The heat transfer is modeled by assuming transient heat conduction across the liquid film surrounding an elongated bubble and sequential conduction/convection within the liquid slug. Embedded submodels for estimating important flow parameters, e.g. bubble velocity and liquid film thickness, are implemented as “building blocks”, thus making the entire modeling framework totally stand-alone. The CFD simulations are performed by utilizing ANSYS Fluent v. 14.5 and the interface between the vapor and liquid phases is captured by the built-in Volume Of Fluid algorithm. Improved schemes to compute the surface tension force and the phase change due to evaporation are implemented by means of self-developed functions. The comparison with the CFD results shows that the proposed method emulates well the bubble dynamics during evaporation, and predicts accurately the time-averaged heat transfer coefficients during the initial transient regime and the terminal steady-periodic stages of the flow.
Thermal Behavior of Rectangular Flux Channels With Discretely Specified Contact Flux and Temperature
Authors: Razavi Masood , Muzychka Yuri S. , Kocabiyik Serpil
Published: November 2015

In this paper, an analytical solution for the thermal behavior of rectangular flux channels with discretely specified boundary conditions is presented. The boundary conditions along the source plane can be a combination of contact temperatures, heat fluxes, and/or adiabatic. Convective cooling is applied along the sink plane, and the edges of the channel are assumed adiabatic. The governing equation of the system is the Laplace equation which is solved using the method of separation of variables and the least squares method. The solution is presented in the form of Fourier series expansion. As a case study, a symmetrical flux channel with a combination of five discretely specified boundary conditions, including temperature, heat flux and adiabatic conditions is considered. Temperature profile along the channel is calculated and compared with the Finite Element Method (FEM) using COMSOL commercial software package [1]. A good agreement is observed between the analytical and FEM results.
Validation Study for VOF Simulations of Boiling in a Microchannel
Authors: Gorlé Catherine , Lee Hyoungsoon , Houshmand Farzad , Asheghi Mehdi , Goodson Kenneth , Parida Pritish R.
Published: November 2015

This paper presents a comparison of Volume-of-Fluid simulation results with experiments [1] for two-phase flow and heat transfer in a micro channel. Mass transfer between the phases is modeled using a reduced-order model, requiring the definition of a time relaxation constant, r. A two-step solution procedure is used, where first a fixed temperature boundary condition is imposed at the heater to avoid overheating of the device during the initial development of the two-phase flow. After obtaining a quasi-steady-state solution this is changed to a heat flux boundary condition to determine the final solution. Results using three different values for r indicate that the value of the constant should vary throughout the domain. A final simulation where r is defined as a function of the streamwise location results in a prediction of the base temperature within 1K of the experimental result, a pressure drop within 30%, and a prediction of the location of transition from subcooled to saturated flow within 2mm.
A Two-Dimensional Microfluidic-Based Tactile Sensor for Tissue Palpation Under the Influence of Misalignment
Authors: Yang Yichao , Shen Jiayue , Hao Zhili
Published: November 2015

This paper reports on a proof-of-concept study of applying a two-dimensional (2D) microfluidic-based tactile sensor for tissue palpation under the influence of misalignment. Two unavoidable misalignment issues, uncertainty in contact point and non-ideal normal contact, severely distort the genuine elasticity distribution of a tissue region, yielding false identification of abnormality. The core of the 2D tactile sensor is one whole microstructure embedded with an electrolyte-enabled 2D resistive transducer array underneath. This unique configuration allows the tactile sensor to interact with a tissue region in a continuous manner that mimics manual palpation: the whole microstructure (fingertip) presses a tissue region and the corresponding deflection distribution is captured concurrently by the embedded transducer array (distributed sensors under the skin). This continuous manner tackles the misalignment issues encountered by an individual sensor or a sensor array, in that any misalignment encountered by the 2D sensor is manifested as an increasing trend of the distributed deflection-depth relations along the tilt direction. Tissue phantoms with embedded nodules and extrusions are prepared and are measured using the 2D tactile sensor, validating the capability of the tactile sensor to identify abnormalities in soft tissue under the influence of misalignment.
Flow and Heat Transfer in Liquid Films Flowing Over Highly Curved Surfaces
Authors: Zeng Zezhi , Warrier Gopinath , Ju Y. Sungtaek
Published: November 2015

Large-scale evaporative cooling is one of the leading sources of fresh water consumption. Dry cooling based on existing heat exchangers, however, has found limited usage due to the high cost and large foot prints/weights. Development of alternative low-cost light-weight heat exchangers for dry cooling is therefore urgently needed. One promising design for such alternative heat exchangers is what we call Direct-contact Liquid-on-String Heat Exchangers (DILSHE). DILSHE consists of a vertically aligned array of inexpensive polymer strings. A nonvolatile liquid flows over the strings, forming thin liquid films. Large surface areas provided by these films enable efficient heat transfer to counter-flowing cooling air. Physics-based design and optimization of DILSHE requires rigorous understanding of flow and heat transfer phenomena of falling liquid films on highly curved surfaces. Formation of travelling beads through the Rayleigh-Plateau or Kapitza instability can enhance heat transfer across liquid-gas interfaces. We have developed a numerical model for liquid-gas flows and heat transfer in the drop-like regime, where the Rayleigh-Plateau instability dominates and the shape of travelling beads is governed mainly by the influence of surface tension. We solve the Young-Laplace equation to obtain the liquid bead shape, which was then used to construct a finite element model. The time-dependent Navier-Stokes equation and the energy equation were then solved to obtain velocity and temperature distributions in the liquid and the surrounding counter-flowing air. The temporal and spatial variations in the temperature of travelling beads are analyzed to evaluate the effective heat transfer coefficients, which are key input parameters for an overall heat exchange model to quantify the heat transfer characteristic of DILSHE. The present work helps build foundation for systematic design of new generations of heat exchangers for dry cooling.
Rack Server Solution in Data Center
Authors: Kang Sheng , Chen Guofeng , Wang Chun , Ding Ruiquan , Zhang Jiajun , Zhu Pinyan , Liu Chao , Ahuja Nishi
Published: November 2015

With the advent of big data and cloud computing solutions, enterprise demand for servers is increasing. There is especially high growth for Intel based x86 server platforms. Today’s datacenters are in constant pursuit of high performance/high availability computing solutions coupled with low power consumption and low heat generation and the ability to manage all of this through advanced telemetry data gathering. This paper showcases one such solution of an updated rack and server architecture that promises such improvements.

The ability to manage server and data center power consumption and cooling more completely is critical in effectively managing datacenter costs and reducing the PUE in the data center. Traditional Intel based 1U and 2U form factor servers have existed in the data center for decades. These general purpose x86 server designs by the major OEM’s are, for all practical purposes, very similar in their power consumption and thermal output. Power supplies and thermal designs for server in the past have not been optimized for high efficiency. In addition, IT managers need to know more information about servers in order to optimize data center cooling and power use, an improved server/rack design needs to be built to take advantage of more efficient power supplies or PDU’s and more efficient means of cooling server compute resources than from traditional internal server fans. This is the constant pursuit of corporations looking at new ways to improving efficiency and gaining a competitive advantage.

A new way to optimize power consumption and improve cooling is a complete redesign of the traditional server rack. Extracting internal server power supplies and server fans and centralizing these within the rack aims to achieve this goal. This type of design achieves an entirely new low power target by utilizing centralized, high efficiency PDU’s that power all servers within the rack. Cooling is improved by also utilizing large efficient rack based fans for airflow to all servers. Also, opening up the server design is to allow greater airflow across server components for improved cooling. This centralized power supply breaks through the traditional server power limits. Rack based PDU’s can adjust the power efficiency to a more optimum point. Combine this with the use of online + offline modes within one single power supply. Cold backup makes data center power to achieve optimal power efficiency. In addition, unifying the mechanical structure and thermal definitions within the rack solution for server cooling and PSU information allows IT to collect all server power and thermal information centrally for improved ease in analyzing and processing.
CFD Analysis of an Air-Cooled Planar Heat Sink With Cross-Connected Alternating Converging-Diverging Channels
Authors: Kanargi Omer Bugra , Yap Christopher , Lee Poh Seng
Published: November 2015

In this study, the thermal and hydraulic performances of an air-cooled planar heat sink with cross-connected alternating converging-diverging channels were investigated. The commercial CFD solver ANSYS Fluent was used to solve the fluid flow and heat transfer from the fins to the air flow for a range of converging-diverging channel expansion ratios, heat generation rates and Reynolds numbers. The converging and diverging channel sections create high and low pressure zones, respectively, in the flow domain and this pressure difference induces secondary flows from the converging channel sections to the diverging channel sections through the cross connections. The observed heat transfer enhancement results from two different phenomena: (1) thermal boundary layer disruption and re-initialization at the cross connections and (2) fluid mixing; where the former reduces the convection resistance in the vicinity of the fin walls by reducing the thermal boundary layer thickness, while the latter allows a more uniform temperature build-up of air in the streamwise direction. Despite the fact that the pressure drop penalty increases due to flow restriction compared to the straight channel heat sinks, it is possible to enhance the Nusselt number for up to 100% with the proposed heat sink design.
Passive Thermosyphon Cooling System for High Heat Flux Servers
Authors: Szczukiewicz Sylwia , Lamaison Nicolas , Marcinichen Jackson B. , Thome John R. , Beucher Peter J.
Published: November 2015

The main aim of the current paper is to demonstrate the capability of a two-phase closed thermosyphon loop system to cool down a contemporary datacenter rack, passively cooling the entire rack including its numerous servers. The effects on the performance of the entire cooling loop with respect to the server orientation, micro-evaporator design, riser and downcomer diameters, working fluid, and approach temperature difference at the condenser have been modeled and simulated. The influence of the thermosyphon height (here from 5 to 20 cm with a horizontally or vertically oriented server) on the driving force that guarantees the system operation whilst simultaneously fulfilling the critical heat flux (CHF) criterion also has been examined. In summary, the thermosyphon height was found to be the most significant design parameter. For the conditions simulated, in terms of CHF, the 10 cm-high thermosyphon was the most advantageous system design with a minimum safety factor of 1.6 relative to the imposed heat flux of 80 W cm−2. Additionally, a case study including an overhead water-cooled heat exchanger to extract heat from the thermosyphon loop has been developed and then the entire rack cooling system evaluated in terms of cost savings, payback period, and net benefit per year. This approximate study provides a general understanding of how the datacenter cooling infrastructure directly impacts the operating budget as well as influencing the thermal/hydraulic operation, performance, and reliability of the datacenter. Finally, the study shows that the passive two-phase closed loop thermosyphon cooling system is a potentially economically sound technology to cool high heat flux servers of datacenters.
The Detection of Minimum BLT Condition With Electrical Capacitance Measurement
Authors: Saito Yasuhiro , Suzuki Satoru , Komuro Takanori
Published: November 2015

Recently, the increase in performance of semiconductor devices has been accompanied by a simultaneous rise in the amount of heat they emit. Thermal interface materials (TIMs) play an important role in connecting different surfaces to ensure efficient transfer of heat [1]. TIMs can be classified into the following three types: Type 1 materials are greases and pastes, Type 2 materials consist of viscoelastic solids such as gap fillers (an example of which is silicone sheeting), and Type 3 materials are non-compressible solids. Typical Type 1 materials consist of liquid and solid fillers which have a uniform diameter [2]. When pressure is applied, they can easily fill the gap between a heat source and a heat sink, and the gap thickness (BLT: Bond Line Thickness) will eventually decrease to the diameter of the filler. It is well known that under these conditions, the total thermal resistance from a heat source to a heat sink can be minimized. In this study, we demonstrate that we can clearly detect whether the TIM reaches this minimum thickness or not, by measuring the electric capacitance with an LCR meter, instead of evaluating heat related parameters. We performed experiments for a Type 1 TIM which has a viscosity of 240 Pa·s. We hope this technology will be effective, especially in cases of mass-production, because electrical measurement is faster, more repeatable, and more accurate than thermal measurement [3].
A Capillary-Wick Heat Pipe Fabricated on a Plastic Board (Fundamental Experiments on Heat Transport Characteristics)
Authors: Koito Yasushi , Maehara Hiroyuki , Shimada Daisuke , Tomimura Toshio
Published: November 2015

A capillary-wick heat pipe having the dimensions of 5.0 mm × 5.0 mm × 100 mm (length) is fabricated on a surface of a plastic board, and the experimental investigations are conducted on the operational characteristics of the heat pipe. Plastics are easy to manufacturing, lightweight, low cost, flexible, and besides, the present study aims at the phase-change heat transfer inside the plastic board. A sintered copper powder and water are used as the wick structure and the working fluid of the heat pipe, respectively. In experiments, an evaporator section of the heat pipe is heated by a heater while a condenser section is water-cooled by a heat sink. A heat input and a liquid volume inside the heat pipe are changed, and the temperature distribution of the heat pipe is measured by thermocouples. Moreover, a one-dimensional thermal circuit model is made to evaluate the effective thermal conductivity of the heat pipe. From the experimental results, the continuous phase-change heat transfer inside the plastic board and its effectiveness are confirmed. It is also revealed that the effective thermal conductivity of the heat pipe is 854 W/(m·K) in maximum under the present experimental conditions.
Experimental Characterization of the Vertical and Lateral Heat Transfer in 3D-SIC Packages
Authors: Oprins Herman , Cherman Vladimir , Van der Plas Geert , De Vos Joeri , Beyne Eric
Published: November 2015

In this paper, we present the experimental characterization of 3D packages using a dedicated stackable test chip. An advanced CMOS test chip with programmable power distribution has been designed, fabricated, stacked and packaged in molded and bare die 3D packages. The packages have been experimentally characterized in test sockets with and without cooling, and soldered to the PCB. Using uniform and localized hot spot power distribution, the thermal self-heating and thermal coupling resistance and the lateral spreading in the 3D packages have been studied. Furthermore, the measurements have been used to characterize the thermal properties of the epoxy mold compound and the die-die interface and to calibrate a thermal model for the calculation of equivalent properties of underfilled μbump arrays. This model has been applied to study the trade-off between the stand-off height reduction and the underfill thermal conductivity increase in order to reduce the inter die thermal resistance.
Thermal Diffusion Tomography for Quantitative Non-Destructive Characterization of Electronic Packages
Authors: Öner Bahar , Ertürk Hakan
Published: November 2015

Thermal management problems in electronic packages have been a challenging problem due to increasing number of transistors in chips and reduction in product size. Thermal interface materials (TIM) help heat dissipation by reducing thermal contact resistance between chip and integrated heat spreader (IHS) and TIM quality is critical for effective removal of heat generated from the package. Therefore, identification of defects within TIM is required during package assembly process development. Imaging techniques such as computerized scanning acoustic microscopy (CSAM) and X-ray tomography are used as non-destructive testing techniques to identify TIM defects qualitatively. More recently, it was shown that IR thermography can be used as a qualitative means of identifying defects as well. Thermal diffusion tomography is a powerful alternative to those techniques due to its lower cost and ease of application. In this study, quantitative characterization of defects in TIM is presented using thermal diffusion tomography. The study is conducted considering a high density interconnect flip chip package that includes spreading effect due to different sized IHS and die. Defect size and location are detected analyzing the measured thermal response of electronic package by solving the resulting inverse problem by Levenberg-Marquardt algorithm as an image reconstruction technique.
Inverse Design of Cooling of Electronic Chips Subject to Specified Hot Spot Temperature and Coolant Inlet Temperature
Authors: Reddy Sohail R. , Dulikravich George S.
Published: November 2015

Most methods for designing electronics cooling schemes do not offer the information on what levels of heat fluxes are maximally possible to achieve with the given material, boundary and operating conditions. Here, we offer an answer to this inverse problem posed by the question below. Given a micro pin-fin array cooling with these constraints:

- given maximum allowable temperature of the material,

- given inlet cooling fluid temperature,

- given total pressure loss (pumping power affordable), and

- given overall thickness of the entire electronic component,

find out the maximum possible average heat flux on the hot surface and find the maximum possible heat flux at the hot spot under the condition that the entire amount of the inputted heat is completely removed by the cooling fluid. This problem was solved using multi-objective constrained optimization and metamodeling for an array of micro pin-fins with circular, airfoil and symmetric convex cross sections that is removing all the heat inputted via uniform background heat flux and by a hot spot. The goal of this effort was to identify a cooling pin-fin shape and scheme that is able to push the maximum allowable heat flux as high as possible without the maximum temperature exceeding the specified limit for the given material. Conjugate heat transfer analysis was performed on each of the randomly created candidate configurations. Response surfaces based on Radial Basis Functions were coupled with a genetic algorithm to arrive at a Pareto frontier of best trade-off solutions. The Pareto optimized configuration indicates the maximum physically possible heat fluxes for specified material and constraints.
Energy Efficiency and Air Quality Considerations in Airside Economized Data Centers
Authors: Klein Levente J. , Bermudez Sergio A. , Marianno Fernando J. , Hamann Hendrik F. , Singh Prabjit
Published: November 2015

Many data center operators are considering the option to convert from mechanical to free air cooling to improve energy efficiency. The main advantage of free air cooling is the elimination of chiller and Air Conditioning Unit operation when outdoor temperature falls below the data center temperature setpoint. Accidental introduction of gaseous pollutants in the data center along the fresh air and potential latency in response of control infrastructure to extreme events are some of the main concerns for adopting outside air cooling in data centers. Recent developments of ultra-high sensitivity corrosion sensors enable the real time monitoring of air quality and thus allow a better understanding of how airflow, relative humidity, and temperature fluctuations affect corrosion rates. Both the sensitivity of sensors and wireless networks ability to detect and react rapidly to any contamination event make them reliable tools to prevent corrosion related failures. A feasibility study is presented for eight legacy data centers that are evaluated to implement free air cooling.
Reduced Order Modeling for Chip-Embedded Micro-Channel Flow Boiling
Authors: Parida Pritish R.
Published: November 2015

The information technology (IT) industry is exploring three-dimensional (3D) stacking of chips to maintain future computing scalability. However, 3D chip stacks require a solution to significant new thermal challenges. Interlayer two-phase evaporative cooling with a chip-to-chip interconnect-compatible dielectric fluid is an enabling technology but faces significant development issues. One such issue is the inability to thermally model a microprocessor with spatially varying heat sources together with a two phase microfluidic convection network. While progress has been made on two-phase conjugate simulations at the chip and channel levels, none of those provide a computationally manageable approach.

In the present study, a reduced physics conjugate heat transfer model has been developed for simulating two-phase flow boiling through chip embedded micron scale cavities. This model has been validated with good accuracy against data available from literature. The validated model was then extended to predict the thermal performance of a state-of-the-art microprocessor chip with embedded two-phase cooling, where significant improvements in device junction temperatures were observed compared to the baseline cooling solution.
Modeling Thermal Spreading Resistance in Via Arrays
Authors: Fish Michael , McCluskey Patrick , Bar-Cohen Avram
Published: November 2015

As thermal management techniques for 3D chip stacks and other high power density electronic packages continue to evolve, interest in the thermal pathways across substrates containing a multitude of conductive vias has increased. To facilitate the use of numerical models that can reduce computational costs and time in the thermal analysis of through-layer via (TXV) structures, much research to date has focused on defining effective anisotropic thermal properties for a pseudo-homogeneous TXV medium using isothermal boundary conditions. While such an approach eliminates the need to model heat flow through individual vias, the resulting properties can be shown to depend on the specific boundary conditions applied to a unit TXV cell. More specifically, effective properties based on isothermal boundary conditions fail to capture the local “micro-spreading” resistance associated with more realistic heat flux distributions and local hot spots on the surface of these substrates.

This work assesses how the thermal spreading resistance present in arrays of vias in interposers, substrates, and other package components can be properly incorporated into the modeling of these arrays. We present the conditions under which spreading resistance plays a major role in determining the thermal characteristics of a via array and propose methods by which designers can both account for the effects of spreading resistance and mitigate its contribution to the overall thermal behavior of such substrate-via systems. Finite element modeling of TXV unit cells is performed using commercial simulation software (ANSYS).

Compactly stated, micro-spreading contributes to the total resistance RT = R1d + (fu + fl)Rsp,max, where 0≤ f ≤ 1 are adjustment factors that depend on the conditions at the upper and lower surfaces of the via array layer and Rsp,max occurs under worst-case conditions.
Development of a Raised-Floor Plenum Design Tool
Authors: VanGilder James W. , Zhang Xuanhang (Simon)
Published: November 2015

Existing tools for analyzing raised-floor-plenum airflow distribution focus on the prediction of tile-by-tile airflow rates and therefore necessarily require the input of a specific floor layout. Such CFD-based tools are also typically expensive, potentially slow and are of limited availability. The present study describes a design tool which takes a somewhat more qualitative approach in which overall tile airflow uniformity and the breakdown of cooling airflow (e.g., through the perforated tiles, raised-floor leakage, and cable cutouts) are estimated from only high-level design information. The design tool’s predictions are based on both CFD simulations performed “off line” and an analytical flow network model and can be used at the concept stage in a construction project or as a learning tool to quickly demonstrate the effect of design tradeoffs on plenum airflow distribution. The applicability of the tool to a real facility is confirmed through measured data and the tool is used to investigate the effect of various design choices on perforated tile airflow uniformity and distribution.
Improved Flexibility of Alumina Ultrathin Barrier Films by Nano-Lamination
Authors: Lewis Ryan , Slaughter Julie , Lee Y. C.
Published: November 2015

Ultra-thin ceramic films deposited by atomic layer deposition (ALD) can provide an excellent oxygen and moisture barrier for diverse applications, such as organic light emitting devices, micro heat pipes, vacuum packaging, and flexible electronics. Due to their nano-scale thickness, these films exhibit a certain degree of flexibility despite being ceramic. Above a critical strain, the ALD films will crack, and this cracking has been shown to be governed by a fracture mechanics model. This paper shows the critical cracking strain of Al2O3 deposited on polymers can be improved by incorporating the ceramic film into a nano-laminate (NL) structure, with alternating layers of ALD Al2O3 and a compliant polymer film. We find that the critical cracking strain depends on the elastic modulus of the polymer film. We experimentally demonstrate a factor of 2 improvement in critical cracking strain using an 8-layer polymer/ALD NL, with 3 different polymers, independent of the polymer thickness.
Development of Heat Transfer Tools for Sizing Flexible Graphite Spreaders in Mobile Applications
Authors: Beyerle Rick , Shives Gary
Published: November 2015

Even as the use of flexible graphite heat spreaders becomes ubiquitous in mobile electronics, numerically quantifying the heat dissipation remains a challenge. The rapid pace of development of mobile devices has deterred the industry from establishing standards, and rules of thumb are few, as are closed-form solutions. Users have requested numerical methods and tools to simplify the selection of flexible graphite heat spreaders from among the standard thicknesses and grades, as well as to quantify the effect of changing heat transfer area and configuration.

In the presence of adjacent layers — adhesives, dielectrics, or still air gaps — the thin nature of the materials and the high, orthogonal thermal conductivity ratios of the graphite combine to create a complex conjugate heat transfer problem. Although the thinnest of these sheets constitute but a tiny fraction of the thickness of a cell phone or tablet, their dominant role in the heat transfer requires that they not be neglected in the calculations. Some CFD software guidelines advise using multiple meshing layers to capture fully the heat transfer in these spreaders, while others (primarily FEA based) provide a plate element that negates the need for discretization. In the former, a fully meshed spreader confounds the goal of a quick calculation, but the flexibility of 3D solution also demands meticulous attention to the details, provides “an answer” that is easy to misinterpret, and in the hands of an unskilled user, invites error.

The goal of this project is to establish the guidelines for computing heat spreading in graphite, including cell dimension ratio, mesh density, spreading radius, and transport capacity and to marry the orthogonal properties of the material with the row-column format of a spreadsheet or matrix software. It also reviews methods for addressing the non-orthotropic situations such as angled plates, and the curved surfaces seen in the case of graphite wraps and flexible hinges. There are cases in which a simple contact resistance value adequately represent a graphite thermal interface material, but others that require an accounting for the lateral conductivity that increases the efficacy of the TIM. Finally, the error of the calculation is assessed for a simple representative geometry.
Fabrication of a Vapor Chamber on a Plastic Board
Authors: Hideyama Fumihiko , Nonoshita Shuto , Koito Yasushi , Tomimura Toshio
Published: November 2015

A vapor chamber is a flat-plate heat pipe, where a cooled (condenser) section is much larger than a heated (evaporator) section, and has been used as a heat spreader to enhance the cooling of electronic devices. An objective of this study is to integrate the vapor chamber into a polycarbonate board. Plastic materials are easy to manufacturing, light weight, low cost, flexible, and then the present study aims at performing a phase-change heat transfer and a heat spreading inside the polycarbonate board. A sintered copper powder and water are used as a wick structure and a working fluid, respectively. In experiments, the heat is applied by a heater while the cooling water is circulated between a thermostatic bath and a cooling jacket. The experiments are conducted changing a liquid volume and a heat input, and the transient temperature distribution of the vapor chamber is measured by thermocouples. For comparison, the experiment is also conducted where the working fluid is not charged into the vapor chamber. Moreover, based on a thermal resistance network, an analytical model of the vapor chamber is made and the analysis is performed on the phase-change heat transfer inside the vapor chamber. From the experimental and analytical results, the heat transfer characteristics of the polymer-based vapor chamber and the effectiveness of the phase-change heat transfer are confirmed.
3D IC With Embedded Microfluidic Cooling: Technology, Thermal Performance, and Electrical Implications
Authors: Zhang Xuchen , Han Xuefei , Sarvey Thomas E. , Green Craig E. , Kottke Peter A. , Fedorov Andrei G. , Joshi Yogendra , Bakir Muhannad S.
Published: November 2015

In this paper, a novel thermal testbed with an embedded micropin-fin heat sink is designed and fabricated. The micropin-fin array has a nominal height of 200 μm and a diameter of 90 μm. Single phase and two phase thermal testing of the micropin-fin array heat sink are performed using deionized (D.I.) water as the coolant below atmospheric pressure. The measured pressure drop is as high as 100 kPa with a mass flux of 1637 kg/m2s at a heat flux of 400 W/cm2 in a two-phase regime. The heat transfer coefficient and the vapor quality are calculated and reported. The impact of microfluidic cooling on the electrical performance of the 3D interconnects is also analyzed. The high aspect ratio through silicon vias (TSVs) used in the electrical analysis have a nominal diameter of 10 μm.
System-Level Packaging of Wide Bandgap Inverters for Electric Traction Drive Vehicles
Authors: Olejniczak Kraig J. , Flint Tom , Simco David , Storkov Sergei , McGee Brad , George Kenny , Killeen Peter , Curbow Austin , Shaw Robert S. , Passmore Brandon , McNutt Ty R.
Published: November 2015

In this paper, we describe the system-level packaging of a 30 kW continuous, 55 kW peak, traction inverter to showcase the electro-thermal-mechanical performance enhancements of silicon carbide (SiC), a wide bandgap (WBG) semiconductor, over silicon. Higher efficiency, larger gravimetric and volumetric power densities, and smaller thermal management system requirements may be achieved through higher operating junction temperatures afforded by SiC versus silicon power devices. By applying advanced system-level packaging techniques, high-temperature control circuitry, utilizing 105°C-rated capacitors, and reducing the number of system interconnects and attaches to enable higher system reliability, a substantial cost reduction from the die level to the system level can be demonstrated by completely eliminating an electric vehicle’s secondary low-temperature cooling loop. The endgame is to reduce the traction inverter size (≥ 13.4 kW [peak]/L), weight (≥ 14.1 kW [peak]/kg), and cost (≤ $182/100,000) relative to output power while maintaining 15-year reliability metrics [1].
Evaluation of Fatigue Crack Initiation and Propagation in Thin Solder Joints Using a Lap-Joint Shear Specimen With High Stiffness Fixtures
Authors: Sayama Toshihiko , Tsuritani Hiroyuki , Okamoto Yoshiyuki , Kinoshita Masayoshi , Mori Takao
Published: November 2015

Fatigue damage in solder joints is one of the most significant factors in the failure of electronic components. Accordingly, many research studies on the fatigue lifetime evaluation of solder joints have been undertaken to improve the reliability of the components. The authors have devised a lap-joint specimen with high stiffness fixtures in order to carry out shear fatigue testing on thin solder joints, which have thickness of a few hundred μm and are manufactured via a reflow process similar to that used in actual printed circuit boards (PCBs). In this work, using the developed lap-joint specimen, the fatigue properties, including crack initiation and propagation of Sn-3.0Ag-0.5Cu solder joints were evaluated under low cycle shear loading conditions with creep deformation. The lap-joint specimen was fabricated by the reflow soldering of two copper adherend, and was assembled with high stiffness loading fixtures. The dimensions of the solder joint are 4 mm (length) × 2 mm (width), with a thickness ranging from 100 to 400 μm. In the shear fatigue test, under the assumption of thermal loading conditions of actual PCBs, the inelastic strain amplitude and total strain rate were set to from 0.5 to 1.2 % and 1×10−4 s−1, respectively. In addition, the fatigue crack initiation lifetime is defined as the number of cycles N20% at which the load amplitude has decreased by 20 % from the initial value. As the first study result, the experimental relations between the fatigue crack initiation lifetime and the inelastic strain range were obtained. Next, in order to apply the experimental data to the evaluation of fatigue crack initiation in actual solder joints via finite element analyses, the lifetime data were related to the calculated inelastic strain at the interface corners of the solder joint of the specimen, where fatigue cracks initiate due to strain concentration. Finally, assuming that the reduction of the load amplitude corresponds linearly to the fatigue crack length, the experimental relations between the fatigue crack propagation rate and J-integral range were also obtained. The experimental data are regarded to be valid, given a comparison to other crack propagation curves for solder obtained by tensile cyclic loading of a flat specimen with a center crack. Consequently, the developed lap-joint specimen with high rigidity is effective for acquiring the material properties regarding fatigue crack initiation and propagation in actual thin solder joints.
Modeling and Parametric Study of Light Scattering, Absorption and Emission of Phosphor in a White Light-Emitting Diode
Authors: Wang Jiaqi , Lo Jeffery C. C. , Lee S. W. Ricky , Yun Feng , Tao Mian
Published: November 2015

In this study, we proposed and demonstrated an effective approach to model and predict spectral power distribution (SPD) for a phosphor-converted light emitting diode (pc-LED). For emission and excitation, broadband diffuse transmittances of 1 mm YAG:Ce phosphor plates with different concentrations were measured by a spectrophotometer. For emission, it was found that transmittance for all wavelengths was almost identical. This result indicates that emission spectrum prediction could be simplified by simulating the radiant power of the peak wavelength only. At the peak wavelength (560 nm), our simulation results, in which optical constants were calculated by the Lorenz-Mie theory, agreed well with our measurements. For excitation, a novel transmittance measurement setup based on an LED goniophotometer was proposed to obtain the absorption coefficient. By adjusting the optical parameter in our ray-tracing model to fit measured transmittances, accurate absorption coefficients were determined. Based on our calculation and measured optical parameters, we simulated the radiant power of the blue light and yellow light of commercial white LED packages. By expanding the total blue and yellow power into linear combinations of wavelengths in both regions, we successfully predicted the SPD of our LED package. Our predicted SPD has good agreement with the measured results.
Improved Air Cooling Heat Pipe Based Thermal Solutions for Heat Sinks in Optical Plug Modules
Authors: Haghighi Ehsan B. , Phan Thanh L. , Wuttijumnong Vijit , Mashiko Koichi , Saito Yuji , Mochizuki Masataka
Published: November 2015

Ever-increasing both data speed and traffic volume in the network telecommunications; as a result, producing more heat loss, challenges the conventional cooling methods. An optical plug module is a transceiver in data communication applications. By increasing the cooling demands, new thermal management solutions are necessary for optical plug modules. This article experimentally studies the heat pipe based cooling solutions for the optical plug modules. Heat pipes can passively transfer part of the produced heat from the hardly accessible places of the modules and expose it to the present active air cooling. Three different heat pipe based arrangements for a four-port optical plug assembly at both free and forced convection were investigated. Based on the results heat pipes helped to reduce heat sinks and total thermal resistance of this assembly on average by 27% and 16%, respectively under airflow rate of 10 ft3/min.
Optimum Design and Operation of Thermoelectric Heat Pump With Two Temperatures
Authors: Yazawa Kazuaki , Shakouri Ali
Published: November 2015

We present a comprehensive analysis and optimization of the thermoelectric (TE) heat pump and refrigeration in contact with two constant-temperature reservoirs, followed by a discussion of their cost effectiveness. In many applications in electronics cooling, the heat source temperature is constrained as well as the gas or liquid cooling heat sink. We optimize the thermoelectric design by changing both the element (leg) thickness and drive current simultaneously in order to achieve maximum energy efficiency, i.e., to obtain the highest coefficient of performance (COP) for the heat pump. Each variable and performance is considered per unit area.

COP vs cooling capacity, which is the heat amount pumped, by changing the driving current, shows a unique characteristic and it looks like the Greek character ‘beta’ in a plot. This ‘beta plot’ gives a global view of the performance of various TE heat pump systems. We discuss the similarity with the graph obtained in power generation in contact with the constant temperature reservoirs when the trade-off between the efficiency and power output is considered. In this plot, the maximum COP is found at a much smaller current compared to the maximum heat cooling capacity Qmax. This Qmax is found when the internal resistance is sqrt (1 + ZT) times the sum of the external resistances, but only when these contacts are symmetric and the net temperature difference is zero. The ratio increases slightly as the net temperature difference increases (heat pumping to a higher temperature). This shows some differences compared to the power generation mode where an impedance match happens when the ratio of internal to external resistances is constant at sqrt (1 + ZT). If the contact thermal resistances with the hot and cold sides are asymmetric, Qmax and the optimum resistance ratio are both reduced when the heat sink resistance increases and they both increase as the heat sink resistance decreases.

TE materials are expensive relative to the other components; hence, it is important to minimize the material use. The COP per cost and cooling capacity per cost are investigated. Similar to power generators, the TE element can be thinner as the fractional area coverage of the TE elements is reduced, while maintaining a constant internal thermal resistance. The most cost effective design is found to be thinner than that of the maximum performance. Also, the ZT value impact for the cost performances is smaller, especially in COP.