bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental Study of Aqueous Binary Mixture Droplet Vaporization on Nanostructured Surfaces
Authors: LaBrie Russell J. , Padilla Jorge , Carey Van P.
Published: November 2015

In this study heat transfer due to vaporization is investigated for low concentration binary mixtures of 2-propanol/water on nanostructured surfaces. The surfaces are comprised of zinc oxide (ZnO) nanocrystals grown by hydrothermal synthesis on a smooth copper substrate having an average roughness of 0.06 μm. Three nanostructured surfaces used in this study differ only in the duration of the hydrothermal synthesis consisting of 4, 10, and 24 hours of surface growth. Surface geometries were observed to be a function of hydrothermal synthesis time with an increase in area coverage, length, and diameter of nanocrystals with increase synthesis time. ZnO nanocrystals exhibit mean diameter of 500–700 nm, mean length of 1.7–3.3 μm and porosities of 0.04–0.58. Individual droplets between 2.5–3.9 mm in diameter consisting of a binary mixture of 2-propanol/water with concentration of either 0.01 M or 0.03 M were deposited at a minimum distance above the surface that would be sufficient for droplets to detach on their own due to gravity onto a nanostructured surface at temperatures between 110–140 °C. High speed video was used to record the deposition and vaporization process and through image analysis it was possible to measure heat transfer coefficients based on the wetted area, as well as other parameters. Through the video analysis it was observed that droplets which are approximately spherical, impact the surface and spread into a thin film with mean film thickness between 65–400 μm which then evaporated by film evaporation without nucleate boiling. Wettability of each of the surfaces was characterized through contact angle measurements from photographs of the droplet profile when the droplet profile was discernible. When profiles were not discernible due to hydrophilicity of some surfaces, contact angles were calculated by utilizing droplet volume and spread area. Contact angle measurements were performed on the surfaces before and after each experiment in order to document changes in wettability as a result of experimentation. Results from this experiment are compared to water droplet vaporization results from a previous experiment in order to determine whether 2-propanol enhances the heat transfer, and found that the heat transfer coefficient was increased by up to 128% in some cases. Heat transfer enhancement was found to be a function of droplet diameter as well as mixture concentration with 3.9 mm 0.01 M 2-propanol/water droplets showing larger enhancement. Potential uses of heat transfer in this application are also discussed.
Arcing and Spacing Requirements for High Voltage Printed Circuit Boards
Authors: Singh Prabjit
Published: November 2015

The electronic hardware miniaturization trend continues unabated. The reduced feature spacing expose the high voltage power supply circuits to arcing. The power MOSFET gate to drain and the drain to source lead gaps are narrow enough for zinc whiskers emanating from under the raised floor zinc plated tiles to arc across the MOSFET leads. Arcing can also occur because of paper cellulose fibers and dust in high relative humidity environments. The physics of arcing will be presented. Paschen’s law of arcing will be described; it will be shown how the law led to a novel way of testing power supplies for propensity to arcing. Examples of application of the test, called the partial vacuum test, to power supplies will be described. Testing the integrity of conformal coating is one useful application of the partial vacuum test. A novel zinc whisker spray test, developed to determine the spacing required to avoid arcing between features at high electric potential between them, will be described and its results will be presented that verify the UL feature spacing guidelines.
Improvement of Thermal Conductivity of Electroplated Copper Interconnections by Controlling Their Crystallinity
Authors: Gotoh Masaru , Suzuki Ken , Miura Hideo
Published: November 2015

Electroplated copper thin films are indispensable for the interconnections in the advanced electronic products, such as TSV (trough silicon via) structures, fine bumps, and thin-film interconnections in various devices and interposers. However, it has been reported that both electrical and mechanical properties of the films vary drastically comparing with those of conventional bulk copper. The main reason for the variation can be attributed to the fluctuation of the crystallinity of grains and grain boundaries in the films. Porous or sparse grain boundaries cause the increase in electrical resistivity and the embrittlement of the films. Thus, the thermal conductivity of the electroplated copper thin films should be varied drastically depending on their micro texture based on Wiedemann-Franz law. Since copper interconnections are used for not only electrical conductor but also thermal heat conductor, it is important to clarify the relationship between the crystallinity and thermal properties of the films.

In this study, the local distributions of the crystallinity and physical properties were investigated experimentally. As the result of the temperature distribution due to local Joule heating along an interconnection, it was suggested that the variation in the quality of the grain boundaries in the electroplated copper thin-films caused the non-uniformity of the resistivity and thus, Joule heating in the thin films. In this study, the effect of the seed layer material on the thermal properties of the electroplated copper thin film was investigated. When a Ru seed layer was deposited as a buffer layer between the electroplated copper thin film and the Ta diffusion barrier layer, both the crystallinity and uniformity of grain boundaries in the electroplated copper films were improved since lattice mismatch between copper and the seed layer metal was decreased. The improvement of the crystallinity increased the long-term reliability of the interconnections under the loads of electromigration and stress-induced migration.
Theoretical and Empirical Energy Impacts of Economization in Data Centers
Authors: Comperchio Dan , Behere Sameer
Published: November 2015

Data center energy consumption can be divided into three broad categories: Information Technology (IT), Electrical, and Mechanical. An efficient data center uses the least amount of non-IT energy, which is typically divided between the mechanical and electrical systems. Mechanical systems generally contribute a large portion of the non-IT energy use by providing cooling from compressor-based equipment [1,2] and because of this, strategies to reduce compressor energy consumption can lead to significant mechanical system energy savings. The most efficient way to reduce compressor energy is through elimination or significant reduction in annual runtime. This is possible with the use of integrated airside or waterside economizers.

This paper demonstrates the impacts of economization in data centers through data collected from four operating facilities over the course of implementing various economizer improvement projects. System architectures include water-cooled centrifugal chiller plant with waterside economization, direct expansion air handling units (AHU) with airside economization, air-cooled centrifugal chillers with integrated waterside economization, and direct expansion computer room air conditioners (CRAC) with evaporative cooling and waterside economization. A systematic and methodical comparison of the baseline and post-conditions is discussed, comparing expected to observed economizer operating conditions. The comparison of multiple real-world scenarios revealed a range of variances in expected operation of economizer sequences to actual observations, indicating a need for close monitoring of system performance by data center operators to fully realize economizer benefits within facilities.
Evaluation of Intensity of Singularity at 3D Piezoelectric Bonded Joints Using a Conservative Integral
Authors: Luangarpa Chonlada , Koguchi Hideo
Published: November 2015

In the present study, a conservative integral based on the Betti reciprocal principle is formulated in order to obtain the intensity of singularity at a vertex of the interface in three-dimensional piezoelectric bi-material bonded joints. To our knowledge, there are few studies on the determination of the intensity of singularity in the three-dimensional piezoelectric bonded joints. In addition, no study on the determination of the intensity of singularity in the 3D piezoelectric bonded joints using the conservative integral has been conducted. Eigenanalysis formulated using a three-dimensional finite element method (FEM) is used to calculate the order of stress singularity, angular variables of mechanical displacements, stresses, electric displacements and electric potential. In order to investigate the influence of an integral area on the accuracy of the results, models with various integral areas are used. The results are compared with those obtained from FEM.
Microstructural Stability of Au-Sn SLID Joints for Harsh Environments
Authors: Soto Paola Rivera , Quintero Pedro O. , Mulero Mellyssa , Ibitayo Dimeji
Published: November 2015

Solid liquid inter diffusion (SLID) is an interconnection technique for electronic packaging, particularly beneficial for high power and harsh environments conditions. It consists of the bonding of two materials with different melting points at a low processing temperature to achieve a high melting point interconnection. The materials investigated in this work are a gold-tin bond attaching a SiC diode to an AlN direct-bond-copper (DBC) substrate. Gold (Au) is the high melting point constituent while the eutectic gold-tin (80 wt.% Au-20 wt.%Sn) offers the low melting point (280°C). This work is aimed at the microstructural evaluation of the joints at different bonding and aging conditions in an effort to get the insights of this interconnection technology from a metallurgical perspective. Four different bonding conditions were used: 315°C-5min, 315°C-10min, 340°C-1min and 340°C-5min; from which a base-line as built condition was assessed by means of metallographical analysis. Furthermore, the samples were aged at 250°C from 1000 to 4000 hours in increments of 1000hrs to study and quantify the microstructural stability and intermetallic (IMC) growth at the interface. This aging experiment has been designed to obtain accelerated information on the kinetics of this reaction so that predictive models can be developed for the real application conditions. The samples were diced, polished and analyzed following standard metallographical techniques; both optical and electronic microscopy (SEM-EDS) was employed. The as-built samples, for the four bonding conditions, presented differences in IMC growth with the thickest layers appearing at the harshest processing conditions. After aging the IMC kept growing and the formation of a new IMC layer was discovered and investigated, furthermore, cracks started to show in some of the samples. It was observed that after 4000 hours some of the cracks extended across the whole interface.
Modeling and Simulation Challenges in Embedded Two Phase Cooling: DARPA’s ICECool Program
Authors: Matin Kaiser , Bar-Cohen Avram , Maurer Joseph J.
Published: November 2015

Modeling and simulation of two-phase phenomena, as well as their impact on electrical performance and physical integrity are critical to the success of embedded cooling strategies. In DARPA’s Intrachip/Interchip Embedded Cooling (ICECool) program, thermal/electrical/mechanical co-simulation and modeling tools are being applied to the analysis and design of RF GaN MMIC (Monolithic Microwave Integrated Circuit) Power Amplifiers (PA) and digital ICs, with the ultimate goal of achieving greater than 3X electronic performance improvement. This paper addresses various simulation strategies and numerical techniques adopted by the DARPA ICECool performers, with attention devoted to co-simulation through coupled iterations of thermal, mechanical and electrical behavior for capturing device characteristics and predicting reliability and “best in class” simulations that can provide an understanding of device behavior during rugged operating conditions impacted by multi-physics environments. The effect of CTE (Coefficient of Thermal Expansion) mismatch on bond and structural integrity, the impact of cooling fluid choice on performance, the factors affecting erosion/corrosion in the microchannels, as well as electro-migration limits and joule heating effects, will also be addressed. A separate discussion of various two-phase issues, including interface tracking, system pressure drops, conjugate heat transfer, estimating near wall heat transfer coefficients, and predicting CHF (Critical Heat Flux) and dryout is also provided.
Local Measurements of Flow Boiling Heat Transfer on Hot Spots in 3D Compatible Radial Microchannels
Authors: Yang Fanghao , Schultz Mark , Parida Pritish , Colgan Evan , Polastre Robert , Dang Bing , Tsang Cornelia , Gaynes Michael , Knickerbocker John , Chainer Timothy
Published: November 2015

Hot spots and temperature non-uniformities are critical thermal characteristics of current high power electronics and future three dimensional (3D) integrated circuits (ICs). Experimental investigation to understand flow boiling heat transfer on hot spots is required for any two-phase cooling configuration targeting these applications. This work investigates hot spot cooling utilizing novel radial microchannels with embedded pin arrays representing through-silicon-via (TSV) interconnects. Inlet orifices were designed to distribute flow in radial channels in a manner that supplies appropriate amounts of coolant to high-power-density cores. Specially designed test vehicles and systems were used to produce non-uniform heat flux profiles with nominally 20 W/cm2 background heating, 200 W/cm2 core heating and up to 21 W/mm2 hot spot (0.2 mm × 0.2 mm) heating to mimic a stackable eight core processor die (20 mm × 20 mm) with two hot spots on each core. The temperatures associated with flow boiling heat transfer at the hot spots were locally measured by resistance temperature detectors (RTDs) integrated between the heat source and sink. At nominal pressure and flow conditions, use of R1234ze in these devices resulted in a maximum hot spot temperature (Ths) of under 63 °C and average Ths of 57 °C at a hot spot power density of 21 W/mm2. A semi-empirical model was used to calculate the equivalent heat transfer rate around the hot spots which can provide a baseline for future studies on local thermal management of hot spots.
Co-Placement for Pin-Fin Based Micro-Fluidically Cooled 3D ICs
Authors: Yang Zhiyuan , Srivastava Ankur
Published: November 2015

3D ICs with through-silicon vias (TSVs) can achieve high performance while exacerbating the problem of heat removal. This necessitates the use of more aggressive cooling solutions such as micropin-fin based fluidic cooling. However, micropin-fin cooling comes with overheads such as non-uniform cooling capacity along the flow direction and restriction on the position of TSVs to where pins exist. 3D gate and TSV placement approaches un-aware of these drawbacks may lead to detrimental effects and even infeasible chip design. In this paper, we present a hierarchical partitioning based algorithm for co-placing gates and TSVs to co-optimize the wire-length and in-layer temperature uniformity, given the logical level netlist and layer assignment of gates. Compared to the wire-length driven gate placement followed by a TSV legalization stage, our approach can achieve up to 75% and 25% reduction of in-layer temperature variation and peak temperature, respectively, with the cost of 13% increase in wire-length.
Numerical and Experimental Characterization of the Transient Effectiveness of a Water to Air Heat Exchanger for Data Center Cooling Systems
Authors: Gao Tianyi , del Valle Marcelo , Ortega Alfonso , Sammakia Bahgat G.
Published: November 2015

The cross flow heat exchanger is at the heart of most cooling systems for data centers. Air/Water or air/refrigerant heat exchangers are the principal component in Central Room Air Conditioning (CRAC) units that condition data room air that is delivered through an underfloor plenum. Liquid/air heat exchangers are also increasingly deployed in close-coupled cooling systems such as rear door heat exchangers, in-row coolers, and overhead coolers. In all cases, the performance of liquid/air heat exchangers in both steady state and transient scenarios are of principal concern. Transient scenarios occur either by the accidental failure of the cooling system or by intentional dynamic control of the cooling system. In either scenario, transient boundary conditions involve time-dependent air or liquid inlet temperatures and mass flow rates that may be coupled in any number of potential combinations. Understanding and characterizing the performance of the heat exchanger in these transient scenarios is of paramount importance for designing better thermal solutions and improving the operational efficiency of existing cooling systems. In this paper, the transient performance of water to air cross flow heat exchangers is studied using numerical modeling and experimental measurements. Experimental measurements in 12 in. × 12 in. heat exchanger cores were performed, in which the liquid (water) mass flow rate or inlet temperature are varied in time following controlled functional forms (step jump, ramp). The experimental data were used to validate a transient numerical model developed with traditional assumptions of space averaging of heat transfer coefficients, and volume averaging of thermal capacitances. The complete numerical model was combined with the transient effectiveness methodology in which the traditional heat exchanger effectiveness approach is extended into a transient domain, and is then used to model the heat exchanger transient response. Different transient scenarios were parametrically studied to develop an understanding of the impact of critical variables such as, the fluid inlet temperature variation and the fluid mass flow rate variation, and a more comprehensive understanding of the characteristics of the transient effectiveness. Agreement between the novel transient effectiveness modeling approach and the experimental measurements enable use of the models as verified predictive design tools. Several studies are designed based on the practical problems related to data center thermal environments and the results are analyzed.
Investigation on Singular Fields at the Vertex of Interface in Piezoelectric Material Joints and its Application
Authors: Koguchi Hideo , Maekawa Tomiya , Luangarpa Chonlada
Published: November 2015

A singular electric flux density field may occur when mechanical compression or electrical load to piezoelectric joints causes a singular electric flux density field is applied in piezoelectric material corners. This means that a large electric displacement occurs arround a singular point. It can be thought that there are many possible applications of this local large electric displacement. Hence, electric singularity characteristics should be investigated to increase the performance of piezoelectric joints.

It is known that the intensity of stress singularity reduces with decreasing the thickness of piezoelectric joints; conversely, the intensity of electric singularity increases under a constant external voltage and load. This implies that there is a relationship between electric singularity and stress singularity. In the present paper, the electric singularity on piezoelectric joints is investigated numerically. Usefulness of piezoelectric joints will then be discussed from the results of analysis.
Convective Heat Transfer Coefficients of Automatic Transmission Fluid Jets With Implications for Electric Machine Thermal Management
Authors: Bennion Kevin , Moreno Gilberto
Published: November 2015

Thermal management for electric machines (motors/generators) is important as the automotive industry continues to transition to more electrically dominant vehicle propulsion systems. Cooling of the electric machine(s) in some electric vehicle traction drive applications is accomplished by impinging automatic transmission fluid (ATF) jets onto the machine’s copper windings. In this study, we provide the results of experiments characterizing the thermal performance of ATF jets on surfaces representative of windings, using Ford’s Mercon LV ATF. Experiments were carried out at various ATF temperatures and jet velocities to quantify the influence of these parameters on heat transfer coefficients. Fluid temperatures were varied from 50°C to 90°C to encompass potential operating temperatures within an automotive transaxle environment. The jet nozzle velocities were varied from 0.5 to 10 m/s. The experimental ATF heat transfer coefficient results provided in this report are a useful resource for understanding factors that influence the performance of ATF-based cooling systems for electric machines.
Local Thermal Measurements of Impinging Liquid Jets With an Angled Confining Wall for Power Electronics Cooling
Authors: Maddox John F. , Knight Roy W. , Bhavnani Sushil H.
Published: November 2015

The local surface temperature, heat flux, heat transfer coefficient, and Nusselt number were measured for an inline array of circular, normal jets of single-phase, liquid water impinging on a copper block with a common outlet for spent flow, and an experimental 2-D surface map was obtained by translating the jet array relative to the sensors. The effects of variation in jet height, jet pitch, confining wall angle, and average jet Reynolds number were investigated. A strong interaction between the effects of the geometric parameters was observed, and a 5° confining wall was seen to be an effective method of managing the spent flow for jet impingement cooling of power electronics. The maximum average heat transfer coefficient of 13,100W=m2K and average Nusselt number of 67.7 were measured at an average jet Reynolds number of 14,000.
Inductive Coupled Plasma Etching of High Aspect Ratio Silicon Carbide Microchannels for Localized Cooling
Authors: Dowling Karen M. , Suria Ateeq J. , Won Yoonjin , Shankar Ashwin , Lee Hyoungsoon , Asheghi Mehdi , Goodson Kenneth E. , Senesky Debbie G.
Published: November 2015

High aspect ratio microchannels using high thermal conductivity materials such as silicon carbide (SiC) have recently been explored to locally cool micro-scale power electronics that are prone to on-chip hot spot generation. Analytical and finite element modeling shows that SiC-based microchannels used for localized cooling should have high aspect ratio features (above 8:1) to obtain heat transfer coefficients (300 to 600 kW/m2·K) required to obtain gallium nitride (GaN) device channel temperatures below 100°C. This work presents experimental results of microfabricating high aspect ratio microchannels in a 4H-SiC substrate using inductively coupled plasma (ICP) etching. Depths of 90 μm and 80 μm were achieved with a 5:1 and 12:1 aspect ratio, respectively. This microfabrication process will enable the integration of microchannels (backside features) with high-power density devices such as GaN-on-SiC based electronics, as well as other SiC-based microfluidic applications.
The Effectiveness of Data Center Overhead Cooling in Steady and Transient Scenarios: Comparison of Downward Flow to a Cold Aisle Versus Upward Flow From a Hot Aisle
Authors: Silva-Llanca Luis , del Valle Marcelo , Ortega Alfonso
Published: November 2015

The most common approach to air cooling of data centers involves the pressurization of the plenum beneath the raised floor and delivery of air flow to racks via perforated floor tiles. This cooling approach is thermodynamically inefficient due in large part to the pressure losses through the tiles. Furthermore, it is difficult to control flow at the aisle and rack level since the flow source is centralized rather than distributed. Distributed cooling systems are more closely coupled to the heat generating racks. In overhead cooling systems, one can distribute flow to distinct aisles by placing the air mover and water cooled heat exchanger directly above an aisle. Two arrangements are possible: (i.) placing the air mover and heat exchanger above the cold aisle and forcing downward flow of cooled air into the cold aisle (Overhead Downward Flow (ODF)), or (ii.) placing the air mover and heat exchanger above the hot aisle and forcing heated air upwards from the hot aisle through the water cooled heat exchanger (Overhead Upward Flow (OUF)). This study focuses on the steady and transient behavior of overhead cooling systems in both ODF and OUF configurations and compares their cooling effectiveness and energy efficiency. The flow and heat transfer inside the servers and heat exchangers are modeled using physics based approaches that result in differential equation based mathematical descriptions. These models are programmed in the MATLAB™ language and embedded within a CFD computational environment (using the commercial code FLUENT™) that computes the steady or instantaneous airflow distribution. The complete computational model is able to simulate the complete flow and thermal field in the airside, the instantaneous temperatures within and pressure drops through the servers, and the instantaneous temperatures within and pressure drops through the overhead cooling system. Instantaneous overall energy consumption (1st Law) and exergy destruction (2nd Law) were used to quantify overall energy efficiency and to identify inefficiencies within the two systems. The server cooling effectiveness, based on an effectiveness-NTU model for the servers, was used to assess the cooling effectiveness of the two overhead cooling approaches.
Perforated Tile Airflow Prediction: A Comparison of RANS CFD, Fast Fluid Dynamics, and Potential Flow Modeling
Authors: Healey Christopher M. , VanGilder James W. , Pardey Zachary M.
Published: November 2015

Fast Fluid Dynamics (FFD), which has its origins in video game and movie animation applications, promises faster solve times than traditional RANS (Reynolds-Averaged Navier Stokes) CFD, is relatively easy to code, and is particularly suited to parallelization. Further, FFD is capable of modeling all relevant airflow physics including momentum, buoyancy and frictional effects which are not included in a standard Potential Flow Model (PFM). The present study is a first attempt to formally evaluate FFD for data center applications in which perforated tile airflow is predicted utilizing two-dimensional plenum models. Comparisons are made to RANS CFD and Potential Flow Modeling (PFM) over a variety of data center configurations based on five basic data center layouts, most of which are based on actual data centers. Results are compared to experimental measurements for one scenario.
Parameter Estimation for Lumped Capacitance Modeling of CRAH Units During Chilled Water Interruption
Authors: Erden Hamza Salih , Khalifa H. Ezzat
Published: November 2015

Lumped capacitance models have been introduced to study transient thermal response of data centers. Chilled water interruption of a Computer Room Air Handling (CRAH) unit is one of several failure scenarios of data center cooling infrastructures. In such a scenario, predicting the transient thermal response of the CRAH unit depends requires the determination of the CRAH lumped capacitance model parameters: the thermal capacitance (thermal mass) and the time constant. In this paper, we propose an experimental methodology to extract sufficient information for the lumped capacitance modeling of CRAH units. The method requires measurements of inlet and exit air temperature, air flow rate and CRAH fan power. If the chilled water supply to a CRAH unit is intentionally interrupted in a data center with multiple redundant CRAH units, sufficient information to estimate the CRAH lumped capacitance parameters can be obtained without disturbing the data center operation.
ALE-FEM for Two-Phase Flows With Heat and Mass Transfer in Microchannels
Authors: Anjos Gustavo R. , Mangiavacchi Norberto , Pontes Jose , Thome John
Published: November 2015

A numerical method is described to study two-phase flows for single and multiple bubbles with phase change. The fluid flow equations are based on the Arbitrary Lagrangian-Eulerian formulation (ALE) and the Finite Element Method (FEM), creating a new two-phase method with an improved model for the liquid-gas interface in microchannels. A successful adaptive mesh update procedure is also described for effective management of the mesh at the two-phase interface to remove, add and repair surface elements, since the computational mesh nodes move according to the flow. The Lagrangian description explicitly defines the two-phase interface position by a set of interconnected nodes which ensures a sharp representation of the boundary, including the role of the surface tension. The methodology proposed for computing the curvature leads to accurate results with moderate programming effort and computational cost and it can also be applied to different configurations with an explicit description of the interface. Such a methodology can be employed to study accurately many problems such as oil extraction and refinement in the petroleum area, design of refrigeration systems, modelling of biological systems and efficient cooling of electronics for computational purposes, being the latter the aim of this research. The obtained numerical results will be described, therefore proving the capability of the proposed new methodology.
Thermodynamic Characterization of a Server Optimized for High Performance Computing Using Water Cooling
Authors: Steinke Mark E. , Kamath Vinod , Parnell Lynn
Published: November 2015

Energy efficiency is an essential element of server design for high performance computers. Traditional HPC servers or nodes that are air cooled enable efficiency by using optimized system design elements that include efficient heat sink design for critical components such as CPUs, Memory, Networking and Disk Subsystems. In addition, airflow optimization is enabled via critical component placement decisions as well as fan and cooling algorithms that have an objective to optimize airflow and maximize system performance. Critical elements that cannot be avoided in traditional air cooled servers are computer center level management of both the airflow requirements and the exhaust heat flux of the servers. An alternative approach shown in this paper uses a novel water cooled design that enables both extreme energy efficiency for heat extraction of the server heat load and allows for lower device operating temperatures for the critical components. Experimental data documented in this paper illustrates the advantages of using non-chilled water to cool the server, allowing 85 to 90 percent of the server heat load to be extracted by water while allowing inlet water temperatures up to 45 degrees Celsius. A comparison is made of the energy consumption needed to cool the server components for both the air cooled and water cooled systems. The base system used for the comparison uses identical system electronics and firmware. The server thermal data shown in the paper include thermal behavior at idle, typical and maximum power consumption states for the server. The data documents the range of boundary conditions that can be tolerated for water cooled server solutions and the comparative advantages of using this technology.
A Computational Study to Compare the Data Center Cooling Energy Spent Using Traditional Air Cooled and Hybrid Water Cooled Servers
Authors: Vallury Aparna , Steinke Mark E. , Kamath Vinod , Parnell Lynn
Published: November 2015

High performance datacenters that are being built and operated to ensure optimized compute density for high performance computing (HPC) workloads are constrained by the requirement to provide adequate cooling for the servers. Traditional methods of cooling dense high power servers using air cooling imposes a large cooling and power burden on datacenters. Airflow optimization of the datacenter is a constraint subject to a high energy penalty when dense power hungry racks each capable of consuming 30 to 40 kW are populated in a dense datacenter environment. The work documented using a simulation model (TileFlow) in this paper demonstrates the challenges associated with a standard air cooled approach in a HPC datacenter. Alternate cooling approaches to traditional air cooling are simulated as a comparison to traditional air cooling. These include models using a heat exchanger assisted rack cooling solution with conventional chilled water and, a direct to node cooling model simulated for the racks.

These three distinct data center models are simulated at varying workloads and the resulting data is presented for typical and maximal inlet temperatures to the racks. For each cooling solution an estimate of the energy spend for the servers is determined based on the estimated PUEs of the cooling solutions chosen.
Eulerian Multiphase Conjugate Model for Chip-Embedded Micro-Channel Flow Boiling
Authors: Parida Pritish R. , Tsuei Hsin-Hua , Chainer Timothy J.
Published: November 2015

The 3D (three dimensional) integration of microelectronic chips into chip stacks is an enabling technology to provide a possible path for increasing computational performance. However, 3D chip stacks require a solution to significant new thermal challenges. As a feasible solution, two-phase cooling utilizing a chip-to-chip interconnect-compatible dielectric fluid can be used. This chip-integrated micrometer scale two-phase cooling technology can be essential to fully optimize the benefits of improved integration density and modularity of 3D stacking of high performance integrated circuits (ICs) for future computing systems; but is faced with significant developmental challenges including high fidelity modeling.

In the present work, an Eulerian multiphase model has been developed for simulating two-phase evaporative cooling through chip embedded microscale cavities. First, the model was used to predict the flow and heat transfer characteristics for coolant R245fa flowing through a single straight micro channel with cross section 100 × 100 um and length 10 mm. The flow is sub-cooled in the initial section of the channel and saturated in the remaining. The results were compared to experimental data available from literature, focusing on the model capability to predict the correct flow pattern, temperature profile and pressure drop. Next, the validated model was extended to the simulation of complex flow geometries expected in microprocessor chip-stacks with chip-to-chip interconnects.
Nondestructive Observation of Thermal Fatigue Crack Propagation in FBGA and Die Attached Solder Joints by Synchrotron Radiation X-Ray Laminography
Authors: Tsuritani Hiroyuki , Sayama Toshihiko , Okamoto Yoshiyuki , Takayanagi Takeshi , Hoshino Masato , Uesugi Kentaro , Hanamura Takuya , Mori Takao
Published: November 2015

The reliability of solder joints on printed circuit boards (PCBs) is significantly affected by thermal fatigue processes due to downsizing and high density packaging in electronic components. Accordingly, there is a strong desire in related industries for development of a new nondestructive inspection technology to detect fatigue cracks appearing in these joints. The authors have applied the SP-μCT, a synchrotron radiation X-ray microtomography system, to the nondestructive observation of such cracks. However, for planar objects such as PCB substrates, reconstruction of CT images is difficult due to insufficient X-ray transmission along the parallel axis of the substrate. In order to solve this problem, a synchrotron radiation X-ray laminography system was developed to overcome the size limits of such specimens. In this work, this system was applied to the three-dimensional, nondestructive observation of thermal fatigue cracks in solder joints, for which X-ray CT inspection has been extremely difficult. The observed specimens included two typical joint structures formed using Sn-3.0Ag-0.5Cu solder: (1) a fine pitch ball grid array (FBGA) joint specimen in which an LSI package is connected to a substrate by solder bumps 360 μm in diameter, and (2) a die-attached specimen in which a 3 mm square ceramic chip is mounted on a substrate. The optical system developed for use in X-ray laminography was constructed to provide a rotation axis with a 30° tilt from the right angle to the X-ray beam, and to obtain X-ray projection images via the beam monitor. The same solder joints were observed successively using the laminography system at beamline BL20XU at SPring-8, the largest synchrotron radiation facility in Japan. In the FBGA type specimen, fatigue cracks were clearly observed to appear at the periphery of the joint interface, and to propagate gradually to the inner regions of the solder bumps as thermal cycling proceeded. In contrast, in the die-attached joint specimen, micro-cracks were observed to appear and propagate through the thin solder layer. An important observation was that these micro-cracks become interconnected prior to propagation of the main fatigue crack. The fatigue crack propagation lifetime was also estimated in both specimens by measuring the crack surface area and calculating the average crack propagation rate through the three-dimensional images. Consequently, the sectional images obtained by the laminography system clearly show the process of crack propagation due to thermal cyclic loading.
Thermal Performance Enhancement of Glass Interposer
Authors: Cho Sangbeom , Joshi Yogendra K.
Published: November 2015

As demands on performance for mobile electronics continue to increase, traditional packaging technology is facing its limit in number of input/outputs (I/Os) and thermal challenges. Glass interposers offer many advantages over previous packaging technology for mobile electronics, including ultra-high electrical resistivity, low loss, and lower cost at processed interposer levels. However, it has two fundamental limitations; brittleness and relatively low thermal conductivity (∼1 W/mK), compared to Si (∼150 W/mK). This paper presents a study on thermal performance enhancement of glass interposer based on thermal modeling, and compares it with silicon interposer. The model captures in-plane and out-of-plane thermal performance enhancement with copper structures incorporated in the interposer. To further study the effect of advanced cooling schemes on interposer technology, an integrated vapor chamber design is evaluated through computational modeling.
Shock Risk Assessment of BGA Components: Importance of Physics Based Metrics
Authors: Rahmani Helia , Vujosevic Milena , Pei Min
Published: November 2015

This study focuses on the implications that the indiscriminate use of empirical shock metrics can have on shock risk assessment of Ball Grid Array (BGA) components in board level tests. Empirical shock metrics discussed are: 1) shock table applied acceleration, 2) top package acceleration and 3) board strain. Computational modeling and fundamental physics considerations were used to prove that the empirical metrics can lead to wrong conclusions about capability of the BGAs and their performance in the field. These empirical metrics can be considered an indirect indicator of solder joint failure, but because there is no universal (setup independent) correlation to solder joint failure, they are not directly transferable. For a metric to be universal it has to have a unique correlation to solder joint damage, i.e. it has to be based on the physics of failure. For the shock failures, such a metric can be solder joint stress or strain. There are two important implications of this study: 1) when indirect metrics are used, the value determined in the system condition should not be directly used as a test condition requirement and 2) BGA capability when measured by indirect metric will vary as a function of board setup. The indirect metrics can still be used, but they must be scaled to account for structural differences among board setups. For that scaling to be valid, it must be rooted in the knowledge of failure physics and the application of physics based metric. The study provides examples on how empirical metrics scaling should be done and discusses the physics behind it.
High Temperature Shear Strength of Cu-Sn Transient Liquid Phase Sintered Interconnects
Authors: Greve Hannes , Moeini S. Ali , McCluskey Patrick , Joshi Shailesh
Published: November 2015

In this paper, we assess the high temperature shear strength of interconnects formed by sinter pastes via a Transient Liquid Phase Sintering (TLPS) process. The joints assessed in this study were formed by the transient liquid phase sintering of Sn and Cu. The shear strength of Cu dies sintered to Cu substrates with Cu-Sn sinter pastes was assessed at temperatures of 200°C and compared to that of specimens attached with Sn3.5Ag solder. In contrast to the shear strength of the sintered specimens, the shear strength of the soldered specimens drops below 10MPa under these conditions. High temperature creep tests were performed at the same temperature level for the soldered and sintered specimens. It was found that the Sn3.5Ag interconnects show extensive creep with short time-to-failure. The joints of sintered specimens did not undergo considerable deformation due to creep. Microstructural analyses have been performed on interconnects formed between Cu dies and Cu substrates and Si diodes on Direct Bond Copper (DBC) substrates. The joint microstructure consists of a matrix of Cu-Sn intermetallic compounds (IMCs) either with or without additional Cu particles dispersed in the matrix. It is demonstrated that the paste-based TLPS interconnects assessed combine short durations for process completion with both high shear strength and high resistance to creep under elevated temperature conditions. This, in combination with high thermal conductivity, shows that transient liquid phase sinter pastes have the potential to be an excellent high temperature and high power interconnect technology.
Physics Based Requirements for Qualification of BGA Components in Temperature Cycling
Authors: Han Ru , Vujosevic Milena , Pei Min
Published: November 2015

This paper discusses a new approach for definition of temperature cycling qualification requirement that accounts for the physics of the deformation process in use condition and in the accelerated temperature cycling test condition. The methodology is used to define solder joint reliability (SJR) requirement for Package on Package (PoP) components. Included in the study is the impact of adhesives on SJR requirements. The approach used is different from standards-based approaches that define the requirements in the way that is often independent of package materials and geometries. Physics based damage metrics and numerical modeling was used to comprehend design, technology, material, and temperature profile and provide an in-depth understanding of package deformation and failure mechanism. This, coupled with a developed fatigue law was then used to translate use conditions to test condition requirement.

The study shows that accelerated test will not accelerate all PoP solder joints equally and that requirements for PoP to board interconnects will be different from requirements for top–to–bottom package interconnects. Similarly, for component with adhesives, when requirements are based on physics, they must be different than requirements for component without adhesive and those requirement should be a function of adhesive thermo-mechanical material properties.

Given rapid changes in technology, explosion of new devices and new use conditions, manufacturers constantly make tradeoffs between performance, cost and reliability. The qualification process needs to be optimized to meet these increasing challenges and qualification based on knowledge of physics presented in this paper is designed to meet these challenges.
Molybdenum Disulfide (MoS2) Nanomechanical Resonators Integrated on Microchannels
Authors: Yang Rui , Li Zhenyu , Feng Philip X.-L.
Published: November 2015

We report on experimental demonstration of multilayer molybdenum disulfide (MoS2) nanomechanical resonators integrated on microchannels, with the potential for resonant operation and sensing applications in microfluidics. Microchannels with width of ∼5μm and depth of ∼1–2μm are fabricated on polydimethylsiloxane (PDMS) substrate. We transfer MoS2 flakes with both singly-clamped cantilever and doubly-clamped membrane structure onto the PDMS channels, and measure both undriven thermomechanical resonances and optically driven responses from the devices. The devices show up to 6 thermomechanical resonances, with highest resonance frequency (fres) of 16.3MHz and quality (Q) factor ∼200. This type of MoS2 nanomechanical resonators, when integrated with microfluidics in microchannels, would make new interesting candidates for biosensing and chemical sensing in fluids.
Mechanism and Model of Bond Formation in Ultrasonic Wire Bonding
Authors: Tszeng T. Calvin
Published: November 2015

Despite being a critical phenomenon of tremendous technological significance in ultrasonic flip-chip and wire bonding processes of today’s microelectronic devices, interfacial bond formation still calls for better understanding at a fundamental level. The goal of the research is to improve these processes through better understanding and modeling of bond formation. This paper presents a micromechanics model that addresses increasing contact area during ultrasonic cyclic loading cycle. The micromechanics model provides interfacial shear stress as boundary condition to FEM simulations of ultrasonic bonding processes. Comparison between preliminary results and experimental data is conducted.
Mechanical Solution for Dealing With Phase Change Material Flow When Mounting Electrical Components
Authors: Grosskreuz Paul J. , Zhou Rui , Bronk Kelly J.
Published: November 2015

A heat sink/electronic component system using a phase change interstitial material (thermal interface material) can be optimized by using an adaptive clamping system that adjusts to the flow of the phase change material as it sets with a rise in temperature. The particular thermal clamping system being evaluated in this white paper is that of a servo motor inverter. This thermal system consists of a standard heat sink-fan forced air cooling system. This concept could be applied to any system requiring a phase change thermal interface material.
Analyzing Temperature Changes in Powered LEDs by Combining Thermal and Radiometric Characterization With Computational Fluid Dynamics
Authors: Marovic Boris , Clark Mathew , Petroski James
Published: November 2015

This paper describes the workflow and improved accuracy of a combined characterization and simulation of an LED luminaire. The achieved measurement results of the thermal and radiometric characterization, the process of implementing these results into a thermal simulation, and the benefit for the luminaire designer are presented.