bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of Three Dimensional Strain on the Electronic Properties of Graphene Nanoribbons
Authors: Yang Meng , Ohnishi Masato , Suzuki Ken , Miura Hideo
Published: November 2015

Graphene has great potential for ultra-sensitive strain sensors applications due to its high mechanical strength and good compatibility with the traditional semiconductor process. In the current study, we investigated the effect of tensile and bending deformations on the electronic states of graphene nanoribbons (GNRs) using density functional theory (DFT) to clarify the underlying mechanism of the piezoresistive properties of graphene. It is found that the electronic structure of armchair graphene nanoribbons (AGNRs) is very sensitive to the tensile deformation. When a uniaxial tensile stress is applied to AGNRs with width Na = 10, the band structure is modified, leading to the change in band gap approximately from 0 eV to 1.0 eV. The band gap values of bent AGNRs decrease significantly when the maximum local dihedral angle exceeds a critical value due to the orbital hybridization. Based on these knowledge, we fabricated a strain sensor using the graphene film grown by thermal chemical vapor deposition (CVD) method on Cu foil. The strain sensor is fabricated directly on the graphene-coated Cu foils by using the standard photolithography process and reactive ion etching (RIE) and then transferred onto a stretchable and flexible polydimethysiloxane (PDMS) substrate. The one-dimensional tensile test and three-dimensional bending test are performed to investigate the piezoresistive properties. A gauge factor 3.4 was achieved under the tensile deformation. The fabricated strain sensor also exhibits good performance to detect bending deformation.
Thermal Spreading Resistance in a Multilayered Orthotropic Circular Disk With Interfacial Resistance and Variable Heat Flux
Authors: Muzychka Y. S.
Published: November 2015

Thermal spreading resistance in a multilayered orthotropic disk is considered. Interfacial resistance between each layer is prescribed by means of a contact conductance hc using a Robin type boundary condition. Orthotropic properties are considered by transforming the orthotropic system into an equivalent isotropic system using stretched coordinates. A recursive modeling approach is presented to account for the effects of two or more layers in the structure from the simple case of a single isotropic layer. This approach simplifies the analysis considerably. Finally, variable heat flux distribution is considered for three special cases: uniform, parabolic, and inverse parabolic. Numerous special cases can be derived from the general result including perfect interfacial contact and perfect sink plane conductance. Additional issues are also discussed in detail. The expressions for the total thermal resistance and spreading resistance can be easily implemented in any mathematical software or coded in Fortran, C, or BASIC. Since the method is strictly analytical, thermal analysts can quickly assess changes in layer properties, material sequence, heat flux distribution, and effects of interfacial contact resistance, with little extra effort.
The Impact of Interfacial Layers on the Thermal Boundary Resistance and Residual Stress in GaN on Si Epitaxial Layers
Authors: Yates Luke , Bougher Thomas L. , Beechem Thomas , Cola Baratunde A. , Graham Samuel
Published: November 2015

The development of gallium nitride (GaN) on silicon (Si) substrates is a critical technology for potential low cost power electronics. These devices can accommodate faster switching speeds, hotter temperatures, and high voltages needed for power electronics applications. However, the lattice mismatch and difference in crystal structure between 111 Si and c-axis hexagonal GaN requires the use of buffer layers in order to grow device quality epitaxial layers. For lateral high electron mobility transistors, these interfacial layers act as a potential source of increased thermal boundary resistance (TBR) which impedes heat flow out of the GaN on Si devices. In addition, these interfacial layers impact the growth and residual stress in the GaN epitaxial layer which can play a role in device reliability. In this work we use optical methods to experimentally measure a relatively low TBR for GaN on Si with an intermediate buffer layer to be 3.8 ± 0.4 m2K/GW. The effective TBR of a material stack that encompasses GaN on Si with a superlattice (SL) buffer is also measured, and is found to be 107 ± 1 m2K/GW. In addition the residual state of strain in the GaN layer is measured for both samples, and is found to vary significantly between them. Thermal conductivity of a 0.8μm GaN layer on AlN buffer is determined to be 126 ± 25 W/m-K, while a 0.84 μm GaN layer with C-doping on a SL structure is determined to be 112 ± 29 W/m-K.
MD Evaluation of the IR Spectra of DNA Bases in the Process of Transport Through Graphene Nanopore
Authors: Kitani Takumu , Zolotoukhina Tatiana
Published: November 2015

Development of the 1D and 2D IR spectroscopy of small organic molecules and clusters opens yet another way of possible identification of small organic molecules in the state of motion in the graphene nanopore scanning device. With the advantage of obtaining qualitative and at least semi-quantitative information of specimens real-time and non-invasively, vibrational spectroscopy techniques, infrared (IR) and Raman have become more and more important in the analysis of biomolecular samples. At present, the sensitivity and spatial resolution of these techniques stands at the challenge of the detection and analysis of biosamples at very low concentration (single molecule) and high spatial resolution (nanometer/sub-nanometer scale). Spectral analysis requires theoretical assignment of vibrational modes to each biomolecule.

We considered vibrational spectra of DNA nucleobase at the time when they are translocated through the graphene nanopore. The Fourier transform of the density of states (DOS) of each base was calculated and the spectra of the base molecules and C atoms of graphene pore edge were obtained. Translocation rate was fixed to have maximum interaction of the base with 1.5 nm pore and single orientation of nucleobases was evaluated relative to molecular plane. Whether interaction of nucleobase and nanopore is able to enhance the signal is still remains unanswered. But we have shown that the spectra of each nucleobase are different and can be considered the fingerprint of the particular molecule.

The interaction forces between pore and base are structure dependent and time-limited by translocation time. In such case, transient correlation functions were utilized for the DOSes of the individual bases and forces on each atom of the particular base were sorted by intensity. The spectra of individual atoms in the bases as well as of whole molecule were compared and frequencies of most intense peaks were related to particular atoms. Molecular dynamics method is used for the DNA base and graphene nanopore calculations with the MM2/MM3 potentials for the base and REBO graphene potential. Interaction potential between the bases can simultaneously give additional information for the electronic transport calculations with possible tra and graphene are of the MM2/MM3 part of the Van der Waals interaction only has been considered. Possibility of base identification by spectral signature is confirmed. Calculated spectra are compared with results of the existing IR measurements for nucleobases.
On Simple Prediction Method for Thermal Contact Resistance Between Wavy Surfaces With Thermal Interface Material Under Low Mean Nominal Contact Pressure (Fundamental Study Based on 1-D Model)
Authors: Tomimura Toshio , Koito Yasushi , Do Taewan , Ishizuka Masaru , Hatakeyama Tomoyuki
Published: November 2015

The thermal contact resistance (TCR) is the crucial issue in the field of heat removal from systems like electronic equipment, satellite thermal control systems, and so on. To cope with the problem, a lot of studies have been done mainly for flat rough surfaces. However, as pointed out so far, there are still wide discrepancies among measured and predicted TCRs, even for similar materials. To investigate the key factors for the abovementioned discrepancies, a fundamental analysis was conducted in our previous study [1] using a simple contact surface model, which was composed of the unit cell model proposed by Tachibana [2] and Sanokawa [3]. Furthermore, by introducing a 2-D microscopic surface model, which consists of random numbers and Abbott’s bearing area curve, the effects of surface waviness and roughness on the temperature fields near the contact interface have been investigated microscopically [4]. In this study, based on a 1-D wavy surface model, a fundamental study has been conducted to predict TCR and the thermal contact conductance (TCC), which is a reciprocal of TCR, between wavy surfaces with the thermal interface material (TIM) under a relatively low mean nominal contact pressure of 0.1–1.0 MPa. From comparison between the calculated and measured results, it has been shown that, in spite of a simple 1-D analysis, the present model predicts the temperature drop at the contact interface, which is obtained as the product of TCR and the heat rate flowing through TIM, within some 10 to 60% error for a TIM with the thermal conductivity of 2.3 W/(m·K) and the initial thickness of 0.5, 1 and 2 mm.
A Numerical Study of Pulsating Heat Pipe Performance
Authors: d’Entremont Brian P. , Thome John R.
Published: November 2015

A pulsating heat pipe (PHP), also known as an oscillating heat pipe (OHP), is a passive thermal transport device which consists of a single meandering microchannel making multiple passes each through an evaporator and condenser. With a sufficient number of such passes, intermittent boiling of liquid slugs within each evaporator pass perturbs flow in adjacent channels leaving the device in a perpetually unstable state of oscillation. A PHP is thus distinguished operationally from a loop thermosyphon by having a motive force other than buoyancy and the ability to operate in all gravitational orientations.

The most successful PHP models to date track liquid slug motion, sensible heating of the slugs, and mass transfer between liquid slugs and vapor plugs due to evaporation and condensation. However, the predictive capabilities of PHP models remain poor and the numbers assigned to evaporation and condensation heat transfer coefficients are generally not well justified by any realistic physical process. The current study applies methods consistent with state of the art prediction methods in microchannel boiling, to obtain results which predict the PHP’s heat transfer performance and the effect of gravitational orientation on performance.
Load Capacity and Thermal Efficiency Optimization of a Research Data Center Using Computational Modeling
Authors: Schaadt Joseph R. H. , Fouladi Kamran , Wemhoff Aaron P. , Pigeon Joseph G.
Published: November 2015

Data centers are most commonly cooled by air delivered to electronic equipment from centralized cooling systems. The research presented here is motivated by the need for strategies to improve and optimize the load capacity and thermal efficiency of data centers by using computational fluid dynamics (CFD). Here, CFD is used to model and optimize the Villanova Steel Orca Research Center (VSORC). VSORC, presently in the design stages, will provide a testing environment as well as the capability to investigate best practices and state of the art strategies including hybrid cooling, IT load distribution, density zones, and hot aisle and cold aisle containment. The results of this study will be used in the overall design and construction of the aforementioned research data center. The objective of this study is to find the optimal operating points and design layout of a data center while still meeting certain design constraints. A focus is on finding both the ideal total supply flow rate of the air conditioning units and the ideal chilled water supply temperature (CHWST) setpoint under different data center design configurations and load capacities. The total supply flow rate of the air conditioning units and the supply temperature setpoint of the chilled water system are varied as design parameters in order to systematically determine the optimal operating points. The study also examines the influence of hot aisle and cold aisle containment strategies in full containment, half containment, and no containment configurations on the determined optimal operating conditions for the modeled research data center.
Data Center Optimization Using Performance Metrics
Authors: Seymour Mark J. , Herrlin Magnus K.
Published: November 2015

Data centers are rapidly growing in size and number and consume an increasing and also significant proportion of energy production. Yet, their mission critical nature means they are constructed and operated at almost any cost. The data center industry is becoming more aware of the need to manage energy alongside managing capacity and availability. A vital element — and one that cannot be ignored — is how well cooling is delivered to IT equipment. Monitoring allows you to understand how well you are currently operating and whether you are within acceptable bounds. Meanwhile, simulation of the airflow and heat transfer allows you to predict future performance and understand current and future cooling issues. The challenge with both approaches is that they provide large volumes of data and interpretation can become a challenging task. Consider two scenarios:

i. One configuration results in a lot of ‘hot spots’ in the data center, resulting in equipment permanently operating in the ASHRAE Allowable range for intake air temperature.

ii. A second configuration has one ‘hot spot’ where equipment is operating above the ASHRAE Allowable range, but the remainder is within the ASHRAE Recommended range.

Which is better? One solution is to use metrics to help understand the performance. However, existing metrics are not always well known and understood. The only metric that is currently in common use is Power Usage Effectiveness (PUE). While PUE is useful as a measure of data center cooling efficiency, it does not address the cool air delivery effectiveness within the IT space. However, high-level cooling delivery metrics such as Rack Cooling Index (RCI), Return Temperature Index (RTI), Supply Heat Index (SHI), and Return Heat Index (RHI) have been recognized for producing key information but are not as widely used as they could be. Other metrics have been developed that give more detailed understanding of delivery performance. Capture Index, or more specifically Cold Aisle Capture Index (CACI) and Hot Aisle Capture Index (HACI) provide a measure of whether cooling systems targeting specific equipment work effectively. Simulation also allows diagnostic performance ‘measurement’ with detailed indices such as Rack and Room Recirculation. Since data center airflow is complex, this paper uses case studies to show how using metrics provides a rapid insight into both performance and what might need to be addressed to improve and optimize performance.
Data Center Cooling Efficiency With Simulation-Based Optimization
Authors: Billet Laurent M. , Healey Christopher M. , VanGilder James W. , Pardey Zachary M.
Published: November 2015

The efficient control of cooling for data centers is an issue of broad economic importance due to the significant energy consumption of data centers. Many solutions attempt to optimize the control of the cooling equipment with temperature, pressure, or airflow sensors. We propose a simulation-based approach to optimize the cooling energy consumption and show how this approach can be implemented with simple power-consumption models. We also provide a real-life case study to demonstrate how energy saving cooling setpoints can be found using calibrated simulations and smooth metamodels of the system.
Thermal Boundary Conductance at Metal-Graphene-Metal Interfaces Using Time-Domain Thermoreflectance Method
Authors: Brown David B. , Bougher Thomas L. , Cola Baratunde A. , Kumar Satish
Published: November 2015

Thermal boundary conductance (TBC) at Ti-graphene-Ti and Au-graphene-Au interfaces was measured using time-domain thermoreflectance (TDTR). Graphene grown on Cu foil by chemical vapor deposition was transferred to metallized Si substrates and TDTR was performed following the deposition of a top metal contact to form the metal-graphene-metal sandwiched structure. The results show that TBC was reduced at Au-graphene-Au interface in comparison to Au-Au interface and the value was similar to TBC measured at Au-HOPG interface. TBC at Ti-graphene-Ti interface was measured to be in the same range as Ti-Ti interface. This result is believed to be due to formation of oxide on surface of Ti, but further investigation is required. To the best of our knowledge, this is the first experimental study of TBC in metal-graphene-metal architecture.
Quantification of Phase Change Material Energy Storage Capability Using Multiphysics Simulations
Authors: O’Connor W. E. , Wemhoff A. P.
Published: November 2015

Organic phase change materials (PCMs) such as paraffins or unsaturated acids use the latent heat of melting for thermal energy storage as a passive cooling mechanism for portable electronics. Researchers have suggested that a PCM’s thermal energy storage capability is linked to its thermal properties, yet this connection has not yet been quantified. This study first uses group theory and known values from literature to obtain the thermophysical properties for a variety of paraffins and unsaturated acids. Then, multiphysics-based finite element analysis (FEA) is applied to determine the influence of these thermophysical properties on the PCM latent heat storage capability for a side heating configuration. The FEA models include melting and re-solidification, natural convection, conduction, and the monitoring of input and output periodic heat fluxes. The phase change was achieved through application of temperature-dependent viscosity and heat capacity relations. The thermal energy storage efficiency is defined as one minus the ratio of integrated output heat flux to the integrated input heat flux. The FEA results are used to provide predictions of thermal energy storage for a variety of PCMs for various aspect ratios under different heating conditions. Insights are gained in relating thermal storage efficiency to the system configuration.
Investigation of Liquid Metal Ohmic Contacts for Graphene Photonic Devices
Authors: Ordonez Richard C. , Acosta Noah , Melcher Jordan , Kamin Nackieb , Garmire David
Published: November 2015

We demonstrate the first contact resistance measurements of graphene–galinstan (g-g) ohmic contacts in an effort to improve the performance of graphene photonic devices. The nobility of carbon materials provide an interesting graphene sensor application to explore an oxidation free liquid metal - semimetal interface that can be used to lower contact resistance at source/drain terminals of a standard graphene phototransistor. Our methods utilize photopolymerization of the reactive monomer Trimethlylolpropane Triacrylate (TMPTA) in order to fabricate micro structures necessary to overlay liquid metal contacts on graphene. With the use of an industry standard transfer length method (TLM), a contact resistance of −124±28Ω was measured at both standard temperature and pressure. The results from our study suggest that liquid metals such as galinstan are comparable alternatives to rigid semiconductor interfaces and demonstrates interesting boundary characteristics that may lead to heavy chemical doping and associated low resistance contacts that are required to increase sensitivity in graphene photonic devices.
Experimental Validation of a Detailed Phase Change Model on a Hardware Testbed
Authors: De Vivero Charlie , Kaplan Fulya , Coskun Ayse K.
Published: November 2015

Continued CMOS scaling accompanied with a stall in the voltage scaling has led to high on-chip power densities. High on-chip power densities elevate the temperatures, substantially limiting the performance and reliability of computing systems. The use of Phase Change Materials (PCMs)1 has been explored as a passive cooling method to manage excessive chip temperatures. The thermal properties of PCMs allow a large amount of heat to be stored at near-constant temperature during the phase transition. This heat storage capability of PCM can be leveraged during periods of intense computation. For systems with PCM, development of new management strategies is essential to maximize the benefits of PCM. In order to design and evaluate these management strategies, it is necessary to have an accurate PCM thermal model. In our recent work, we proposed a detailed phase change thermal model, which we integrated into a compact thermal simulation tool, HotSpot. In this paper, we build a hardware testbed incorporating a PCM unit on top of the chip package. We then validate the accuracy of our previously proposed thermal model by comparing the HotSpot simulation results against the measurements on the testbed. We observe that the error between the measured and simulated temperatures is less than 4°C with 0.65 probability. Finally, we implement a soft PCM capacity sensor that monitors the remaining PCM latent heat capacity to be used for development of thermal management policies. We evaluate a set of thermal management policies on the testbed. We compare policies that adjust the sprinting frequency based on current temperature against the policies that take action based on the remaining PCM capacity.
RFIC Transformer With 12x Size Reduction and 15x Performance Enhancement by Self-Rolled-Up Membrane Nanotechnology
Authors: Huang Wen , Li Moyang , Gong Songbin , Li Xiuling
Published: November 2015

Two types of on-chip RFIC transformers based on CMOS compatible strain-induced self-rolled-up membrane (S-RuM) nanotechnology, with extremely small footprint, are demonstrated. The rolled-up transformers, with their 3D tubular form factors, dramatically reduce the substrate parasitic effects and push the maximum working frequency into millimeter wave bands with a coupling coefficient, k, as high as 0.92. The 3D stand-up nature also allows the tube transformers to be less susceptible to residue stress in the substrate and thus compatible with flexible platforms for wearable RF applications. The demonstrated samples with a turn ratio, n, of 5.5:1 only occupies 805 μm2 on-chip area (s) which is 12x smaller than that of the best planar transformer with the same turn ratio, and its figure of merit n·k/s, is therefore ∼ 6046/mm2, enhanced by 15x.
Characterization of Carbon Nanotube Forest Interfaces Using Time Domain Thermoreflectance
Authors: Bougher Thomas L. , Taphouse John H. , Cola Baratunde A.
Published: November 2015

Forests comprised of nominally vertically aligned carbon nanotubes (CNTs) are excellent candidates for thermal interface materials (TIMs) due to their theoretically predicted outstanding thermal and mechanical properties. Unfortunately, due to challenges in the synthesis and characterization of these materials reports of the thermal conductivity and thermal contact resistance of CNT forests have varied widely and typically fallen far short of theoretical predictions. In particular, the micro- and nano-length scales characteristic of the heat transfer in CNT forests pose significant challenges and may lead to misreported results. Here we examine the ability of a popular and well-established thermal metrology technique, time-domain thermoreflectance (TDTR), to resolve the properties of CNT forest TIMs. The characteristic heating frequencies of TDTR (1–10 MHz) are used to probe heat transfer at length scales spanning ∼0.1–1 μm, applicable for measuring the contact resistance between the CNT forest free tips and an opposing substrate. We identify the range of CNT forest-opposing substrate interface resistances that can be resolved with TDTR, and simultaneously demonstrate the effectiveness of several processes developed to reduce the resistance of these interfaces. The limitations of characterizing CNT forests with TDTR are discussed in terms of uncertainty and sensitivity to parameters of interest.
Fabrication, Packaging, and Catheter Assembly of 2D CMUT Arrays for Endoscopic Ultrasound and Cardiac Imaging
Authors: Moini Azadeh , Nikoozadeh Amin , Choe Jung Woo , Khuri-Yakub Butrus T. , Chang Chienliu , Stephens Doug , Smith L. Scott , Sahn David
Published: November 2015

Ultrasound is increasingly in demand as a medical imaging tool and can be particularly beneficial in the field of intracardiac echocardiography (ICE). However, many challenges remain in the development of a 3D ultrasound imaging system.

We have designed and fabricated a quad-ring capacitive micromachined ultrasound transducer (CMUT) for real-time, volumetric medical imaging. Each CMUT array is composed of four concentric, independent ring arrays, each operating at a different frequency, with 128 elements per ring. In this project, one ring will be used for imaging. A large (5mm diameter) lumen is available for delivering other devices, including high intensity focused ultrasound transducers for therapeutic applications or optical fibers for photoacoustic imaging.

We address several challenges in developing a 3D imaging system. Through wafer vias are incorporated in the fabrication process for producing 2D CMUT arrays. Device integration with electronics is achieved through solder bumping the arrays, designing a flexible PCB, and flip chip bonding CMUT and ASICs to the flexible substrate. Finally, we describe a method for integrating the flex assembly into a catheter shaft. The package, once assembled, will be used for in-vivo open chest experiments.
Experimental Characterization of Underfill Materials Exposed to Moisture Including Preconditioning
Authors: Chowdhury Promod R. , Chhanda Nusrat J. , Suhling Jeffrey C. , Lall Pradeep
Published: November 2015

Microelectronic encapsulants exhibit evolving properties that change significantly with environmental exposures such as isothermal aging and high humidity conditions. In this work, the material behavior changes occurring in underfill materials subjected to moisture exposures in an humidity chamber have been characterized using 60 × 3 × 0.5 mm uniaxial test specimens which were cured with production equipment using the same conditions as those used in actual flip chip assembly. After curing, the samples were divided into two groups and subjected to different preconditioning: (1) no preconditioning, (2) prebaking at 85 C for 24 hours. The fabricated and preconditioned uniaxial test specimens were then exposed in an adjustable thermal and humidity chamber to combined hygrothermal exposures at 85 C and 85% RH for various durations (0, 1, 3, 10, 30, 60 days). After the moisture exposures, a microscale tension-torsion testing machine was used to evaluate the complete stress-strain behavior of the material at room temperature (25 C). In addition, the viscoelastic mechanical response of the underfill encapsulant has also been characterized via creep testing at room temperature for several applied stress levels after the moisture exposures. From the recorded results, it was found that the moisture exposures strongly degrade the mechanical properties of the tested underfill including the initial elastic modulus, ultimate tensile stress, and tensile creep rate. Prebaking was found to increase the initial material properties, but the degradations due to subsequent moisture exposures occurred in a similar manner.
Measurement and Simulation of Moisture Induced Die Stresses in Flip Chip on Laminate Assemblies
Authors: Nguyen Quang , Roberts Jordan C. , Suhling Jeffrey C. , Jaeger Richard C. , Lall Pradeep
Published: November 2015

In this work, an investigation has been performed on hygrothermally induced die stresses in flip chip assemblies caused by moisture absorption by the underfill encapsulant. Silicon test chips were first applied to perform a variety of measurements of moisture and thermally induced die stresses in flip chip on laminate assemblies. The sample die stresses were first measured after underfill encapsulation and cure, and then subsequently after long term storage (10 years) at room temperature and ambient humidity. The assemblies were then exposed to and 85 °C and 85% RH high humidity harsh environment for various durations, and the die stresses were evaluated as a function of the exposure time. Finally, reversibility tests were conducted to see whether the effects of moisture uptake were permanent. After long term storage, the experimental measurements showed that the normal stresses in the flip chip die relaxed significantly, while the shear stresses exhibited only small variations. In addition, the 85/85 hygrothermal exposure had strong effects, generating tensile die normal stress changes of up to 30 MPa in the flip chip assemblies. Thus, the initial compressive die normal stresses due to flip chip assembly were found to relax significantly during the moisture exposure. Upon fully redrying, it was observed that the moisture-induced stress changes were fully recovered.

The results of the experimental measurements were subsequently correlated with predictions from finite element numerical simulations. When performing moisture diffusion modeling, the conventional method is to use a thermal analogy based on the similarity of governing equations of heat transfer and moisture diffusion. However, this method has some drawbacks including giving incorrect results when dealing with time- and temperature-dependent problems or discontinuities in the moisture concentrations at material boundaries. In this study, we have used a new feature in ANSYS v14 to perform coupled multi-physics simulations of the moisture diffusion process without the aforementioned limitations. The simulation results were found to show strong correlations with experimental measurements.
Characterization of Die Stresses in Plastic Packages Subjected to Moisture and Thermal Exposures
Authors: Nguyen Quang , Rahim M. Kaysar , Roberts Jordan C. , Suhling Jeffrey C. , Jaeger Richard C.
Published: November 2015

Stress sensing test chips are a powerful tool for measuring in-situ stresses in electronic packages. In this study, we have applied (111) silicon test chips to perform a variety of measurements of die stresses in plastic packages. In particular, stresses were characterized in 240 pin Quad Flat Packs (QFPs) subjected to various thermal and moisture loadings. The utilized 10 × 10 mm sensor chips incorporated optimized eight-element piezoresistive rosettes that were capable of measuring the complete state of stress at the die surface (including the interfacial shear stresses).

The fabricated test chips were initially used to measure die stresses in the QFPs after molding and post mold bake. Measurement results were correlated with finite element simulations of the molding process. Subsequently, the effects of thermal cycling on the measured die stress distributions for selected packages were investigated. After these initial measurements, the samples were stored at room temperature and ambient humidity for 17 years. The samples were then re-measured after this long term storage to evaluate the degree of die stress relaxation that had occurred. Several packages were then exposed to a harsh high temperature and high humidity environment (85 C, 85% RH) for various time durations, and allowed to absorb moisture. The die stresses at several locations were characterized as a function of time during the hygrothermal exposure. The weight variations in each sample were also measured during the 85/85 exposure to gauge the moisture uptake, and reversibility tests were conducted to see whether the effects of moisture uptake were permanent. Using these measurements and numerical simulations, valuable insight has been gained on moisture induced failure phenomena in plastic packages.

Good agreement was found between the predicted and measured die normal stress distributions occurring after molding of the QFP. The magnitudes of the in-plane normal and shear stresses were found to have decreased by up to 30% after moderate levels of thermal cycling. After long term storage, the experimental measurements showed that the die normal stresses in the QFPs relaxed significantly (up to 40%), while the die shear stresses exhibited only small variations. In addition, the 85/85 hygrothermal exposures had strong effects, generating tensile die normal stress changes of up to 130 MPa. Upon fully redrying in reversibility tests, it was observed that the moisture-induced normal stress changes were not recovered. Good correlations were observed between the variations of sample weight (increases in moisture content) and the variations of the die normal and shear stress changes.
Chlorine-Ion Related Corrosion in Cu-Al Wirebond Microelectronic Packages
Authors: Lall Pradeep , Luo Yihua , Nguyen Luu
Published: November 2015

The increasing price of gold has resulted in industry interest in use of copper as alternative wire bonds interconnect material. Copper wire has the advantages of the lower cost, lower thermal resistivity, lower electrical resistivity, higher mechanical strength and higher deformation stability over the gold wire. In spite of the upside above, the Cu-Al wire bond is susceptible to the electrolytic corrosion and the reliability of Cu-Al wire bond is of great concern. Typical electronic molding compounds are hydrophilic and absorb moisture when exposed to humid environmental conditions. EMC contain ionic contaminants including chloride ions as a result of the chemical synthesis of the subcomponents of the resin, etching of metallization and the decomposition of the die-attach glue. The presence of moisture in the operating environment of semiconductor package makes the ion more mobile in the EMC. The migration of chloride ions to the Cu-Al interface may induce electrolytic corrosion inside the package causing degradation of the bond interface resulting in eventual failure. The rate at which the corrosion happens in the microelectronic packages is dependent upon the rate at which the ions transport through the EMC in addition to the reaction rate at the interface. In this effort, a multiphysics model for electrolytic corrosion in the presence of chloride has been presented. The contaminant diffusion along with the corrosion kinetics has been modeled. In addition, contaminated samples with known concentration of KCl contaminant have been subjected to the temperature humidity conditions (130°C/100RH)The resistance of the Cu-Al interconnects in the PARR test have been monitored periodically using resistance spectroscopy. The diffusion coefficients of chloride ion at various temperatures in the molding compound have been measured using inductively coupled plasma. Measured diffusion coefficients have been incorporated into the COMSOL multiphysics model. Moisture ingress into the EMC has been quantified through measurements of the weight gain in the EMC. Predictions from the COMSOL multiphysics model have been correlated with experimental data.
Evaluation of Cooling Technologies for Xeon Phi™ Based High Performance Computing Clusters
Authors: Sarangi Suchismita , Kuhn Will A. , Rider Scott , Wright Claude , Krishnan Shankar
Published: November 2015

Efficient and compact cooling technologies play a pivotal role in determining the performance of high performance computing devices when used with highly parallel workloads in supercomputers. The present work deals with evaluation of different cooling technologies and elucidating their impact on the power, performance, and thermal management of Intel® Xeon Phi™ coprocessors. The scope of the study is to demonstrate enhanced cooling capabilities beyond today’s fan-driven air-cooling for use in high performance computing (HPC) technology, thereby improving the overall Performance per Watt in datacenters. The various cooling technologies evaluated for the present study include air-cooling, liquid-cooling and two-phase immersion-cooling. Air-cooling is evaluated by providing controlled airflow to a cluster of eight 300 W Xeon Phi coprocessors (7120P). For liquid-cooling, two different cold plate technologies are evaluated, viz, Formed tube cold pates and Microchannel based cold plates. Liquidcooling with water as working fluid, is evaluated on single Xeon Phi coprocessors, using inlet conditions in accordance with ASHRAE W2 and W3 class liquid cooled datacenter baselines. For immersion-cooling, a cluster of multiple Xeon Phi coprocessors is evaluated, with three different types of Integrated Heat Spreaders (IHS), viz., bare IHS, IHS with a Boiling Enhancement Coating (BEC) and IHS with BEC coated pin-fins. The entire cluster is immersed in a pool of Novec 649 (3M fluid, boiling point 49 °C at 1 atm), with polycarbonate spacers used to reduce the volume of fluid required, to achieve target fluid/power density of ∼ 3 L/kW. Flow visualization is performed to provide further insight into the boiling behavior during the immersion-cooling process.

Performance per Watt of the Xeon Phi coprocessors is characterized as a function of the cooling technologies using several HPC workloads benchmark run at constant frequency, such as the Intel proprietary Power Thermal Utility (PTU), and industry standard HPC benchmarks LINPACK, DGEMM, SGEMM and STREAM. The major parameters measured by sensors on the coprocessor include total power to the coprocessor, CPU temperature, and memory temperature, while the calculated outputs of interest also include the performance per watt and equivalent thermal resistance. As expected, it is observed that both liquid and immersion cooling show improved performance per Watt and lower CPU temperature compared to air-cooling. In addition to elucidating the performance/watt improvement, this work reports on the relationship of cooling technologies on total power consumed by the Xeon-Phi card as a function of coolant inlet temperatures. Further, the paper discusses form-factor advantages to liquid and immersion cooling and compares technologies on a common platform. Finally, the paper concludes by discussing datacenter optimization for cooling in the context of leakage power control for Xeon-Phi coprocessors.
Thermal Evaluation Process on Smart Watches
Authors: Han Angel Qian , Kang Nanbo
Published: November 2015

A smart watch is one of the most popular wearable devices now. The end user cares about the electronic performance and safety. Along with the battery life and security, thermal safety is the most common concern. We show how to meet the ergonomic standard of user, predict the thermal performance on the typical sceneries. A thermal simulation software Flotherm or Icepack can be applied to the design stage.
CMUTs in Permanent Contact Operation for High Output Pressure
Authors: Ho Min-Chieh , Hoffmann Maik , Unger Alexander , Park Kwan Kyu , Kupnik Mario , Khuri-Yakub Butrus T.
Published: November 2015

We present the operation of capacitive micromachined ultrasonic transducers (CMUTs) in permanent contact mode as an efficient transducer. The gap height of our transducers is chosen to be slightly smaller than the static deflection of the plate due to the pressure difference between the ambient and the vacuum cavity. Thus, the plates are in contact with the bottom of the cavities even with no dc bias applied. The devices were fabricated based on the thick box process. High-temperature assisted direct wafer bonding technique was used to fabricate devices with such large cell size (radii ∼ 2000 μm) featuring low frequencies ∼100–150 kHz. Extensive acoustic characterization was performed to demonstrate the behavior of such CMUTs in terms of displacement profile, output pressure and acoustic pitch-catch response. A maximum sound pressure of ∼145 dB (SPL) at the transducer surface is measured at 240 V dc and 10 V ac with 100 cycles of burst signal. This is a great improvement from conventional CMUTs (with deeper gap height, operating at 55 kHz), which requires 350 V dc and 200 V ac in order to achieve an output pressure of 129 dB (SPL) at the transducer surface. The results presented in this paper demonstrate that operating CMUTs in permanent contact mode indeed enhances the device output pressure, and provides a good candidate for efficient ultrasonic transducers.
Integrate Monolithic Nanostructures in Microchannels to Enhance Flow Boiling Heat Transfer of HFE-7000
Authors: Yang Fanghao , Li Xiaochuan , Li Wenming , Li Chen
Published: November 2015

Two-phase microchannel heat sink is promising in cooling high power electronics with dielectric fluids. Compared to water, dielectric fluids can assure system safety in case of working fluid leakage. However, two-phase heat transfer of these hydrofluorocarbon refrigerants is restricted by their relatively low thermal conductivities and low latent heats. Numerous nanoscale/submicron structures have been developed to enhance the single and two-phase heat transfer in microchannels; but these techniques usually require nanoparticle seeds in multi-step wet processes or nanolithography to integrate these nanostructures. Therefore, most of these techniques were time-consuming and costly. In this study, we present a plasma etching method using a modified Bosch process to create silicon tips with nanoscale scallops in microchannels. This is a rapid and cost-effective method to integrate large density of nucleation sites without involving nanolithography method or using nanoparticle seeds. Then, these silicon tip arrays were aligned with side walls of microchannels. As a result, flow boiling heat transfer of a dielectric refrigerant, HFE-7000, is substantially enhanced in a microchannel heat sink (five parallel channels: 10 mm L × 220 μm W × 250 μm H). Compared to plain-wall microchannels, the average junction temperature can be reduced up to 10 °C at a heat flux of 55 W/cm2 and the equivalent thermal resistance of microchannel heat sink is reduced up to 31% at a mass flux of 1018 kg/m2·s.
Singulation by Plasma Etching: Integration Techniques to Enable Low Damage, High Productivity Dicing
Authors: Barnett Richard , Thomas Dave , Ansell Oliver
Published: November 2015

Plasma dicing has rapidly gained traction as a viable alternative to conventional blade and laser techniques for wafer singulation. This has been due mostly to the significant benefits plasma dicing delivers in relation to the quality and reliability of devices singulated in this manner.

Key to the successful integration of plasma dicing, into the established hierarchy of singulation techniques, is how the ancillary parts of the process flow can be utilized or adapted to accommodate it. More importantly, is the ease at which this can happen and also, how implementation can be achieved in a cost effective manner.
Develop an Excel-Based Modeling Tool to Predict Weld and HAZ Cooling Rate and Hardness for Pipeline Welding
Authors: Yang Yu-Ping , Cao Zhenning , Gould Jerry , McGaughy Tom , Jennings Jon
Published: November 2015

A Microsoft Excel-based screening tool was developed to allow an engineer with weld process knowledge to predict cooling rate and hardness during welding procedure qualifications to screen a combination of materials and welding process parameters quickly to meet requirements of fabrication and design codes. The material properties for commonly used pipeline steels have been built into a database coupled with the screening tool. The Excel-based tool includes a physics-based laser and arc welding solution which was developed based on Rosenthal’s mathematical equations for a point heat source to predict thermal cycles by inputting welding parameters. A reflecting heat source scheme was adapted to model the boundary conditions and plate thickness effect on cooling rate. The Excel-based tool also includes a microstructure model which was developed based on the Ashby model. The microstructure model can be used to predict the distributions of individual phases such as ferrite, bainite, and martensite along with a hardness map across the weld and heat-affected-zone (HAZ) regions by integrating with the thermal model.
Operational Lessons Learned Based on Comprehensive Temperature Measurements at Primary Circuit Components
Authors: Reese Sven H. , Seichter Johannes , Klucke Dietmar , Karabaki H. Ertugrul
Published: November 2015

In nuclear power plants operated by E.ON, thermocouples have been installed since commissioning of the plants at fatigue relevant locations, especially at primary circuit components. Temperature measurement planes have been retro-fitted, further developed and the positions of the measurement planes have been reassigned and optimized continuously based on operational lessons learned. Comprehensive surveillance activities yield to a significant amount of information which is used to analyze the component specific health status. Additionally this information can be used to optimize plant’s behavior in the context of operational excellence.

An evaluation of temperature measurement is needed from the regulatory point of view and reported periodically in the context of long term fatigue evaluation being a significant part of the German ageing management process and break preclusion concept. Beyond that, the detailed information of temperature transients, gained by these measurements, allows the engineer to analyze thermal loadings of monitored components. Subsequently this information is used to optimize operation of the plants by minimizing fatigue relevant transients. The more detailed the temperature transient information is the more complex analytical and numerical models have to be in order to comprehensively consider relevant effects. Therefore numerical Finite-Element models of primary circuit components have been developed allowing the engineer to analyze temperature loading (e.g. stratification or plug-flow events) in detail and to draw conclusions being used to optimize the plant.

In the context of this publication examples of pressurized light water reactors will be discussed in detail showing the ability of the detailed evaluation process and the effectiveness of the evaluation procedure:

• Operational lessons learned from temperature measurement evaluation.

• Minimizing stratification events in the surge line by optimizing the point in time when main coolant pumps are turned off.

• Temperature loading of the auxiliary spray line during start-up phase.
Application of Extended Finite Element Method (XFEM) to Stress Intensity Factor Calculations
Authors: Shim Do-Jun , Uddin Mohammed , Kalyanam Sureshkumar , Brust Frederick , Young Bruce
Published: November 2015

The extended finite element method (XFEM) is an extension of the conventional finite element method based on the concept of partition of unity. In this method, the presence of a crack is ensured by the special enriched functions in conjunction with additional degrees of freedom. This approach also removes the requirement for explicitly defining the crack front or specifying the virtual crack extension direction when evaluating the contour integral.

In this paper, stress intensity factors (SIF) for various crack types in plates and pipes were calculated using the XFEM embedded in ABAQUS. These results were compared against handbook solutions, results from conventional finite element method, and results obtained from finite element alternating method (FEAM). Based on these results, applicability of the ABAQUS XFEM to stress intensity factor calculations was investigated. Discussions are provided on the advantages and limitations of the XFEM.