bluuebunny/crossref_metadata_2025 huggingface dataset:
Data Center Airflow Prediction With an Enhanced Potential Flow Model
Authors: VanGilder James W. , Zhang Xuanhang (Simon) , Healey Christopher M.
Published: January 2014

Potential flow models (PFM) have been implemented for a variety of applications, including data center airflow and temperature estimation. As an approximate solution to the data center room physics, potential flow models have great value in their simplicity and the limited computational effort required providing estimates. However, potential flow models lack the ability to capture the effects of buoyancy, which can affect airflow patterns within data centers. We show how this effect can be simulated within PFM; resulting in a model we call Enhanced PFM (EPFM). This model is only marginally more complex to implement than PFM and retains much of the properties of the original PFM, specifically its simplicity and stability. Solution time, about double that of PFM, is still only a small fraction of that of CFD, while empirical tests show a marked improvement in the prediction of key data center temperatures.
Transient Compact Modeling of 3D Stacked Die Inter-Tier Microfluidic Cooling Under Non-Uniform Heat Flux
Authors: Wan Zhimin , Joshi Yogendra
Published: January 2014

Three-dimensional (3D) chip stacking is a promising approach for high performance microsystems. Such vertical integration could increase the clock frequency and reduce the signal delay. During operation, the input signal and the resulting input power change with time. Realistic power maps on active chip tiers produce highly non-uniform heat flux patterns with hotspots, which change with time. In this paper, a transient compact model of a multi-layer chip stack subjected to time dependent power map was developed. Inter-tier single phase microfluidic cooling with a pin fin array was used to enhance heat transfer. The validity of the compact model was confirmed by comparison with full computational fluid dynamics/heat transfer (CFD/HT) modeling. It was found that the transient compact model ran thousand times faster than the CFD/HT model. The maximum deviation between the two models was 1 °C. The compact model was then used to analyze the transient thermal response of a 4 layer dual core stack of Penryn microprocessors when a total power of 172.4 W with a 300 W/cm2 hot spot was suddenly applied. The hotspot temperature rise approaches 35 °C, at a steady state time of about 0.015 s. The temperature rise of bottom tier was always higher than the other three tiers during the transient process because it has only single side cooling, while other tiers have double sided cooling. Also, different power maps and flow patterns, including parallel and counter flow for different layers were investigated. It was found that controlling the flow in different layers individually instead of applying the same flow conditions could achieve better thermal performance and energy savings.
Application of Synchrotron Radiation X-Ray Laminography to Nondestructive Evaluation of the Fatigue Crack Propagation Process in Flip Chip Solder Joints
Authors: Tsuritani Hiroyuki , Sayama Toshihiko , Okamoto Yoshiyuki , Takayanagi Takeshi , Uesugi Kentaro , Hoshino Masato , Mori Takao
Published: January 2014

The reliability and lifetime of micro-joints on printed circuit boards (PCBs) is significantly affected by fatigue processes, including fatigue crack initiation and propagation to failure. Accordingly, the industries producing electronic devices and components strongly desire the development of a new nondestructive inspection technology, which detects micro-cracks appearing as thermal fatigue fractures in these joints. Accordingly, the authors have demonstrated that the micro-cracks in the micro-solder joints can be observed using the SP-μCT synchrotron X-ray micro tomography system. However, in order for such solder joint micro-cracks to be observable by SP-μCT, the observation object must have a diameter of less than roughly 1 mm. In this investigation, we applied a synchrotron radiation X-ray laminography system to three-dimensionally and nondestructively evaluate the fatigue crack propagation process in flip chip solder micro-joints. X-ray laminography is a technique for nondestructively observing planar objects. The optical system developed for use in X-ray laminography was constructed to provide the rotation stage with a 20° tilt from the horizontally incident X-ray beam. For this reason, X-rays were sufficiently transmitted through the planar object, in all directions. The observed specimens had a flip chip structure, in which a 10.04 mm square LSI chip is connected to a 52.55 mm (length) × 30.0 mm (width) FR-4 substrate by 120 μm diameter Sn-3.0wt%Ag-0.5wt%Cu lead-free solder bumps. A thermal cycle test was carried out, and specimens were collected at fixed cycle numbers. The same solder joints were observed successively using the synchrotron radiation X-ray laminography system at beamline BL20XU at SPring-8, the largest synchrotron radiation facility in Japan. An X-ray beam energy of 29.0 keV was selected to obtain laminography images with high contrast among component. The obtained laminography images clearly show the evolution of cracks, voids, and the Ag3Sn phase due to the thermal cyclic loading of the solder joints. In addition, the surface area of the same fatigue cracks was also measured, to quantify the crack propagation process. However, the surface area change measured by laminography differed from the crack propagation results obtained by standard SP-μCT. This difference may be due to an inability to observe some micro-cracks, due to crack closure to beneath than the detection limit of synchrotron radiation X-ray laminography. Consequently, these results demonstrate the possibility that nondestructive observation of fatigue cracks in the solder bumps on a large size electronic substrate by synchrotron radiation X-ray laminography, although its detection ability for narrow cracks may be limited, compared to SP-μCT.
High Field Breakdown of Carbon Nanotube Network Transistors
Authors: Gupta Man Prakash , Behnam Ashkan , Estrada David , Pop Eric , Kumar Satish
Published: January 2014

We develop and employ a self-consistent electro-thermal model to study the high field breakdown of carbon nanotube (CNT) network thin film transistors (CN-TFTs). We investigate the effects of the CNT alignment angle and length distribution on the breakdown process caused by excessive self-heating. We examine relevant breakdown characteristics such as the peak current and corresponding voltage and power in relation to these two network parameters. We find that the breakdown behavior can significantly vary with respect to the CNT length and alignment distribution even when the network density is kept the same. Results suggest that an optimum alignment (∼ 65°) can be found for a network with constant CNT lengths to obtain higher current/power without setting off an early breakdown. When both CNT length and alignment angle are varied, we find that networks with higher average CNT length have lower optimum alignment such that doubling the average CNT length lowers the optimum alignment angle by half. Therefore these network parameters need to be carefully selected to achieve greater thermal reliability and higher electrical performance.
Thermodynamic Modeling of Data Center Cooling Systems
Authors: Wemhoff A. P. , del Valle M. , Abbasi K. , Ortega A.
Published: January 2014

The Villanova Thermodynamic Analysis of Systems (VTAS) is a flow network tool for modeling the thermodynamics, fluid mechanics, and heat transfer inherent to an entire data center system, including contributions by individual servers, the data center airspace, and the heating, ventilating, and air conditioning (HVAC) components. The framework for the VTAS design simulation involves the coupling of steady-state energy flows between components via a novel loop formulation. Models for all of the main components of the data center are included in VTAS as MATLAB functions. The application of VTAS towards an example legacy air cooled data center show that the largest portions of exergy destruction pertain to the server and room airflow components.
Experimental Study of Damage Mechanism of Carbon Nanotube as Nano-Component of Electronic Devices Under High Current Density
Authors: Sasagawa Kazuhiko , Fujisaki Kazuhiro , Unuma Jun , Azuma Ryota
Published: January 2014

Carbon nanotube (CNT) has a great tolerance to high current density which is a cause of electromigration (EM). Therefore, CNT is expected to use as the materials of nanoscale components of electronic devices. The damage mechanisms of CNT are regarded as the effects of oxidation by Joule heating and/or the EM by high-density electron flows. In this study, we investigated the damage mechanism of CNT structures used as nano-component of electronic devices. An EM acceleration testing system was designed using the CNT structures collected at the gap of thin-film electrodes. The EM tests were conducted under the several kinds of current density conditions and the surrounding environments. An indicator of lifetime was determined by voltage measurements during the acceleration tests and their fracture phenomena were evaluated by means of microscopic observations. As the results, the amounts of lifetime of CNT were longer in the lower oxygen concentrations than in the air condition. In the microscopic studies, it was confirmed that the local evaporation of carbon atoms due to oxidation appeared at the cathode side of the CNT structures under low current density, and the center area of CNT under high current density. Both types of damage morphologies induced by oxidation and EM were observed at the damaged CNT. The results showed the dominant damage mechanism alternated between oxidation and EM depending on current density under oxygen rich conditions.
Comparative Numerical Analysis of Angled Branching Microchannel Heat Sink
Authors: Chen Cheng , Samadiani Emad , Sammakia Bahgat
Published: January 2014

Branching microchannel networks have been considered as an effective liquid cooling system for electronic components. In this paper, an asymmetric angled branching design of microchannel network is introduced and compared with the parallel branching cooling system. Full CFD models based on finite-element analysis are established to analyze their thermal performances under uniform heat distribution. Performances of angled branching network and a series of parallel networks are compared for a pressure range from 0.5psi to 4psi. Results indicate that the angled branching network dissipates heat better at lower provided pressure drop due to its small flow resistance and flexibility in fluid transportation. On the other hand, a lower peak temperature is noted for the parallel branching network at higher pressure head.
Improvement of the Reliability of Thin-Film Interconnections Based on the Control of the Crystallinity of the Thin Films
Authors: Asai Osamu , Furuya Ryosuke , Fan Chuanhong , Suzuki Ken , Miura Hideo
Published: January 2014

Electroplated copper thin films have started to be applied to not only interconnections in printed wiring boards, but also thin film interconnections and TSV (Through Silicon Via) in semiconductor devices because of its low electric resistivity and high thermal conductivity. Thus, the electrical reliability of the electroplated copper interconnections was discussed experimentally.

The relationship between the electrical properties and crystallographic quality (crystallinity) of electroplated copper thin-film interconnections was investigated. The crystallinity of the grains and grain boundaries of the interconnections was evaluated on the basis of the image quality (IQ) value obtained by electron back-scatter diffraction (EBSD) analysis. The electrical properties of the interconnections vary significantly depending on their crystallinity. The crystallinity also changed drastically as functions of electroplating conditions and the annealing temperature after electroplating.

Although the electro migration (EM) resistance of the annealed interconnection is improved, the stress-induced migration (SM) is activated by a high residual tensile stress after annealing caused by the strong constraint of the shrinkage of the film during recrystallization. To improve its electrical reliability without heat treatment after the electroplating, the effects of the seed layer under the interconnections on the crystallinity of the electroplated film was investigated. As a result, the crystallinity was improved by changing the seed layer from Cu to Ru. In addition, the decrease in current density during electroplating also improves the crystallinity. Therefore, both introducing the Ru seed layer in addition to decreasing the current density during electroplating is effective for developing highly reliable copper interconnections.
Inelastic Thermal Stress Simulation for Si Chip and Insulator Around TSV in 3D SiP
Authors: Kinoshita Takahiro , Kawakami Takashi , Wakamatsu Takeshi , Shima Shunpei , Matsumoto Keiji , Kohara Sayuri , Yamada Fumiaki , Orii Yasumitsu
Published: January 2014

Stresses and mechanical strength of brittle materials (Si chip and insulator) around TSV (Through Silicon Via) structures in 3D SiP (Three Dimensional System in Package) were discussed under device operation condition and reflow process condition by using a large scale simulator ADVENTURECluster®, which was based on FEM (Finite Element Method), for ensuring the reliability of 3D SiP.

In case of the device operation, the equivalent stress of TSV were lower than yield stress of copper, and the maximum principal stress of Si and insulator were also lower than its bending strength. In case of the reflow process, the equivalent stress of TSV were over the yield stress of copper, and the maximum principal stresses of Si and insulator were closed to its bending strength. In addition, steep stress elevations were shown at edge part of Si and insulator. It will be a singular stress field by stress concentration. Its stress singularity was evaluated and the local strength of Si chip and insulator was discussed.
Highly Sensitive 2D Strain Sensor Using Carbon Nanotube
Authors: Kawakami Hiroshi , Ohnishi Masato , Suzuki Ken , Miura Hideo
Published: January 2014

A new highly sensitive strain measurement method has been developed by applying the strain-induced change of the electronic conductivity of CNTs. It is reported that most multi-walled carbon nanotubes (MWCNTs) show metallic conductivity and they are rather cheap comparing with single-walled carbon nanotubes (SWCNTs). However, it was found that the electric conductivity of MWCNTs changes drastically under uniaxial strain because of the drastic change of their band gap. Therefore, the authors have developed a highly sensitive strain sensor which can detect the local strain distribution by using MWCNTs. In order to design a new sensor using MWCNT, it is very important to control the shape of the MWCNTs under strain. Thus, a method for controlling the shape of the MWCNTs was developed by applying a chemical vapor deposition (CVD) technique. It was found that the shape of the grown MWCNT could be controlled by changing the average thickness of the catalyst and the deposition temperature of the MWCNT. The electrical resistance of the grown MWCNT changed almost linearly with the applied strain, and the maximum strain sensitivity obtained under the application of uniaxial strain was about 10%/1000-μstrain (gauge factor: 100). A two-dimensional strain sensor, which consists of area-arrayed fine bundles of MWCNTs, has been developed by applying MEMS technology. Under the application of compressive strain, the electric resistance was confirmed to increase almost linearly with the applied strain.
Transparent Electrodes From Graphene/Single Wall Carbon Nanotube Composites
Authors: Sojoudi Hossein , Reiter Fernando , Graham Samuel
Published: January 2014

A transparent conductive electrode comprised of alternating layers of graphene grown by chemical vapor deposition (CVD) and metallic single wall nanotubes (M-SWNTs) is presented. It was found that the addition of two single-layer graphene sheets enhances the conduction pathways in the M-SWNT film, yielding up to a 75% decrease in the sheet resistance with little sacrifice in the optical transmittance. Enhancements in the electrical properties of the films were made through a heat treatment process followed by nitric acid and thionyl chloride doping, yielding a sheet resistance of 70 Ω/sq with a transmittance of 78% at 550 nm. Composite films having undergone an annealing step were found to have stable electrical properties upon exposure to atmospheric conditions while doped films demonstrated limited stability.
A Parametric Numerical Study in Cylindrical Oblique Fin Minichannel
Authors: Fan Yan , Lee Poh Seng , Jin Li-Wen , Chua Beng Wah , Mou Na-Si , Jagirdar Mrinal
Published: January 2014

A novel cylindrical oblique fin minichannel heat sink was proposed to cool cylindrical heat sources using forced convection scheme. In this paper, parametric numerical study was employed to understand the importance of the various dimensions of the oblique fin heat sinks and their heat transfer performance and pressure drop. Three dimensional conjugated heat transfer simulations were carried out using Computational Fluid Dynamics (CFD) method based on laminar flow to determine its performance in the oblique fin heat sink. 214 parametric studies were performed by varying the oblique angle from 20° to 45°, secondary channel gap from 1mm to 5mm and Reynolds number from 200 to 900. Their thermal performance was compared for a constant heat flux of 1 W/cm2. The results show that the flow is main channel directed in shorter secondary channel structure while the flow becomes secondary channel directed in longer secondary channel structure. Secondary flow becomes more effective in heat transfer when increasing the secondary channel gap. When the oblique angle increases, more flow will be diverted into secondary channel and improve flow mixing to enhance the heat transfer. The best configuration in this paper was suggested based on the numerical data point. The overall performance can be improved up to 110% at Reynolds number of 900 compared with conventional straight fin minichannel. Therefore, this is the attractive candidate for future cylindrical heat sinks.
Analytical Modeling of Temperature Distribution in Interposer-Based Microelectronic Systems
Authors: Choobineh Leila , Agonafer Dereje , Jain Ankur
Published: January 2014

Heterogeneous integration in microelectronic systems using interposer technology has attracted significant research attention in the past few years. Interposer technology is based on stacking of several heterogeneous chips on a common carrier substrate, also referred to as the interposer. Compared to other technologies such as System-on-Chip (SoC) or System-in-Package (SiP), interposer-based integration offers several technological advantages. However, the thermal management of an interposer-based system is not well understood. The presence of multiple heat sources in various die and the interposer itself needs to be accounted for in any effective thermal model. While a finite-element based simulation may provide a reasonable temperature prediction tool, an analytical solution is highly desirable for understanding the fundamentals of the heat transfer process in interposers. In this paper, we describe our recent work on analytical modeling of heat transfer in interposer-based microelectronic systems. The basic governing energy conservation equations are solved to derive analytical expressions for the temperature distribution in an interposer-based microelectronic system. These solutions are combined with an iterative approach to provide the three-dimensional temperature field in an interposer. Results are in excellent agreement with finite-element solutions. The analytical model is utilized to study the effect of various parameters on the temperature field in an interposer system. Results from this work may be helpful in the thermal design of microelectronic systems containing interposers.
Effect of Electrode Material on Joint Strength of Soldered Joints With Sn-Bi and Sn-Bi-Sb Lead-Free Solder Balls
Authors: Hirata Akihiro , Shohji Ikuo , Tsuchida Tetsuyuki , Ookubo Toshikazu
Published: January 2014

The aim of this study is to investigate the joint properties of Sn-58mass%Bi and Sn-57mass%Bi-0.5mass%Sb low-melting lead-free solder balls on the electroless Ni/Au and Ni/Pd/Au plated Cu electrodes fabricated with a lead-free plating solution. Compared with the conventional Ni plating solution containing lead, the soldered joints with Ni/Au and Ni/Pd/Au electrodes fabricated with the lead-free plating solution showed comparable joint properties. In the joints with Ni/Pd/Au electrodes, ball shear force increased when the Pd layer is dissolved into solder by multi reflows. In both joints with Ni/Au and Ni/Pd/Au electrodes, a part of the Ni layer was dissolved into solder and thus the (Ni,Cu)3Sn4 intermetallic compound layer formed at the joint interface. Ball shear force decreased upon aging due to the growth of the (Ni,Cu)3Sn4 layer at the joint interface.
Effect of Additives in an Electrolyte on Mechanical Properties of Electrolytic Copper Foil
Authors: Nagayama Takuya , Yoshida Hiroaki , Shohji Ikuo
Published: January 2014

The effect of additives in electrolyte on mechanical properties of electrolytic copper foil was investigated. Bis-(3-sulfopropyl)-disulfide disodium salt (SPS), animal protein of low molecular (PBF) and hydroxyethyl cellulose (HEC) were added in electrolyte as additives. The additive amount of SPS was changed in this study. The addition of SPS is effective to improve tensile strength and hardness of electrolytic copper foil. With increasing the additive amount of SPS, the grain of electrolytic copper became finer and thus its hardness and elastic modulus increased. On the other hand, fatigue properties improved when the additive amount of SPS decreased and the grain size of electrolytic copper became relative large.
Analysis of Single-Phase Multi-Layer Heat Sinks Using a Porous Media Approach: Influence of Spatially Varying Porosity
Authors: Podhiny John J. , Ortega Alfonso
Published: January 2014

Prior analyses and experiments have demonstrated that varying or scaling the number of fluid channels in each layer of a stacked multi-layer heat sink yields distinct advantages over traditional single-layer designs which use channels with high aspect ratios. Specifically, a design which implements scaling in order to vary the porosity (or equivalently, the number of channels) from one layer to the next allows a given thermal performance to be realized at a lower pressure drop than the corresponding non-scaled design. While previous studies use porous media theory for their analytical foundation just as the current studies do, they also focused on heat exchangers with machined channels, and have consequently been limited to discrete variation of the porosity (or number of channels) in each layer. Extending this approach by allowing for continuous porosity variation provides a generalized and powerful design method for scaled multi-layer heat exchangers by mathematically modeling them using two-equation volume averaged quantities. This approach also yields insight into the fundamental design parameters which control the performance of this class of heat exchangers, and suggests that their performance-governing parameters are similar to those which govern fin performance.

This paper focuses primarily on the thermal behavior of these liquid cooled heat sinks which employ various scaling functions to define the spatial variation of porosity. Comparisons are made with available closed-form solutions as well as results available from prior studies of discretely-scaled designs. The results indicate that scaling laws which have not been investigated previously can likely yield further performance improvements relative to prior designs.
Microstructural Analysis of Low-Cycle Fatigue Damage Process of Sn-Ag-Cu Solder Joint
Authors: Kontani Hiroyuki , Kariya Yoshiharu , Fumikura Tomoya
Published: January 2014

In this study, the relationship between microstructural change and fracture in the process of low-cycle fatigue of Sn-Ag-Cu solder joint was investigated using the solder ball of 630μm and 100μm in diameter by analysis of crystallographic orientation by means of EBSD. The 630μm specimen has subgrain boundaries formed by dynamic recovery in the stress concentration region, and the subgrain boundaries become high-angle random grain boundaries by additional cycles. The fatigue crack stably propagates along the random grain boundary in the stress concentration region. In contrast, the 100μm specimen has subgrain boundaries and high-angle random grain boundaries formed across the entire joint area. Since the occurrence of grain boundary fracture across the entire joint area by the connection of high energy grain boundaries, the crack propagation life of the 100μm specimen shortens without the stable crack growth compared to the 630μm specimen.
Impact of Cold Aisle Containment on Thermal Performance of Data Center
Authors: Muralidharan Bharathkrishnan , Shrivastava Saurabh K. , Ibrahim Mahmoud , Alkharabsheh Sami A. , Sammakia Bahgat G.
Published: January 2014

The use of air containment systems has been a growing trend in the data center industry and is an important energy saving strategy for data center optimization. Cold Aisle Containment (CAC) is one of the most effective passive cooling solutions for high density heat load applications. Cold Aisle Containment provides a physical separation between the cold air and the hot exhaust air by enclosing the cold aisle, preventing hot air recirculation and cold air bypass. This separation provides uniform inlet air temperatures to the servers, which can further contribute to overall data center efficiency.

This paper includes the thermal test data for a data center lab with and without a CAC set up. The paper quantifies the thermal impact of implementing a CAC system over an open Hot Aisle/Cold Aisle (HA/CA) arrangement for three different cabinet heat load conditions at two different CRAC (Computer Room Air Conditioner) return air set point conditions. It studies the advantages of CAC over standard HA/CA arrangement. A case study has been presented showing a cooling energy savings of 22% with the use of a CAC system over a standard HA/CA arrangement.
Cooling Energy Reduction During Dynamically Controlled Data Center Operation
Authors: Parida Pritish R. , Chainer Timothy J. , Schultz Mark D. , David Milnes P.
Published: January 2014

Data Center power usage has been growing rapidly as new internet applications are developed, requiring computing resources such as high-density cloud computing server farms. All of the electricity consumed by data centers is ultimately turned into heat. This heat is removed using fans, pumps, chillers and air conditioning equipment, which comprise roughly a quarter of the total data center power consumption.

As part of a US Department of Energy cost shared grant, a data center test facility was designed and constructed with the goal to demonstrate the potential to reduce cooling energy use to less than 5% of the total IT and facilities energy usage by utilizing liquid cooling of the electronics rack. The system was characterized to develop temperature-based energy-aware servo control algorithms for further cooling energy reductions. These algorithms were implemented to dynamically reduce the data center cooling power consumption while controlling to a specified temperature under varying outdoor temperature and workload conditions. This work illustrates the energy-centric controls that were implemented to achieve dynamic system control in the most energy efficient manner.
Transient Liquid Phase Sintered Joints for Power Electronic Modules
Authors: Greve Hannes , McCluskey F. Patrick
Published: January 2014

Pastes consisting of micron-sized particles of a low melting point metal (i.e. Sn) and a high melting point metal (e.g. Ag, Cu) embedded in organic binder have been developed to attach silicon or wideband gap semiconductor devices to metallic or ceramic substrates for power electronic applications requiring operation at high temperatures. The attachment is made by a pressure-less, low temperature transient liquid phase sintering (LT-TLPS) process in air. Process time and temperature, along with binder type and amount are adjusted to minimize the formation of voids in the joints. Test samples consisting of copper dice on copper substrates joined by these LT-TLPS sinter pastes have been manufactured for shear testing. A shear fixture for high-temperature testing has been designed, and shear tests have been performed at temperatures of 25°C, 125°C, 250°C, 400°C, and 600°C. The influence of process time, process temperature, and the ratio of low-melting point metal (Sn) to high-melting point metal (Ag, Cu) on the shear strength at each temperature has been assessed. It has been shown that the shear strength of TLPS sinter joints remains high up to the melting point of the dominant intermetallic phase of the joint. The joints show no softening below the melting point of these phases. AgSn sinter joints show only limited change in shear strength up to 400°C. CuSn joints exhibit high shear strength up to 600°C for high copper ratios. While process times of 5–15 minutes are sufficient to drive the sintering reaction to near completion, extended curing improves the strength of the sinter joints even more. Failure analyses for joints of different compositions have been conducted along with cross-sectioning of sintered but non-sheared specimens to correlate reliability to microstructure.
A Revised Anand Constitutive Model for Lead Free Solder That Includes Aging Effects
Authors: Motalab Mohammad , Basit Munshi , Suhling Jeffrey C. , Lall Pradeep
Published: January 2014

Traditional finite element based predictions for solder joint reliability during thermal cycling accelerated life testing are based on solder constitutive equations (e.g. Anand viscoplastic model) and failure models (e.g. energy dissipation per cycle model) that do not evolve with material aging. Thus, there will be significant errors in the calculations with lead free SAC alloys that illustrate dramatic aging phenomena. In this study, we have developed a revised set of Anand viscoplastic stress-strain relations for solder that include material parameters that evolve with the thermal history of the solder material. The effects of aging on the nine Anand model parameters have been examined by performing stress-strain tests on SAC305 samples that were aged for various durations (0–6 months) at temperature of 100 C. The stress-strain data were measured at three strain rates (.001, .0001, and .00001 1/sec) and five temperatures (25, 50, 75, 100, and 125 C). The mechanical tests have been performed using both water quenched (WQ) and reflowed (RF) samples (two unique specimen microstructures). In the case of the water quenched samples, there is rapid microstructural transitioning during the brief time that occurs between placing molten solder into the glass tubes and immersing the tubes in water bath. On the other hand, the reflowed samples are first cooled by water quenching, and then sent through a reflow oven to re-melt the solder in the tubes and subject them to a desired temperature profile matching that used in PCB assembly.

As expected, the observed mechanical properties of water quenched samples were better (higher in magnitude) than the corresponding mechanical properties of the reflowed samples. Although the differences in elastic modulus between the water quenched and reflowed samples are small, significant differences are present for the yield and ultimate tensile stresses (for each aging condition). For both the water quenched and reflowed specimens, significant degradation of the mechanical properties has been observed with aging. Using the measured stress-strain and creep data, mathematical expressions have been developed for the evolution of the Anand model parameter with aging time. Our results show that 2 of the 9 constants remain essentially constant during aging, while the other 7 show large changes (30–70%) with up to 6 months of aging. The revised Anand constitutive equations for solder with aging effects have also been incorporated into commercial finite element codes (ANSYS and ABAQUS).
Aging Induced Mechanical Property Degradation and Microstructure Evolution of SAC+X Lead Free Solder Alloys
Authors: Cai Zijie , Suhling Jeffrey C. , Lall Pradeep , Bozack Michael J.
Published: January 2014

The microstructure, mechanical response, and failure behavior of lead free solder joints in electronic assemblies are constantly evolving when exposed to isothermal aging and/or thermal cycling environments. In our prior work on aging effects, we have demonstrated that large degradations occur in the material properties (stiffness and strength) and creep behavior of Sn-Ag-Cu (SAC) lead free solders during aging. These effects are universally detrimental to reliability and are exacerbated as the aging temperature and aging time increases. Conversely, changes due to aging are relatively small in conventional Sn-Pb solders.

In our current work, we are exploring several doped SAC+X alloys in an attempt to reduce the aging induced degradation of the material behavior of SAC solders. The doped materials are lead free SAC solders that have been modified by the addition of small percentages of one or more additional elements (X). Using dopants (e.g. Bi, In, Ni, La, Mg, Mn, Ce, Co, Ti, Zn, etc.) has become widespread to enhance shock/drop reliability, wetting, and other properties; and we have extended this approach to examine the ability of dopants to reduce the effects of aging and extend thermal cycling reliability. In this paper, we concentrate on presenting the results for SAC+X (X = Zn, Co, Ni). The enhancement of aging resistance for the doped lead free solders was explored. Comparisons were made to the responses of non-doped SAC lead free solder alloys.

The effects of aging on mechanical behavior have been examined by performing stress-strain and creep tests on solder samples that were aged for various durations (0–6 months) at elevated temperature (100 °C). Variations of the mechanical and creep properties (elastic modulus, yield stress, ultimate strength, creep compliance, etc.) were observed and modeled as a function of aging time and aging temperature. Our findings show that the doped SAC+X alloys illustrate reduced degradations with aging for all of the aging temperatures considered. Also, the stress-strain and creep mechanical properties of doped solders are better than those of reference solders after short durations of aging. After long term aging, doped solder alloys were found to have more stable behaviors than those of the standard SAC alloys. A parallel microstructure study has shown that less degradation and coarsening of the phases occurs in doped solder materials relative to non-doped solders after severe aging.
Effect of Isothermal Aging and High Strain Rate on Material Properties of Innolot
Authors: Lall Pradeep , Limaye Geeta , Shantaram Sandeep , Suhling Jeff
Published: January 2014

Industry migration to lead-free solders has resulted in a proliferation of a wide variety of solder alloy compositions. The most popular amongst these are the Tin-Silver-Copper (Sn-Ag-Cu or SAC) family of alloys like SAC105, SAC305 etc. Recent studies have highlighted the detrimental effects of isothermal aging on the material properties of these alloys. SAC alloys have shown up to 50% reduction in their initial elastic modulus and ultimate tensile strength within a few months of elevated temperature aging. This phenomenon has posed a severe design challenge across the industry and remains a road-block in the migration to Pb-free. Multiple compositions with additives to SAC have been proposed to minimize the effect of aging and creep while maintaining the melting temperatures, strength and cost at par with SAC. Innolot is a newly developed high-temperature, high-performance lead-free substitute by InnoRel™ targeting the automotive electronics segment. Innolot contains Nickel (Ni), Antimony (Sb) and Bismuth (Bi) in small proportions in addition to Sn, Ag and Cu. The alloy has demonstrated enhanced reliability under thermal cycling as compared to SAC alloys. In this paper, the high strain rate material properties of Innolot have been evaluated as the alloy ages at an elevated temperature of 50°C. The strain rates chosen are in the range of 1–100 per-second which are typical at second level interconnects subjected to drop-shock environments. The strain rates and elevated aging temperature have been chosen also to correspond to prior tests conducted on SAC105 and SAC305 alloys at this research center. This paper presents a comparison of material properties and their degradation in the three alloys — SAC105, SAC305 and Innolot. Full field strain measurements have been accomplished with the use of high speed imaging in conjunction with Digital Image Correlation (DIC). Ramberg-Osgood non-linear model parameters have been determined to curve-fit through the experimental data. The parameters have been implemented in Abaqus FE model to obtain full-field stresses which correlates with contours obtained experimentally by DIC.
Device Simulation for Effects of Mechanical Stress on Electrical Performances of nMOSFETs: The Impacts of Stress-Induced Change of Intrinsic Carrier Density
Authors: Koganemaru Masaaki , Tada Naohiro , Ikeda Toru , Miyazaki Noriyuki
Published: January 2014

This paper discusses a numerical model for analysing the effects of mechanical stress on semiconductor devices. In other words, drift-diffusion device simulation is conducted using a physical model incorporating the effects of mechanical stress. Then, each impact of the stress-induced physical phenomena is analysed. In our previous study, three physical phenomena that were attributed to mechanical stress have been modeled in our electron mobility model, i.e., the changes in relative population, the momentum relaxation time and the effective mass of electrons in conduction-band valleys. In addition, in this study, the stress-induced change of intrinsic carrier density is modeled. Stress-induce variations of drain current characteristics on n-type Metal Oxide Semiconductor Field Effect Transistors (nMOSFETs) are evaluated using a drift-diffusion device simulator including above mentioned physical models. It is demonstrated that the impact of stress-induced change of intrinsic carrier density is small for our evaluated nMOSFETs.
Prognostics of Damage Accrual in SSL Luminaires and Drivers Subjected to HTSL Accelerated Aging
Authors: Lall Pradeep , Sakalaukus Peter , Davis Lynn
Published: January 2014

This paper will show an investigation of off-the-shelf luminaires with the focus on the LED electronic drivers, specifically the aluminum electrolytic capacitors (AECs), that have been aged using high temperature shelf life (HTSL) testing of 135°C in order to prognosticate the remaining useful life of the luminaires. Luminaires have the potential of seeing excessive temperatures when being transported across the country or being stored in non-climate controlled warehouses. They are also being used in outdoor applications in desert environments that see little or no humidity but will experience extremely high temperatures during the day. This makes it important to increase our understanding of what effects being stored at high temperatures for a prolonged period of time will have on the usability and survivability of these devices. The U.S. Department of Energy has made a long term commitment to advance the efficiency, understanding and development of solid-state lighting (SSL) and is making a strong push for the acceptance and use of SSL products. In this work, the four AECs of three different types inside each LED electronic driver were studied. The change in capacitance and the change in equivalent series resistance (ESR) of the AECs were measured and considered to be a leading indication of failure of the LED system. These indicators were used to make remaining useful life predictions to develop an algorithm to predict the end of life of the AECs. The luminous flux of a pristine downlight module was also monitored using each LED electronic driver that was subjected to HTSL through the progression of the testing to determine a correlation between the light output of the lamp and the failing components of the LED electronic driver. Prognostic and Health Management (PHM) is a useful tool for assessment of the remaining life of electrical components and is demonstrated for AECs in this work.
Comparison of Prognostic Health Management Algorithms for Assessment of Electronic Interconnect Reliability
Authors: Lall Pradeep , Lowe Ryan , Goebel Kai
Published: January 2014

This paper compares three prognostic algorithms applied to the same data recorded during the failure of a solder joint in ball grid array component attached to a printed circuit board. The objective is to expand on the relative strengths and weaknesses of each proposed algorithm. Emphasis will be placed on highlighting differences in underlying assumptions required for each algorithm, details of remaining useful life calculations, and methods of uncertainty quantification. Metrics tailored specifically for Prognostic Health Monitoring (PHM) are presented to characterize the performance of predictions. The relative merits of PHM algorithms based on a Kalman filter, extended Kalman filter, and a particle filter all demonstrated on the same data set will be discussed. The paper concludes by discussing which algorithm performs best given the information available about the system being monitored.
Dynamic Modeling of a Two-Phase On-Chip Cooling System Applied on Parallel High Performance Microprocessors
Authors: Lamaison Nicolas , Marcinichen Jackson B. , Thome John R.
Published: January 2014

Transient modeling and control of two-phase on-chip microevaporator cold plates of a liquid pump cooling cycle is studied. The purpose is to cool down multiple micro-processors in parallel and their auxiliary electronics (memories, DC/DC converters, etc.) in series. The cooling system is composed of multiple on-chip microevaporators in parallel, a condenser, a liquid accumulator, a liquid pump and all piping joining these components. In order to achieve high heat transfer and chip temperature uniformity, two-phase flow of HFC134a is considered for the coolant. The dynamics of the system are relevant aspects to be studied since the heat dissipated by the microprocessors is changing continuously. Thus, a new simulation code has been developed here to emulate the operation during transients. Such transient simulations allow us to verify whether critical heat flux (CHF) conditions are reached during heat load disturbances and to track the available heat at the condenser for energy recovery purposes.

Presently, a case study with four microprocessors cooled in parallel flow is simulated considering different levels of uniform heat flux (36, 30, 25 and 10 Wcm−2), which showed the robustness of the predictive-corrective solver used. For a desired exit mixing vapor quality of 30%, at an inlet pressure and subcooling of respectively 16 bar (saturation temperature of 57.9 °C) and 2 K, the resulting distribution of the mass flow rates in the microevaporators were 3.6, 4.0, 4.5 and 7.4 kg/h (largest flow rate for lowest heat load) and the total pressure drop over the entire section was 0.6 kPa. The CHF and maximum chip temperature remained below of the critical limits. Preliminary comparisons with experimental tests showed errors in the predictions of mean chip temperature and mixing vapor quality to be within ±10%.
CFD Simulation of Wave Soldering Process (Case of Simple 2-Dimensional Configurations)
Authors: Inagaki Kazunori , Nakagawa Shinji , Hatakeyama Tomoyuki , Ishikura Koichi , Koizumi Katsuhiro
Published: January 2014

Wave soldering process with a simple model has been studied numerically. Wave soldering process is one of major soldering process used in the manufacture of electronics. The purpose of this study is to clarify how a lifting velocity and an arrangement of parts affect the solder volume on the joint after wave soldering process. Open source computational fluid dynamics software, OpenFOAM, is used. Solder is assumed to be kept at the constant temperature over the melting point. Flow of melting solder and surrounding air is simulated and the dynamic movement of the interface between two fluids is captured using VOF method. Two dimensional arrangements of the parts were adopted. The dynamic movement of the solder surface is visualized from numerical results. The size of a rear land affects the solder volume on the front parts. It is confirmed that the changes of the volume is closely related to the solder surface configuration at the moment of the detachment from the solder bath.
Transient Thermal Response of Servers Through Air Temperature Measurements
Authors: Erden Hamza Salih , Khalifa H. Ezzat , Schmidt Roger R.
Published: January 2014

Transient CFD analysis of data centers requires appropriate representations of the transient thermal characteristics of servers. Thermal conductance and thermal capacitance are two determining characteristics for the response of servers under unsteady conditions. Previous studies proposed tests that require detailed temperature and thermal capacitance measurements for each of the server component, requiring access to individual components inside the server. In this paper, we propose a method for obtaining the transient thermal characteristics of a server from server inlet and outlet temperatures under transient operating conditions.
Improved Cooling of Electromagnetics by Directed Airflow
Authors: Downing Scott , Fain Adam , Majumdar Pradip
Published: January 2014

The transformers in aircraft power conversion are often very heavy and represent a significant fuel or range penalty. Being thermally sized, improved cooling methods would allow downsizing and thereby reduced weight. Since the conductive paths in these metal “dense” devices are good, the controlling thermal resistance is typically the convective coefficient. The goal of this study was to optimize the convective air cooling across transformers by parametrically testing candidate shroud geometries to minimize average and hot spot surface temperatures with minimal fan power. Experimental results from a low velocity wind tunnel were well correlated by CFD modeling, providing confidence in continued shroud development with only CFD or experimentally. A simple cubic test block was selected as surrogate to generalize and simplify both test methods and results. A new “goodness” parameter was developed that included both the heat transfer performance and pressure penalty for a comparative index of shroud designs.

The work was divided into two phases. Phase-A used numerical modeling to study a variety of different shapes to select the best for experimental testing. Phase-B included the testing and further parametric evaluation with CFD studies. A parameter was developed that quantified the effect of conductive spreading on the test article surface.

Presented here are the results of these studies, where several general shroud shapes emerged as high performance in comparative evaluation. Following the down select, specific geometrical dimensions relative to the duct and mock transformer sizes were further optimized.