bluuebunny/crossref_metadata_2025 huggingface dataset:
Characterization of Non-Conductive Adhesives
Authors: Farley Daniel , Dasgupta Abhijit , Caers J. F. J. M.
Published: April 2009

This study aims to establish a global-local modeling methodology for determining the residual contact stress developed during fabrication of flip-chip-on-flex (FCOF) microelectronics systems. The assembly consists of a silicon die with gold bumps bonded with a non-conductive adhesive (NCA) on to gold-plated copper bumps on a flex substrate. Manufacturing variabilities cause a nonuniformity in the bump heights, leading to some bumps that are “tall” and some that are “short.” The fabrication process needs to achieve a significant amount of compressive initial contact stress in all the bumps, to achieve an acceptable level of electrical contact resistance. Furthermore, this stress level forms the initial condition for cyclic relaxation of the stress (and corresponding progressive loss of contact resistance) due to temperature cycling throughout the life cycle of the assembly. A key issue to be investigated is the nonuniformity of the contact stresses due to the variabilities in the height of the metal contact bumps. The method is demonstrated for a selected NCA. The fabrication process consists of mechanical compression to bring all the bumps into contact, thermal curing of the adhesive during which it undergoes chemical shrinkage, removal of the mechanical compressive force and cool-down to room temperature. The modeling complexities include the geometric complexity, as well as nonlinearities due to elastic-plastic properties and large deformations of the metal bumps, evolution of contact surfaces between the two bumps, and nonlinear thermomechanical properties of the adhesive as it cures. Modeling strategies used to capture the nonlinearities include “contact elements” to prevent interpenetration at the contact surfaces, elastic-plastic models to account for metal plasticity, “element birth and death” to account for the solidification of the polymer NCA. The entire bonding process is modeled with a global-local model to reduce the computational complexity. The results of the global model serve as the input for the local model. Key findings include: the accuracy of the simulation is very sensitive to the accuracy of the gold and flex constitutive models used; the inclusion of viscoelastic properties for the epoxy has a significant effect on simulations; and better stress development comes from a higher concentration of short bumps than tall bumps.
Survey of Environmental Data for COTS (Commercial Off-the-Shelf) Used in Military Systems
Authors: Ramot Medy , Weiss Eran , Cohen Niv
Published: April 2009

COTS — Commercial Off-The-Shelf items — used in electronic military systems present advantages of being state-of-the-art technology, low cost and readily available on the market. Demand to reduce design lead-time and minimize cost bring on an increasing trend to use COTS in military applications. COTS manufacturers design the parts for moderate environmental conditions, which characterizes consumer goods, computers, telecommunications, and home entertainment industries. Military systems require reliable, high quality performance and survival in harsh environment. Therefore, in order to asses the potential risks of COTS implementations in military applications, it is necessary to be acquainted with the parts environmental specifications. This paper presents a survey of mechanical and environmental specifications as presented in various data sheets, by the manufacturers. Observations and concerns results of observations will be presented about issues that have to be addressed in the process of using COTS in military systems.
Performance Characterization of a Thermal Management Concept for High-Density, High-Speed Parallel Optical Interconnects
Authors: Vukovic Alex , Savoie Michel
Published: April 2009

Parallel optical interconnects (POIs) offer capacity advantages for high-density telecomm requirements. Once an array of multi-channel, high-capacity POIs is placed on a board, shelf or telecom rack, it creates a challenging thermal management task. To develop a thermal management concept for high dissipating racks (over 10 kW), capable of keeping a maximum temperature of 80°C for optical components, with tight temperature uniformity requirements of ±1°C between a receiver (Rx) and transmitter (Tx), a fully functional two-shelf telecommunication rack has been built. A thermal management solution was proposed and was based on the implementation of heat pipes embedded into each board. The performance of embedded heat pipes is characterized for densely packaged, high-speed optics applications requiring temperature uniformity and stringent temperature limits. The proposed solution completely meets requirements of power-dense, high speed POIs at the board/shelf level for the typical telecommunication rack. Moreover, it evolves into an enabling strategy for the reliable control of power-dense, temperature-sensitive optical components.
Artificial Neural Network Trained, Genetic Algorithms Optimized Thermal Energy Storage Heatsinks for Electronics Cooling
Authors: Kanesan Jeevan , Arunasalam Parthiban , Seetharamu Kankanhalli N. , Azid Ishak A.
Published: April 2009

A thermal response model for designing thermal energy storage heatsink utilized for electronics cooling is developed in this paper. In this study, thermal energy storage (TES) heatsink made out of aluminum with paraffin as the phase change material (PCM) is considered. By using numerical simulation, stabilization time and maximum operating temperature to transition temperature difference is obtained for varying fin thicknesses, fin height, number of fins and PCM volume. The numerical simulation results were then compared with existing experimental work. The numerical results matched the melting temperature variation obtained by the experimental work. The validated numerical results are then used to train the artificial neural networks (ANN) to predict stabilization time and maximum operating temperature to transition temperature difference for new fin thicknesses, fin height, number of fins and PCM volume. Finally the optimization of the fin thickness, fin height, number of fins and PCM volume of the thermal energy storage heatsink is obtained by embedding the trained ANN as a fitness function into genetic algorithms (GA). The objective of optimization is to maximize stabilization time and to minimize maximum operating temperature to transition temperature difference. Finally the optimized results for the TES heatsink is used to build a new computer model for numerical analysis. The final optimized model results and the validated preliminary model results are then compared. The final results will show a significant improvement from the validated model. Further the study will show that by combining ANN and GA, a superior tool for optimization is realized.
Experimental Validation of Channeled Wall Passive Cooling Enhancement
Authors: Yazawa Kazuaki , Nishino Yasushi , Nakagawa Shinji , Ishizuka Masaru
Published: April 2009

Passive cooling enhancement with least additional mass of the material on the external wall can be a method for least energy thermal management for the consumer electronics. Previously, the channeled wall enhancement was found to have potentially four fold performance compare to the single wall in the analysis work, in which the enhancement factor with optimum shape with appropriate thermal conductivity of the material was presented. This method is assumed promising to help the energy efficient and quieter thermal design without sacrificing the valuable flat cosmetic surface of equipment. In this paper, flow regime in a channel composite wall is observed experimentally with PIV method as well as thermal resistance of the wall is observed in the same apparatus. These observation results are discussed with numerical analysis. The discussion is summarized with cooling enhancement factor based on the allowable temperature with comparing to the single wall. The result provides the strong evidence for the optimization on proposed analytic models.
Thermal-Resistance Measurements on Mechanical Gap Fillers
Authors: Kolodner Paul , Hodes Marc , Ewes Ingo , Holmes Paul
Published: April 2009

This paper discusses spring-loaded mechanical structures that can be used to make thermal connections between an object to be cooled, such as an integrated circuit, and a dissipative structure, like a cooling plate or heat sink. These metal structures are flexible and resilient, adapting to variations in orientation of the two objects to be coupled. Precision experiments and computations demonstrate that they have much lower thermal resistance than elastomeric “gap-filler” pads that are usually used to perform this function.
Effect of Localized Hotspot on the Thermal Performance of Two-Phase Microchannel Heat Exchanger
Authors: Prasher Ravi S. , Dirner John , Chang Je-Young , Myers Alan , Chau David , Prstic Suzana , He Dongming
Published: April 2009

Microchannel heat exchangers using two-phase convective boiling is one of the most promising future technologies for the cooling of microprocessors. Heat generation on microprocessors is highly non-uniform due to the presence of multiple time-varying localized hotspots. Previous literature has primarily been focused on the performance of microchannels under uniform heating conditions. In this paper we report the performance of microchannel heat exchanger under both uniform and hotspot (non-uniform) heating conditions. We performed these experiments using a novel test setup where the test chip has multiple temperature sensors, one heater to provide uniform heating and a hotspot heater of size 400 μm × 400 μm. We report some of the preliminary results on the thermal performance of the heat exchanger. Results show that fluctuation in the wall temperature is different under hotspot heating conditions as compared to the uniform heating conditions.   This paper was also originally published as part of the Proceedings of the ASME 2005 Heat Transfer Summer Conference.
Thermometry of Polycrystalline Silicon Structures Using Raman Spectroscopy
Authors: Abel Mark R. , Graham Samuel
Published: April 2009

Raman spectroscopy was investigated as a method for the temperature and stress measurement in thermal MEMS devices. Calibrations of the Stokes Raman shift and the Stokes to anti-Stokes intensity ratio for doped samples were performed in order to calculate the temperature on simple polysilicon structures. Straight and serpentine micro-heaters of various sizes were fabricated from 2 micron doped polysilicon films on thick sacrificial oxide layers. Operating temperatures were measured at a range of input powers for devices attached to the oxide layer as well as released structures. Measurements show that all devices can exceed 400°C with the released devices requiring much less power, as expected. Temperature measurements using the Stokes shift method were compared to the conventional intensity ratio method in order to deduce the effects of thermal stresses on the temperature measurements. Using this method, it was found that thermal stresses could be qualitatively determined simultaneously with temperature in silicon MEMS devices. The effects of stress, however, results in less than a 10% difference in temperature over all of the input powers tested in this study.
Development of Piezoelectric RF-MEMS Switch Driven by Low Operating Voltage
Authors: Kanno Isaku , Suzuki Takaaki , Endo Hironobo , Kotera Hidetoshi
Published: April 2009

This paper presents the possibility of piezoelectric RF-MEMS switches for low voltage operation. The switches we fabricated consist of micro-cantilevers using PZT thin films with the length of 490 μm and the width of 87 μm. The cantilevers are actuated as unimorph actuators that can be deflected by applying voltage between upper and lower electrodes. We could obtain large tip deflection of 3 μm even at the low voltage of 5.0V, which is well compatible with conventional IC drivers. This result indicates that the RF-MEMS switches using piezoelectric PZT thin films is advantageous to the low voltage switching devices in RF components compared with conventionally proposed electrostatic ones.
A Viscoplastic Constitutive Model and Estimation Method of Material Constants for Lead-Free Solder Alloys
Authors: Ohguchi Ken-Ichi , Sasaki Katsuhiko
Published: April 2009

A viscoplastic constitutive model for lead-free solder alloys was proposed. The proposed model is the elasto-plastic-creep type which comprises the sum of the plastic, transient creep, and steady-state creep strains. To determine the material constants employed by the proposed constitutive model, a simple method to estimate the constants was also proposed. The method was applied to the lead-free solder alloy of Sn-3.0Ag-0.5Cu. Then, all the material constants for Sn-3.0Ag-0.5Cu employed by the constitutive model were estimated only by three tensile tests and one subsequent stress relaxation test. Using the constitutive model with the estimated constants, the ratcheting deformations of Sn-3.0Ag-0.5Cu under different strain rates were simulated. As a result, the simulations could predict the strain rate effect on the ratcheting deformation well. Therefore, the FEM analysis employing the constitutive model with the estimation method for material constants must be useful for the faster selection of the optimum lead-free solder alloys for electronic devices mounting because the material constants in the model can be estimated without conducting the long-time creep tests.
Empirical Optimization of High Density Plate-Fin Heat Sink for Constant Pumping Power in Low Profile Cooling Application
Authors: Yazawa Kazuaki , Yoshida Tatsuro , Nakagawa Shinji , Ishizuka Masaru
Published: April 2009

Since the VLSI processors are increasing power in accordance with exponential law, cooling solutions for such as personal computers have been evolving for over a decade. Recent heat sinks are designed with high dense fins and low profile to adapt to a high heat flux source within a slim enclosure. To achieve such compact cooling solution, thin fin and small gap is desirable. In addition, the pumping power is also limited by the allowable narrow space for fans. Thus it is important to minimize the thermal resistance for given pumping power that we define the optimum. Due to the lack of literatures on topic of low profile and high dense fins experiments, an apparatus was specially built to measure the thermal and fluid dynamic performance at the same time. Since such a high dense fin arrangement requires extra space on the sides by manufacturing reasons, the impact of bypass flow needs to be considered. The experiments are carefully carried out and the results are precisely compared with numerical analysis. The numerical model aiming to find the optimum for given pumping power is discussed with extrapolating the data points. This report is concluded with the best configuration of plate fins of low profile heat sinks for a given fan performance.
Effects of Acoustic Resonance on Hydrodynamics and Evaporation of a Pendant Liquid Droplet
Authors: Moon Jong Hoon , Kang Byung Ha , Kim Ho-Young
Published: April 2009

One of the fascinating prospects is the possibility of new hydrodynamics technology on micro-scale system since oscillations of micro-droplets are of practical and scientific importance. In this paper, nonlinear and forced oscillations of supported viscous droplet were focused. The droplet has a free contact line with solid plate and inviscid fluid. Natural frequencies of a pendant droplet have been investigated experimentally by imposing the acoustic wave while the frequency is being increased at fixed amplitude. The evaporation process of oscillating droplet with thermofoil has been also observed to investigate analyzing the resonance effect on the thermal characteristics of droplet. It is found that a pendant droplet shows the resonant behaviors at each mode similar to the theoretical analysis. During imposing the acoustic wave, the pendant droplet makes a rotating motion in its longitudinal axis which is a new shape oscillation mode. The evaporation rate of a pendant droplet at resonant frequency is significantly enhanced.
Computational Method for Characterization of a Microchannel Heat Sink Involving Two-Phase Flow
Authors: Kelkar Kanchan M. , Patankar Suhas V. , Kang Sukhvinder
Published: April 2009

Microchannel heat sinks are being increasingly considered for the cooling of electronic equipment because of their ability to absorb high heat fluxes directly from the heat-dissipating components in a compact manner with a low thermal resistance. In this study, a computational method is presented for the analysis of conjugate heat transfer and two-phase flow in a heat sink containing a single microchannel. It involves a two-domain solution of the three-dimensional conduction within the solid region and the one-dimensional two-phase momentum and energy transfer within a microchannel. The nonlinear coupling between the two domains that occurs through the heat exchange at the walls of the microchannels is handled using an iterative calculation. Analysis of the flow and heat transfer in the microchannel is based on the homogenous flow assumption that is deemed to be accurate for the flow of low surface tension coolants such as methanol, isobutane, and HFC’s. Representative single and two-phase correlations are used for the calculation of the friction factor and the heat transfer coefficient. The computational model is applied for the prediction of the performance of a microchannel heat sink over a range of mass flow rates. The results of the analysis show the important physical effects that govern the performance of the microchannel heat sink involving two-phase flow. These include the acceleration of the flow in the microchannel in the two-phase region that influences the pressure drop through it and the two-phase enhancement of heat transfer that determines the temperature field within the solid region.   This paper was also originally published as part of the Proceedings of the ASME 2005 Heat Transfer Summer Conference.
Prediction of Microelectronic Substrate Warpage Using Homogenized Thermomechanical Finite Element Models
Authors: Hutapea Parsaoran , Grenestedt Joachim L. , Modi Mitul , Mello Michael , Frutschy Kristopher
Published: April 2009

High-density microelectronic substrates, used in organic CPU packages, are comprised of several polymer, fiber-weave, and copper layers and are filled with a variety of complex features such as traces, micro-vias, Plated-Through-Holes (PTH), and adhesion holes. When subjected to temperature changes, these substrates may warp, driven by the mismatch in Coefficients of Thermal Expansion (CTE) of the constituent materials. This study focused on predicting substrate warpage in an isothermal condition. The numerical approach consisted of three major steps: estimating homogenized (effective) thermomechanical properties of the features; calculating effective properties of discretized layers using the effective properties of the features; and assembling the layers to create 2D Finite Element (FE) plate models and to calculate warpage of the whole substrates. The effective properties of the features were extracted from 3D unit cell FE models, and closed-form approximate expressions were developed using the numerical results, curve fitting, and some simple bounds. The numerical approach was applied to predict warpage of production substrates, analyzed, and validated against experimentally measured stiffness and CTEs. In this paper, the homogenization approach, numerical predictions, and experimental validation are discussed.
Liquid Packaged MEMS Tunable Capacitors With Fast Response Times and Reduced Vibration Sensitivity
Authors: Borwick Robert L. , Stupar Philip A. , DeNatale Jeffrey
Published: April 2009

A new developed liquid packaging approach dramatically improves the performance of MEMS tunable capacitors. Previous devices suffered from slow response times due to mechanical ringing and poor vibration immunity. By packaging the device in the appropriate liquid, critical damping has been achieved and response times as fast as 50ms have been demonstrated.
Quantitative Evaluation of Crystal Rotation Behavior Around the Fatigue Crack in Copper Films
Authors: Shimizu Kenichi , Torii Tasiyuki
Published: April 2009

Using a fatigue testing method by which fatigue cracks can be initiated and propagated in a film adhered to cover a circular through-hole in a base plate subjected to pull-pull cyclic loads, annealed rolled pure copper films of 100μm thickness were fatigued. In order to discuss about the correlation between fatigue crack propagation and the change of crystal orientation, crystal orientation on the surface of the film materials was measured before and after fatigue testing. The crystallo-graphic information of these films was analyzed using the EBSD (Electron Back-scatter Diffraction) system and the quantitative evaluation method for the crystal rotation angle, the rotation axis and the rotation direction with fatigue testing was developed based on the analysis of crystal orientation matrix. As a result, the crystal rotation angle near the fatigue crack is larger than that apart from the crack and the crystal rotation was quantitatively larger around the transgranular crack than the intergranular crack propagated along the annealing twin boundary.
Exergy-Based Optimization Strategies for Multi-Component Data Center Thermal Management: Part I — Analysis
Authors: Shah Amip J. , Carey Van P. , Bash Cullen E. , Patel Chandrakant D.
Published: April 2009

As heat dissipation in data centers rises by orders of magnitude, inefficiencies such as recirculation will have an increasingly significant impact on the thermal manageability and energy efficiency of the cooling infrastructure. For example, prior work has shown that for simple data centers with a single Computer Room Air-Conditioning (CRAC) unit, an operating strategy that fails to account for inefficiencies in the air space can result in suboptimal performance. To enable system-wide optimality, an exergy-based approach to CRAC control has previously been proposed. However, application of such a strategy in a real data center environment is limited by the assumptions inherent to the single-CRAC derivation. This paper addresses these assumptions by modifying the exergy-based approach to account for the additional interactions encountered in a multi-component environment. It is shown that the modified formulation provides the framework necessary to evaluate performance of multi-component data center thermal management systems under widely different operating circumstances.
A Basic Study of Characteristics Between Stress and Deflection on Strength Test of Bio-Ceramics
Authors: Nakayama Mitsuyuki , Suzuki Hideto
Published: April 2009

Comparing the mechanical characteristics of bio-ceramics with the elaborate bone, there are two important conditions required for it. First, the strength must be three times larger than that of the elaborate bone. Second, Young’s modulus must be two or three times larger than that. However, the biomaterial which satisfies these conditions hasn’t developed. So, it is the main purpose of this research to clarify the relation between stress and deflection on strength test of bio-ceramics, and to get the fundamental data for improvement. In this paper, it is discussed how such quantity as deflection, stress and stress increasing rate are related. As the result, the following characteristics were found. First, deflection and the breaking stress increase as an average grain diameter of bio-ceramics becomes small. Second, the behavior of the stress increasing rate to deflection can be classified into two types. These are the fluctuation type and the stable type. Third, the experimental equations are obtained against the maximum deflection. One is for the breaking stress, and the other is for the maximum stress increasing rate. From the discussion, it was found that the characteristics of stress and deflection depended on such material properties of bio-ceramics as Young’s modulus, an average grain diameter, and micro-structure of the biomaterials.
Effect of the Device Structure in Electro-Thermal Analysis of Si CMOS
Authors: Hatakeyama Tomoyuki , Fushinobu Kazuyoshi , Okazaki Ken
Published: April 2009

Numerical calculation of submicron silicon MOSFET and CMOS device is performed. In order to have a higher degree of integration, the distance between two MOSFETs in CMOS structure can be decreased. But decreasing the distance between two MOSFETs results in an electrical interaction. In this research, by comparing the calculation result of n-type and p-type MOSFET and that of CMOS, we examine the interaction mechanism between n-type and p-type MOSFET in CMOS device when the distance between n-type and p-type MOSFET is decreased. From the calculated results, we investigate that the reason of the interaction between two MOSFET in CMOS is the forward bias at the p-n junction of substrate. Furthermore, we can estimate the distance, at the case of interaction, from the results of n-type and p-type MOSFET separately model, not from the results of CMOS model.
Multidisciplinary Placement Optimization of Heat Generating Electronic Components on Printed Circuit Boards Using Artificial Neural Networks
Authors: Suwa Tohru , Hadim Hamid A.
Published: April 2009

A multidisciplinary placement optimization methodology for heat generating electronic components on printed circuit boards (PCBs) is presented. The methodology includes thermal, electrical and placement criteria involving junction temperature, wiring density, line length for high frequency signals, and critical component location which are optimized simultaneously using the genetic algorithm. A board-level thermal performance prediction methodology which is based on a combination of a superposition method and artificial neural networks (ANNs) is developed for this study. Two genetic algorithms with different thermal prediction methods are used in a cascade in the optimization process. The first genetic algorithm is based on simplified thermal network modeling and it is mainly aimed at finding component locations that avoid any overlap. Compact thermal models are used in the second genetic algorithm leading to more accurate thermal prediction which improves the placement optimization obtained using the first algorithm. Using this optimization methodology, large calculation time reduction is achieved without losing accuracy. To demonstrate the capabilities of the present methodology, a test case involving component placement on a PCB is presented.
Subtractive Laser Processing of Low Temperature Inkjet Printed Micro Electric Components of Functional Nano-Ink for Flexible Electronics
Authors: Ko Seung Hwan , Chung Jaewon , Choi Yeonho , Hwang David J. , Grigoropoulos Costas P. , Poulikakos Dimos
Published: April 2009

The low temperature fabrication of electrical components on flexible substrate is presented in this paper. As an additive process, combined with a CAD tool, a drop-on-demand (DOD) inkjetting system was used to print electrical components from gold nano-particles suspended in Alpha-Terpineol solution on flexible polymer substrates. In a subtractive process, Nd:YAG pulsed laser was irradiated to produce finer electrical components. Continuous Ar laser was irradiated locally to evaporate carrier solution as well as to sinter gold nanoparticles. It is demonstrated for the first time, that the melting temperature depression of gold nanoink, combined with local laser heating and ablation can be used to fabricate micro electric components on low melting temperature polymer substrate to realize flexible electronics.
Numerical Modeling of Heat Transfer and Phase Transition in Programming the Ovonic Unified Memory Cells
Authors: Small Evan , Sadeghipour Sadegh M. , Asheghi Mehdi
Published: April 2009

An Ovonic Unified Memory (OUM) cell is a semiconductor device that stores data by a thermally induced phase transition between polycrystalline (set) and amorphous (reset) states in a thin film of chalcogenide alloy. The small volume of active media acts as a programmable resistor switching between a high (amorphous) and low (crystalline) resistance state. The change in the film resistivity (>40X dynamic range) caused by this rapid, reversible structural change is measured to detect the state of the cell (set or reset) for read out. OUM can benefit from a simulator capable of predicting the electrical, thermal, and crystallization behavior for design and optimization, particularly at the present stage of the development. This paper reports on the efforts being made to prepare such a numerical simulator, using an existing finite element computer code as the source for thermal and electrical modeling, and a custom crystallization code for phase prediction. Heat generation in the device is by Joule heating and is achieved by passage of the electric current, which is obtained from the electrical simulation. This result appears in the heat source term of the heat transfer equation that is solved for thermal modeling. As the first attempt the Ohmic current-voltage relation was implemented successfully to simulate set and reset in a two dimensional model of OUM. Solution of the drift-diffusion equation is now underway to capture the semiconductor behavior of the I-V curve. A good progress is made however, still more works needs to be done to fully implement the drift diffusion equation.
Indirect Spray Evaporative Thermal Management for Semiconductor Burn-In
Authors: Benjamin Michael A. , Odar Andrew M. , Steinthorsson Erlendur , Cotten Charles B.
Published: April 2009

Semiconductor burn-in testing is one of several quality assurance tests conducted during High Volume Manufacturing (HVM) of semiconductor logic devices. The goal of burn-in is to induce “infant mortality” component failures. To accelerate infant mortality defects, semiconductor devices are subjected to stressing techniques that induce heat levels, typically, 100%–300% greater than end use environment heat loads. For this work, an indirect spray cooling method was developed and experimentally evaluated. In the indirect method, sprays are sealed within a spraycap (evaporator) that is thermally connected with the heated surface by way of a thermal interface material. The test fluid is the perfluorocarbon HFE-7000 that has a boiling point of 34°C at 1 atm. pressure. Experiments were run at a spraycap nominal pressure of 1 atm. with about 16°C of liquid subcooling at the inlet. Tests were performed on a lidded Thermal Test Vehicle (TTV) device (1.2 cm2 die size) to measure the thermal solution maximum power, dynamic control, repeatability, and the effect of applied force. Time varying test patterns (thermal loads) are simulated by changing TTV power in 20 W steps up to 200 W. The pertinent output measurements for performance evaluation are TTV power and junction temperatures (Tj), thermocouple measurements in the heat path, coolant flow rate, and applied force to the TTV. From these measurements, resultant parameters of thermal resistance and heat transfer coefficients are calculated. Maximum TTV power maintaining Tj at or below 105°C was shown to approach 240 W. Thermal controllability of the system was demonstrated for a Tj of 105 °C over the TTV power range of 30 W to 200 W. Performance was extremely stable and very repeatable even when the spraycap exit quality was 100%. The thermal solution demonstrated good repeatability during a limited cycle test. Contact force of approximately 10 lbf (45 N) was found to minimize the thermal resistance of the solution, and no significant improvement is realized beyond that force level.
Porosity, Specific Surface Area and Effective Thermal Conductivity of Anisotropic Open Cell Lattice Structures
Authors: Balantrapu Kiran , Sarde Deepti Rao , Herald Christopher M. , Wirtz Richard A.
Published: April 2009

Open-cell box-lattice structures consisting of mutually orthogonal thermally conductive cylindrical ligaments can be configured to have wide ranging porosity, a large specific surface area and effective thermal conductivity in a particular direction together with specified structural characteristics. Thermal and mechanical properties can be tuned (and anisotropy introduced) by specification of different filament diameter and pitch for the vertical and horizontal filaments. Analytical models for porosity, specific surface area and effective thermal conductivity of lattice structures having different ligament diameters and pitches (anisotropy) are developed. The models show that all three of these quantities are functions of three dimensionless lengths.   This paper was also originally published as part of the Proceedings of the ASME 2005 Heat Transfer Summer Conference.
Solder Joint Reliability Prediction of Flip Chip Packages Under Shock Loading Environment
Authors: Loh Wei Keat , Garner Luke J.
Published: April 2009

Solder joint reliability under shock loading condition has been a concern over the years especially with constant reductions in the characteristic dimensions of the package and the solder joint. With the impeding transition to lead free solder, attention has moved from temperature cycle failure to mechanical shock failure. A method has been developed that employs a specially designed shock test board (STB) to characterize the solder joint performance as a function of board surface strain. This unique test board can be adapted to a wide range of test and boundary conditions. Using this board, a range of shock inputs is tested to establish correlation of the dynamic response and solder joint damage severity. The effects of package size, board thickness, and solder ball pitch are included in this paper to highlight the critical design factors for solder joint reliability in shock. This method can be used to characterize solder joint performance at the component level, early in development. One can also provide board strain based design limits guide system design and prevent late discovery of solder joint issues. Modal analysis based finite element models have been used to supplement understanding of the board response. The model demonstrates good correlation of the strain on the board and the stresses developed in the solder joint. The modeling data also suggested that the maximum principal stress at the solder joint face is a good failure criterion as it maps well with the actual failure locus found.
Experimental Thermal Characterization of Pin Fin Heat Sinks With Top and Side Bypass
Authors: Lei Ning , Ortega Alfonso
Published: April 2009

Extensive tests were conducted to obtain thermal characteristics of pin fin heat sinks with pins of square cross-section under different top and side bypass conditions. A consistent set of aluminum heat sinks with constant planform area and variable pitch were utilized. The heat sinks were tested in an adjustable cross-section wind tunnel, which provides 0 to 1 top and 0 to 10 side clearance ratios. The base temperature of the heat sinks was measured under constant heat flux. The overall thermal resistance and average heat transfer coefficient of the heat sinks were calculated and the influence of top and side bypass was explored. The thermal experimental data were consistent with the previous hydraulic experimental data. With the increase of flow velocity, flow transition from laminar to turbulent flow was revealed by the thermal resistance data. It was found that the average heat transfer coefficient depends strongly on the flow velocity through the heat sinks. Heat sinks with side clearance only result in smaller overall thermal resistance compared to top clearance only at the same clearance ratio. In some cases, the heat sinks with a small side clearance had better thermal performance than the same heat sinks without clearance. The empirical equation for infinitely long tube bundles of circular cross-section was used to correlate the thermal experimental data.
A New Modified Crack Surface Displacement Extrapolation Method for Analysis of Package Delamination
Authors: Hu Guojun , Tay Andrew A. O.
Published: April 2009

Based on a new second-order analysis of the bimaterial interface crack, a modified crack surface displacement extrapolation method (MCSDEM) for evaluating the stress intensity factor (SIF) and mode mixity at a crack has been developed. Unlike existing methods which require extrapolation of values of SIFs with the concomitant uncertainties, the new method engenders a constant SIF over a considerable distance from the crack tip. The new method is also much less sensitive to mesh density than existing methods. The new MCSDEM is then used to investigate the effects of temperature, moisture diffusion and interface vapor pressure on the delamination of the interface between the leadframe pad and the encapsulant of a plastic quad flat pack (PQFP) during solder reflow for both eutectic solder and leadfree solder. Using a 160-leaded PQFP as a test vehicle, the effect of initial delamination size varying from 0.1 mm to 3.5 mm was studied. The results show that the relative contributions of temperature, moisture and interface vapor pressure is qualitatively different for solder reflow of leadfree soldered and eutectic soldered components.
Thermal Performance and Key Challenges for Future CPU Cooling Technologies
Authors: Sauciuc Ioan , Prasher Ravi , Chang Je-Young , Erturk Hakan , Chrysler Gregory , Chiu Chia-Pin , Mahajan Ravi
Published: April 2009

Over the past few years, thermal design for cooling microprocessors has become increasingly challenging mainly because of an increase in both average power density and local power density, commonly referred to as “hot spots”. The current air cooling technologies present diminishing returns, thus it is strategically important for the microelectronics industry to establish the research and development focus for future non air-cooling technologies. This paper presents the thermal performance capability for enabling and package based cooling technologies using a range of “reasonable” boundary conditions. In the enabling area a few key main building blocks are considered: air cooling, high conductivity materials, liquid cooling (single and two-phase), thermoelectric modules integrated with heat pipes/vapor chambers, refrigeration based devices and the thermal interface materials performance. For package based technologies we present only the microchannel building block (cold plate in contact with the back-side of the die). It will be shown that as the hot spot density factor increases, package based cooling technologies should be considered for more significant cooling improvements. In addition to thermal performance, a summary of the key technical challenges are presented in the paper.   This paper was also originally published as part of the Proceedings of the ASME 2005 Heat Transfer Summer Conference.
Wafer Level Package Challenges: Fabrication Methodology, Packaging Infrastructure and Die-Shrink Considerations
Authors: Solberg Vern
Published: April 2009

Although the concept of wafer level packaging (“WLP”) is not new, the industry has yet to widely adopt WLP for volume manufacturing. Before the industry can do so, it must first overcome several challenges. The following are considered to be of primary concern: 1. High volume WLP fabrication methodology must become widely available. While several companies have demonstrated redistribution techniques that could prove practical for volume manufacturing of WLP, most of these techniques are proprietary and not available for wide adoption. 2. WLP manufacturers must leverage the existing assembly infrastructure as much as possible. Otherwise, WLP manufacturers will not be able to keep costs down. Unfortunately, with respect to testing and handling, the existing infrastructure will likely have to be extended. 3. WLP poses the new problem of designing a die-sized package with a footprint that remains constant despite die-shrink and other changes that are likely to occur from one generation of a die to the next. During the planning phase of the product developers must maintain two significant features: the contact size and pitch selected for the wafer level packaged IC must remain consistent from one die generation to the other, and the array pattern must accommodate efficient conductor routing on the circuit board. In this paper, the author will discuss these issues and explore a wafer level package methodologies, that addresses key aspects of the existing package assembly infrastructure that could be leveraged or extended, and the criteria for defining a practical contact array pattern with consideration of die shrink projections.
A Novel Scheme in Thermal Modeling of Printed Circuit Boards
Authors: Liu Wenjun , Lee Min-Young , Shabany Younes , Asheghi Mehdi
Published: April 2009

This paper presents a simple yet novel analytical approach to model the heat conduction in a Printed Circuit Board (PCB) by taking advantage of the large thermal conductivity contrast between the copper and glass-epoxy layers. The model provides a compact expression for the effective thermal resistance of a PCB and captures an approximate 2-dimensional temperature distribution within the PCB copper layer using simple one-dimensional fin equations in successive copper-glass epoxy layers. The results for effective thermal resistance and temperature distributions in copper layers agree within ±10% of those predicted using finite element (FEM) simulations. The present approach can significantly improve the system level thermal modeling and design of single and multi-component PCBs.
An Alternative Method for the Cooling of Power Microelectronics Using Classical Refrigeration
Authors: Chiriac Victor , Chiriac Florea
Published: April 2009

Classical refrigeration using vapor compression has been widely applied over the last decades to large-scale industrial systems, with few known applications to the microelectronics cooling field, due to the small size limitation. The present study proposes an efficient mechanical refrigeration system to actively cool the electronic components populating a Printed Circuit Board in High-Power Microelectronics System. The proposed system includes several miniaturized components — compressor, evaporator, condenser — part of a refrigeration system designed to fit the smaller scale power electronics. The system is thermally optimized to reach high COP (Coefficients of Performance). An array of micro-channels is used for the evaporator/condenser units. A previous study indicated that the R-134s refrigerant provides the best COP/feasibility ratio, while being the most suitable for microelectronics applications [1]. The present study develops an analytical model of the proposed small scale vapor compression refrigerator using the R-134a refrigerant. The refrigeration system is thermally optimized for cooling powers ranging from 20–100W, with the COP of the system reaching values up to 4.5. In the final section of the study, the efficiency of the proposed system is further compared to existing active cooling techniques using Thermoelectric Coolers (TEC). The advantages of the proposed system are highlighted, establishing a baseline performance vs. size relationship for vapor-compression refrigerators, to serve as the basis for comparison for future miniaturized refrigeration systems.
Thermal Performance of Thermally Enhanced PBGA Packages
Authors: Neelakantan Sriram , Joiner Bennett
Published: April 2009

Enhancement of thermal performance of a PBGA (plastic ball grid array) is often accomplished by embedding metal layers in the substrate and the inclusion of a copper spreader in the mold compound. The thermal effectiveness of the substrate planes in spreading the heat is frequently compromised by the necessity to separate the various voltages on the power plane and the required clearance regions around the electrical vias. Determining the appropriate effective conductivity has frequently been addressed by engineering judgment. Detailed simulation of the entire package, inclusive of structural detail in all the substrate planes, is very time consuming and inefficient. In the paper, the determination of the effective conductivity is accomplished with a simplified finite element representation of each metal layer capturing the connectivity of the traces or the metal patterns in the planes. The appropriate conductivities are then used in a detailed CFD model. The two stage process was chosen to model the performance of the thermally enhanced PBGA with and without a copper spreader placed at the top of the mold compound. A test case of a 516 27×27 mm PBGA was used. The model results are compared against experimental measurements. The CFD model was used to determine the performance with heat sinks in natural and forced convection environments and the results compared to experimental measurements.
Mechanical Modeling of a Solder Thermal Interface Material: Implications for Thermo-Mechanical Reliability
Authors: Subramanian Sankara J.
Published: April 2009

This paper addresses cracking in solder thermal interface materials (STIMs) used in electronic packages under accelerated testing or service conditions. Finite-element models of various packages have been built to study the deformation in the STIM through a few cycles of accelerated testing. Two commonly observed failure modes — center/off-center brittle interfacial cracking, and cohesive corner cracking — were looked at. The success of the modeling approach was evaluated by comparison with thermal impedance data, as well as with CSAM images showing the extent of cracking in the STIM. It is shown that the models agree qualitatively with experimental data, both in terms of failure locations, as well as in terms of rank ordering different packages in terms of STIM degradation.
On Measurement of Effective Silicon Backend Strength Using Bump Pull/Shear Techniques
Authors: Sane Sandeep , Tandon Shalabh , Chandran Biju , Alazar Tsgereda , Sorenson Leonard R.
Published: April 2009

Integrating a low-K ILD layer within silicon is key to reducing RC delays. However, low-K ILD materials typically have low mechanical strength, making their incorporation with lead free interconnects an industry-wide challenge. It is well known that conversion to lead free first level interconnects increases die backend stresses due to the higher melting temperature and increased solder stiffness. The paper will focus on the measurement of the effective silicon backend strength after subjecting the dice to different fabrication and assembly steps. The effective strength will also be evaluated post reliability stress exposure to eventually understand the life of these films. The paper will describe how a commercially available Dage 4000 tool was modified for this application. Bump pull was carried out using a 100μm tweezers, while bump shear used 1mil (25.4μm) wide stylus. Static and dynamic calibration was first carried out to ensure repeatability and reproducibility of the results. Peak force and failure modes were used as metrics to compare the effectiveness of different experimental legs. Traditional failure analysis approach of mechanical polishing, or when needed, use of FIB for sample preparation, with subsequent SEM/EDX analysis was utilized to understand the failure mechanism. Data suggests that shear and pull lead to different failure modes. Bump shear mainly led to failure at the bump/polyimide interface and did not necessarily correspond to the weakest layer or interface in the silicon backend. Whereas bump-pull, which applies tensile force to the stack up, lead to failures in the weakest layer, typically the low-K ILD, in the silicon backend. Hence, bump pull provided the advantage over shear as it allowed evaluation of the weakest interface in the stack up. Two case studies are discussed to demonstrate on how bump pull/shear metrologies were used to understand the impact of different assembly/FAB process variables and highly accelerated steam test (HAST) reliability stress on silicon backend strength. First case study shows influence of assembly flux on silicon backend strength, while second case study describes impact of HAST on different FAB backend processes.