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Über dieses Buch

MEMS and Nanotechnology, Volume 4 represents one of eight volumes of technical papers presented at the Society for Experimental Mechanics Annual Conference on Experimental and Applied Mechanics, held at Uncasville, Connecticut, June 13-16, 2011. The full set of proceedings also includes volumes on Dynamic Behavior of Materials, Mechanics of Biological Systems and Materials, Mechanics of Time-Dependent Materials and Processes in Conventional and Multifunctional Materials; Optical Measurements, Modeling and, Metrology; Experimental and Applied Mechanics, Thermomechanics and Infra-Red Imaging, and Engineering Applications of Residual Stress.

Inhaltsverzeichnis

Frontmatter

Integrated process feasibility of hard-mask for tight pitch interconnects fabrication

Abstract
As scaling continues beyond nano-technology, integrated circuit reliability is gaining increasing concerns in IC (Integrated Circuit) fabrication technology with decreasing transistor gate size, and the impact of trace interconnect failure mechanisms on device performance and reliability will demand much more from integration schemes, interconnect materials, and processes. An optimal low-k dielectric material and their related deposition, pattern lithography, etching and cleaning are required to form dual-damascene interconnect patterns fabrication processes. As technology nodes advance to nanotechnology, metal hard-mask such as TiN is used to gain better etching selectivity and profile controlling to the low-k materials during the pattern etching process. A hard-mask scheme approach of interconnects patterning of wafer fabrication is the ability to transfer patterns into under layers with tightest optimal dimension control. Employing a hard-mask scheme in the fabrication process, successfully achieved lithography patterning, dry etch selectivity in high aspect ratio interconnects comparison with a non hard-mask process were discussed. An optimal planarization treatment of photo-resist, good etch selectivity, a feasible manufacturing integrated process of hard mask dual damascene scheme, optimal profile controlling the critical interconnects and good electrical device performances were studied for tight pitch damascene interconnect architecture.
Chun-Jen Weng

Thermoelectric Effects in Current Induced Crystallization of Silicon Microstructures

Abstract
We have observed melting of nanocrystalline silicon microwires self-heated through single high-amplitude microsecond voltage pulses which leads to growth from melt upon resolidification. The resolidified regions form two single-crystal domains for wires with sub-micrometer widths. The current densities (J) involved in this process are ~ 1-10 MA/cm2 for suspended wires, and ~ 10-100 MA/cm2 for wires on oxide. These extremely high current densities and the resulting high temperatures (~ 1700 K) and temperature gradients (~ 1 K/nm) along the microwires give rise to strong thermoelectric effects. The thermoelectric effects are characterized through capture and analysis of light emission from the self-heated wires biased with lower magnitude AC voltages (J < 5 MA/cm2). The hottest spot on the wires consistently appears closer to the lower potential end for n-type, and the higher potential end for p-type microwires. Experimental light emission profiles are used to verify the linear thermoelectric models and material parameters used for simulations. Good agreement between these experimental and simulated profiles indicates that the linear models can be used to predict the thermal profiles for current induced crystallization of microstructures. However, the linear models are expected to be insufficient to fully explain the thermoelectric processes for higher current densities and stronger thermal gradients that are generated by high-amplitude short duration pulses.
Gokhan Bakan, Niaz Khan, Helena Silva, Ali Gokirmak

Evaluation of Resistance Measurement Techniques in Carbon Black and Carbon Nano-tubes Reinforced Epoxy

Abstract
Two different resistance measurement techniques are used in an epoxy material reinforced separately with carbon black (CB) micro-particles and carbon nano-tubes (CNTs) to evaluate the effectiveness of both the techniques and type of reinforcement on damage detection under uni-axial tensile loading. Two techniques, namely traditional four-point probe (FPP) and fourcircumferential ring probe (FCRP) are employed and a constant current is applied through outer probes. The resulting voltage drop between inner probes is measured using a commercial high resolution electrometer based system. Since current density distribution in both techniques is different, the measured change in resistance (both qualitatively and quantitatively) is also different. In addition to change in current density due to different techniques, the size of conductive reinforcement also has significant impact on both current distribution and further change in resistance. CB reinforced epoxy showed very high percentage change in resistance against CNTs reinforced epoxy for both techniques. It was identified that CNTs reinforced epoxy showed no significant difference for both FPP and FCRP methods. However, for CB reinforced epoxy, significant difference in percentage change in resistance was observed for both resistance measurement methods.
Venkat K. Vadlamani, Vijaya B. Chalivendra, Arun Shukla, Sze Yang

A nano-tensile tester for creep studies

Abstract
Free-standing metallic thin films are increasingly used as structural components in MEMS. In commercial devices, long-term reliability is essential, which requires determining time-dependent mechanical properties of these films. The uniaxial tensile test is a preferred method due to uncomplicated determination of the stress and strain state. However, at the MEMS-scale this method is not straightforward: specimen handling and loading, force and deformation measurement need careful consideration. Here we discuss the challenges of the application and measurement of nano-Newton forces, nanometer deformations and micro-radians rotation alignment ensuring negligible bending in on-chip tensile test structures during long periods. We then present a novel tensile-testing instrument with in-situ capabilities in SEM and Optical Profilometry. The design solutions to measure these small forces and deformations whilst ensuring a uniaxial stress state will be presented.
L. I. J. C. Bergers, J. P. M. Hoefnagels, E. C. A. Dekkers, M. G. D. Geers

The Measurement of Cyclic Creep Behavior in Copper Thin Film Using Microtensile Testing

Abstract
A micro-tensile testing for studying the cyclic fatigue mechanical properties of freestanding copper thin film with thickness of sub-micrometer application for MEMS was performed to observe its mechanical response under tension-tension fatigue experiments with a variety of mean stress conditions at cyclic loading frequencies up to 20 Hz. Tensile sample loading was applied using a piezoelectric actuator. Loads were measured using a capacitance gap sensor with a mechanical coupling to the sample. The experiments were carried out with feedback to give load control on sputter deposited 300, 500 and 900 nm copper thin films. Loading cycles to failure reached over 10^6 at low mean load with a trend of decreasing cycles to failure with increasing mean load as anticipated. The cyclic fatigue results provided clear evidence for a cyclic creep rate dependent and change in failure mechanism from crack formation to extended plasticity as the mean load is decreased.
K.-S. Hsu, M.-T. Lin, C.-J. Tong

New Insight into Pile-up in Thin Film Indentation

Abstract
A new method of accurately and reliably extract the actual Young’s modulus of a thin film on a substrate has been developed. The method is referred to as the discontinuous elastic interface transfer model. The method has been shown to work exceptionally well with films and substrates encompassing a wide range of elastic moduli and Poisson ratios. The advantage of the method is that it does not require a continuous stiffness method and can use the standard Oliver and Pharr analysis and the use of a predictive formula for determining the modulus of the film as long as the film thickness, substrate modulus and bulk Poisson ratio of the film are known. However, when there is much pile-up during the indentation process in a softer film, the experimental data does not follow the predictive formula but instead follows a similar model with a single Poisson ratio between the film and the substrate.
B. C. Prorok, B. Frye, B. Zhou, K. Schwieker

Measuring Substrate-Independent Young’s Modulus of Thin Films

Abstract
Substrate influence is a common problem when using instrumented indentation (also known as nano-indentation) to evaluate the elastic modulus of thin films. Many have proposed models in order to be able to extract the film modulus (E f ) from the measured substrate-affected modulus, assuming that the film thickness (t) and substrate modulus (E s ) are known. Existing analytic models work well if the film is more compliant than the substrate. However, no analytic model accurately predicts response when the modulus of the film is more than double the modulus of the substrate. In this work, a new analytic model is reviewed. Using finite-element analysis, this new model is shown to be able to accurately determine film modulus (E f ) over the domain 0.1 < E f /E s < 10. Finally, the new model is employed to determine the Young’s modulus of low-k and silicon carbide films on silicon.
Jennifer Hay

Analysis of Spherical Indentation of an Elastic Bilayer Using a Modified Perturbation Approach

Abstract
Accurate mechanical property measurement of films on substrates by instrumented indentation requires a solution describing the effective modulus of the film/substrate system. Here, a first-order elastic perturbation solution for spherical indentation on a film/substrate is presented. Finite element method (FEM) simulations were conducted for comparison with the analytical solution. FEM results indicate that the new solution is valid for a practical range of modulus mismatch, especially for a stiff film on a compliant substrate. It also shows that effective modulus curves for the spherical punch deviates from those of the flat punch when the thickness is comparable to contact size. The work is applicable in tribological films and other engineering systems requiring hard, protective coatings.
Jae Hun Kim, Andrew Gouldstone, Chad S. Korach

Nano-indentation Studies of Polyglactin 910 Monofilament Sutures

Abstract
Nano-indentation studies using atomic force microscopy (AFM) were introduced to investigate the effects of hydrolysis degradation on mechanical properties of polyglactin 910 monofilament sutures. The polyglactin 910 sutures were immersed in phosphate buffered saline (PBS) solution of pH 5, 7.4, 10 without enzyme and pH 7.4 with esterase enzyme. After that, the samples were incubated at 37 oC under an oscillation of 80 rpm. Samples were removed for testing after 7, 14, 21 and 28 days. The effects of degradation on gradation of Young’s modulus values across fiber cross-section were studied by doing progressive nano-indentation from center to surface of cross section of the sutures. Results indicate that the pH 7.4 condition hydrolysis degradation did not have a significant impact on variation of Young’s modulus values of the polyglactin 910 sutures from the center to the surface after different degradation times. And the Young’s modulus from the original samples to sutures after 4 weeks degradation have a decreasing trend, but not including the first week. Then the SEM, FTIR and Tensile test were conducted to investigate the mechanical and chemical properties of polyglactin 910 monofilament sutures.
Leming Sun, Vijaya Chalivendra, Paul Calvert

Analytical approach for the determination of nanomechanical properties for metals

Abstract
A modified form of the two-slope method used for the determination of mechanical properties of a material is presented in this paper. Modified expressions for the determination of slopes of the loading and unloading curves make use of the energy based parameters which are independent of the indentation size. A correction factor is also introduced to account for the inward radial displacement of material’s surface points which has important implications on the accuracy of the mechanical properties. Mechanical properties obtained after these modifications compares well with the experimental results. The elastic modulus and hardness obtained by the proposed method precisely describe the elasto-plastic behavior of the metals considered in this study which further confirms the accuracy of the method described herein. The proposed method enhances our understanding of the behavior of a material at very small scale of length and may be extended to determine the mechanical properties of materials other than metals.
Kaushal K Jha, Nakin Suksawang, Arvind Agarwal

Advances in Thin Film Indentation

Abstract
A new method to accurately and reliably extract the actual Young’s modulus of a thin film on a substrate by indentation was developed. The method involved modifying the discontinuous elastic interface transfer model to account for substrate effects that were found to influence behavior even a few nanometers into a film several hundred nanometers thick. The method was shown to work exceptionally well for all 25 different combinations of 5 films on 5 substrates that encompassed a wide range of compliant films on stiff substrates to stiff films on compliant substrates. A predictive formula was determined that enables film modulus to be calculated as long as one knows the film thickness, substrate modulus and bulk Poisson’s ratio of the film and substrate. The calculated values of film modulus were verified with prior results that employed the membrane deflection experiment and resonance-based methods. The greatest advantages of the method are that the standard Oliver and Pharr analysis can be used and that it does not require the continuous stiffness method, enabling any indenter to be employed. The film modulus then can be accurately determined by simply averaging a handful of indents on a film/substrate composite.
B. Zhou, K. Schwieker, B. Frye, B. C. Prorok

Cyclic Nanoindentation Shakedown of Muscovite and Its Elastic Modulus Measurement

Abstract
A series of cyclic loading nanoindentation experiments with varied maximum loads (F max) of 0.05 to 2.0 mN were performed on a nanostructured, layered muscovite with loading direction normal to its basal plane. A critical load (e.g., 0.5 mN) exists that leads to distinct load-displacement curves: when F max is greater than this load, the loading/unloading curves, after a few initial cycles, become characteristic closed hysteresis loops, suggesting that shakedown process occur quickly; otherwise, only nonlinear elastic, completely overlapped hysteresis loops were observed. These phenomena result in two representative elastic moduli that depend on indentation depth. For F max (e.g., 0.05 and 0.1 mN) smaller than the critical load, the obtained elastic modulus at the nonlinear elastic state is nearly 88 GPa, which agrees with the reported Young’s modulus of the material; However, when F max exceeds the critical value, the measured modulus decreased to a lower constant value around 55 GPa, close to the bulk modulus of muscovite. The transition from a higher, true Young’s modulus to a lower bulk modulus can be attributed to the three dimensional confinement around the indenter tip after plastic shakedown at relatively larger depth, where significant alteration to the originally layered structure of muscovite has taken place.
Hang Yin, Guoping Zhang

Assessment of Digital Holography for 3D-Shape Measurement of Micro Deep Drawing Parts in comparison to Confocal Microscopy

Abstract
Fast and accurate measurement of the 3D-shape of mass fabricated parts is increasingly important for a cost effective production process. For quality control of mm or sub-mm sized parts tactile measurement is unsuitable and optical methods have to be employed. This is especially the case for small parts which are manufactured in a micro deep drawing process. From a number of measurement techniques capable to measure 3D-shapes, we choose Digital Holography and confocal microscopy for further evaluation. Although the latter technique is well established for measuring of micro parts, its application in a production line suffers from insufficient measurement speed due to the necessity to scan through a large number of measurement planes. Digital Holography on the other hand allows for a large depth of focus because one hologram enables the reconstruction of the wave field in different depths. Hence, it is a fast technique and appears superior to standard microscopical methods in terms of application in a production line. In this paper we present results of shape recording of micro parts using Digital Holography and compare them to measurements performed by confocal microscopy. The results prove the suitability of Digital Holography as an inline quality control instrument.
Nan Wang, Claas Falldorf, Christoph von Kopylow, Ralf B. Bergmann

Full-Field Bulge Testing Using Global Digital Image Correlation

Abstract
The miniature bulge test is an acknowledged method for characterizing freestanding thin films. Nevertheless, some discrepancies in the quantitative results from such tests can be found in literature, explained in part by erroneous assumptions in the analytical description used to compute the global stress and strain from the membrane pressure and deflection. In this research, a new method is presented which renders the analytical description obsolete. A specialized Global Digital Image Correlation technique on high resolution, confocal microscopy, surface height maps of bulged membranes, has been developed. This method is able to capture full-field continuous deformation maps, from which local strain maps are computed. Additionally, local stress maps are derived from full-field curvature maps and the applied pressure. The local stress-strain maps allow the method to be used on inhomogeneous, anisotropic membranes as well as on exotic membrane shapes.
Jan Neggers, Johan Hoefnagels, François Hild, Stéphane Roux, Marc Geers

Experimental Investigation of Deformation Mechanisms Present in Ultrafine-Grained Metals

Abstract
Ultrafine-grained (UFG) metals possess grain sizes on the order of hundreds of nanometers and display a remarkable capacity for high strength, high ductility, and enhanced superplasticity. This paper presents the preparatory steps necessary for a highresolution experimental investigation into the deformation mechanisms active in UFG metals. A new experimental methodology is used, in which an optical metrology known as Digital Image Correlation (DIC) is combined with scanning electron microscopy to track the quantitative development of full-field strains on the length scale of the microstructure. The micro-scale field of view and use of a SEM for image capture require the development of novel specimen patterning methods and of image distortion corrections prior to experimentation. The results obtained through the combined SEM and DIC approach, and corresponding pre- and post-mortem electron backscatter diffraction (EBSD) analysis, enable the analysis of the real-time, micro-scale evolution of Lagrangian strains at an unprecedented spatial resolution, and the quantification of the surface deformations inside grains and across grain boundaries as the material is subjected to thermo-mechanical loading.
Adam Kammers, Samantha Daly

Characterization of a Variation on AFIT’s Tunable MEMS Cantilever Array Metamaterial

Abstract
Metamaterials are devices with embedded structures that provide the device with unique properties. Several applications for metamaterials have been proposed including electromagnetic cloaks, lenses with improved resolution over traditional lenses, and improved antennas. This research addresses an obstacle to practical metamaterial development, namely the small bandwidth of current metamaterial devices. This research characterizes the effectiveness of several metamaterial designs. The basic design incorporates a microelectromechanical systems (MEMS) variable capacitor into a double negative (DNG) metamaterial structure. One set of devices is fabricated with the MEMS capacitor in the gap of the split ring resonator (SRR) of the DNG metamaterial. Applying voltage to the MEMS device changes the effective capacitance, thereby adjusting the resonant frequency of the device. Additionally, similar devices with three possible capacitor layouts are examined with stripline measurements and computer models. Recommendations for design improvements are provided. The initial capacitor layout with MEMS capacitors in the split ring gaps is recommended for future design iterations with adjusted gap capacitance values.
Matthew E. Jussaume, Peter J. Collins, Ronald A. Coutu

MEMS for real-time infrared imaging

Abstract
This project investigates an innovative approach to imaging with Micro Electro-Mechanical Systems (MEMS) based devices. By using a Linnik interferometer and advanced phase unwrapping algorithms for processing data, the feasibility of generating high-resolution grayscale images in real-time was proven with an array of individually addressable MEMS micro-mirrors. Further investigations on a thermal imaging detector consisting of an array of pixels defined by surface micromachined bimaterial beam structures were carried out. A thermal loading fixture was manufactured and incorporated into the interferometer setup, which was also optimized to provide high measuring resolution. Interferometric images were collected at several temperatures in order to determine the beams’ response as a function of temperature, which successfully demonstrated the suitability of the detector to imaging with high-sensitivity and with a linear response. Experimental results were used with analytical and computational models to further predict the thermo-mechanical characteristics of the beams and to perform parametric investigations and optimization of their design. Further developments will consist of integrating the detector into a highly advanced, completely mechanical, imaging device having mK thermal resolution. The availability of such device will greatly improve current thermal imaging technology.
I. Dobrev, Marc Balboa, Ryan Fossett, C. Furlong, E. J. Harrington

New Insights into Enhancing Microcantilever MEMS Sensors

Abstract
Damping effects on different geometries were investigated by testing them in air at different pressure levels, ranging from the atmospheric pressure of 105 Pa to 10-2 Pa. The resulting responses of these geometries followed the same trend as the analytical plot for the rectangular shape structure. As the relative resonant frequency of the structure is proportional to the intrinsic resonant frequency, measured at the lowest pressure levels achieved by the AFM system, different shapes showed different amount of responses as a function of pressure. As the intrinsic resonant frequency of the triangular shape was the highest, its relative resonant frequency was the highest; the modified geometry showed intermediate responses among the three geometries.
S. Morshed, B. C. Prorok

A Miniature MRI-Compatible Fiber-optic Force Sensor Utilizing Fabry-Perot Interferometer

Abstract
Magnetic resonance imaging provides superior imaging capability because of unmatched soft tissue contrast and inherent three-dimensional visualization. Force sensing in robot-assisted systems is crucial for providing tactile feedback and measuring tissue interaction forces in needle-based percutaneous procedures in MRI. To address the issues imposed by electromagnetic compatibility in the high-field MRI and mechanical constraints due to the confined close-bore space, this paper proposes a miniaturized fiber optic force sensor utilizing Fabry-Perot interferometry. An opto-electromechanical system is designed to experimentally validate the optical model of the sensor and evaluate its sensing capability. Calibration was performed under static and dynamics loading conditions. The experimental results indicate a gage sensitivity on the order of 40 (mV/με) of the sensor and a sensing range of 10 Newton. This sensor achieves high-resolution needle insertion force sensing in a robust and compact configuration in MRI environment.
Hao Su, Michael Zervas, Cosme Furlong, Gregory S. Fischer

Micromechanical Structure With Stable Linear Positive And Negative Stiffness

Abstract
We introduce a novel micromechanical structure that exhibits two regions of stable linear positive and negative stiffness. Springs, cantilevers, beams and any other geometry that display an increasing return force that is proportional to the displacement can be considered to have a “Hookean” positive spring constant, or stiffness. Less well known is the opposite characteristic of a reducing return force for a given deflection, or negative stiffness. Unfortunately many simple negative stiffness structures demonstrate either unstable buckling which can require extraneous moving constraints during deflection, so as not to deform out of useful shape, or are highly nonlinear such as the disk cone spring. In MEMS, buckling caused by stress at the interface of silicon and thermally grown SiO2 causes tensile and compressive forces that will warp structures if the silicon layer is thin enough. The structure presented here utilizes this effect but overcomes its limitations and empirically demonstrates linearity in both regions. The structure is manufactured using only common micromachining techniques and can be made in situ with other devices.
Jeffrey P. Baugher, Ronald A. Coutu

Terahertz Metamaterial Structures Fabricated by PolyMUMPs

Abstract
We present a novel approach for the fabrication of terahertz (THz) metamaterial structures utilizing PolyMUMPs, a foundry process commonly used in the fabrication of microelectricalmechanical systems (MEMS) devices. The structure has an alternating composition consisting of three polysilicon layers and two silicon dioxide layers each with a unique thickness. A split ring resonator (SRR) structure was fabricated with dimensions to support resonance around 5 THz. The structures were arrayed to cover a 1 cm2 area. The backside of the samples was polished to improve the transmission characteristics of the material during Fourier transform spectroscopy measurements. The data indicates a transmission null around 3.7 THz due to the periodic arrangement of the SRR structures. These results are encouraging for future use of PolyMUMPs in terahertz metamaterial designs which is ideal for the repeatability the manufacturing process lends to the design.
Elizabeth A. Moore, Derrick Langley, Ronald A. Coutu

Investigations Into 1D and 2D Metamaterials at Infrared Wavelengths

Abstract
Investigations are made into the characterization of 1D metamaterials consisting of stacks of metal and dielectric. These stacks are modeled and designed to have a permittivity approaching zero. Simulation, fabrication and testing are conducted to verify the design of the layered material. These stacks are fabricated using magnetron sputtering and tested using Fourier Transform Infrared Spectroscopy (FTIR). Comparison between modeled and measured reflection and transmission are used to determine if the fabricated structure is behaving like a homogeneous material. Collected results indicate that a homogeneous structure was structures were formed, one with a possible low permittivity.
Jack P. Lombardi, Ronald A. Coutu

MEMS integrated metamaterials with variable resonance operating at RF frequencies

Abstract
Metamaterials are engineered materials with integrated structures designed to produce a resonant response at specific frequencies. The capacitive and inductive properties of metamaterials effect the overall refractive index of the media in which an RF signal propagates by generating a resonant frequency response. Incorporating microelectromechanical systems (MEMS) into the structure adds the ability to tune metamaterials and generate a variable resonance. In this investigation, a resonant response is achieved for the 1 – 4 GHz range with tuning. Employing processing techniques to create microelectronic devices, different metamaterial designs are fabricated on quartz substrates. Using these modifications, a design which provides the ideal shift in resonance is selected to incorporate into future RF systems. This paper reports on the modeling, design, fabrication and testing of various designs of metamaterials incorporated with MEMS.
Derrick Langley, Elizabeth A. Moore, Ronald A. Coutu, Peter J. Collins

Creep measurements in free-standing thin metal film micro-cantilever bending

Abstract
Creep is a time-dependent deformation mechanism that affects the reliability of metallic MEMS. Examples of metallic MEMS are RF-MEMS capacitors/switches, found in wireless/RF applications. Proper modeling of this mechanism is yet to be achieved, because size-effects that play a role in MEMS are not well understood. To understand this better, a methodology is setup to study creep in Al-Cu alloy thin film micro-cantilevers micro-fabricated in the same MEMS fabrication process as actual RF-MEMS devices. The methodology entails the measurement of time-dependent deflection recovery after maintaining cantilevers at a constant deflection for a prolonged period. Confocal profilometry and a simple mechanical setup with minimal sample handling are applied to control and measure the deformation. Digital image correlation, leveling and kinematics-based averaging algorithms are applied to the measured surface profiles to correct for various errors and improve the precision to yield a precision < 7% of the surface roughness. A set of measurements is presented in which alloy microstructure length scales at the micrometer-level are varied to probe the nature of this creep behavior.
L. I. J. C. Bergers, J. P. M. Hoefnagels, M. G. D. Geers

MEMS Reliability for Space Applications by Elimination of Potential Failure Modes through Analysis

Abstract
As the design of Micro-Electro-Mechanical System (MEMS) devices matures and their application extends to critical areas, the issues of reliability and long-term survivability become increasingly important. This paper reviews some general approaches to addressing the reliability and qualification of MEMS devices for space applications. The failure modes associated with different types of MEMS devices that are likely to occur, not only under normal terrestrial operations, but also those that are encountered in the harsh environments of space, will be identified.
Rohit Soni

Analysis and Evaluation Methods Associated with the Application of Compliant Thermal Interface Materials in Multi-chip Electronic Board Assemblies

Abstract
Increased demands on large scale server system packaging density have driven the need for new, more challenging electronic component cooling solutions. One such application required the development of a large form-factor printed circuit board assembly with multiple power transformer devices to be cooled via a common heat spreader. Thermally coupling the multiplicity of devices to the heat spreader was completed using a compliant thermal interface material. Given the mechanical tolerance range, the strain rate dependency of the interface material and the mechanical load limitations of the electronic devices, finite element analysis and empirical evaluation techniques were applied to ensure the anticipated interface gaps were established and that the initial and residual mechanical loading effects were understood. A characterization of the thermal interface material’s mechanical properties was completed for analysis input. Coupling this input with the geometric and stiffness properties of the assembly’s structural elements provided predictions of both the initial as well as the residual mechanical assembly loads. Once completed, experiments using pressure sensitive film and piezoresistive film load cells were completed to correlate with the acquired analytical predictions.
John Torok, Shawn Canfield, David Edwards, David Olson, Michael Gaynes, Timothy Chainer

Hierarchical Reliability Model for Life Prediction of Actively Cooled LED-Based Luminaire

Abstract
The interest in light-emitting diodes (LEDs) for illumination applications has been increasing continuously over the last decade due to two key attributes of long lifetime and low energy consumption compared to the conventional incandescent light and compact fluorescent light. Although LEDs are attractive for lighting applications due to the aforementioned advantages, unique technical challenges, such as the extreme sensitivity of luminous output and useful lifetime to LED junction temperature, need to be overcome for their large-scale commercialization.
Bong-Min Song, Bongtae Han, Avram Bar-Cohen, Rajdeep Sharma, Mehmet Arik

Direct Determination of Interfacial Traction-Separation Relations in Chip-Package Systems

Abstract
Microelectronic devices are multilayered structures with many different interfaces. Their mechanical reliability is of utmost importance when considering the implementation of new materials. The cohesive interface modeling approach has the capability of modeling crack nucleation and growth, provided interfacial parameters such as strength and toughness of the system are available. These parameters are obtained through the extraction of traction-separation relations, through indirect either hybrid numerical/experimental methods or direct experimental methods. The direct method promises to determine the parameters in an unambiguous manner. All methods of extracting traction-separation relations require some local feature of the crack-tip region to be measured.
Shravan Gowrishankar, Haixia Mei, Kenneth M. Liechti, Rui Huang
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