Mechanical Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2795
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Item Integrated Measurement Technique To Measure Curing Process-dependent Mechanical And Thermal Properties Of Polymeric Materials Using Fiber Bragg Grating Sensors(2009) Wang, Yong; Han, Bongtae; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)An innovative technique based on a fiber Bragg grating (FBG) sensor is proposed to measure the critical mechanical and thermal properties of polymeric materials. The properties include (1) chemical shrinkage evolution during curing, (2) modulus evolution during curing, (3) glass transition temperature (4) coefficient of thermal expansion (CTE), and (5) visco-elastic properties. Optimum specimen configurations are proposed from the theoretical analysis. Then an efficient numerical procedure is established to determine the material properties from the measured Bragg wavelength (BW) shift. The technique is implemented with various polymeric materials. The measured quantities are verified through a self-consistency test as well as the existing testing methods such as a warpage measurement of a bi-material strip, and a TMA measurement. The evolution properties obtained at a curing temperature are extended further by combining them with the conventional isothermal DSC experiments. Based on the existing theories, the evolution properties can be predicted at any temperatures. The proposed technique greatly enhances the capability to characterize the mechanical properties and behavior of polymeric materials. Since the specimen preparation is very straightforward, the proposed method can be routinely practiced and the measurement can be completely automated. It will provide a much-needed tool for rapid but accurate assessment of polymer properties, which, in turn, will enhance the accuracy of predictive modeling for design optimization of a microelectronics product at the conceptual stage of product development.Item EVOLUTION OF THE MICROSTRUCTURE AND VISCOPLASTIC BEHAVIOR OF MICROSCALE SAC305 SOLDER JOINTS AS A FUNCTION OF MECHANICAL FATIGUE DAMAGE(2009) Cuddalorepatta, Gayatri; Dasgupta, Abhijit; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The effect of mechanical cycling fatigue damage and isothermal aging histories on the evolution of the constitutive and fatigue responses, and microstructure of microscale SAC305 solder joints is investigated. In particular, the study examines if joint dependent behavior should be expected from as-fabricated and cycled microscale SAC305 joints that exhibit an initial non-homogenous coarse-grained Sn microstructure. In addition, the ability of traditionally used macroscale constitutive models based on continuum mechanics to represent the viscoplastic constitutive behavior of the non-homogenous as-fabricated microscale SAC305 specimens is explored. Insights into the effect of key microstructural features and dominant creep mechanisms influencing the measured viscoplastic behavior of SAC305 are provided using a multi-scale mechanistic modeling framework. Modified lap-shear microscale SAC305 specimens are characterized using the thermomechanical microscale test setup (TMM). Microscale SAC305 solder specimens show significant piece-to-piece variability in the viscoplastic constitutive properties under identical loading histories in the as-fabricated state. The mechanical response is strongly influenced by the grain microstructure across the entire joint, which is non-repeatable and comprises of very few highly anisotropic Sn grains. The statistical non-homogeneity in the microstructure and the associated variability in the mechanical properties in the microscale SAC305 test specimen are far more significant than in similar Sn37Pb specimens, and are consistent with those reported for functional microelectronics solder interconnects. In spite of the scatter, as-fabricated SAC305 specimens exhibit superior creep-resistance (and lower stress relaxation) than Sn37Pb. Macroscale creep model constants represent the non-homogeneous behavior of microscale joints in an average sense. Macroscale modeling results show that the range of scatter measured from macroscale creep model constants is within the range of scatter obtained from the stress relaxation predictions. Stress relaxation predictions are strongly sensitive to the inclusion or exclusion of primary creep models. The proposed multiscale framework effectively captures the dominant creep deformation mechanisms and the influence of key microstructural features on the measured secondary creep response of microscale as-fabricated SAC305 solder specimens. The multiscale model predictions for the effect of alloy composition on SAC solders provide good agreement with test measurements. The multiscale model can be extended to understand the effects of other parameters such as aging and manufacturing profiles, thereby aiding in the effective design and optimization of the viscoplastic behavior of SAC alloys. Accumulated fatigue damage and isothermal aging are found to degrade the constitutive and mechanical fatigue properties of the solder. The scatter gradually decreases with an increasing state of solder damage. Compared to the elastic-plastic and creep measurements, the variability in the fatigue life of these non-homogenous solder joints under mechanical fatigue tests is negligible. Recrystallization is evident under creep and mechanical fatigue loads. Gradual homogenization of the Sn grain microstructure with damage is a possible reason for the observed evolution of scatter in the isothermal mechanical fatigue curves. The yield stress measurements suggest that SAC305 obeys a hardening rule different from that of isotropic or kinematic hardening. The measured degradation in elastic, plastic and yield properties is captured reasonably well with a continuum damage mechanics model from the literature.Item Evaluation of solder-joint reliability for a 10mm Quad Flat Leadless package with top-side paddle using classical models for a leadless device and accelerated life testing(2009) Levin, Mark Alan; Barker, Dr. Donald; Reliability Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The standard QFN package consists of a leadless perimeter array and a bottom solderable thermal paddle. The thermal performance of the package can be improved by moving the paddle to the topside. The soldered surface area of the package reduces by about 80% with a top-side paddle. The soldered-joint life will also reduce due to the significant thermal coefficient of expansion mismatch between the QFN package and the circuit board. The solder-joint reliability of a large QFN package with top-side paddle is not well understood. This thesis evaluates the solder-joint reliability of a 10mm square leadless QFN package with top-side paddle. The analysis includes several classical models for a leadless package and compares modeling results to accelerated reliability testing. The accelerated tests include the influence mold compound and lead finish play on solder-joint life and ways to improve solder-joint reliability.Item Single and Multiresponse Adaptive Design of Experiments with Application to Design Optimization of Novel Heat Exchangers(2009) Aute, Vikrant Chandramohan; Azarm, Shapour; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Engineering design optimization often involves complex computer simulations. Optimization with such simulation models can be time consuming and sometimes computationally intractable. In order to reduce the computational burden, the use of approximation-assisted optimization is proposed in the literature. Approximation involves two phases, first is the Design of Experiments (DOE) phase, in which sample points in the input space are chosen. These sample points are then used in a second phase to develop a simplified model termed as a metamodel, which is computationally efficient and can reasonably represent the behavior of the simulation response. The DOE phase is very crucial to the success of approximation assisted optimization. This dissertation proposes a new adaptive method for single and multiresponse DOE for approximation along with an approximation-based framework for multilevel performance evaluation and design optimization of air-cooled heat exchangers. The dissertation is divided into three research thrusts. The first thrust presents a new adaptive DOE method for single response deterministic computer simulations, also called SFCVT. For SFCVT, the problem of adaptive DOE is posed as a bi-objective optimization problem. The two objectives in this problem, i.e., a cross validation error criterion and a space-filling criterion, are chosen based on the notion that the DOE method has to make a tradeoff between allocating new sample points in regions that are multi-modal and have sensitive response versus allocating sample points in regions that are sparsely sampled. In the second research thrust, a new approach for multiresponse adaptive DOE is developed (i.e., MSFCVT). Here the approach from the first thrust is extended with the notion that the tradeoff should also consider all responses. SFCVT is compared with three other methods from the literature (i.e., maximum entropy design, maximin scaled distance, and accumulative error). It was found that the SFCVT method leads to better performing metamodels for majority of the test problems. The MSFCVT method is also compared with two adaptive DOE methods from the literature and is shown to yield better metamodels, resulting in fewer function calls. In the third research thrust, an approximation-based framework is developed for the performance evaluation and design optimization of novel heat exchangers. There are two parts to this research thrust. First, is a new multi-level performance evaluation method for air-cooled heat exchangers in which conventional 3D Computational Fluid Dynamics (CFD) simulation is replaced with a 2D CFD simulation coupled with an e-NTU based heat exchanger model. In the second part, the methods developed in research thrusts 1 and 2 are used for design optimization of heat exchangers. The optimal solutions from the methods in this thrust have 44% less volume and utilize 61% less material when compared to the current state of the art microchannel heat exchangers. Compared to 3D CFD, the overall computational savings is greater than 95%.Item End-of-Life and Constant Rate Reliability Modeling for Semiconductor Packages Using Knowledge-Based Test Approaches(2009) Yang, Liyu; Bernstein, Joseph B; Reliability Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)End-of-life and constant rate reliability modeling for semiconductor packages are the focuses of this dissertation. Knowledge-based testing approaches are applied and the test-to-failure approach is approved to be a reliable approach. First of all, the end-of-life AF models for solder joint reliability are studied. The research results show using one universal AF model for all packages is flawed approach. An assessment matrix is generated to guide the application of AF models. The AF models chosen should be either assessed based on available data or validated through accelerated stress tests. A common model can be applied if the packages have similar structures and materials. The studies show that different AF models will be required for SnPb solder joints and SAC lead-free solder joints. Second, solder bumps under power cycling conditions are found to follow constant rate reliability models due to variations of the operating conditions. Case studies demonstrate that a constant rate reliability model is appropriate to describe non solder joint related semiconductor package failures as well. Third, the dissertation describes the rate models using Chi-square approach cannot correlate well with the expected failure mechanisms in field applications. The estimation of the upper bound using a Chi-square value from zero failure is flawed. The dissertation emphasizes that the failure data is required for the failure rate estimation. A simple but tighter approach is proposed and provides much tighter bounds in comparison of other approaches available. Last, the reliability of solder bumps in flip chip packages under power cycling conditions is studied. The bump materials and underfill materials will significantly influence the reliability of the solder bumps. A set of comparable bump materials and the underfill materials will dramatically improve the end-of-life solder bumps under power cycling loads, and bump materials are one of the most significant factors. Comparing to the field failure data obtained, the end-of-life model does not predict the failures in the field, which is more close to an approximately constant failure rate. In addition, the studies find an improper underfill material could change the failure location from solder bump cracking to ILD cracking or BGA solder joint failures.Item Evaluation of Environmental Tests for Tin Whisker Assessment(2009) Panashchenko, Lyudmyla; Osterman, Michael D; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Tin whiskers are electrically conductive crystalline structures of tin that over time may grow outward from tin-rich surfaces and present a reliability hazard to electronic systems. While the problem has been known for decades, no satisfactory explanation of whisker growth mechanisms exists, leaving the industry to create whisker-assessment tests based on empirical data gathered under various environmental storage conditions controlled for temperature, humidity and temperature cycling. The long-term predictability of these environmental storage tests has not been addressed and the accuracy of these tests in foreseeing whisker growth is unclear. In this thesis, different tin finishes are assessed for whisker growth in accordance with existing environmental test standards and compared to growth seen in ambient storage conditions. The results indicate that environmental tests may over-predict, under-predict, or show little distinguishable growth as compared to ambient-stored tin finishes. In conclusion, environmental tests are not a reliable method of assessing future whisker growth.Item Electrostatic Gas-Liquid Separation from High Speed Streams--Application to Advanced On-Line/On- Demand Separation Techniques(2009) Alshehhi, Mohamed Saeed; Ohadi, Michael M; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The separation of suspended droplets from gases has been one of the basic scientific and technical problems of the industrial era and this interest continues. Various industrial applications, such as refrigeration and HVAC systems, require control of fine droplets concentrations in moving gaseous mediums to maintain system functionality and efficiency. Separating of such fine droplets can be achieved using electrostatic charging as implemented in electrostatic precipitators (ESPs). They use electrostatic force to charge and collect solid particles. The objective of the present work was to study the feasibility of using wiretube electrostatic separator on the removal of fine water and oil droplets from air stream based on corona discharge ionization process. A parametric study was conducted to find key parameters affecting the separation process. This goal was approached by simulating the charging and separation phenomena numerically, and then verifying the modeling findings through experiments. The numerical methodology simulated the highly complex interaction between droplets suspended in the flow and electrical field. Two test rigs were constructed, one for air-water separation and the other for air-oil separation. A wiretube electrostatic separator was used as the test section for both test rigs. The separation performance was evaluated under different electric field and flow conditions. Finally, based on the results, a novel air-water separator prototype was designed, fabricated and tested. The numerical modeling results qualitatively showed acceptable agreement with the experimental data in terms of the trend of grade efficiency based on droplets size. Both numerical modeling results and experimental data showed that with a proper separator design, high separation efficiency is achievable for water and oil droplets. Based on the experimental data, at flow velocity of 5 m/s and applied voltage of 7.0 kV, the maximum separation efficiency for water and oil was 99.999 % and 96.267 %, respectively. The pressure drop was as low as 100 Pa and maximum power consumption was 12.0 W.Item MOLECULAR DYNAMICS SIMULATIONS OF LASER INDUCED SHOCK RESPONSE IN REACTIVE Ni/Al NANOLAMINATES(2009) Meissner, Alexander Blacque; Zachariah, Michael; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)To characterize the self-propagating, high-temperature exothermic alloying reactions of Ni/Al nanoscaled multilayered films induced by laser pulse shock loading, classical molecular dynamics simulations were performed. In the current work, a novel technique was developed to facilitate the energy input and distribution into nanolaminate thin films. The laser pulse shock loading technique enables the initial shock response of the material to be captured as well as the late-time mass diffusion controlled alloying reaction and Ni3Al formation. Shock compression raises the temperature, pressure, and density of the Ni and Al layers which triggers the Ni to diffuse into the Al and initiate the self-propagating alloying reaction. Thermodynamic states, enthalpy of reaction, and global reaction rates of the laminated films were obtained. It was determined that the series of complex rarefaction and reflection waves play a significant role in altering the thermodynamic state of the laminate. Attributes of the rarefaction and reflection waves are controlled by the geometry and thickness of the alternating layers. The dependence of layer thickness on the temperature, pressure, enthalpy of reaction, and global reaction rate was investigated and characterized.Item Numerical Investigations of Gaseous Spherical Diffusion Flames(2009) Lecoustre, Vivien Renaud Francis; Sunderland, Peter B.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Spherical diffusion flames have several unique characteristics that make them attractive from experimental and theoretical perspectives. They can be modeled with one spatial dimension, which frees computational resources for detailed chemistry, transport, and radiative loss models. This dissertation is a numerical study of two classes of spherical diffusion flames: hydrogen micro-diffusion flames, emphasizing kinetic extinction, and ethylene diffusion flames, emphasizing sooting limits. The flames were modeled using a one-dimensional, time-accurate diffusion flame code with detailed chemistry and transport. Radiative losses from products were modeled using a detailed absorption/emission statistical narrow band model and the discrete ordinates method. During this work the code has been enhanced by the implementation of a soot formation/oxidation model using the method of moments. Hydrogen micro-diffusion flames were studied experimentally and numerically. The experiments involved gas jets of hydrogen. At their quenching limits, these flames had heat release rates of 0.46 and 0.25 W in air and in oxygen, respectively. These are the weakest flames ever observed. The modeling results confirmed the quenching limits and revealed high rates of reactant leakage near the limits. The effects of the burner size and mass flow rate were predicted to have a significant impact on the flame chemistry and species distribution profiles, favoring kinetic extinction. Spherical ethylene diffusion flames at their sooting limits were also examined. Seventeen normal and inverse spherical flames were considered. Initially sooty, these flames were experimentally observed to reach their sooting limits 2 s after ignition. Structure of the flames at 2 s was considered, with an emphasis on the relationships among local temperature, carbon to oxygen atom ratio (C/O), and scalar dissipation rate. A critical C/O ratio was identified, along with two different sooting limit regimes. Diffusion flames with local scalar dissipation rates below 2 1/s were found to have temperatures near 1410 K at the location of the critical C/O ratio, whereas flames with greater local scalar dissipation rate exhibited increased temperatures. The present work sheds light on important combustion phenomenon related to flame extinction and soot formation. Applications to energy efficiency, pollutant reduction, and fire safety are expected.Item Methodology for Detection and Assessment of the Impact Of Informal Processes On Organizational Output(2009) Ross, Lesa Moore; Mosleh, Ali; Reliability Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This research focuses on the detection and assessment of informal processes within an organization. Informal processes are defined as activities that are not formalized with respect to the inputs, resources, and/or controls; or an activity that deviates from a formal process. Informal processes affect all aspects of an organization's business. Informal processes cannot be eliminated (nor should they necessarily be). The question becomes how can we identify the informal processes and assess their impact on our system/s safety? The research reported in this paper is aimed at providing an answer to this question. A theoretical foundation in the area of organizational culture, structures and practices culminating in the SoTeRiA (Socio-Technical Risk Analysis) framework provides the general model for this research. A comprehensive methodology for the detection, identification and assessment of informal processes is presented which will allow an organization to benefit from positive informal processes, while resolving detrimental informal processes to preclude their use. Two detection methods have been developed - an indirect detection method (questionnaire completed by a management representative) and a direct detection method (process audit). A methodology has been developed to be utilized as a guideline in the performance of process audits that encompasses process element identification, process interactions, and the usage of document trees. A methodology for the assessment of the impact of informal processes on an organization has been developed that will enable businesses and organization's to have more accurate and complete data from which to make their decisions regarding the state of the organization. To assess the impact of informal processes, Bayesian Belief Networks were utilized to determine the probability of the process output failure with the inclusion of informal processes and then after the informal processes were brought into the formal system. The application of this methodology has proven that when either informal processes that are beneficial to an organization are brought into the formal system, or detrimental informal processes are eliminated, the probability of the output failure decreases. The methodology presented provides a comprehensive approach to the understanding, detection, and assessment of informal processes in an organization.