Mechanical Engineering

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HIGH ACCELERATIONS PRODUCED THROUGH SECONDARY IMPACT AND ITS EFFECT ON RELIABILITY OF PRINTED WIRING ASSEMBLIES
(2010) Douglas, Stuart Taylor; Dasgupta, Abhijit; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The focus of this thesis is the investigation of extremely high accelerations through secondary impact and its effect on reliability of printed wiring assemblies. The test equipment consists of a commercially available drop system and a commercially available attachment termed a Dual Mass Shock Amplifier (DMSA), which extends the impact acceleration range to as much as 30,000 Gs by utilizing secondary impact dynamics. Further secondary impacts between the test vehicle and fixture are intentionally generated in simulation and tested experimentally to imitate board 'slap' phenomena in product assemblies, and to generate even further amplification of the acceleration at various locations on the test specimen. In this thesis a detailed description of the test equipment and modeling techniques are provided. Model complexity ranges from simple analytic closed-form rigid-body mechanics to detailed nonlinear dynamic finite element analysis. The effects of different equipment design parameters (table mass, spring stiffness, table clearance) are investigated through parametric modeling. The effects of contact parameters (constraint enforcement algorithms, stiffness, damping) on model accuracy are explored. Test fixtures for high shock accelerations are discussed and used for board level reliability testing of printed wire assemblies containing WLCSP49s and MEMS microphones.
Latching Microelectromechanical Shock Sensor Systems: Design, Modeling, and Experiments
(2010) Currano, Luke Joseph; Balachandran, Balakumar; Yu, Miao; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Latching shock sensors are acceleration threshold sensors that trigger when the acceleration level exceeds the designed acceleration threshold. The latching mechanism provides a mechanical memory, which keeps the sensor in a triggered, or latched, state until the sensor is reset. The attractive feature of this type of sensor is that it does not require power during monitoring; power is only needed to query and reset the sensor. Several devices have been presented in the literature, but with limited experimental data and models that provide little to no insight into the dynamics of the latching event. The aim of this work is to further the understanding of the physics and design of micromechanical latching shock sensors by conducting a combination of careful experiments and development of original reduced-ordermodels. These efforts enable one to obtain a detailed picture of the latching dynamics for the first time. Latching shock sensors have been designed, fabricated, and experimentally evaluated in this work. The model predictions have been compared to the experimental results to verify the validity, including a quantitative comparison of the position of the shock sensor during a latching event captured via high-speed videography. This is the first time a latching event has been imaged in this class of sensors, and the first time, the model predictions of position versus time histories have been validated through experiments. The models have also been used to conduct detailed numerical studies of the shock sensor, amongst other things to predict a latch "bounce" phenomenon during an acceleration event. To understand more thoroughly how the various design parameters affect the latching threshold of the sensor, various parametric and optimization studies have also been conducted with the reduced-order models to guide designs of future latching acceleration threshold sensors.
LEAD-FREE ELECTRONICS USE AND REPAIR DYNAMIC SIMULATION
(2009) Chaloupka, Andrew Charles; Sandborn, Peter A; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The conversion from tin-lead to lead-free electronics has increased concern amongst engineers about the reliability of electronic assemblies. In order to communicate the impact of the conversion in terms of cost and availability, a simulation of electronic systems at the LRU level to and through a repair facility was created. The model includes the effects of repair prioritization, multiple possible failure mechanisms, no-fault-founds, and un-repairable units. Example analyses were performed on electronic assemblies that use SAC and SnPb solder using a repair process modeled after a NSWC Crane Aviation Repair Process. The case studies revealed that LRUs exposed to usage profiles characteristic of aerospace and high performance applications, high thermal cycling temperatures with short dwell times, SAC exhibited significantly increased repair costs when compared to tin-lead. Prioritizing LRUs and increasing the rate of deployment had no significant impact on the cost or availability metrics for the cases considered.
Some mechanics challenges and solutions in flexible electronics
(2009) Tucker, Matthew Brody; Li, Teng; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Flexible electronics is an emerging field with potential applications such as large area flexible displays, thin film solar panels, and smart prosthesis, to name a few. Promising future aside, there are challenges associated with flexible electronics including high deformability requirements, needs for new manufacturing techniques and high performance permeation barriers. This thesis aims to explore possible solutions to address these challenges. First, a thin stiff film patterned with circular holes is proposed as a deformable platform to be used in flexible electronics in either component and circuit level. Second, we explore possible pathways to improve the quality of transfer printing, a nanofabrication technique that can potentially enable roll-to-roll printing of flexible devices. Third, we investigate the failure mechanism of multilayer permeation barriers for flexible electronics and offer an improved design to achieve better mechanical reliability.
EVALUATION OF THERMAL INTERFACE MATERIALS AND THE LASER FLASH METHOD
(2009) Khuu, Vinh; Khuu, Vinh P; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Thermal interface materials (TIMs) are used to reduce the interfacial thermal resistance between the chip and the heat sink, which has become a bottleneck to heat removal in a variety of electronic applications. Degradation in thermal performance of the TIM can contribute to unacceptably high chip temperatures, which can significantly impact device or system performance during operation. While progress has been made in recent years in the development of tools to measure beginning-of-life thermal performance, characterizing the long-term performance of the TIM can be crucial from a life cycle stand point since TIMs may experience harsh operating conditions, including high temperature and high humidity, for extended periods of time in typical applications. The laser flash method is one approach for measuring thermal conductivity that has an advantage over more commonly used techniques because of the non-contact nature of the measurement. This technique was applied to 3-layer structures to investigate the effects of thermal cycling and elevated temperature/humidity on the thermal performance of select polymer TIMs in pad form, as well as an adhesive and a gel. While most samples showed little change (less than 10% in thermal resistance) or slight improvement in the thermal performance, one thermal putty material showed degradation due to temperature cycling resulting from bulk material changes near the glass transition temperature. Scanning acoustic microscope images revealed delamination in one group of gap pad samples and cracking in some putty samples due to temperature cycling. Finite element simulations and laser flash measurements performed to validate the laser flash data indicated that sample holder plate heating, an effect previously unexamined in the literature, can lead to inaccurately high TIM thermal conductivity values due to suppression of the sample temperature rise during the laser flash measurement. This study proposed a semi-empirical methodology to correct for these effects. Simulated laser flash test specimens had bondlines that showed little thickness variation (usually within the measurement error) due to clamping by the sample holder plates. Future work was proposed to refine the laser flash sample holder design and perform additional validation studies using thermal test vehicles based on nonfunctional packages.
The Effects of Environmental Stresses on the Reliability of Flexible and Standard Termination Multilayer Ceramic Capacitors
(2009) Brock, Garry Robert; Pecht, Michael G; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Flexible termination capacitors were designed to reduce stresses transmitted to the ceramic dielectric of a capacitor and thereby prevent flex cracking. Two studies were conducted to examine the reliability of flexible termination multilayer ceramic capacitors (MLCCs) subjected to environmental stresses. The first study used temperature-humidity-bias to compare the effects of termination type (standard vs. flexible), presence of a conformal coating (acrylic coating vs. no coating), and voltage bias level. In situ monitoring demonstrated similar failure statistics between the flexible and standard termination capacitors, presence of conformal coating, and voltage bias level. Upon removal from THB conditions recovery occurred only in the standard termination MLCCs. Flexible termination capacitors at the rated voltage bias were found to have more permanent failures after exposure to THB testing as compared to standard termination capacitors. Failure analysis indicated that silver and palladium migration between electrodes was the failure mechanism in the biased flexible termination capacitors. In the second study flexible and standard termination MLCCs experienced a storage test in which they were exposed to elevated temperature and humidity conditions. It was found that the standard termination MLCCs had a lower reliability with the majority of the MLCCs failing compared to the flexible termination MLCCs where only one MLCC failed. Nearly all failures were for insulation resistance with few capacitors failing for other parameters. Subsequent bake-out of the MLCCs showed some recovery, however more failures were still occurring in the standard termination MLCCs compared to the flexible termination MLCCs. X-ray photoelectron spectroscopy and cross-sectioning were used to examine the failure mechanisms of the capacitors. A bulk migration of silver into the dielectric was determined to be one of the failure mechanisms in the capacitors.
PROGNOSTICS OF SOLDER JOINT RELIABILITY UNDER VIBRATION LOADING USING PHYSICS OF FAILURE APPROACH
(2009) Gu, Jie; Pecht, Michael G; Barker, Donald; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Physics-of-failure (PoF) is an approach that utilizes knowledge of a product's life cycle loading and failure mechanisms to perform reliability modeling, design, and assessment. Prognostics is the process of predicting the future reliability of a system by assessing the extent of deviation or degradation of a product from its expected normal operating states. When prognostics is combined with physics-of-failure models, it is possible to make continuously updated reliability predictions based on the monitoring of the actual environmental and operational conditions of each individual product. A literature review showed that the research on prognostics of solder joint reliability under vibration loading is very limited. However, personal portable electronic products are no longer used exclusively in a benign office environment. For example, any electronic component (throttles, brakes, or steering) in an automobile should be able to survive in a vibration environment. In this thesis, a methodology was developed for monitoring, recording, and analyzing the life-cycle vibration loads for remaining-life prognostics of solder joints. The responses of printed circuit boards (PCB) to vibration loading were monitored using strain gauges and accelerometers, and they were further transferred to solder strain and stress for damage assessment using a failure fatigue model. Damage estimates were accumulated using Miner's rule after every mission and then used to predict the life consumed and the remaining life. The results were verified by experimentally measuring component lives through real-time daisy-chain resistance measurements. This thesis also presents an uncertainty assessment method for remaining life prognostics of solder joints under vibration loading. Basic steps include uncertainty source categorization, sensitivity analysis, uncertainty propagation, and remaining life probability calculation. Five types of uncertainties were categorized, including measurement uncertainty, parameter uncertainty, model uncertainty, failure criteria uncertainty, and future usage uncertainty. Sensitivity analysis was then used to identify the dominant input variables that influence model output. After that, a Monte Carlo simulation was used for uncertainty propagation and to provide a distribution of accumulated damage. From the accumulated damage distributions, the remaining life was then able to be predicted with confidence intervals. The results showed that the experimentally measured failure time was within the bounds of the uncertainty analysis prediction.
A Study of Nanometer Semiconductor Scaling Effects on Microelectronics Reliability
(2009) White, Mark; Bernstein, Joseph B.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The desire to assess the reliability of emerging scaled microelectronics technologies through faster reliability trials and more accurate acceleration models is the precursor for further research and experimentation in this relevant field. The effect of semiconductor scaling on microelectronics product reliability is an important aspect to the high reliability application user. From the perspective of a customer or user, who in many cases must deal with very limited, if any, manufacturer's reliability data to assess the product for a highly-reliable application, product-level testing is critical in the characterization and reliability assessment of advanced nanometer semiconductor scaling effects on microelectronics reliability. This dissertation provides a methodology on how to accomplish this and provides techniques for deriving the expected product-level reliability on commercial memory products. Competing mechanism theory and the multiple failure mechanism model are applied to two separate experiments; scaled SRAM and SDRAM products. Accelerated stress testing at multiple conditions is applied at the product level of several scaled memory products to assess the performance degradation and product reliability. Acceleration models are derived for each case. For several scaled SDRAM products, retention time degradation is studied and two distinct soft error populations are observed with each technology generation: early breakdown, characterized by randomly distributed weak bits with Weibull slope Beta=1, and a main population breakdown with an increasing failure rate. Retention time soft error rates are calculated and a multiple failure mechanism acceleration model with parameters is derived for each technology. Defect densities are calculated and reflect a decreasing trend in the percentage of random defective bits for each successive product generation. A normalized soft error failure rate of the memory data retention time in FIT/Gb and FIT/cm2 for several scaled SDRAM generations is presented revealing a power relationship. General models describing the soft error rates across scaled product generations are presented. The analysis methodology may be applied to other scaled microelectronic products and key parameters.
Development of a Test Methodology to Determine Dynamic Fracture Strength in Microfabricated MEMS Structures
(2009) Emmel, Rachel Lauren; Barker, Donald; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
As micro-electro-mechanical systems (MEMS) are becoming more and more common in both military and consumer electronics, virtual qualification of these devices remains an important design tool. To model MEMS devices subjected to high shock loading, the dynamic fracture strength of the most widely used MEMS material, single crystal silicon (SCSi), is needed. Minimal research has been performed to determine this material property and the research that has been completed suggests that fracture strength varies considerably with processing parameters. Therefore, an efficient and inexpensive testing method to determine the dynamic fracture strength of processed SCSi has been developed. Experimentation with SCSi MEMS structures has also been carried out using this new testing method. A probabilistic Weibull distribution for bending of DRIE processed SCSi around the <110> directions was created as a design for reliability tool. Additional experiments demonstrated that the fracture strength for bending of DRIE processed SCSi around the <100> directions is greater than 1.1 GPa. Suggestions for subsequent work that focuses on the bending of SCSi around the <100> directions are also presented.
Quality and Reliability of Elastomer Sockets
(2009) Lopez, Leoncio D.; Pecht, Michael G.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Integrated Circuit (IC) sockets provide hundreds to thousands of electrical interconnects in enterprise servers, where quality and reliability are critical for customer applications. The evaluation of IC sockets, according to current industry practices, relies on the execution of stress loads and stress levels that are defined by standards, having no consideration to the physics of failure (PoF), target operating environment, or contact resistance behavior over time. In a similar manner, monitoring of contact resistance during system operation has no considerations to the PoF or environmental conditions. In this dissertation a physics of failure approach was developed to model the reliability of elastomer sockets that are used in an enterprise server application. The temperature and relative humidity environment, at the IC socket contact interface, were characterized as a function of external environmental conditions and microprocessor activity. The study applied state-of-the-art health monitoring techniques to assess thermal gradients on the IC socket assembly and to establish an operating profile that could be used for the development of a PoF model. A methodology was developed for modeling and monitoring contact resistance of electrical interconnects. The technique combined a PoF model with the Sequential Probability Ratio Test (SPRT). In the methodology the resistance behavior is characterized as a function of temperature. The effective a-spot radius was extracted from the characterization data and modeled with a power-law. A PoF model was developed to estimate the resistance of an elastomer contact, based on the effective a-spot radius and the ambient temperature. The methodology was experimentally demonstrated with a temperature cycle test of the elastomer socket. During the evaluation the difference between estimated and observed resistance values were tested with the SPRT. The technique was shown to be very accurate for modeling contact resistance and to be highly sensitive for the detection of resistance degradation. A qualitative reliability model was developed for the mean contact resistance of an elastomer socket, using fundamental material properties and user defined failure criteria. To derive the model, the resistance behavior of contacts under nominal mechanical load was studied as a function of time and temperature. The elastomer contact was shown to have a very complex resistance behavior, which was modeled by multiple statistical distributions. It was shown that elastomer sockets, in spite of experiencing stress relaxation at the macroscale (elastomer), can exhibit decreases in contact resistance, a result of stress redistribution at the microscale (Ag particles), which increases Ag-Ag particle stress and the effective contact area.

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