Bumiller, Elissa (M.S. Mechanical Engineering)

Electrochemical Migration on FR-4 Printed Wiring Board Surfaces

Field failures due to electrochemical migration (ECM) are one of the most vexing issues in electronics products and systems. This is especially true in the telecommunications industry, as benign environments (no temperature or power cycles) and high reliability electronic components reduce the probability of other failure mechanisms. The occurrence of ECM is expected to continue to rise as continued outsourcing reduces original equipment manufacturer control over the assembly process and reductions in pitch reduce migration distances and hinder the ability to effectively clean areas under components and connectors. In a more specific set of circumstances, the growth of the telecom market to equatorial countries, such as Southeast Asia and Latin America, increases the likelihood of equipment being subjected to environments that will accelerate field failures due to ECM.

The threat of ECM is primarily a function of halide contamination on the board, which occurs as a byproduct of the soldering process. This thesis examines the effect of chloride contamination on the potential of FR-4 boards to fail due to ECM. Four common metallizations (HASL, ENIG, ImSn, ImAg) were used for surface insulation resistance testing. All test specimens were examined visually for evidence of ECM. This study concludes that voltage gradient not voltage bias is a primary factor in ECM. Also, the concentration of metal ions in the electrolytic solution controls whether the failure mechanism is ECM or electrolytic corrosion.

Casey, Paul (M.S. Mechanical Engineering)

Managementof Lead-Free Solder Alloy Intellectual Property for Lead-Free Electronics

The number of patents making lead-free solder compositions proprietary has been increasing sharply since the mid-1990s. Patents protecting lead-free metallurgy may prove to be a problem in the shift to lead-free assembly, and will become more significant as volume manufacturing with lead-free solders is spurred by European and Japanese lead bans. In contrast to the common perception that the greatest impediments to lead-free technology are technical problems, patent-related snags are investigated. They may be in the form of current patent applications, potential overlap in patents, patent breadth, regional issues, and the culmination of these in the potential for infringement. Moreover, the inherent variability of lead-free patent claims also makes infringement a concern, even in situations where compositions appear to be distinct. In order to deal with these problems, a patent management scheme was developed and realized with software. Case studies were conducted with lead-free alloy families of significance to the electronics industry. The approach used generally applies not only to solder alloys, but to fields of invention that are comprised of quantifiable compositions of matter.

Gupta, Shirish (M.S. Mechanical Engineering)

Temperature and Rate Dependent Partitioned Constitutive Relationships for 95.5PB2SN2.5AG Solder Alloy

One of the biggest challenges for power electronic devices is to be reliable in harsh environments. The operating temperatures in typical applications can go as high as 200^oC. The die attachment material of a power electronic device is one of the weak links in the system. The eutectic Sn-Pb solder alloy, which is the most commonly used permanent interconnect in electronics packaging cannot fulfill these service requirements, hence there is a need to find suitable replacements.

Durability characterization is essential in order to accurately predict the reliability of the solder alloy chosen for the die attach material under life cycle loads. A large number of models are available, which can be used to determine the life of die attach in small signal and power modules, however the shortfall of these models is the lack of test data for all but the most common (e.g. eutectic Sn-Pb solder) die attach materials. Hence, relevant constitutive properties must be measured, as they are essential for quantitative characterization of damage accumulated in the die attach, the knowledge of which is essential for accurate durability assessment.

The aim of this study is to determine the relevant constitutive properties for high temperature high lead 95.5Pb2Sn2.5Ag solder alloy (Indalloy 163), by implementation of the direct local measurement technique developed by Kwon et al. [2003]. Temperature and loading rate dependent mechanical and constitutive properties of the afore mentioned solder alloy have been obtained by modeling the experimental data gathered by conducting monotonic, isothermal, constant strain rate tests at a range of temperatures and strain rates utilizing miniature single-lap shear specimens, with a partitioned form of the general constitutive equation.

Lantz, Leon (Ph.D. Mechanical Engineering)

The Measurement of Ion Diffusion in Epoxy Molding Compounds Used to Encapsulate Microelectronic Devices

Plastic encapsulation has almost completely replaced hermetic ceramic packages for the protection of semiconductor integrated circuit devices. The plastic encapsulant is not a hermetic seal and is permeable to moisture, gases and ions. When the absorbed moisture is combined with ions, there is an opportunity for electrolytic corrosion of the metal surfaces of the device and package elements.

Information regarding the diffusion of ions through commercial encapsulants has not been reported in the literature. In this dissertation three analytical techniques were used to measure ion diffusion in commercial encapsualants. Diffusion cell measurements and time-of-flight secondary ion mass spectroscopy showed that the ions diffuse through the encapsulant as a front. A Fickian diffusion profile was also detected at the leading edge of the diffusing ion front by dynamic secondary ion mass spectroscopy depth profiling.

Diffusion coefficients were calculated for moisture and ions in a commercial encapsulant. The diffusion coefficient for chloride ions in the encapsulant were eight to nine orders of magnitude smaller than the diffusion coefficients for moisture. We concluded that the slow ion diffusion rate in the commercial encapsulants was due to the binding of the ions to the polymer matrix. At the slow ion diffusion rates observed in these studies, ionic contaminants present on the surface of the encapsulant will not diffuse to the surface of the encapsulated semiconductor device in the projected life of the device.

Diffusion coefficients for the diffusion of chloride ions were calculated for temperatures from 75 to 200oC. A change in the slope of an Arrhenius plot of the diffusion coefficients as a function of temperature was observed which corresponded to the glass transition temperature of the encapsulant. This increase in ion diffusion above the glass transition temperature has implications for the assignment of acceleration factors for accelerated tests, such as HAST tests, which may be run at temperatures above the glass transition temperature of the encapsulant. A mathematical model for ion diffusion in the encapsulants is presented.

Mishra, Satchidananda (M.S. Mechanical Engineering)

Life Consumption Monitoring for Electronics

Life consumption monitoring (LCM) is a method to assess product's reliability based on its remaining life in a given life cycle environment. In life consumption monitoring, product's reliability can be assessed by comparing the remaining life of the product with estimated total life. The life consumption monitoring process involves continuous or periodic measurement, sensing, recording, and interpretation of physical parameters associated with a system's life cycle environment to quantify the amount of system degradation.

This thesis explains a life consumption monitoring methodology for electronic products. It presents two case studies to estimate the remaining lives of two identical circuit card assemblies in an automobile under-hood environment using the life consumption monitoring approach. Failure modes and effects analysis (FMEA) revealed that temperature and vibration are the dominant environmental factors, which could potentially cause malfunction of the circuit card assembly. Temperature sensor and accelerometers are used along with a data logger to monitor and record the environmental loads during the experiment. A data simplification scheme is developed to make the raw sensor data suitable for further processing. Physics-of-failure (PoF) reliability assessment models are used to estimate the remaining life of the circuit card assembly based on the simplified data. Performances of the test board assemblies are monitored through resistance monitoring. The life cycle environment and results for the case studies are compared with each other.

Pollack, Deborah (M.S. Mechanical Engineering)

Validity of Constitutive Properties of Mechanical Deformation Analysis of Eutectic Solder

Thermal cycling produces thermo-mechanical deformations in electronic assemblies, resulting in fatigue damage in surface mount solder interconnections. Accurate prediction of the interconnection stress is a critical step to design optimization for enhanced interconnection reliability, which requires the well-defined constitutive properties of solder interconnection.

Tin-Lead eutectic solder has been widely used in electronic systems. Numerous researchers have tested it extensively and many constitutive models are available in the literature. Among the various models, the unified approach is accepted most extensively by industry, because of its simplicity and availability in commercial finite element analysis (FEA) packages. However, variations in the unified models published constants are extremely large. These variations are attributed to differences in test methods, specimen preparation, and inherent complex nature of the unified approach, which makes modeling prediction uncertain.

In this research, a simple FEA using a commercial package (ANSYS) is conducted to investigate the performance of the unified model. A whole-field real time experimental analysis is performed, on a specimen with controlled solder geometry, to document thermally induced deformations as a function of thermal hysterisis. A FEA for the configuration identical to the specimen geometry is followed using the three different constants of the unified model available in the literature. The numerical predictions are compared with the experimental data to determine validity of the constants.

Rajagopal, Subramanian (M.S. Mechanical Engineering)

Transition of Low-Volume Complex Electronics System Industries to Lead-free Electronics

The European Parliament's Waste Electrical and Electronic Equipment legislation requires manufacturers to bear responsibility for the mandatory collection, reuse and recycling of electronics products. The Restriction of the use of Hazardous Substances in electrical and electronic equipment legislation bans lead and other hazardous substances in electronics products by July 2006. This thesis work is a systematic effort to identify and analyse the risks to low-volume complex electronic system industries due to the transition to lead-free electronics, determine the relevance and significance of lead-free legislation exemptions on low-volume complex electronic system industries and to develop recommendations to mitigate the risks of lead-free transition. There are four areas of risks faced by the LVCES industries due to the lead-freetransition. First, there is a lack of reliability data of lead-free electronics subject to harsh environmental conditions, long-term storage and long-term usage. Second, the compatibility of lead- free electronics with low-volume complex electronic systems is unknown. Third, is the availability and traceability of lead-free parts. Fourth, is the logistical and technical challenges during repair and rework. The lead-free legislation contains a number of exemptions. The underlying basis of the lead-free legislation exemptions has been studied. The relevance and significance of these exemptions to the low-volume complex electronic system industries has been analyzed along with how various sectors of the low-volume complex electronic system industries can respond to the lead-free legislation exemptions in their product development. Developed recommendations to mitigate the risks of lead-free transition in low-volume complex electronic system industries.

Rawat, Anoop (M.S. Mechanical Engineering)

Thermomechanical Characterization and Evaluation of Conformal Coatings

Conformal coatings are thin layers of synthetic resins or organic polymers applied to printed circuit boards and electronic components for protection against environmental, mechanical, electrical, and chemical problems including contaminants such as dust, dirt, fungus, moisture, chemicals, thermomechanical stress, mechanical shock, and vibration. They are "conformal" in that they conform to the contours of the assembly. Major types of conformal coatings include urethane, silicone, acrylics, epoxies, and parylene. Parylene is vacuum-deposited and forms a pore free uniform coating. The three basic forms of parylene are classified as types N, D, and C. This thesis presents a study aimed at the characterization of the thermomechanical properties (elastic modulus and coefficient of thermal expansion) of Parylene N as a function of temperature, relative humidity, and time (isothermal aging) in order to understand and analyze the effect of these materials on the durability of fragile surface mount components and interconnects. A model for the aging of parylene relating elastic modulus of parylene with temperature, relative humidity, and time has been developed. In addition, the moisture absorption characteristics of parylene N were also studied. This study has used the coatings in dimensions that are close to the 'as coated' dimensions in actual applications where coatings are applied over printed circuit boards and components.

Stadterman, Thomas (Ph.D. Mechanical Engineering)

Practical Assessment Methodologies for Circuit Cards Under Mechanical Shock Loading

Currently many military contractors do not specifically evaluate electronics in a shock environment. They rely on either past experience, or the attenuation of shock pulses from shock mounts without actually evaluating the effect of shock on the electronic equipment. A practical assessment methodology for assessing and evaluating electronics in shock environment is needed, because the development of electronics for military equipment is constrained by schedule requirements. If shock assessment is actually to be performed in design, an approach that uses little or no testing and relatively simplistic finite-element analysis is needed.

The objective for this dissertation was to develop a practical assessment approach for examining circuit card reliability under mechanical shock loading. To develop this approach, a combination of experimentation and modeling was used to characterize failure mechanisms and sites. Because of electrodynamic shaker-table limitations, a drop-impact tester was developed that provided dynamic three- or four-point bend tests on circuit cards. The tester produced repeatable strain responses in the circuit card, which resembled half-sine wave pulses. To characterize circuit board fracture, circuit cards were tested using a shaker table, the drop-impact tester and a static three-point bend tester. Test results and finite-element analysis were used to develop failure models based on failure risk due to strain in the outer layer of the board. To characterize component failure, the drop-impact tester was used to fracture components, leads, or solder joints. Failure models were developed for specific component types in terms of failure risk based on inertial loading or stress from the circuit board bending.

Using results of the circuit board and component research, a practical assessment approach for circuit cards subjected to shock loading was developed. This approach recommends specific finite- element analyses and testing to determine or validate board response. The practical assessment approach was successfully demonstrated on two US Army circuit cards. This practical assessment approach will allow electronic circuit card designers to quickly evaluate circuit cards with minimal testing and finite-element analysis. It also provides information necessary for circuit card redesign to improve failure risk for shock loading.

Shetty, Vidyasagar (M.S. Mechanical Engineering)

Remaining Life Assessment Process of Electronic Systems

The remaining life assessment (RLA) process is a reliability process, which predicts amount of life left in a systems. Remaining life assessment is performed on hardware, which has already seen operational life. This thesis details the remaining life assessment process in detail and also provides a case study of remaining life assessment performed on the Shuttle Remote Manipulator Systems (SRMS). The electronics of Shuttle Remote Manipulator Systems was designed in the 1970s with a target application life of ten years. They have performed without any failures for over 20 years. The remaining life assessment process was done do investigate if the life of the SRMS could be extended until the year 2020.

Stellrecht, Eric (M.S. Mechanical Engineering)

Hygroscopic Swelling in Plastic Encapsulated Microelectronics: An Experimental Study Using Moir?Interferometry

Plastic encapsulated microcircuits (PEMs) dominate the market share of microcircuit sales worldwide due to advantages over hermetic packages in terms of size, weight, and cost. In spite of these many advantages, one important disadvantage of PEMs is that the polymeric mold compound absorbs moisture when exposed to a humid environment.

Various failure mechanisms caused by moisture ingress in PEMs, including popcorning and delamination, have been studied extensively. It has also been shown that the mold compound experiences hygroscopic, or moisture induced, swelling when moisture is absorbed. Limited data on hygroscopic swelling is available in the literature due to the difficulty associated with ascertaining hygroscopic swelling values.

In this thesis, a whole-field experimental method to characterize the hygroscopic swelling of mold compounds is proposed. The method utilizes moir?interferometry, which measures in-plane displacements over a relatively large gage length with submicron measurement sentsitivity. Deformations caused by the absorption of moisture in mold compounds are analyzed to determine the coefficient of hygroscopic swelling (CHS). The large gage lenth mollifies the point-to-point variation within the sample, and the high sensitivity provides the measurement accuracy required for the small hygroscopic deformation. Representative results obtained from a commercially available mold compounds are presented.

The experimental procedure developed in this thesis is also used to perform the analysis of the hygroscopic swelling induced deformations in two commonly used electronics packages. The results indicate that the stress- induced strain along the mold compound/silicon chip interface cased by hygroscopic swelling is more that twice than the stress-induced strain cased by a thermal excursion of 60^oC. These results demonstrate that hygroscopic swelling must be taken into account in environments where plastic packages are exposed to a high temperature/high relative humidity environemt.

Valentin, Ricky (Ph.D. Mechanical Engineering)

A Rapid Failure Assessment Approach for Insertion Mounted Solder Interconnects Under Thermomechanical Cycling

During use, insertion mounted solder joints can be subjected to a variety of loading conditions that can lead to failure. The intention of this dissertation is to demonstrate relationships between applied thermal environment and the associated life cycle of the insertion mount systems by using sophisticated analytical simulations and associated life predictive analyses. In order to achieve the objective, a rapid approach was developed for calculating loads at critical locations of an insertion-mounted structure subjected to cyclic thermomechanical loading. The analytical calculation of stresses from the elastic foundation theory and concentric cylinder theory were also used in an iterative creep process. The analytical solution of the insertion mount systems permitted the examination of the multiaxial fatigue behavior at critical locations for peripheral and area array packages experiencing thermomechanical loading. Comparisons were made with ANSYS finite- element analysis software, experimental results, and field results.

Vijayaragavan, Niranjan (M.S. Mechanical Engineering)

Physics of Failure Based Reliability Assessment of Printed Circuit Boards Used Permanent Downhole Monitoring Sensor Gauges

"Intelligent well" completions used for real-time downhole monitoring of pressure and temperature in an oil reservoir. The life cycle of permanent downhole gauges is primarily characterized by high temperature, vibration and shock environments. Reliable operation of about 5 years is required from these systems. This thesis work is a systematic effort to study the reliability of the printed wiring boards used in the permanent downhole monitoring applications, considering their entire life cycle, for operation at temperatures close to 150?C, using the physics of failure methodology. A methodology to address life cycle load considerations with electronics used for oil and gas extraction and production applications has been developed. Significant degradation mechanisms at the component and at the board level have been identified using numerical simulation. Accelerated thermal testing was performed to assess the reliability of the boards at a temperature of 150?C for a period of 5 years. This thesis work has been performed on a permanent downhole gauge used by Schlumberger, called Hyper Permanent Quartz Gauge (HPQG), for their permanent monitoring applications.

Wang, Zhaoyang (Ph.D. Mechanical Engineering)

Development and Application of Computer-aided Fringe Analysis

Photo-mechanics methods have matured and emerged as important engineering tools. Although numerous image-processing algorithms have been developed to complement interferometric measurement techniques, these algorithms have been implemented originally for classical interferometry and the extensions to the general photo-mechanic fringe analysis have been limited. In this dissertation, the existing computer-aided digital fringe image analysis and processing techniques are investigated; the most appropriate fringe processing schemes and their limitations are identified. To make the computer-aided fringe analysis practical to the real engineering problems, the existing schemes are improved and a series of new fringe analysis techniques are developed. Among these new techniques, the self-adaptive fringe filtering scheme considers not only the orientations of the local fringes but also the local fringe densities; the enhanced random phase shifting algorithm can detect the phase shift amounts and the full-field fractional fringe orders and their gradients accurately. Based on the study, a Windows GUI-based expert software system is developed for interferogram fringe analysis and processing. This expert system includes all the algorithms presented in this dissertation.

Selected but original applications of the computer-aided fringe analysis are presented. They include: (1) development of infrared diffraction interferometer for co-planarity of high-density solder bump patterns; the infrared light enables the regularly spaced solder bump arrays to produce well-defined diffracted wavefronts, (2) development of an inverse method to determine elastic constants using circular disc and moir interferometry; this method use a non-linear over-deterministic approach to determine elastic constants simultaneously, and (3) applications to out-of-plane shape and warpage measurement and in-plane displacement and strain measurements of electronic packaging components.

Wu, Ji (Ph.D. Mechanical Engineering)

Electrical Characterization and Reliability Assessment of Lead-Free Solder Coated Electrical Contacts

Elimination of lead from electronics is a global phenomenon driven by both legislation and market forces. The European Union has set July 2006 as the deadline for eliminating tin-lead solder from most electronic products. Extensive studies on lead-free solders for device and product interconnections are going on worldwide. However lead-free solders are also expected toreplace tin-lead solders as contact finishes for electronic contacts.

In this study, the electrical performance of tin-silver-copper and tin- copper coatings was investigated and compared with tin-lead eutectic coating. X-ray Photoelectron Spectroscopy (XPS) analysis was conducted to understand the chemistry of the surface films on lead-free and tin-lead alloy coatings. The contact resistance after different aging conditions, including mixed flowing gas, steam, and dry heat aging was examined. Energy Dispersive Spectroscopy (EDS) analysis was conducted to determine the nature of the surface films in the aged samples.

Fretting corrosion was studied for tin-silver-copper and tin-copper lead-free alloy coatings and compared with tin-lead coating. Fretting corrosion experiments were conducted and compared at different temperatures (25ºC, 50ºC, 80ºC), and under difference normal forces (20g, 50g, 80g) at room temperature. Time to failure of each test was recorded. A Weibull distribution was used to best fit the experimental data. Mean time to failure (MTTF), and time to 1% and 10% failure were calculated for each material at each fretting condition. Comparisons were made among the materials.

Wu, Xin (Ph.D. Mechanical Engineering)

Developing Highly Accurate and Stable Open-region Electromagnetic Simulations

In open-region electromagnetic simulation problems, the computational domain has to be truncated by an artificial boundary condition to model the infinite condition strongly affects the accuracy of overall numerical simulation. For a class of advanced problems demanding high accuracy, such as in the modeling of medical detection devices, indoor wire less communication systems and remote sensing equipments, the received signal can be several orders of magnitude less than the transmitted signal. Furthermore, wide-band simulations require long running times for transients-based simulations, which increase the potential for instability. Therefore, accuracy and stability of absorbing boundary conditions are identified as critical in the design of numerical algorithms compatible with advanced applications.

In this work, the theory of Concurrent Complementary Operators Method (C-COM) in both transient and frequency-domain numerical simulations is investigated. The C-COM is based on the basic premise of primary reflection cancellation. The C-COM applications to numerically derived ABCs in finite difference time-domain (FDTD)method, and to the frequency domain ABCs in both finite difference frequency domain (FDFD) method and finite element method (FEM) method are developed. Extensive numerical experiments are conducted showing dramatic increase in accuracy when the C-COM is applied in comparison to previous published techniques.

Previous works that addressed the boundary instability arising from the application of the absorbing boundary condition used either the von Neumann analysis or the Gustafsson-Kreiss-Sundstrom (GKS) analysis. These earlier works, however, did not explain the inconsistencies that have been observed between the theoretical predictions and numerical experiments. This thesis presents a new stability analysis applicable to boundary conditions. This new analysis, referred to as Coupled Stability Analysis, is based on the fundamental assumption that absorbing boundary conditions are not perfect, and therefore, generate waves that reflect back into the computational domain. It is found that this analysis yields results that are fully consistent with those obtained from numerical experiments. As an important consequence of this analysis, and contrary to earlier conjectures, we show that Higdons absorbing boundary condition of order 3 (and possibly, higher orders) to be unconditionally unstable. The theoretical predictions of the CSA are supported by previous numerical findings and several numerical experiments presented here.

Yang, Yubing (Ph.D. Mechanical Engineering)

Reliability Assessment of Optical Fibers Under Tension and Bending Loads

The mechanical failure of optical fibers must be avoided to ensure reliability of fiber-based systems. The first stress event in a fiber's lifetime is the proof test. The proof test will alter the fiber's strength distribution for all subsequent processing and applications. Thus it is critical to know the fiber strength distribution after proof test (post-proof strength distribution). It is generally assumed that the proof test truncates the strength distribution at the proof test stress level. But, many users are concerned because they know that theoretically it has been shown that after proof test the strength of fiber may be much less than the proof test stress level, and that the minimum post-proof strength is determined only by the unloading rate during the proof test. But this theoretical result is not consistent with historical field data. Historically no one has documented failures stresses below the proof stress level. This dissertation resolves this apparent contradiction by reviewing the theory and conducting a probabilistic assessment.

As optical fibers are used more and more in computer and switching gear backplanes, a new potential mechanical reliability problem arises due to the necessary bends introduced in optical fibers. Previous researchers were concerned the uniform stress optical fibers saw in long haul underground applications, but bending places a non-uniform stress along the fiber surface. So it is inaccurate to borrow fiber usage mechanical guidelines from long-haul application. The dissertation reviews existing theories and then develops a new analytic approach to assess the mechanical reliability of optical fibers under bending loads and static fatigue conditions. This new analytic approach is verified through a simple static two-point bend experiment. Finally the newly developed reliability assessment method is used to develop new guidelines for bending application and examples are presented to show how the approach can be used to attack some very common mechanical reliability problems with optical fibers.