CALCE EPSC Graduate Student Theses and Dissertation Abstracts (2012)

Chauhan, Preeti (Ph.D)
Thermal Cycling Reliability of SAC305, SAC105, SN100C and SnPb Solders under Isothermal Aging
Solder joints on printed circuit boards provide electrical and mechanical connections between electronic devices and metalized patterns on boards. These solder joints are often the cause of failure in electronic packages. Solders age under storage and operational life conditions, which can include temperature, mechanical loads, and electrical current. Aging occurring at a constant temperature is called isothermal aging. Isothermal aging leads to coarsening of the bulk microstructure and increased interfacial intermetallic compounds at the solder-pad interface, which both have reliability implications. For example, coarsening of the solder bulk degrades the creep properties of solders, whereas voiding and increased thickness of interfacial intermetallic compounds increases the mechanical weakness in the solder joint. Isothermal aging is commonly conducted in industry before carrying out reliability tests. Industry guidelines on solder interconnect reliability test methods recommend preconditioning the solder assemblies by isothermal aging before conducting reliability tests. The guidelines assume that isothermal aging simulates a “reasonable use period,” but do not relate the isothermal aging levels with specific use conditions. Studies on the effects of isothermal aging on the thermal cycling reliability of tin-lead and tin-silver-copper solders are limited in scope and results have been contradictory.
The objective of this research is to provide the relation between isothermal aging (temperature and duration) and the thermal cycling reliability of select Sn-based solders. The Sn-based solders with 3%, 1%, and 0% silver content that have replaced tin-lead are studied and compared against tin-lead solder. The activation energy and growth exponents of the Arrhenius model for the intermetallic growth in the solders are provided. The aging metric, in terms of phase size in the solder bulk and interfacial intermetallic compound thickness at the solder–pad interface, is established. Based on the findings of thermal cycling tests on aged solder assemblies, recommendations are made for isothermal aging of solders before thermal cycling tests.

Dai, Jun (Ph.D)
Risk Assessment and Mitigation of Telecommunication Equipment under Free Air Cooling Conditions
Currently, about 40% of the total energy in data centers is consumed by the cooling infrastructures. One way to save some of this energy is through free air cooling (FAC), which utilizes outside air as the primary cooling medium, instead of air conditioning. Due to the substantial energy savings, FAC has been adopted by leading companies such as Microsoft and Google. Some standards, such as the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 90.1 (2010 version), have been updated to facilitate its adoption. The implementation of free air cooling will change the operating environment, which may adversely affect the performance and reliability of telecom equipment. This thesis reviews the challenges and risks posed by free air cooling. The increased temperature, uncontrolled humidity, and possible contamination can cause certain failure mechanisms, such as conductive anodic filament and corrosion, to be more active. If the local temperatures of hot spots go beyond the recommended operating conditions, the performance of the equipment can be significantly affected. This is the first study focusing on the impact of FAC on telecom equipment performance. A methodology is developed which uses the performance variations under traditional air conditioning to create a baseline, compares the performance variation under FAC conditions with that baseline, and then determines the impact of FAC on telecom equipment performance. This method can help data centers determine an appropriate operating environment based on their service requirements when FAC is implemented. This thesis also develops a multi-stage (design, test, and operation) approach to assess and mitigate the reliability risks of free air cooling in data centers that are already in operation. A case study is presented to show the implementation process and verify that it can mitigate reliability risks by detecting anomalies and providing early warning of failures. This approach does not interrupt data center services or consume additional useful life of telecom equipment. It allows the implementation of FAC in data centers which were not originally designed for this cooling method.

Han, Sung-Won (Ph.D)
Assessment of Electrical Shorting and Metal Vapor Arcing Potential of Tin Whiskers
Tin whiskers are conductive crystal hair-like growths that form unpredictably from tin and tin alloy surfaces. In electronic systems, the growth of tin whiskers presents a reliability concern due to their potential to create electrical shorts and metal vapor arcs. Concern over tin whiskers is increasing due to the development of smaller electronics and the increased use of pure tin and lead-free tin alloys. While much attention has been placed on developing tests to assess tin whisker growth propensity and mitigation strategies to prevent whisker failure, few studies have focused on the electrical characteristics of whiskers that cause product failures. With regard to electrical shorts, tin whisker mechanical bridging does not guarantee current flow between two differently electrically biased conductors. Due to surface films, the voltage difference between the conductors must exceed a threshold level in order to produce a current flow through the tin whisker to the bridged conductive surface. However, the influence of contact force and the presence of surface contaminants on the breakdown voltage of tin whiskers have not been adequately investigated. Whiskers also present a safety concern, since whisker-induced electrical shorts can initiate metal vapor arcs, which are capable of melting the metal and incinerating the plastics used in electronic products. The potential for metal vapor arc formation is influenced by several factors, including whisker geometry, bias voltage, and ambient pressure. Previous studies have demonstrated metal vapor arc formation using gold and tin wires; however, material and geometry differences between these test articles and actual tin whiskers may influence metal arc formation. Furthermore, the combined effects of several factors have not yet been studied, and a practical guide for assessing the potential for tin whisker–induced metal arc formation has not been provided. This work provides the characteristics and assessment of tin whisker-induced electrical shorting and metal vapor arcing. To determine the propensity for electrical shorting, the breakdown voltage of tin whiskers was measured using gold- and tin-coated probes to characterize the influence of two different contact materials on breakdown voltage. The effect of contact force on breakdown voltage and the current-voltage characteristics that are related to the failure mode and the possibility of electrical shorting by tin whiskers were also investigated. To assess the propensity of metal vapor arcing by tin whiskers, the effect of physical and electrical factors, such as whisker geometry, bias voltage, and pressure, on the formation of vapor arcing was investigated. Based on the experimental evidence, an arc current metric as a function of bias voltage and resistance was developed to characterize the potential for vapor arc formation by tin whisker. Finally, a logistic regression model that can assess the likelihood of vapor arc formation by tin whiskers and be used as a guideline for circuit design in terms of minimizing the vapor arc propensity via tin whiskers was developed.

Nelson III, Raymond (Ph.D)
Concurrent Multi-Part Multi-Event Design Refresh Planning Models Incorporating Solution Requirements and Part-Unique Temporal Constraints
Technology obsolescence, also known as DMSMS (Diminishing Manufacturing Sources and Material Shortages), is a significant problem for systems whose operational life is much longer than the procurement lifetimes of their constitute components. The most severely affected systems are sustainment-dominated, which means their long-term sustainment (life-cycle) costs significantly exceed the procurement cost for the system. Unlike high-volume commercial products, these sustainment-dominated systems may require design refreshes to simply remain manufacturable and supportable. A strategic method for reducing the life-cycle cost impact of DMSMS is called refresh planning. The goal of refresh planning is to determine when design refreshes should occur (or what the frequency of refreshes should be) and how to manage the system components that are obsolete or soon to be obsolete at the design refreshes.
Existing strategic management approaches focus on methods for determining design refresh dates. While creating a set of feasible design refresh plans is achievable using existing design refresh planning methodologies, the generated refresh plans may not satisfy the needs of the designers (sustainers and customers) because they do not conform to the constraints imposed on the system.
This dissertation develops a new refresh planning model that satisfies refresh structure requirements (i.e., requirements that constrain the form of the refresh plan to be periodic) and develops and presents the definition, generalization, synthesis and application of part-unique temporal constraints in the design refresh planning process for systems impacted by DMSMS-type obsolescence.

Oh, Hyunseok (Ph.D)
Prognostics of Ball Bearing based on Vibration and Acoustic Emission Analysis
Ball bearings are used to support rotating shafts in machines such as wind turbines, aircraft engines, and desktop computer fans. There has been extensive research in the areas of condition monitoring, diagnostics, and prognostics of ball bearings. As the identification of ball bearing defects by inspection interrupts the operation of rotating machines and can be costly, the assessment of the health of ball bearings relies on the use of condition monitoring techniques. Fault detection and life prediction methods have been developed to improve condition-based maintenance and product qualification. However, intermittent and catastrophic system failures due to bearing problems still occur, resulting in loss of life and increasing maintenance and warranty costs. Issues such as false and missed alarms, delays in fault detection, and inaccurate life prediction of ball bearings are problems for industry. This research focuses on the prognostics of ball bearings based on vibration and acoustic emission analysis to provide early warning of failure and predict the life in advance. The sensitivity of vibration and acoustic emission signals to detect ball bearing degradation was investigated from correlation analysis of the signals collected from the condition monitoring of ball bearings and the information obtained from failure analysis of the ball bearings. An anomaly detection method based on the sequential probability ratio test was developed to minimize the cost resulting from false and missed alarms and delay of anomaly detection time in a product qualification scenario. A prognostic method based on Bayesian filter and distribution adaptation was developed to provide the distribution of time-to-failure of ball bearings in advance. Case studies will be presented to demonstrate the application of the developed methods with desktop computer fans. The prognostic method developed in this research can be extended as a general method to predict the life of a component or system.

Sinha, Koustav (Ph.D)
Mechanics of Non Planer Interfaces in Flip-Chip Interconnects
With the continued proliferation of low cost, portable consumer electronic products with greater functionality, there is an increasing demand for electronic packaging that is smaller, lighter and less expensive. Flip chip is an essential enabling technology for these products. From a structural perspective, these are multi-layered structures fabricated from highly dissimilar materials. Often, the interfaces between these materials are where failure is most likely to occur when the device is subjected to thermomechanical loading. Thus, understanding and being able to predict the behavior of critical interfaces in a device is directly related to the reliability of the system. To mitigate this it's important to make the interfacial bonding robust by optimizing the bond formation process parameters. This study is on two different interconnect types - the first one involves gold-gold interface in adhesively bonded flip-chip-on-flex packages based on a non conductive adhesive (NCA) bonding process and the second on the crack behavior at the solder-IMC interface in other flip chip joints. The effect of various bonding parameters, geometric features and material properties on the strength behavior of these joints is quantified. This can help optimize the process variables to get the desired strength at these critical interfaces and therefore result in a reliable product.

Bakhshi, Roozbeh (M.S)
Reliability Assessment of Microvias in High Density Interconnect Printed Circuit Boards
With the introduction of hand held electronics such as laptops, cell phones, and tablets, the numbers of input/output pads and the input/output pad density on printed circuit boards have increased. This evolution requires the printed circuit board industry to increase the interconnection density of the printed circuit boards, which is achieved by introducing high density interconnect layers in printed circuit boards that utilize microvias as the connection between the layers. The microvias have a higher wiring density, finer lines and spaces, smaller vias, and higher connection pad density than the plated through holes that are employed in conventional printed circuit boards technology. Microvias have aspect ratios (ratio of height to diameter) of less than 1:1, compared to plated through holes, which could have an aspect ratio of 6:1 or even as high as 20:1. When high density interconnect technology was introduced, microvias were used to connect two layers of board, but with advancements in technology and shrinking package sizes, stacked microvia technology has been developed. The filling of the microvia is usually copper, although some epoxy can also be used. In the copper-filled stacked microvias, some voiding in the microvias has been observed due to manufacturing process defects. The microvias can contain only one void or is some cases multiple voids. The objective of this study is to investigate the effect of these voiding in stacked microvias on the reliability of high density interconnect printed circuit boards and compare it to the non-voided stacked microvias. The stacked microvias in this study are used to connect three layer or four layers of the printed circuit board. Liquid to liquid thermal shock testing was performed as the accelerated testing method. This study identifies the different failure modes that occur and the corresponding failure mechanisms and sites.

Fritzler, Thomas (M.S)
Scintillation Conditioning of Tantalum Capacitors with Manganese Dioxide Cathodes
The advantages of tantalum capacitors with manganese dioxide cathodes are high volumetric efficiencies of more than 100,000 CV/g (Capacitance x voltage/gram), stability of their capacitance over their rated temperature range within 5%, and the ability to self-heal after dielectric breakdown. Dielectric breakdown of tantalum capacitors with manganese dioxide can be problematic under large surge currents, since the parts can ignite and damage surrounding components and circuitry. Since tantalum capacitors are increasingly used in low impedance applications due to their long life, volumetric efficiency, small size, and the development of tantalum capacitors with even higher CV products, the potential damage due to surge current failures is expected to increase. Scintillation testing is a method that activates the self-healing mechanism in tantalum capacitors. In preliminary experiments, the deliberate activation of self-healing yielded up to 100% higher breakdown voltages in weak parts that had an increased risk of ignition failure. This improvement results in a better performance under surge current conditions. This study demonstrates that scintillation conditioning reduces surge current failures in tantalum capacitors with manganese dioxide cathodes. Tantalum capacitors with MnO2 cathodes from two manufacturers are subjected to scintillation conditioning and compared to non-conditioned populations in a surge current test. To ensure that the activation of the self-healing mechanism has no detrimental effect on the reliability of the parts, a life test is conducted. The results show that the conditioning method increases the breakdown voltage of self-healed tantalum capacitors by up to 25% under surge current conditions, which mitigates the risk of ignition failures. No detrimental effect on the life of the conditioned samples was observed.

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