CALCE EPSC Graduate Student Theses and Dissertation Abstracts (2014)

Shrivastava, Anshul (Ph.D)
Reliability Evaluation of Liquid and Polymer Aluminum Electrolytic Capacitors

Liquid aluminum electrolytic capacitors are known for their reliability problems. They are considered as the weakest link in the power electronics system. The liquid electrolyte of these capacitors is the single most important component which affects the reliability of these capacitors. The principal ingredients of the liquid electrolyte are solvent, water, solute and additives such as corrosion inhibitors and hydrogen absorbers. Usually, the primary solvent used in liquid electrolyte of aluminum electrolytic capacitors is ethylene glycol or γ-butyrolactone. The effect of liquid electrolyte solvent on the failure mechanisms observed in liquid aluminum electrolytic capacitors is missing. Effect of ripple current on the observed failure mechanisms is unknown. Polymer aluminum (PA) capacitors were introduced as the polymer electrolyte is conductive and solid therefore, it does not evaporate and the equivalent series resistance (ESR) of the PA capacitors is low. Manufacturers advise not to use PA capacitors in elevated temperature-humidity environments. But, the Failure modes and mechanisms of polymer aluminum electrolytic capacitors in elevated temperature-humidity are unknown.
In this study, life testing of liquid aluminum electrolytic capacitors chosen based on primary solvent of the electrolyte was performed. For γ-butyrolactone solvent based capacitors, the failure mechanisms observed causing decrease in capacitance were evaporation of electrolyte and decrease in surface area of the aluminum oxide dielectric layer. The observed ESR increase was due to evaporation of electrolyte. For ethylene glycol solvent based capacitors, ESR increase was observed due to ester and amide formation, along with decrease in concentration of the carboxylic acid salts in the electrolyte and evaporation of electrolyte. The failure mechanisms observed in life tests with and without ripple current were the same.
PA capacitors were tested at elevated temperature-humidity of 85ºC, 85%RH and Highly Accelerated Stress Test (HAST) condition of 110ºC, 85%RH. PA capacitors failed due to increase in ESR and increase in leakage current. Iron particles in dielectric layer from the manufacturing process of PA capacitors caused the high leakage current failure. This is a new failure mechanism which has not been reported in the literature. Failure modes observed in 85ºC, 85%RH and HAST tests were same therefore, HAST tests can be used as rapid assessment test for PA capacitors in elevated temperature-humidity environment.

Patel, Chandradip (Ph.D)
Performance Assessment of Mems Gyroscope and Shock Durability Evaluation af SAC305-X Solders for High Temperature Applications

Recent advances in MEMS technology have resulted in relatively low cost MEMS gyroscopes. Their unique features compared to macro-scale devices, such as lighter weight, smaller size, and less power consumption, have made them popular in many applications with environmental conditions ranging from mild to harsh. This dissertation aims to address a gap in the literature on MEMS gyroscopes by investigating the effects of elevated temperatures on the performance of MEMS gyroscopes. MEMS gyroscopes are characterized at room and elevated temperatures for both stationary and rotary conditions. During the test, MEMS gyroscopes are subjected to five thermal cycles at each of four temperature ranges (viz. 25degC to 85degC, 25degC to 125degC, 25degC to 150degC and 25degC to 175degC). A simulation model is developed in MATLAB Simulink to simulate the temperature effect on the MEMS gyroscope. Simulation results show good agreement with experimental results and confirm that Young's modulus and damping coefficient are the dominant factors responsible for temperature-dependent bias at elevated temperatures. Solder interconnects are one of the weakest elements in MEMS devices. Thus, the reliability of solder interconnects is separately studied in this dissertation. Though, SAC305 (96.5%Sn3.0%Ag0.5%Cu) is the industry preferred solder in combined thermal cycling and shock/drop environments, it exhibits better thermal cycling reliability than drop/shock reliability. One of the ways to improve drop/shock reliability of SnAgCu solder is by microalloy addition of various dopants such as Mn, Ce, Ti, Y, Ge, Bi, Zn, In, Ni, Co etc. Thus, the second part of this dissertation aims to evaluate the shock durability of SAC305 and SAC305-X (where X refers to two different concentrations of Mn and Ce dopants). High temperature isothermal aging tests are conducted on selected solders using QFN44, QFN32 and R2512 package types at 185degC and 200degC up to 1000 hours. Isothermal aging test results showed that interfacial IMC growth reduction can be achieved by microalloy addition of selected dopants in SAC305 on both copper and nickel leaded package types. Shock durability of selected solders is examined on as-reflowed and thermally aged test boards. Mechanical shock is performed using a custom shock machine that utilizes a shock pulse of 500G with a 1.3 millisecond duration. The shock test results showed that the mechanical shock reliability of SAC305 was significantly improved on both as-reflowed and thermally aged test boards by microalloy addition of one of the selected dopant in SAC305.

Chang, Moon-Hwan (Ph.D)
Prognostics-based Qualification of White Light-Emitting Diodes (LEDs)

Light-emitting diode (LED) applications have expanded from display backlighting in computers and smart phones to more demanding applications including automotive headlights and street lightening. With these new applications, LED manufacturers must ensure that their products meet the performance requirements expected by end users, which in many cases require lifetimes of 10 years or more. The qualification tests traditionally conducted to assess such lifetimes are often as long as 6,000 hours, yet even this length of time does not guarantee that the lifetime requirements will be met.

This research aims to reduce the qualification time by employing anomaly detection and prognostic methods utilizing optical, electrical, and thermal parameters of LEDs. The outcome of this research will be an in-situ monitoring approach that enables parameter sensing, data acquisition, and signal processing to identify the potential failure modes such as electrical, thermal, and optical degradation during the qualification test. To detect anomalies, a similarity-based-metric test has been developed to identify anomalies without utilizing historical libraries of healthy and unhealthy data. This similarity-based-metric test extracts features from the spectral power distributions using peak analysis, reduces the dimensionality of the features by using principal component analysis, and partitions the data set of principal components into groups using a KNN-kernel density-based clustering technique. A detection algorithm then evaluates the distances from the centroid of each cluster to each test point and detects anomalies when the distance is greater than the threshold. From this analysis, dominant degradation processes associated with the LED die and phosphors in the LED package can be identified. When implemented, the results of this research will enable a short qualification time.

Prognostics of LEDs are developed with spectral power distribution (SPD) prediction for color failure. SPD is deconvoluted with die SPD and phosphor SPD with asymmetric double sigmoidal functions. Future SPD is predicted by using the particle filter algorithm to estimate the propagating parameters of the asymmetric double sigmoidal functions. Diagnostics is enabled by SPD prediction to indicate die degradation, phosphor degradation, or package degradation based on the nature of degradation shape of SPD. SPDs are converted to light output and 1976 CIE color coordinates using colorimetric conversion with color matching functions. Remaining useful life (RUL) is predicted using 7-step SDCM (standard deviation of color matching) threshold (i.e., 0.007 color distance in the CIE 1676 chromaticity coordinates).

To conduct prognostics utilizing historical libraries of healthy and unhealthy data from other devices, this research employs similarity-based statistical measures for a prognostics-based qualification method using optical, electrical, and thermal covariates as health indices. Prognostics is conducted using the similarity-based statistical measure with relevance vector machine regression to capture degradation trends. Historical training data is used to extract features and define failure thresholds. Based on the relevance vector machine regression results, which construct the background health knowledge from historical training units, the similarity weight is used to measure the similarity between each training unit and test unit under the test. The weighted sum is then used to estimate the remaining useful life of the test unit.

Paradee, Gary (M.S.)
Fatigue Properties of Graphene Interconnects on Flexible Substrates

his thesis represents the first determination of the fatigue behavior of Graphene as interconnect material electronic components on flexible substrates. The potential application of this interconnect material is for displays on flexible substrates where fatigue resistance is required due to the stress placed on the interconnect during mechanical bending. As the display is cyclically deformed (fatigued) during normal operation, cracks in the interconnect layer initiate and propagate leading to the lineout failure condition. The major contribution of this work is to show that Graphene is a superior interconnect material to the present state of the art Indium Tin Oxide (ITO) due to its electrical, optical and mechanical properties. The experimental approach in this thesis is based on Graphene samples which were fabricated on Silicon Nitrite (Si3N4)/Polyethylene Naphthalate (PEN) substrates. For comparison, both patterned and uniform ITO films ITO films on Si3N4/PEN were fabricated. The results of the in-depth characterization of Graphene are reported and based on Atomic Force Microscopy (AFM), Raman Spectroscopy and Scanning Electron Microscopy (SEM) are reported. The fatigue characteristics of ITO were determined at stress amplitudes ranging from 2000 MPa to 400 MPa up to 5000 cycles. The fatigue characteristics of Graphene were determined at stress amplitudes ranging from 80 GPa to 40 GPa up to 5000 cycles. The fatigue S-N curves were determined and showed that Graphene's endurance limit is 40 GPa. Beyond the endurance limit, there is no observable high cycle or low cycle fatigue indication for Graphene on a flexible substrate such as PEN. The microstructural analysis by SEM and AFM did not reveal normal fatigue crack growth and propagation. This thesis presents the first comprehensive behavior of Graphene in a bending fatigue stress environment present in numerous flexible electronic applications. The design and stress environments for safe operation has been defined.

Akman, Josh (M.S.)
Numerical Parametric Study of the Thermomechanical Effect of Encapsulation on a Welded Beam Lead Component

Encapsulation of components and assemblies has become widespread in the design of electronic products, providing protection from the environment and enhancing reliability. In this thesis, computational simulations are used to parametrically investigate the thermomechanical role played by the encapsulant when a beam-lead component is welded to slender copper busbars, encapsulated in a polymeric encapsulant, and subjected to temperature cycling. The parametric studies are conducted in two phases, using simplified two-dimensional finite element models. In the first phase, a parametric design space is generated to systematically vary the encapsulant's thermomechanical properties, namely the Young's modulus and Coefficient of Thermal Expansion. A gull wing geometry is introduced into the lead of the component as a stress relief feature. In this case, a ramp thermal loading profile is used to understand the physics of this design and to provide relative comparisons between different combinations of the encapsulant's material properties within the design space. Response surface models are generated over the design space. In the second phase, a Taguchi Design of Experiments (DOE) approach, based on orthogonal arrays, is used to analyze the effects of multiple design parameters under cyclic thermal loading. This includes encapsulant properties (a subset of the properties investigated in the first phase), encapsulant dimensions, lead geometry and dimensions, and busbar dimensions. Lead geometry is considered with and without stress relief features. The loading used in this phase is three temperature cycles between -40oC and 90oC. The primary areas of concern (response variables) in both studies are the component lead and interconnect regions. Deformation and stress states in these critical regions are compared. Main factor effects and selected parameter interactions are computed in accordance with the Taguchi orthogonal arrays, to understand the dominant parameters and parameter interactions for cyclic thermomechanical stresses in this encapsulated assembly.

Pearl, Adam (M.S.)
Effect of Palladium Thickness and Extended Isothermal Aging on the Reliability of Solder Interconnects Formed on ENEPIG Surface Finish

Surface finishes for copper on printed wiring boards play an important role in the reliability of electrical interconnects. Electroless Nickel/Electroless Palladium/Immersion Gold (ENEPIG), developed in the mid-1990s to alleviate the “black-pad” problem created by Electroless Nickel/Immersion Gold (ENIG) surface finish, has gained interest for critical system applications. This thesis investigates the effect of palladium layer thickness and extended isothermal aging on the reliability of both tin-lead and tin silver copper solder interconnects under temperature cycling, vibration cycling, and drop loading conditions. Chip array ball grid array (CABGA) packages soldered onto ENEPIG-finished PCBs are subjected to the three previously listed conditions. Reliability and failure analyses are conducted to determine the overall effect of palladium layer thickness and isothermal aging on the reliability of these solder interconnects.

Bilger, Christopher John (M.S.)
Mechanical and electrical properties of metal-carbon connections for battery applications

Material selection and processing techniques were investigated to form carbon-metal bonds. Mechanical and electrical characterization was performed to more fully comprehend the bonding mechanisms and properties. Utilizing carbon fibers as a primary conduction medium, the specimens from the processes investigated were utilized with lithium-ion cells to further characterize the electrical performance. Electroplating nickel onto the ends of the carbon fibers provides a relatively simple processing technique which improves fiber adhesion to nickel tabs by over 4.7 times when compared to conductive silver epoxy and over 5 times greater than a 1 inch immersion of carbon fiber into a SAC305 solder ingot. Additionally, a reduction of electrical resistance by 0.7 times over the solder ingot is achieved with the electroplating technique. The results of the electroplating are achieved by using about 25% less available contact area than the solder ingot and are scalable for usage in electrical circuits.

Parsa, Ehsan (M.S.)
Effect of Encapsulation on Electrolyte Leakage in Aluminum Electrolytic Capacitors Under Constant Thermal and Electrical Loading

This study focuses on the influence of encapsulation (with silicone elastomer potting compound) on electrolyte leakage in aluminum electrolytic capacitors. Experiments were conducted on potted capacitors at constant elevated temperature and rated DC voltage, and results were compared to those from a control batch of unpotted capacitors. The weight, ESR and capacitance were periodically monitored. Encapsulation was found to decelerate electrolyte loss rate and ESR degradation. There was an increasingly discernible deceleration of capacitance degradation but the magnitude did not reach statistically significant thresholds within the test period. A simplified axisymmetric finite element model was constructed for theoretical understanding of the electrolyte loss process. The experimental measurements were used to guide the selection of the material properties in the model. The model addresses several possible sources of non-uniformities in the mass flux density in the test specimen: (i) radial nonuniformity of mass transport properties of the rubber seal; and (ii) delamination between the potting compound and the capacitor leads. This model was then used: (i) to conduct parametric investigation of the effect of mass transport properties of the potting compound; and (ii) in conjunction with the experimental results to estimate the electrolyte mass loss from the capacitor through the rubber seal.

Kang, Stephen Junho (M.S.)
Adhesion Strength Measurement of Multilayer Structures with Vertical Crack by Four Point Bending Test

Current microelectronic packages consist of multilayer systems. Adhesion strength is one of the most important factors to the reliability of these systems. Previous studies have used four point bending tests as a method for characterizing the energy release rate to obtain the adhesion strength of bilayer systems. An extension of this work is proposed in this study, where a four point bending test of multilayer structures with a vertical crack is used to measure the adhesion strength, assisted by the presence of a predefined area. The predefined area allows for a weak adhesion horizontal accurate pre-crack which permits crack propagation under loading as well as reducing scatter within the values of critical loads. A numerical analysis is conducted to compute the energy release rate from the critical loads using the concept of the J-integral. Two sets of multilayer specimens were fabricated and tested in the study: one for investigating crack front behavior relative to the compliance change in the load-displacement profile by using transparent substrates, and the other using the previous set as a guideline for testing metal substrates under certain environmental conditions. Experimental results along with visual evidence support the consistent behavior between crack front behavior and compliance change. This correlation can be used as a baseline for testing other electronic packages for interfacial failure.

Levy, Jared Michael (M.S.)
Simulation and Analysis of Energy Consumption for an Energy-Intensive Academic Research Building

The University of Maryland's Jeong H. Kim Engineering Building is a state-of-the-art academic research facility. This thesis describes an energy analysis and simulation study that serves to identify energy saving opportunities and optimum operation of the building to achieve its goals of high energy efficiency and substantial CO2 emission reduction. A utility analysis, including a benchmarking study, was completed to gauge the performance of the facility and a detailed energy model was developed using EnergyPlus to mimic current operation. The baseline energy model was then used to simulate eight energy efficiency measures for a combined energy savings of 16,760 MMBtu, reducing annual energy use by 25.3%. The simple payback period for the proposed measures as a single project is estimated to be less than one year. Due to the high-tech and unique usage of the Kim Engineering Building, including cleanrooms and research labs, this thesis also contributes to the development of energy consumption benchmarking data available for such facilities.

Allison, Hannah (M.S.)
A Simulation Approach to Modeling Contingency Strategies for Managing Electronic Part Supply Chain Disruptions

Due to the nature of the manufacturing and support activities associated with long life cycle products, parts need to be dependably and consistently available. However, the parts that comprise long life cycle products are susceptible to a variety of supply chain disruptions. In order to minimize the impact of these unavoidable disruptions to product production and support, manufacturers can implement proactive mitigation strategies. Careful selection of the mitigation strategy (second sourcing and/or buffering) is key, as it can dramatically impact the part total cost of ownership. This thesis developed a simulation model that performs tradeoff analyses and identifies a near-optimal combination of second sourcing and buffering for specific part and product scenarios. In addition, this thesis explores the effectiveness of traditional analytical models when compared to a simulation-based approach for the selection of an effective optimal disruption mitigation strategy. Several case studies were performed that: 1) tested the impact of popular analytical limiting assumptions, and 2) implemented realistic disruption data in the context of real part management. The first set of case studies demonstrated that the simulation model is capable of overcoming significant scenario restrictions prevalent within traditional analytical models: finite horizon (including non-zero WACC), fixed support costs, and unreliable backup suppliers are essential components for determining the effective optimal disruption mitigation strategy for a given disruption scenario. The second set of case studies demonstrates the importance of proper mitigation strategy selection in real electronic part supply chain scenarios. The results from the case studies not only justified the need for a simulation-based approach to disruption modeling, but also helped to cement the simulation model as an effective decision making tool for electronic part distributors.

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