A. James Clark School of Engineering
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The collections in this community comprise faculty research works, as well as graduate theses and dissertations.
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Item The Effect of Package Geometry on Moisture Driven Degradation of Polymer Aluminum Capacitors(2016) Bevensee, Helmut Manfred; Azarian, Michael H.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Polymer aluminum electrolytic capacitors were introduced to provide an alternative to liquid electrolytic capacitors. Polymer electrolytic capacitor electric parameters of capacitance and ESR are less temperature dependent than those of liquid aluminum electrolytic capacitors. Furthermore, the electrical conductivity of the polymer used in these capacitors (poly-3,4ethylenedioxithiophene) is orders of magnitude higher than the electrolytes used in liquid aluminum electrolytic capacitors, resulting in capacitors with much lower equivalent series resistance which are suitable for use in high ripple-current applications. The presence of the moisture-sensitive polymer PEDOT introduces concerns on the reliability of polymer aluminum capacitors in high humidity conditions. Highly accelerated stress testing (or HAST) (110ºC, 85% relative humidity) of polymer aluminum capacitors in which the parts were subjected to unbiased HAST conditions for 700 hours was done to understand the design factors that contribute to the susceptibility to degradation of a polymer aluminum electrolytic capacitor exposed to HAST conditions. A large scale study involving capacitors of different electrical ratings (2.5V – 16V, 100µF – 470 µF), mounting types (surface-mount and through-hole) and manufacturers (6 different manufacturers) was done to determine a relationship between package geometry and reliability in high temperature-humidity conditions. A Geometry-Based HAST test in which the part selection limited variations between capacitor samples to geometric differences only was done to analyze the effect of package geometry on humidity-driven degradation more closely. Raman spectroscopy, x-ray imaging, environmental scanning electron microscopy, and destructive analysis of the capacitors after HAST exposure was done to determine the failure mechanisms of polymer aluminum capacitors under high temperature-humidity conditions.Item 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.Item Evaluating the Susceptibility of Electronic Components Assembled with Leaded Solder to Flexural Failures, with High Rate Considerations(2008-11-20) Watkins, James Robert; Barker, Donald B; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Multilayer ceramic capacitors (MLCCs) are subjected to high strain rate flexural loading via drop tower and air gun tests, with PWB strain rates ranging from 1/s to 10/s. Three MLCC part sizes, three different manufacturers, and standard and flexible termination parts are included in the study. Standard termination capacitors failed via the well documented flex crack failure mechanism. However in all cases this crack followed a vertical path not typical of this failure mechanism. Flexible termination capacitors failed via a newly discovered failure mechanism involving delamination in the end cap metallization between the silver filled epoxy and the nickel-tin plating. For size 1206 parts, this delamination was seen in both end caps, and the part detached from the test board. For size 0603 parts, this delamination occurred in one end cap, while the opposite end cap fractured though the ceramic in a manner similar to a flex crack. Size 0603 parts also failed via the vertical flex cracks documented in standard termination parts. All of the documented failures of MLCC devices at PWB strain rates of ≥ 1/s occurred at maximum PWB strain values greater than an order of magnitude lower than those seen in lower strain rate testing. This rate dependency of MLCC part failures has vast implications for products intended for high rate environments. Additionally, when the PWB strain rate was increased along with PWB maximum strain, flexible termination capacitors performed worse than their standard termination equivalents. This brings to issue the role of these next generation parts in portable consumer electronic devices as well as other designs with high rate implications. Ball grid array (BGA) devices are subjected to four point bend tests via a servo-hydraulic testing machine at PWB strain rates ≤ 0.1/s. The resulting BGA data is found to adhere reasonably well to the Coffin-Manson low cycle fatigue relationship. Independently generated BGA data that differs with respect to many testing variables is plotted alongside the experimental data. The high correlation of the data set indicates the possibility of creating a BGA mechanical failure model that is independent of failure site, package type, and test specifications.