Mechanical Engineering
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Item COPPER CORROSION IN THE FLOWERS OF SULFER TEST ENVIRONMENT(2015) Mahadeo, Dinesh Michael; Pecht, Michael G; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Sulfur, present in the environment in the form of sulfur dioxide and hydrogen sulfide, can produce failure in electronics. In particular, copper, which is used extensively in electronic products, is subject to corrosion in the presence of sulfur. This thesis examines the corrosion of copper under the Flowers of Sulfur (FoS) test at varying temperatures and durations. The FoS test setup, described in ASTM B809, was initially designed to evaluate surface finish porosity, but this setup may have boarder application. To expand the applicability of the FoS test, it is important to characterize the test environment. To this end, a systematic study of copper corrosion was conducted through weight gain measurements of copper coupons that were subjected to FoS test environments. From the test results, a model was developed that correlates copper sulfide thickness to temperature and time under the FoS test. This model can be used to determine test conditions given a target field environment.Item IMPACT OF DUST ON THE RELIABILITY OF PRINTED CIRCUIT ASSEMBLIES(2013) Song, Bo; Pecht, Michael G; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Dust is a ubiquitous component of the environments in which we live and work. It can deposit on printed circuit assembly to act as a source of ionic contamination. Two common consequences of dust contaminations in the printed circuit boards are loss of impedance (i.e., loss of surface insulation resistance) and electrochemical migration between traces and component leads. Both failure mechanisms involve the contamination forming a current leakage path on a printed circuit board. Based on studies on ionic contaminations, researchers have argued that the impact of dust in these two failure mechanisms is dependent on its pH, its hygroscopic compositions, and the critical relative humidity of the salts in it. However, due to the lack of experimental results and the complexity of dust compositions, the argument is not substantiated. Very few papers concerning the impact of different natural dusts on these two failure mechanisms can be found in the literature. In practice, mixtures of Arizona dust and salts are used as a substitute for dust in experiments. In this research, natural dusts were collected from four locations: natural outdoor and indoor dust samples from Massachusetts, U.S., natural outdoor dust from Tianjin, China, and the ISO standard test dust (Arizona test dust). Loss of impedance in dust contaminated printed circuit boards was investigated under controlled temperature (20ºC to 60ºC) and relative humidity (50% to 95%) ranges. The impact of dust on electrochemical migration and corrosion was evaluated under temperature-humidity-bias tests (50ºC, 90% RH, and 10 VDC). In addition to the conventional DC measurement where only resistive data can be obtained, electrochemical impedance spectroscopy were adopted to obtain nonlinear equivalent circuit models of the electrochemical process, which helps to understand the underlying physics-of-failure. The variation of impedance with relative humidity exhibited a transition range. Below the range, the impedance was constant, and above it, the impedance degraded by orders of magnitude. The value of the transition range decreased with an increase of dust deposition density. The equivalent circuit modeling showed that the dominant resistive path gradually shifted from the bulk to the interfacial with the increase of temperature from 20 ºC to 60 ºC. There were big variations among different dusts, which were quantified using the degradation factor introduced in the research, the critical transition range, and time-to-failure. This result demonstrated that a single salt or a mixture of compounds can not be representative of all dusts. It also indicated that using the ISO standard test dust in place of natural dust samples for reliability evaluation could lead to inaccurate results. Dust should be collected from the field in order to evaluate its impact. It is showed in this thesis that some critical characteristics of dust can be used to classify different dusts for the failure mechanisms of interest. Moisture sorption capability of dust can be used to classify different dusts regarding the loss of impedance failure. The dust with the highest moisture sorption capability had the highest degradation factor. Ion species/concentration or conductivity of dust aqueous solution can be used to classify dust regarding the electrochemical migration related failures. Dust with the highest ion concentration and conductivity had the lowest time-to-failure. The underlying principals behind those critical characteristics were described and discussed based on the physics-of-failure.Item CREEP CORROSION OVER PLASTIC ENCAPSULATED MICROCIRCUIT PACKAGES WITH NOBLE METAL PRE-PLATED LEADFRAMES(2005-08-18) Zhao, Ping; Pecht, Michael; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Field failures were observed to be caused by the bridging of corrosion products across lead fingers. This phenomenon was identified as creep corrosion and was the motivation for this work. This dissertation advances the state of knowledge on the creep corrosion process and the strategies for mitigation. A range of plastic encapsulated packages with noble metal pre-plated leadframes, from different vendors, with different package attributes, were used in this study. Creep corrosion on the mold compound surface was reproduced in an accelerated manner using mixed flowing gas (MFG) testing in laboratory conditions. Of the three most widely-used industry-standard MFG testing conditions, Telcordia Outdoor was found to be the most effective environment to induce and promote creep corrosion over the mold compound; Battelle Class III environment can also induce the similar effects of creep corrosion on mold compound, but in a limited rate on selective packages; Telcordia Indoor was found to induce no creep corrosion on the mold compound over a 30 day test time. In both Telcordia Outdoor and Battelle Class III environments, packages attributes and applied pre-conditionings were found to have no significant influence on the creep corrosion effect. Creep corrosion over the mold compound showed a dendritic formation. The thickness of the corrosion product layer on the mold compound tended to increase with increased exposure time. Creep corrosion products were electrically conductive and were able to bridge the adjacent leads, thereby causing electrical shorts. The corrosion products were found to consist primarily of copper oxides, copper chlorides, and copper sulfides. No distinct differences in corrosion products composition was observed on the lead versus the mold compound surfaces. Conformal coating was identified as an effective mitigation strategy to eliminate creep corrosion on noble metal pre-plated leadframe packages.