Monotonic and fatigue testing of spring-bridged freestanding microbeams application for MEMS

Microelectromechanical systems (MEMS) technologies are developing rapidly with increasing study of the design, fabrication and commercialization of microscale systems and devices. Accurate knowledge on the mechanical behaviors of thin film materials used for MEMS is important for successful design and development of MEMS. Here a novel electroplating spring-bridge micro-tensile specimen integrates pin-pin align holes, misalignment compensate spring, load sensor beam and freestanding thin film is demonstrated and fabricated. The specimen is fit into a specially designed micro-mechanical apparatus to carry out a series of monotonic tensile testing on sub-micron freestanding thin films. Certain thin films applicable as structure or motion gears in MEMS were tested including sputtered gold, copper and tantalum nitride thin films. Metal specimens were fabricated by sputtering; for tantalum nitride film samples, nitrogen gas was introduced into the chamber during sputtering tantalum films on the silicon wafer. The sample fabrication method involves three steps of lithography and two steps of electroplating copper to hold a dog bone freestanding thin film. Using standard wet etching or lift off techniques, a series of microtensile specimens were patterned in metal thin films, holes, and seed layer for spring and frame structure on the underlying silicon oxide coated silicon substrate. Two steps of electroplating processing to distinct spring and frame portion of the test chip. Finally, chemical etched away the silicon oxide to separated electroplated specimen and silicon substrate.Comment: Submitted on behalf of EDA Publishing Association (http://irevues.inist.fr/EDA-Publishing

Design and fabrication of electromagnetic micro-relays using the UV-LIGA technique

This dissertation reports a research effort to microfabricate an electromagnetic relay for power applications using a multilayer UV-LIGA process. A mechanically wrapped coil was used and very simple design for the magnetic circuit was adopted to increase the design flexibility and performances. The broad material selection and the capability of making high aspect ratio microstructures of the UV-LIGA make the technology best suited for fabricating microelectromechanical power relays. Fabrication of the device required significant advances in the optical lithography of SU-8 negative photoresist. Research proved that aspect-ratios up to 40:1 in isolated open field structures of thickness between 1 and 1.5 mm can be obtained a standard broadband UV source. The principal factor in this achievement is the reduction of internal stress during the post-exposure bake process that eliminates large plastic deformations present during standard bake procedures. Another challenging issue associated with producing high aspect ratio microstructures is the development narrow groves and deep holes in SU-8 lithography. To overcome this obstacle, megasonic agitation was applied to the developer bath, which resulted in faster development rates, more uniform development, and the ability to produce structures with higher aspect ratios. To date, this process has been used to achieve 100:1 aspect ratio open field features and 45:1 intact cylinder arrays. A multi-layer SU-8 optical lithography and metal electrodeposition process was developed to fabricate the relay. The design required implementation of high aspect ratio lithographic processing techniques to produce a tall nickel magnetic core and insulated magnetic cup in which a pre-wrapped solenoid would be placed for electromagnetic driving. After insertion of the solenoid a Ni-Fe actuator was bonded to the relay base to complete the device. To better understand the fatigue life of electroplated microstructures, a theoretical model was developed determine the possible fracture mechanics properties and fatigue life of LIGA fabricated nickel and nickel-iron alloys for use in microsystems applications. The prototype micro-relays were tested for the dynamic characteristics and power capacity. The experimental results have confirmed that reasonably large current capacity and fast response speed can be achieved using electromagnetic actuation and the multi-layer UV-LIGA fabrication process developed

The electrodeposition and characterisation of compositionally modulated tin-cobalt alloy coatings as lead-free plain bearing material

Traditionally, lead-based bearing overlays dominate the commercial automotive market and it has been proven that an excellent combination of properties can be attained through their use. However, lead is a toxic metal and a cumulative poison in humans. According to the European Union End-of-Life Vehicle (ELV) Directive proposed in 1997, vehicles that registered in'all the member states after 1st July 2003 should contain no lead, mercury, cadmium and hexavalent chromium. In this study, a new sulphate-gluconate electrolyte was used to produce multilayer SnCo coatings, aimed at a lead-free overlay for future market use. Tin-cobalt compositionally modulated alloy (CMA) coatings produced from sulphategluconate electrolytes have been previously examined as a potential replacement for lead-free bearing overlays [1]. However, some obstacles may exist which limit their potential use on an industrial scale. For example, long electroplating times are required to produce a thick coating which is very undesirable from an industrial viewpoint, and also the possible elemental interdiffusion occurring in the coating system under engine operating temperatures could rapidly deteriorate the coating properties. In addition, there is an increasing demand from automotive industry to further improve bearing overlay properties, for example for high performance and high compression ratio engines... cont'd

Reliability of Copper-Filled Stacked Microvias in High Density Interconnect Circuit Boards

The electronics industry strives to produce affordable, lightweight, and reliable products with higher performance. At the electronic component level, this translates to components with increased I/Os and reduced footprints, and on the package substrate and printed circuit board (PCB) level, to the use of high density interconnects (HDIs). HDI technology makes use of microvias as interconnects between different conductor layers. According to IPC standards, microvias are blind or buried vias that are equal to or less than 150 μm in diameter. Advances in miniaturized electronic devices have led to the evolution of microvias from single-level to stacked structures that intersect multiple HDI layers. A stacked microvia is usually filled with electroplated copper to make electrical interconnections and provide structural support. A challenge for HDI circuit board processing is to fabricate microvias without generating defects in the deposited copper structures. Firstly, the copper plating process can easily generate voids in microvias. When voids are present, localized stress concentrations within the electrodeposited copper structure can degrade the reliability of microvias. Secondly, poor quality of electroless copper (a process step following microvia hole drilling and prior to electrolytic copper plating, that makes the microvia hole conductive) results in inferior bonding between the base of the microvia and the target pad underneath the microvia. Microvia base and target pad interface separation is a common failure observed in HDI circuit boards. The objectives of this dissertation are to determine the effects of voids on the lifetime of copper-filled stacked microvias, and to develop an analytical model that the electronics industry can use to predict microvia fatigue life and assess risks associated with production and use of the latest generation of HDI circuit boards. The dissertation also aims to quantitatively address the factors that affect microvia interface separation. A parametric study was conducted to investigate the effects of voids on the thermo-mechanical reliability of copper-filled stacked microvias using 3-D finite element analysis and strain-based fatigue life estimation. It was found that microvia fatigue life is affected by geometrical void characteristics, such as shape, size, and location; microvia aspect ratio; and material properties of dielectric layers. Large voids decrease the lifetime of microvias—for example, a 16% conical void results in a microvia fatigue life that is only 1.4% of that of a non-voided microvia. Moreover, microvia aspect ratio and z-axis coefficient of thermal expansion (CTE) of the HDI dielectric material are critical parameters for the lifetime. The fatigue life of a voided microvia of 0.25 aspect ratio is more than two orders of magnitude longer than the fatigue life of a voided microvia of 0.75 aspect ratio with the same void size. An increase of the z-axis CTE by 40% (from 50 ppm/°C to 70 ppm/°C) decreases the microvia fatigue life by 95%. As an outgrowth of this study, a microvia virtual qualification method was proposed. Using the combination of finite element analysis and fatigue life estimation, the required amount of HDI board reliability testing will be reduced, cutting overall development time and cost. The factors that affect microvia fatigue life were examined, and a design of experiment (finite element simulation) was performed to quantify the effects of those factors on microvia lifetime in terms of cycles to failure. A second-order regression life prediction model was developed using response surface mothed (RSM) to predict cycles to failure of copper-filled stacked microvias under thermal loading. The life prediction model accounts for not only the microvia design parameters and material properties, but also voiding defects introduced during the manufacturing process. The model can predict cycles-to-failure of microvias without voids and with voids of different sizes. The electronics industry can use this model as a convenient and inexpensive tool for HDI design and process validation. This is the first known regression model for copper-filled stacked microvia life prediction. Finally, the factors that affect microvia interface separation were quantitatively addressed. Finite element modeling was used to simulate microvias with imperfect electroless copper layers. This study revealed how thermal loadings and structure flaws (in terms of initial crack length) affect the chance of microvia interface separation

Compliant Chip-to-Package Interconnects for Wafer Level Packaging

Ph.DDOCTOR OF PHILOSOPH

Technology and Advanced Development for a Non-Toxic Orbital Maneuvering System and Reaction Control System for Orbiter Upgrade

NASA intends to pursue technology applications to upgrade the Space Shuttle Orbiter OMS and RCS systems with non-toxic propellants. The primary objectives of an upgraded OMS/RCS are improved safety and reliability, reduced operations and maintenance costs while meeting basic OMS/RCS operational and performance requirements. The OMS/RCS has a high degree of direct interaction with the crew and requires subsystem and components that are compatible with integration into the orbiter vehicle with regard to external mold-line, power and thermal control The non-toxic propulsion technology is also applicable to future Human Exploration and Development of Space (HEDS) missions. The HEDS missions have similar requirements for attitude control and lander descent/ascent propulsion and which will emphasize the use of In-Situ Resource for propellants. When used as a regenerative coolant as in the Shuttle Orbiter OMS combustion chamber, non-toxic fuels such as ethanol are limited in their cooling capacity by the bulk temperature rise permitted to prevent film boiling or possible coking. Typical regeneratively cooled chambers are constructed from highly conductive copper, which maximizes heat transfer, or from low conductivity materials like stainless steel that can also exacerbate cooling problems. For an ethanol cooled application the heat transfer into the fluid must be controlled to reduce the fuel coolant bulk temperature rise. An approach to provide this control is the subject of this report. This report is being issued to document work done by Aerojet on NASA contract NAS 8-98042. Specifically, this project investigates of the use of ethanol, a designated non-toxic fuel, as a coolant for the Space Shuttle Orbital Maneuvering System Engine combustion chamber. The project also addresses a cost reducing fabrication technique for construction of such a combustion chamber. The study contained three major sub-tasks: an analytical investigation and trade study which included layout of a flight type chamber concept, the fabrication and evaluation of formed platelet liner panels and the preparation and testing of mechanical properties specimens representative of a novel hot gas wall concept

Electrochemical fatigue crack treatment

Fatigue is responsible for at least 50% of all mechanical and 90% of all metallic failures. Fatigue cracks often start at stress concentrations, and without timely and appropriate remediation, tend to exhibit relatively fast propagation that leads to property damage and sometimes serious accidents. The objective of this research was to develop a new method of fatigue crack treatment in steel structures and estimate its efficiency and limitations. The method was based on placing fatigue cracks under compression by depositing nickel onto the surfaces of the cracks. The proposed method was applied to ASTM E399 compact-tension specimens machined from ASTM A36 steel. This study found that the method was able to arrest fatigue crack propagation. Fatigue crack arrest period varied from 2,000 to 30,000 cycles. The fatigue life of the specimens was extended for up to 55,000 cycles. In many cases the re-initiation life of fatigue cracks after treatment was similar to the crack initiation life obtained from the V-shaped starter notches. A power law relationship was developed that successfully correlates the fatigue crack arrest life and the stress intensity factor range applied for post-treatment load cycling. Fatigue crack packing with nickel resulted in significant reduction of stress concentration factors of the cracks. Spectroscopic analysis confirmed the presence of nickel in electrochemically treated fatigue cracks. The amount of nickel deposited was found to be non-uniform along the length of the cracks. This study found that an elastic finite element analysis (FEA) supported the notion of compressive stresses being developed at the crack tip and of a significant reduction in the stress concentration factor of the fatigue crack due to application of this treatment method. FEA has also supported the expectation that increasing the dosage of crack packing material would tend to result in a longer crack re-initiation period. The proposed method of electrochemical fatigue crack treatment was found to be beneficial in terms of improved corrosion resistance of treated specimens as long as the treatment was uniform and continuous. The average general corrosion rate of nickel-plated ASTM A36 steel specimens that were uncracked was 80% lower than that of the non-plated cases. The discontinuous deposition observed within cracks was expected to promote localized corrosion of the A36 base metal. Future work will examine the use of deposition candidates with less of a tendency toward dissimilar metal corrosion

Bibliography of Lewis Research Center technical contributions announced in 1976

Abstracts of Lewis authored publications and publications resulting from Lewis managed contracts which were announced in the 1976 issues of STAR (Scientific and Technical Aerospace Reports) and IAA (International Aerospace Abstracts) are presented. Research reports, journal articles, conference presentations, patents and patent applications, and these are included. The arrangement is by NASA subject category. Citations indicate report literature (identified by their N-numbers) and the journal and conference presentations (identified by their A-numbers). A grouping of indexes helps locate specific publications by author (including contractor authors), contractor organization, contract number, and report number

Design and testing of an orthogonal LCP interconnect for RF applications in high vibration environments

A new design is presented for a wideband orthogonal interconnect between two perpendicular printed wiring boards, employing novel geometries and materials to minimize stress under cyclic loading. This will ensure fatigue survivability in high vibration environments, opening the door to vertical interconnection in RF circuit design. This is, to the best of knowledge, the first complete design and prototype for an orthogonal interconnect at the board level for broadband RF circuits. An analytical approach is used to define the driving parameters in the stress distribution within a smooth curve joining two perpendicular surfaces using analytical geometries, and Finite Element Analysis is used to finalize the design and ensure all constituent materials in the interconnect are subjected to stresses below their fatigue strength at 10 million cycles at full deflection. A manufacturing process is then proposed using thermoforming to shape the Liquid Crystal Polymer base material into the desired geometry, as well as an assembly solution to mount the interconnect to an RF signal feed card. Finally, a test setup is designed allowing for high cycle fatigue testing within the order of hours, including the capability to monitor performance of the interconnect by tracking DC continuity through a simulated application using a single post design. The prototype interconnect is tested to failure and is shown to survive 18 million cycles of a typical loading application before failure of the LCP springs occurs in the mode predicted by the initial FEA model.M.S