Microcracking in piezoelectric materials by the Boundary Element Method

A 3D boundary element model for piezoelectric polycrystalline micro-cracking is discussed in this contribution. The model is based on the boundary integral representation of the electro-mechanical behavior of individual grains and on the use of a generalized cohesive formulation for inter-granular micro-cracking. The boundary integral formulation allows to address the electro-mechanical boundary value problem in terms of generalized grain boundary and inter-granular displacements and tractions only, which implies the natural inclusion of the cohesive laws in the formulation, the simplification of the analysis pre-processing stage, and the reduction of the number of degrees of freedom of the overall analysis with respect to other popular numerical methods

MODELING THE PHYSICS OF FAILURE FOR ELECTRONIC PACKAGING COMPONENTS SUBJECTED TO THERMAL AND MECHANICAL LOADING

This dissertation presents three separate studies that examined electronic components using numerical modeling approaches. The use of modeling techniques provided a deeper understanding of the physical phenomena that contribute to the formation of cracks inside ceramic capacitors, damage inside plated through holes, and to dynamic fracture of MEMS structures. The modeling yielded numerical substantiations for previously proposed theoretical explanations. Multi-Layer Ceramic Capacitors (MLCCs) mounted with stiffer lead-free solder have shown greater tolerance than tin-lead solder for single cycle board bending loads with low strain rates. In contrast, flexible terminations have greater tolerance than stiffer standard terminations under the same conditions. It has been proposed that residual stresses in the capacitor account for this disparity. These stresses have been attributed to the higher solidification temperature of lead free solders coupled with the CTE mismatch between the board and the capacitor ceramic. This research indicated that the higher solidification temperatures affected the residual stresses. Inaccuracies in predicting barrel failures of plated through holes are suspected to arise from neglecting the effects of the reflow process on the copper material. This research used thermo mechanical analysis (TMA) results to model the damage in the copper above the glass transition temperature (Tg) during reflow. Damage estimates from the hysteresis plots were used to improve failure predictions. Modeling was performed to examine the theory that brittle fracture in MEMS structures is not affected by strain rates. Numerical modeling was conducted to predict the probability of dynamic failure caused by shock loads. The models used a quasi-static global gravitational load to predict the probability of brittle fracture. The research presented in this dissertation explored drivers for failure mechanisms in flex cracking of capacitors, barrel failures in plated through holes, and dynamic fracture of MEMS. The studies used numerical modeling to provide new insights into underlying physical phenomena. In each case, theoretical explanations were examined where difficult geometries and complex material properties made it difficult or impossible to obtain direct measurements

Two-Scale Simulation of Drop-Induced Failure of Polysilicon MEMS Sensors

In this paper, an industrially-oriented two-scale approach is provided to model the drop-induced brittle failure of polysilicon MEMS sensors. The two length-scales here investigated are the package (macroscopic) and the sensor (mesoscopic) ones. Issues related to the polysilicon morphology at the micro-scale are disregarded; an upscaled homogenized constitutive law, able to describe the brittle cracking of silicon, is instead adopted at the meso-scale. The two-scale approach is validated against full three-scale Monte-Carlo simulations, which allow for stochastic effects linked to the microstructural properties of polysilicon. Focusing on inertial MEMS sensors exposed to drops, it is shown that the offered approach matches well the experimentally observed failure mechanisms

A Micro-milling cutting force and chip formation modeling approach for optimal process parameters selection

Las últimas décadas evidencian una demanda creciente por componentes miniaturizados con dimensiones reducidas y tolerancias estrechas, lo cual ha conllevado al desarrollo de la micro y nanotecnología. El micro-fresado, dentro de los procesos de micro-mecanizado, tiene el potencial de ser uno de los procesos de remoción de material más costo-efectivos y eficientes debido a su facilidad de aplicación, variedad de materiales de trabajo y flexibilidad geométrica. Se enfrenta a unos retos complejos debido al efecto de tamaño, vibraciones y otros factores incontrolables. Este estudio analiza dicho proceso orientado hacia desarrollar una mejor comprensión de la mecánica del micro-corte para ser aplicada en la optimización de parámetros de proceso. Se propone un acercamiento al modelado híbrido en forma novedosa, que permite una evaluación numérica a priori para evaluación de fuerzas y esfuerzos, combinado con experimentación para evaluar parámetros relevantes a la industria (formación de rebabas, desgaste de herramientas, entre otros).DoctoradoDoctor en Ingeniería Mecánic

Computational Design of the Electrical and Mechanical Performance of Steerable MEMS Antennas

This thesis describes the origins, improvements, and variations of a broadband microwave antenna that can be beam-steered by a micro-electromechanical system (MEMS). The steerable MEMS antenna of this work was comprised of a planar antenna on top of a Silicon membrane. The membrane is etched to create a gimbal hinge structure and a platform which supported the antenna and gave it one or two degrees of freedom of rotation. The antennas presented were broadband and fed by a coplanar waveguide (CPW) transmission line which traversed the hinge structure. The antenna\u27s orientation in space was designed to be changed through electrostatic actuation of the antenna platform\u27s hinges. The goal of this thesis was to improve on the initial design and performance of the prototypic antenna. The best variation of the prototype antenna could rotate ±4.0° in two degrees of freedom under 800 VDC of actuation voltage and had a bandwidth of 1.55. The mechanical and electrical aspects of the device were studied and analyzed concurrently. Three variations of the MEMS antenna platform were design and modeled; Generations 1 - 3 (G1 - G3). The G1 platform was an optimized version of the prototypic MEMS platform. The G2 platform could rotate in two dimensions but had much thinner hinges and a more robust antenna platform. The G3 platform was a one degree of freedom version of the G2 platform. A new antenna shape was selected and optimized for integration with the three generations of antenna platforms; the planar inverted cone antenna (PICA). The G3 platform had the best overall electrical and mechanical performance. Two additional antennas were simulated on the G3 platform; a cylindrical dielectric resonator antenna (C-DRA) and a teardrop dielectric resonator antenna (Td-DRA). The three best antenna variations on the G3 platform were simulated to have maximum actuation angles ranging from 10 - 13° and have bandwidths of 3.62 (PICA), 1.70 (C-DRA), and 1.78 (Td-DRA)

Condition Assessment and Fault Prognostics of Microelectromechanical Systems.

International audienceMicroelectromechanical systems (MEMS) are used in different applications such as automotive, biomedical, aerospace and communication technologies. They create new functionalities and contribute to miniaturize the systems and reduce their costs. However, the reliability of MEMS is one of their major concerns. They suffer from different failure mechanisms which impact their performance, reduce their lifetime and their availability. It is then necessary to monitor their behavior and assess their health state to take appropriate decision such as control reconfiguration and maintenance. These tasks can be done by using Prognostic and Health Management (PHM) approaches. This paper addresses a condition assessment and fault prognostic method for MEMS. The paper starts with a short review about MEMS and presents some challenges identified and which need to be raised to implement PHM methods. The purpose is to highlight the intrinsic constraints of MEMS from PHM point of view. The proposed method is based on a global model combining both nominal behavior model and degradation model to assess the health state of MEMS and predict their remaining useful life. The method is applied on a microgripper, with different degradation models, to show its effectiveness

Gold Thermocompression Wafer Bonding

Thermocompression bonding of gold is a promising technique for the fabrication and packaging microelectronic and MEMS devices. The use of a gold interlayer and moderate temperatures and pressures results in a hermetic, electrically conductive bond. This paper documents work conducted to model the effect of patterning in causing pressure non-uniformities across the wafer and its effect on the subsequent fracture response. A finite element model was created that revealed pattern-dependent local pressure variations of more than a factor of three. This variation is consistent with experimental observations of bond quality across individual wafers A cohesive zone model was used to investigate the resulting effect of non-uniform bond quality on the fracture behavior. A good, qualitative agreement was obtained with experimental observations of the load-displacement response of bonds in fracture tests.Singapore-MIT Alliance (SMA

Anisotropic Vapor HF etching of silicon dioxide for Si microstructure release

Damages are created in a sacrificial layer of silicon dioxide by ion implantation to enhance the etch rate of silicon-dioxide in liquid and vapor phase hydrofluoric acid. The etch rate ratio between implanted and unimplanted silicon dioxide is more than 150 in vapor hydrofluoric acid (VHF). This feature is of interest to greatly reduce the underetch of microelectromechanical systems anchors. Based on the experimentally extracted etch rate of unimplanted and implanted silicon dioxide, the patterning of the sacrificial layer can be predicted by simulation

Review of the Synergies Between Computational Modeling and Experimental Characterization of Materials Across Length Scales

With the increasing interplay between experimental and computational approaches at multiple length scales, new research directions are emerging in materials science and computational mechanics. Such cooperative interactions find many applications in the development, characterization and design of complex material systems. This manuscript provides a broad and comprehensive overview of recent trends where predictive modeling capabilities are developed in conjunction with experiments and advanced characterization to gain a greater insight into structure-properties relationships and study various physical phenomena and mechanisms. The focus of this review is on the intersections of multiscale materials experiments and modeling relevant to the materials mechanics community. After a general discussion on the perspective from various communities, the article focuses on the latest experimental and theoretical opportunities. Emphasis is given to the role of experiments in multiscale models, including insights into how computations can be used as discovery tools for materials engineering, rather than to "simply" support experimental work. This is illustrated by examples from several application areas on structural materials. This manuscript ends with a discussion on some problems and open scientific questions that are being explored in order to advance this relatively new field of research.Comment: 25 pages, 11 figures, review article accepted for publication in J. Mater. Sc

Micro-manufacturing : research, technology outcomes and development issues

Besides continuing effort in developing MEMS-based manufacturing techniques, latest effort in Micro-manufacturing is also in Non-MEMS-based manufacturing. Research and technological development (RTD) in this field is encouraged by the increased demand on micro-components as well as promised development in the scaling down of the traditional macro-manufacturing processes for micro-length-scale manufacturing. This paper highlights some EU funded research activities in micro/nano-manufacturing, and gives examples of the latest development in micro-manufacturing methods/techniques, process chains, hybrid-processes, manufacturing equipment and supporting technologies/device, etc., which is followed by a summary of the achievements of the EU MASMICRO project. Finally, concluding remarks are given, which raise several issues concerning further development in micro-manufacturing