Wuthrich / Abou Ziki Micromachining Using Electrochemical Discharge Phenomenon

Fundamentals and Application of Spark Assisted Chemical Engraving
2. Auflage 2014
ISBN: 978-0-323-26564-5
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 218 Seiten

Reihe: Micro and Nano Technologies

Micro-machining is an advanced manufacturing technique of growing importance, and adoption of micro-machining using electrochemical discharges (Micro-ECDM) has increased steadily in recent years. Among new developments is the interest of industry in Micro-ECDM. However, the potential of the technology is not being fully utilized and there is no comprehensive reference book available today covering it. Micromachining Using Electrochemical Discharge Phenomenon, Second Edition fills this gap. It is unique in its detailed coverage of all aspects of the Micro-ECDM process, as well as Spark Assisted Chemical Engraving (SACE). As such, it covers technologies such as chemical etching, micro-drilling, and other material removal mechanisms, high aspect ratio machining, design and construction of the machining apparatus, and a wide range of applications. The new edition compares Micro-ECDM and SACE with other micromachining technologies such as laser machining and traditional EDM. ECDM is used for machining of electrically non-conductive materials. Micro-ECDM/SACE is mainly applied to glass and the book focuses on glass, but the authors also present new results on other materials such as ceramics. In addition, techniques to modify material properties for the machining process are explained. The authors discuss machining strategies including the latest developments in micro-texturing of glass micro-channels and reports on developments in controlling and analysis aspects of machining. This book is a unique reference for engineers and industrial researchers involved in development, design and use of micromachining, chemical micro-drilling or chemical engraving techniques and equipment. - Only all-encompassing reference coving Micro-ECDM and SACE available on the market - Covers a wide range of applications, including applications in the MEMS industry and the Medical Devices and Medical Diagnostics industries - New edition includes expanded sections on comparing Micro-ECDM/SACE with other micromachining technologies

Autoren/Hrsg.

Wuthrich, Rolf

Abou Ziki, Jana D.

Inhaltsverzeichnis

1;Front Cover;1
2;Micromachining Using Electrochemical Discharge Phenomenon: Fundamentals and Application of Spark Assisted Chemical Engraving;4
3;Copyright;5
4;Contents;6
5;Preface;10
6;CHAPTER 1 - MACHINING WITH ELECTROCHEMICAL DISCHARGES—AN OVERVIEW;12
6.1;1.1 SPARK-ASSISTED CHEMICAL ENGRAVING;13
6.2;1.2 SACE AS A MICROMACHINING TECHNOLOGY;19
6.3;1.3 SCOPE OF THE BOOK;20
7;PART 1 ELECTROCHEMICAL DISCHARGES;22
7.1;CHAPTER 2 - HISTORICAL OVERVIEW OF ELECTROCHEMICAL DISCHARGES;24
7.1.1;2.1 DISCOVERY AND EARLY APPLICATIONS;24
7.1.2;2.2 THE WEHNELT INTERRUPTER;26
7.1.3;2.3 SPECTRUM OF THE ELECTROCHEMICAL DISCHARGES;31
7.1.4;2.4 NATURE OF THE ELECTROCHEMICAL DISCHARGES;33
7.1.5;2.5 CONTACT GLOW DISCHARGE ELECTROLYSIS;39
7.2;CHAPTER 3 - GAS-EVOLVING ELECTRODES;46
7.2.1;3.1 INTRODUCTION TO ELECTROCHEMISTRY;46
7.2.2;3.2 BUBBLE FORMATION DURING ELECTROLYSIS;54
7.2.3;3.3 BUBBLE LAYER;57
7.2.4;3.4 THE BUBBLE DIFFUSION REGION;59
7.2.5;3.5 THE BUBBLE ADHERENCE REGION;60
7.2.6;3.6 BUBBLE EVOLUTION ON A GAS-EVOLVING ELECTRODE;67
7.2.7;3.7 MEAN STATIONARY CURRENT–VOLTAGE CHARACTERISTICS;69
7.3;CHAPTER 4 - THE GAS FILM—A KEY ELEMENT;74
7.3.1;4.1 FORMATION OF THE GAS FILM;75
7.3.2;4.2 GAS FILM LIFETIME;86
7.3.3;4.3 SHAPE OF THE GAS FILM;90
7.3.4;4.4 DISCHARGE ACTIVITY INSIDE OF THE GAS FILM;90
7.3.5;4.5 ACTIVE ELECTRODE TEMPERATURE;100
7.3.6;4.6 CONTROLLING THE GAS FILM;103
8;PART 2 MICROMACHINING WITH ELECTROCHEMICAL DISCHARGES;106
8.1;CHAPTER 5 - MATERIAL REMOVAL MECHANISM;108
8.1.1;5.1 GENERAL CONSIDERATIONS;108
8.1.2;5.2 MACHINING AT LOW DEPTHS;113
8.1.3;5.3 MACHINING AT HIGH DEPTHS;122
8.1.4;5.4 CHEMICAL CONTRIBUTIONS;124
8.1.5;5.5 SUMMARY;127
8.2;CHAPTER 6 - COMMON MACHINING STRATEGIES;128
8.2.1;6.1 GENERAL OVERVIEW;128
8.2.2;6.2 GRAVITY-FEED DRILLING;130
8.2.3;6.3 CONSTANT VELOCITY-FEED DRILLING;145
8.2.4;6.4 2D AND 3D MACHINING;155
8.2.5;6.5 WIRE ELECTROCHEMICAL DISCHARGE MACHINING;163
8.3;CHAPTER 7 - CONTROLLING THE MACHINING PROCESS;166
8.3.1;7.1 PROCESS ANALYSIS;167
8.3.2;7.2 PROMOTING ETCHING;168
8.3.3;7.3 CONTROLLING THE HEAT GENERATED;177
8.3.4;7.4 CONTROLLING THE TOOL–WORKPIECE GAP;189
8.3.5;7.5 SEARCHING FOR PROCESS CONTROL SIGNALS;189
8.3.6;7.6 SUMMARY;191
8.4;CHAPTER 8 - DESIGNING A SACE MICROMACHINING SET-UP;192
8.4.1;8.1 GENERAL DESIGN RULES;192
8.4.2;8.2 DRILLING SET-UPS;196
8.4.3;8.3 2D MACHINING SET-UPS;200
8.4.4;8.4 COMMERCIAL MACHINES;202
8.5;CHAPTER 9 - OUTLOOK;204
9;REFERENCES;206
10;Index;214

Leseproben

Chapter 1 Machining with Electrochemical Discharges—An Overview
Abstract
The last century also saw the birth of micromachining, in particular micromachining of silicon. A huge variety of micromachining techniques are available for silicon. A similar situation exists for electrically conductive materials, where, in particular, electrochemical machining (ECM) and electrical discharge machining (EDM) are two powerful tools available. However, several electrically nonconductive materials are also of great interest for many applications. Glass and composite materials are two examples. The technical requirements for using glass in microsystems are growing. Medical devices requiring biocompatible materials is only one of many examples. Various techniques are available to micromachine glass. However, one of the main limiting factors in incorporating glass into microdevices is its limited machinability. A similar situation exists for other hard-to-machine materials, such as ceramics and composite materials. A possible answer to these issues could be spark-assisted chemical engraving (SACE) or electrochemical discharge machining (ECDM). This chapter presents a short overview of SACE and places it into the context of other micromachining technologies. Keywords
Electrochemical discharge; Mechanical machining; Micromachining; SACE; Thermal machining Chapter Outline 1.1 Spark-Assisted Chemical Engraving 2 1.1.1 What is SACE? 2 1.1.2 Machining Examples 3 1.1.3 A Short Historical Overview 5 1.2 SACE as a Micromachining Technology 8 1.2.1 Mechanical Machining 8 1.2.2 Chemical Machining 8 1.2.3 Thermal Machining 8 1.3 Scope of the Book 9 Since the very beginning of history, and even prehistory, humanity has invested a lot of effort in developing the skill of processing materials. There is no need to present the fundamental importance of the capability of machining in any technology. Any new technology requires new machining skills. In the last century, the need for using more and more specialized materials (e.g., silicon, composites, ceramics) greatly increased the already large arsenal of machining technologies. The last century also saw the birth of micromachining, in particular micromachining of silicon. At present, a huge variety of micromachining techniques are available for silicon. A similar situation exists for electrically conductive materials, where, in particular, electrochemical machining (ECM) and electrical discharge machining (EDM) are two very powerful tools available. However, several electrically nonconductive materials are also of great interest for many applications. Glass and composite materials are two examples. The technical requirements for using glass in microsystems are growing. Medical devices requiring biocompatible materials is only one of many examples. The importance of glass is also growing in the field of microelectromechanical systems (MEMS). The term MEMS refers to a collection of microsensors and actuators. MEMS emerged in the 1990s with the development of processes for the fabrication of integrated circuits. In particular, Pyrex® glass is widely used because it can be bonded by anodic bonding (also called field-assisted thermal bonding or electrostatic bonding) to silicon. Glass has some very interesting properties, such as its chemical resistance or biocompatibility. It is amorphous and can therefore be chemically attacked in all directions. As glass is transparent, it is widely used in optical applications or in applications where optical visualization of a process is needed. Some promising applications for glass in the MEMS field are microaccelerometers, microreactors, micropumps, and medical devices (e.g., flow sensors or drug delivery devices). A representative example in which glass-to-silicon bonding is used is bulk micromachined accelerometers (Wolffenbuttel, 1995). In this case, glass serves several functions: • provides a seal and the desired damping; • can be used as a capacitor when a metal plate is placed on it; • can be an overload protection. The use of glass is also very common in sensors other than accelerometers using capacitive sensing technology. 1.1. Spark-Assisted Chemical Engraving
Various techniques are available to micromachine glass. However, one of the main limiting factors in incorporating glass into microdevices is its limited machinability. A similar situation exists for other hard-to-machine materials, such as ceramics and composite materials. A possible answer to these issues could be spark-assisted chemical engraving (SACE), or electrochemical discharge machining (ECDM). 1.1.1. What is SACE?
SACE makes use of electrochemical and physical phenomena to machine glass. The principle is explained in Figure 1.1 (Wüthrich and Fascio, 2005). The workpiece is dipped in an appropriate electrolytic solution (typically sodium hydroxide or potassium hydroxide). A constant DC voltage is applied between the machining tool or tool-electrode and the counter-electrode. The tool-electrode is dipped a few millimeters in the electrolytic solution and the counter-electrode is, in general, a large flat plate. The tool-electrode surface is always significantly smaller than the counter-electrode surface (by about a factor of 100). The tool-electrode is generally polarized as a cathode, but the opposite polarization is also possible. When the cell terminal voltage is low (lower than a critical value called critical voltage, typically between 20 and 30 V), traditional electrolysis occurs (Figure 1.2). Hydrogen gas bubbles are formed at the tool-electrode and oxygen bubbles at the counter-electrode, depending on their polarization and the electrolyte used. When the terminal voltage is increased, the current density also increases and more and more bubbles are formed. A bubble layer develops around the electrodes. As presented in Chapter 3, the density of the bubbles and their mean radius increase with increasing current density. When the terminal voltage is increased above the critical voltage, the bubbles coalesce into a gas film around the tool-electrode. Light emission can be observed in the film when electrical discharges, the so-called electrochemical discharges, occur between the tool and the surrounding electrolyte. The mean temperature of the electrolytic solution increases in the vicinity of the tool-electrode to about 80–90 °C. Machining is possible if the tool-electrode is in the near vicinity of the glass sample (Figure 1.3). Typically, the tool-electrode has to be closer than 25 µm from the workpiece for glass machining to take place.
FIGURE 1.1 Principle of SACE technology: the glass sample to be machined is dipped in an electrolytic solution. A constant DC voltage is applied between the tool-electrode and the counter-electrode. Reprinted from Wüthrich and Fascio (2005) with permission from Elsevier.
FIGURE 1.2 Successive steps toward the electrochemical discharge phenomena: (a) 0 V; (b) 7.5 V; (c) 15 V; (d) 40 V. Two electrodes are dipped into an electrolyte. The terminal voltage is progressively increased from 0 to 40 V. At around 25 V a gas film is formed around the cathode, and at around 30 V the electrochemical discharges are clearly visible. Reprinted from Wüthrich and Fascio (2005) with permission from Elsevier. However, the process is not as simple as it seems on first sight. The gas film around the tool-electrode is not always stable. Microexplosions may occur, destroying the machined structure locally. During drilling of holes, the local temperature can increase to such an extent that heat-affected zones or even cracking can result. 1.1.2. Machining Examples
SACE technology can be used for flexible glass microstructuring. Channel-like microstructures and microholes can be obtained. Two examples are illustrated in Figure 1.4. The channel microstructure was machined with a cylindrical 90-µm-diameter tool-electrode at an applied voltage of 30 V. Machining was done in one step with a tool speed of 0.05 mm s-1. The channels are about 100 µm wide and 200 µm deep. The microhole illustrates the possibility of machining relatively deep structures. In this case the microhole is 1 mm deep.
FIGURE 1.3 Close-up view of micromachining with electrochemical discharges.
FIGURE 1.4 Micrographs of a SACE-machined channel-like structure (left) and a microhole (right) in Pyrex® glass. Reprinted from Wüthrich and Fascio (2005) with permission from Elsevier. The most interesting characteristic of SACE is its flexibility. No mask is needed, and just as in traditional machining, the desired...