Copper Wire Bonding Preeti S. Chauhan • Anupam Choubey ZhaoWei Zhong • Michael G. Pecht Copper Wire Bonding Preeti S. Chauhan Anupam Choubey Center for Advanced Life Cycle Industry Consultant Engineering (CALCE) Marlborough, MA, USA University of Maryland College Park, MD, USA Michael G. Pecht Center for Advanced Life Cycle ZhaoWei Zhong Engineering (CALCE) School of Mechanical University of Maryland & Aerospace Engineering College Park, MD, USA Nanyang Technological University Singapore ISBN 978-1-4614-5760-2 ISBN 978-1-4614-5761-9 (eBook) DOI 10.1007/978-1-4614-5761-9 Springer New York Heidelberg Dordrecht London Library of Congress Control Number: 2013939731 # Springer Science+Business Media New York 2014 This work is subject to copyright. 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Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com) Foreword The wire bonding industry is undergoing a paradigm shift from gold to copper wire bonding—a change that is as significant as the European Union Restriction of Hazardous Substances (EU RoHS) (no-lead) mandate was to soldering. This change to copper wire bonding is driven mostly by economic considerations, and thus, the risks in the use of this technology must be carefully considered. Since the introduction of transistors in the 1950s, interconnections between semiconductors and lead frames or substrates have been made with gold wires. At the time, gold was relatively affordable and easy to bond. While the industry did go through a period of Al-wedge bonding, gold ball bonding technology was used almost universally through the 1980s. Due to the steep decline in the US oil imports in the 1980s, however, the price of gold rose from around US$200 to more than US$800 per troy ounce. As a result, the industry searched for a less expensive alternative, and copper emerged as the metal of choice. Within 3 years, most major semiconductor companies had established copper ball bonding development programs. By the early 1990s, numerous papers and patents on substituting copper for gold bonds had been published. The major advantages of copper wire bonding include having a lower, more stable cost than gold, increased stiffness (minimizing wire sweep), Al–Cu intermetallic growth rates much lower than those for Al–Au, and improved electrical conductivity. However, there were many problems with copper wire bonding, such as the hardness of copper (which can cause cratering and Al pad squeeze), ball-neck fatigue failures in plastic encapsulation during temperature cycling, oxidation of wire surfaces, decreased tail bondability, and corrosion. Furthermore, by 1998, the price of gold had declined to about US$250/oz, and many copper wire development programs were discontinued. Renewed interest in copper ball bonding developed around 2006 when the global recession began. Since gold has been considered a safe haven for investors in troubled times, gold prices quickly increased to US$1,800 per troy ounce by 2012. Thus, again, economic events forced major technological changes in the wire bonding industry. A great deal of progress has been rapidly achieved in substituting copper for gold. For example, most copper wire is now coated with v vi Foreword Pd for better bondability. Additionally, bonding machines have been tailored for copper wire bonding. Many studies detailing new Cu bonding technology have been published, and now this entire book is available on the bonding technology. The success of these efforts is shown in projections that by 2015, one-third of all small-diameter wire bonds (out of the multiple trillions made) will be copper. This book presents a comprehensive discussion of copper wire bonding, from its fundamental concepts to its use in safety-critical applications. Readers will find herein a wealth of information, including a practical how-to approach to implement this exciting new technology. Gaithersburg, MD George G. Harman NIST Fellow, Emeritus Preface Wire bonds form the primary interconnections between an integrated circuit chip and the metal lead frame in semiconductor packaging. Wire bonding is considered to be a more cost-effective and flexible interconnect technology than flip–chip interconnects. As of 2013, more than 90 % of semiconductor packages were interconnected by wire bonding. Gold (Au) wire has been used for wire bonding in the electronics industry for more than 50 years because of its high tensile strength and electrical conductiv- ity, high reliability, and ease of assembly. However, due to its high cost and continuously rising market prices, alternative wire bonding materials have been considered. Copper (Cu) is one of the most preferred alternative materials for wire bonding because of its cost advantages over Au. For example, in March 2013, the cost of Au hovered around US$1,610/oz, compared to US$3.45/lb of Cu. Cu wire also offers advantages in terms of higher mechanical strength, lower electrical resistance, slower intermetallic growth (with an aluminum (Al) pad), and higher thermal conductivity than Au. The higher electrical and thermal conductivity of Cu, compared to Au, enables the use of smaller diameter wire for equivalent current carrying capacity or thermal conductivity. Replacing Au wire with Cu wire in the wire bonding process presents many challenges. Parameter adjustments for ball bond formation, stitch bond formation, and looping profile are needed. Cu is harder than both Au and Al, and therefore bonding parameters, including bonding force, must be kept under tight control. Since Cu wire is highly prone to oxidation, inert gas such as nitrogen or forming gas must be used during the bonding process. In some cases, wire manufacturers have used palladium (Pd)-coated Cu wire, which is more resistant to oxidation than bare Cu wire. Also, since bare Al pads run the risk of being damaged by Cu wires due to the high hardness of Cu and the high bonding force required, the industry has adopted Al pads that are thicker than those used in Au wire bonding, as well as pads with nickel (Ni)-based finishes. Some semiconductor companies have adopted Cu wire bonding technology into their assembly and test sites, and are running Cu wire bonding production across a wide range of package types. For example, in May 2012, Texas Instruments shipped around 6.5 billion units with Cu wire bonding technology in its analog, embedded vii viii Preface processing, and wireless products. Texas Instruments also reported that all seven of its assembly and test sites are running Cu wire bonding production across a wide range of package types. Because of the ongoing trends towards Cu wire bonding, the differences between Au and Cu wire bonding need to be understood in order to modify the manu- facturing processes and reliability tests. The bonding metallurgies, process variations, and reliability of Cu and PdCu wires bonded on various surface finishes need to be evaluated. This book provides an understanding of Cu wire bonding technology, including the bonding process, bonding tools and equipment, PdCu wires, surface finishes, wire bond–pad metallurgies, wire bond evaluation techniques, and reliability tests on Cu wire-bonded parts. The book is organized into nine chapters. Chapter 1 gives an introduction to Cu wire bonding technology. The advantages of Cu over Au, such as lower cost, higher mechanical strength, and higher electrical and thermal conductivity, are discussed. The chapter describes the adoption of Cu wire bonding in the semiconductor industry, as well as future projections for its usage. Chapter 2 presents the wire bonding process, including the influence of process parameters on the wire bonds and bond process optimization. Bonding parameters such as ultrasonic power, ultrasonic generator current, electric flame-off current, firing time, bonding force, and temperature are discussed. The potential defects and failures that could arise during the bonding process and the bonding damage induced by tools are explained. Chapter 3 explains the wire bonding metallurgies for Cu and PdCu wires. The most common variations are bare Cu wires, PdCu wires with Al bond pads, and Ni/Au bond pads. The interfacial metallurgies of bare Cu wire on Al-, Ni-, and Cu-based bond pads are examined. Comparisons are made between the interfacial intermetallics, Au–Al, Cu–Al, and Cu–Au, at the bond–pad interface. The growth rates and electrical, mechanical, and thermal properties of the intermetallics are presented.