Electroless Plating : Fundamentals And Applications Glenn 0. Mallory Juan B. Hajdu Edftors Reprint Edition Sponsored and published by American Electroplaters and Surface Finishers Society American Electroplaters and Surface Finishers Society The International Technical and Educational Society for Surface Finishing International Headquarters in Orlando, Florida Library of Congress Card Number 90-081578 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the publisher, American Electroplaters and Surface Finishers Society. 12644 Research Parkway, Orlando, Florida 32826-3298. Manufactured in the United States of America Published in the United States of America by Noyes PublicationsMnlliam Andrew Publishing, LLC 13 Eaton Avenue, Norwich, New York 13815 10 98 7 6 5 4 3 ABOUT THE EDITORS Glenn 0. Mallory Glenn 0. Mallory has more than 30 years' experience in the surface finishing industry, with specialization in electroless deposition processes. In 1986, Mr. Mallory formed the Electroless Technologies Corporation (ETC) in Los Angeles, CA. ETC is an independent R&D organization which licenses its technical developments. Prior to that he was vice president of R&D for electroless nickel development for the Allied-Kelite Division of Witco Chemical Corporation in Los Angeles. Mr. Mallory is the author of many papers and patents on electroless deposition. He was elected a fellow of the Institute of Metal Finishing in 1974, and received the AES Silver Medal for a paper published in Plating and Surface Finishing in 1975. Mr. Mallory received his BS degree from UCLA and his MS from California State University, Los Angeles. Juan B. Hajdu Juan B. Hajdu has extensive experience in surface treatments and electroless plating. During his30-yearcareer he has been affiliated with ENTHONE, Inc., in New Haven, CT. He joined ENTHONE in 1961 as a research chemist and is currently Vice President, Technology, for ENTHONE-OMI, Inc., a subsidiary of Asarco Inc. Mr. Hajdu obtained his PhD at the University of Buenos Aires, Argentina, then joined Pantoquimica, S.A., an affiliate of ENTHONE. He has written many papers on electroless deposition and electrodeposition, and is the inventor or co-inventor on some 20 patents. In 1966, Mr. Hajdu received the AES Gold Medal for work published in Plating concerning plating on plastics, and in 1970, the Eugene Chapdelaine Memorial Award for his work in zinc plating. 539 viii CONTRIBUTORS MICHAEL J. ALEKSINAS, Fidelity Chemical Products Co., Newark, NJ DONALD W. BAUDRAND, CEF, Allied-Kelite Division, Witco Chemical Corp., PETER BERKENKOTTER, Western Digital Corp., Santa Clara, CA DR. PERMINDER BINDRA, International Business Machines Corp., Endicott, ROBERT CAPACCIO, P.E., Mabbett, Capaccio & Associates, Boston, MA DR. JOSEPH COLARUOTOLO, Occidental Chemical Corp., Berwyn, PA JOHN G. DONALDSON, CEF, Surface Finishing Engineering, Tustin, CA DR. E.F. DUFFEK, Adion Engineering Co., Cupertino, CA DR. NATHAN FELDSTEIN, Surface Technology Inc., Trenton, NJ DR. JUAN B. HAJDU, Enthone-OM1 International, Inc., West Haven, CT DR. N. KOURA, Science University of Tokyo, Machida City, Japan JOHN J. KUCZMA, JR., Elnic, Inc., Nashville, TN DAVID KUNCES, CEF, Fidelity Chemical Products Corp., Newark, NJ JOHN KUZMIK, MacDermid, Inc., Waterbury, CT HARRY J. LITSCH, CEF-SE, Litsch Consultants, Bethlehem, PA GLENN 0. MALLORY, Electroless Technologies Corporation, Los Angeles, DR. YUTAKA OKINAKA, AT&T Bell Laboratories, Murray Hill, NJ KONRAD PARKER, Consultant, Park Ridge, IL FRED PEARLSTEIN, CEF, Temple University, Philadelphia, PA W.H. SAFRANEK, CEF, American Electroplaters and Surface Finishers PHILLIP D. STAPLETON, The Stapleton Co., Long Beach, CA DR. DONALD STEPHENS, Consultant, Westlake Village, CA FRANK E. STONE, Inland Specialty Chemical Corporation, Tustin, CA DIANE M. TRAMONTANA, Occidental Chemical Corp., Grand Island, NY DR. ROLF WEIL, Stevens Institute of Technology, Hoboken, NJ DR. JAMES R. WHITE, International Business Machines Corp., Austin, TX CATHERINE WOLOWODIUK, AT&T Bell Laboratories, Murray Hill, NJ Melrose Park, IL NY CA Society, Orlando, FL PREFACE The term "electroless plating" describes the methods of depositing metals and alloys by means of electrochemical reactions. However, chemical plating is the more accurate term that can be used to denote the several means of metal deposition without the application of electric current from an external source. Hence, immersion deposition, as well as electroless deposition, covered in this book, are two forms of chemical plating. In usage, the term "electroless plating," as coined by Abner Brenner, has come to be synonymous with autocatalytic plating. In this process, the chemical reaction proceeds continuously on selected surfaces, providing the means to produce uniform coatings with unique properties on a wide variety of substrates. The practice of electroless plating is a relatively young art, developed over the past fifty years for a large number of applications. Several major industries, such as printed circuit boards, hard memory disks and electroplated plastics were made possible by the development of electroless technology. This book describes the chemical principles of the major electroless processes and the practical applications of these techniques in industry. Of the different electroless processes available, electroless nickel and electroless copper have gained the largest industrial use and are discussed extensively. Other electroless plating processes and related subjects are discussed in individual chapters. A limited number of techniques, mentioned in the literature, that have no experimental proof or applications background were not included. It is important to note here that electroless plating is a fast-growing field and the references should be updated continuously. Two points should be made on editorial decisions. As a result of our intention to cover both principles and applications of electroless plating, some subjects required a theoretical approach, while other subjects demanded pragmatic and descriptive treatment. For this reason, the authors had very few constraints on style, format, units and the general outlay of their chapters. In addition to electroless plating, immersion plating is reviewed. While this process is not based strictly on chemical reduction, it is closely related to electroless plating in industrial applications. The editors would like to express their gratitude to the many persons who have made this book a reality: First of all to the authors for their cooperation and patience; to the staff and authorities of the American Electroplaters and Surface Finishers Society for their help and support; and to the members of the Electroless Finishing Committee, eSpeCiallY to our colleagues, Michael Aleksinas, Dr. Moe El-Shazly, David Kunces, CEF, and Fred Pearlstein, CEF, in reviewing the manuscripts. Special thanks are also due Harry Litsch, CEF-SE, for preparing the index. No work on the subject of electroless plating should be published without acknowledging the industry's lasting debt to the pioneering work of Dr. Abner Brenner. We hope this book will fill the void which has existed for a complete reference on electroless deposition, and that you will find it a most useful addition to your library. Glenn 0. Mallory Editor Juan B. Hajdu Editor vii CONTENTS Preface Contributors Chapter 1 The Fundamental Aspects of Electroless Nickel Plating Glenn 0. Mallory Chapter 2 Composition and Kinetics of Electroless Nickel Plating Glenn 0. Mallory Chapter 3 Troubleshooting Electroless Nickel Plating Solutions Michael J. Aleksinas Chapter 4 Properties of Electroless Nickel Plating Rolf Weil and Konrad Parker Chapter 5 Equipment for Electroless Nickel John Kuczma, Jr. Chapter 6 Test Methods for Electroless Nickel Phillip Stapleton Chapter 7 Surface Preparation for Electroless Nickel Plating Juan Hajdu Chapter 8 Engineering Applications of E!ectroless Nickel Joseph Colaruotolo and Diane Tramontana Chapter 9 Electronic Applications of Electroless Nickel E.F. Duffek, D. W. Baudrand, CEF, and J.G. Donaldson, CEF Chapter 10 Electroless Deposition of Alloys Fred Pearlstein, CEF Chapter 11 Composite Electroless Plating Nathan Feldstein vii viii 1 57 101 111 139 169 193 207 229 261 269 Chapter 12 Fundamental Aspects of Electroless Copper Plating Perminder Bindra and James R. White Chapter 13 Electroless Copper in Printed Circuit Fabrication Frank E. Stone Chapter 14 Plating on Plastics John J. Kuzmik Chapter 15 Electroless Plating of Gold and Gold Alloys Yutaka Okinaka Chapter 16 Electroless Plating of Platinum Group Metals Yutaka Okinaka and Catherine Wolowodiuk Chapter 17 Electroless Plating of Silver N. Koura Chapter 18 Electroless Cobalt and Cobalt Alloys Section I W.H. Safranek, CEF Section II Peter Berkenkotter and Donald Stephens 289 331 377 401 421 441 463 Chapter 19 51 1 Chemical Deposition of Metallic Films from Aqueous Solutions David Kunces, CEF Chapter 20 Waste Treatment of Electroless Plating Solutions Robert Capaccio, P.E. 519 Index 529 About the Editors 539 Chapter 1 The Fundamental Aspects Of Electroless Nickel Plating Glenn 0. Mallory The chemical deposition of a metal from an aqueous solution of a salt of said metal has an electrochemical mechanism, both oxidation and reduction (redox), reactions involving the transfer of electrons between reacting chemical species. The oxidation of a substance is characterized by the loss of electrons, while reduction is distinguished by a gain of electrons. Further, oxidation describes an anodic process, whereas reduction indicates a cathodic action. The simplest form of chemical plating is the so-called metal displacement reaction. For example, when zinc metal is immersed in a copper sulfate solution, the zinc metal atoms (less noble) dissolve and are spontaneously replaced by copper atoms from the solution. The two reactions can be represented as follows: Oxidation: Zno - Zn” + 2e-, anodic, Eo = 0.76 V Reduction: Cu” + 2e- - Cuo, cathodic, Eo = 0.34 V Owera// reaction: ZnO + Cu” - Zn” + Cuo, Eo = 1.1 V As soon as the displacement reaction begins, the surface of the zinc substrate becomes a mosaic of anodic (zinc) and cathodic (copper) sites. The displacement process continues until almost the entire substrate iscovered with copper. At this point, oxidation (dissolution) of thezincanodevirtually stops and copper deposition ceases. Chemical plating by displacement yields deposits limited to only a few microns in thickness, usually 1 to 3 pm. Hence, chemical plating via the displacement process has few applications. In order to continuously build thick deposits by chemical means without consuming the substrate, it is essential that a sustainable oxidation reaction be employed as an alternative to the dissolution of the substrate. The deposition reaction must occur initially and exclusively on the substrate and subsequently continue to deposit on the initial deposit. The redox potential for this chemical process is usually more positive than that for a metal being deposited by immersion. The chemical deposition of nickel metal by hypophosphite meets both the oxidation and redox potential criteria without changing the mass of the substrate: 1 2 ELECTROLESS PLATING Reduction: Nit' + 2e- - Ni" E" = -25 mV Oxidation: H2PO; + H?O - HpO; + 2H' + 2e- E" = +50 mV Ni'* + H2PO; + H?O - Ni" + H2PO; + 2H' E" = +25 mV which is the sum of the oxidation and reduction equations. This reaction does not represent the true electroless plating reaction, since EN deposition is accompanied by hydrogen evolution. Figure 1.1 shows the difference between immersion and electroless deposition by comparing deposit thickness vs. time. The term electrolessplating was originally adopted by Brenner and Riddell (1) to describe a method of plating metallic substrates with nickel or cobalt alloys without the benefit of an external source of electric current. Over the years, the term has been subsequently broadened to encompass any process that continuously deposits metal from an aqueous medium. In general, electroless plating is characterized by the selective reduction of metal ions only at the surface of a catalytic substrate immersed into an aqueous solution of said metal ions, with continued deposition on the substrate through the catalytic action of the deposit itself. Since the deposit catalyzes the reduction reaction, the term autocatalytic is also used to describe the plating process. Fig. 1.1-Thickness vs. time-comparison between electroless and immersion deposition. The fundamental Aspects of Electroless Nickel Plating 3 In 1844, Wurtz (2) observed that nickel cations were reduced by hypo- phosphite anions. However, Wurtz only obtained a black powder. The first bright metallic deposits of nickel-phosphorus alloys were obtained in 191 1 by Breteau (3). In 1916, Roux (4) was issued the first patent on an electroless nickel plating bath. However, these baths decomposed spontaneously and formed deposits on any surface that was in contact with thesolution, even the wallsofthecontainer. Other investigators studied the process, but their interest was in the chemical reaction and not the plating process. In 1946, Brenner and Riddell (1) published a paper that described the proper conditions for obtaining electroless deposition as defined above. Over the years, the process has been investigated further and expanded by many workers to its present state of development. THE ELECTROLESS NICKEL PLATING BATH: COMPONENTS Electroless nickel (EN) plating is undoubtedly the most important catalytic plating process in use today. The principal reasons for its widespread commercial and industrial use are to be found in the unique properties of the EN deposits. The chemical and physical properties of an EN coating depend on its composition, which, in turn, depends on the formulation and operating conditions of the EN plating bath. Typically, the constituents of an EN solution are: A source of nickel ions A reducing agent Suitable complexing agents Stabilizers/inhibitors Energy The Nickel Source The preferred source of nickel cations is nickel sulfate. Other nickel salts, such as nickel chloride and nickel acetate, are used for very limited applications. The chloride anion can act deleteriously when the EN plating bath is used to plate aluminum, or when the EN deposit is used as a protective coating over ferrous alloys in corrosion applications. The use of nickel acetate does not yield any significant improvement in bath performance or deposit quality when compared to nickel sulfate. Any minor advantages gained by nickel acetate are offset by its higher cost vs. the cost of nickel sulfate. The ideal source of nickel ions is the nickel salt of hypophosphorus acid, Ni(H2P02)?. The use of nickel hypophosphite would eliminate the addition of sulfate anions and keep to a minimum the buildup of alkali metal ions while replenishing the reactants consumed during metal deposition. The concentration of nickel ions and its relationship to the reducing agent and complexing agent concentrations will be discussed in a succeeding chapter. [...]... 60H- 2 2H2PO; + 2H20+ 2H- ~ 3 1 The reduction of nickel ion in this mechanism proceeds as follows: Ni" + 2H- - (Ni'* + 2e- + 2H) - Nil' + HI [I41 The Fundamental Aspects of Electroless Nickel Plating 9 The hydride ion can also react with water or a hydrogen ion: Acid H' + H- - H2 Alkaline H2O + H- - Hz + OH' According to Lukes, the hydrogen that appears as hydride ion was originally bonded to phosphorus... Gorbunova et al for the reaction mechanism of nickel-boron plating consists of three main steps: Reduction of nickel BH; + 4H20 B(0H); + 4H + 4H' + 462Ni" + 4e- 2Ni0 BH; + 2Ni'2 + 4H20 - 2Ni0 Reduction of boron BH; + H' BH, + H 2 - + B(0H)i + 2H2 + 4H' -. B + 5/2H2 Hydrolysis of borohydride BH; + 4Hz0 B(0H); + 4H + 4H' + 4 6- - P91 - B(0H); + 4Hz [411 Mallory (17) has also investigated the reduction of... H ~ R + H The Fundamental Aspects of Electroless Nickel Plating - ROH (2) Oxidation R + OH- (3) Recombination H+H+H? (4) Oxidation H + OH- - H 2 0+ e (5) Metal deposition M'" + ne - M" (6) Hydrogen evolution 2H20+ 2e 19 +e Cathodic - H2 + 20H- In acid media, reactions 4 and 6 in this set of equations become: (4a) Oxidation H ++e (sa) Hydrogen evolution 2H' + 2e - H2 Subsequent discussion in this section... electrode in a hypophosphite-free nickel-containing catholyte It should be noted that deMinjer’s experiments were run under acidic conditions (pH 4.2 to 4.3) We refer the reader to Ref 25 for the details of her experiments The Fundamental Aspects of Electroless Nickel Plating 21 0, U 2 CI 0 0, Q, Q, 0 c E E Q, * ui > > E m CI c Q, L 0 P Q, U P L 0 Q, iJ - - -. _ - - I7 “ Fig 1.2-Schematic representation... 53 4-6 7-1 0 0.499 1.57 38 4.75 1 2-1 4 1.24 59 9.8 6-1 0 - 32 8.0 8-1 1 1.16 An explicit understanding of the reaction mechanisms that govern electroless nickel deposition is necessary from both theoretical and practical viewpoints Knowledge of the mechanisms of the reaction of a reducing agent with nickel ions can lead to the solution of a series of problems-development of methods to increase the plating. .. follows: N-B bond cleavage Reduction of hydrolized nickel with BHhds I - ~ I N i + ~w B , H - ~ Nio~ BH(OH)hd, + 2H + ~ ] + BH(OH)hd, [i: f ] NiOH& + BH3.* bi:**OH] 'OH - NiOH.d, + B(OH)3+ H - Nio+ BHzOH BHz(OH )- Nio +H 1581 [591 + B(OH)3+ 2H + 1571 1601 The sums of the above equations, including the ionization of water, is: - 3NT2+ 2RzNHBH3 6H20 3Ni0+ 2RzNH; + 2B(OH)3+ 3Hz+ 4H' + 1611 Boron reduction - R2NH... connected, he observed “a very evident increase in gas evolution 22 ELECTROLESS PLATING Fig 1.3-Schematic representation of two-chamber system used to study electroless silver on the electrode in the DMAB-containing solution” No gassing was produced on the electrode in the silver-ion solution Therefore, hydrogen evolution in electroless silver plating with DMAB is the product of the anodic oxidation of the... illustrates the cleavage of the N-B bond, the oxidation of BH,, generation of electrons and evolution of hydrogen gas in acid or neutral solutions: (1) (CH,)zNHBHj (CH,)2NH + BHI 4 7H20 - (2) BH3 6H2(OH)+ 3H' + 20H- + H + e (water dissociation 3H20= 3H' + 30H-) (3) BH2(OH)+ 20H(4) BH(OH)z+ OH- - BH(0H) + OH- + H + e - B(OH)3 + H+e Adding steps 1 through 4: lH2O (CH3)zNHBHi + H' - (CHI)NH~ + B(OH)I + 3H' +... sequence of reactions: 14 ELECTROLESS PLATING Ionization of water 4H20 4H' + 40H- - Coordination of hydroxyl ions to solvated nickel ion -2 2Ni(H20)i' + 40H- [ NiGc +4H20 1431 Reaction of hydrolized nickel species with borohydride ion - Ni" + BH2(OH)I+ H NiOHad, BH30H+ [451 The BHz(0H)Z species reacts with the second hydrolyzed nickel ion in a similar manner: [qeeoH]BH2(OH) 2- NiOHad, BH(0H); + H + +... studies on electroless nickel plating by Randin and Hintermann (9) The chemical reduction of nickel at a catalytic surface can be represented by the following reactions: lonization of water at catalytic nickel surface: 2H20 - 2H' + 20H- Coordination of hydroxyl ions to solvated nickel ion: [201 10 r ELECTROLESS PLATING 1 Reactions of hydrolized nickel species with hypophosphite: NiOH,h + H2PO; - Nio+ H2PO; . substrate: 1 2 ELECTROLESS PLATING Reduction: Nit' + 2e- - Ni" E" = -2 5 mV Oxidation: H2PO; + H?O - HpO; + 2H' + 2e- E" = +50 mV Ni'* + H2PO;. nickel-boron plating consists of three main steps: Reduction of nickel BH; + 4H20 - B(0H); + 4H + 4H' + 4 6- 2Ni" + 4e- - 2Ni0 BH; + 2Ni'2 + 4H20 - 2Ni0. mechanism proceeds as follows: Ni" + 2H- - (Ni'* + 2e- + 2H) - Nil' + HI [I41 9 The Fundamental Aspects of Electroless Nickel Plating The hydride ion can also