ELECTRODEPOSITION AND CHARACTERIZATION OF SN-BI LEAD-FREE SOLDER ALLOYS GOH YINGXIN THESIS SUBMITTED IN FULFILMENT OF THE REQUIREMENT FOR THE DEGREE OF DOCTOR OF PHILOSOPHY FACULTY OF ENGINEERING UNIVERSITY OF MALAYA KUALA LUMPUR 2015 i UNIVERSITI MALAYA ORIGINAL LITERARY WORK DECLARATION Name of Candidate: GOH YINGXIN (I.C/Passport No: Registration/Matric No: KHA 100056 Name of Degree: DOCTOR OF PHILOSOPHY Title of Project Paper/Research Report/Dissertation/Thesis (“this Work”): ELECTRODEPOSITION AND CHARACTERIZATION OF SN-BI LEAD-FREE SOLDER ALLOYS Field of Study: NANO MATERIALS I do solemnly and sincerely declare that: (1) I am the sole author/writer of this Work; (2) This Work is original; (3) Any use of any work in which copyright exists was done by way of fair dealing and for permitted purposes and any excerpt or extract from, or reference to or reproduction of any copyright work has been disclosed expressly and sufficiently and the title of the Work and its authorship have been acknowledged in this Work; (4) I do not have any actual knowledge nor do I ought reasonably to know that the making of this work constitutes an infringement of any copyright work; (5) I hereby assign all and every rights in the copyright to this Work to the University of Malaya (“UM”), who henceforth shall be owner of the copyright in this Work and that any reproduction or use in any form or by any means whatsoever is prohibited without the written consent of UM having been first had and obtained; (6) I am fully aware that if in the course of making this Work I have infringed any copyright whether intentionally or otherwise, I may be subject to legal action or any other action as may be determined by UM. Candidate’s Signature Date Subscribed and solemnly declared before, Witness’s Signature Date Name: Designation: ii ABSTRACT Tin-bismuth eutectic alloy (Sn-58 wt.% Bi) is emerging as a potential lead-free solder alternative. The low melting temperature (138oC) of this alloy makes it a suitable candidate for higher level interconnections and for soldering heat sensitive components. Demands for high density interconnections led to the development of electroplating technique in soldering. In this work, a methane sulfonic acid (MSA) based plating bath is developed for the electrodeposition of Sn-Bi solder alloys. Composition control of the alloy is hampered by the large potential gap between the two elements. Electrolyte additives were incorporated to assist the simultaneous deposition of Sn and Bi. The effects of hydroquinone (HQ) and gelatin on the electroplating characteristics of Sn–Bi alloys were investigated. The effects of current density on the deposition behavior and composition of Sn-Bi alloy were investigated by galvanostatic studies. Microstructure and composition characterizations of as-electrodeposited and reflowed Sn-Bi alloys obtained at different current densities were performed by field emission scanning electron microscopy (FESEM) coupled with energy dispersive X-ray spectroscopy (EDS). The melting temperature of the near-eutectic Sn–Bi deposit was obtained by differential scanning calorimetry (DSC). The crystallinity of Sn–Bi deposits was also characterized with X-ray diffraction (XRD). The application of electroplated Sn-Bi solder alloys as solder joint materials were evaluated through their shear behaviour. The effects of reflow temperature on the microstructure and shear strength of Sn-Bi/Cu solder joints were studied. In the plating bath without additives and with either one of the additives, polarization studies showed that Bi deposits at about 25 to 45mV; while Sn deposits at about 410 to 420mV. The addition of HQ has a mild adsorption effect on the electrode surface. On the other hand, gelatin possesses mild complexing effect on Bi deposition. The synergistic effect of HQ and gelatin imposed a strong iii adsorption and complexing effect to reduce the large potential gap between Bi and Sn to only 243mV. With both HQ and gelatin added to the bath, co-deposition of Sn-Bi was achieved. Characterization of the electrodeposits indicated that Bi content in the alloys decreases with increasing current density. Near-eutectic Sn–60.75 wt.% Bi alloy was deposited from the bath containing both HQ and gelatin at a current density of 18mA cm-2. The extrapolated onset temperature from DSC analysis is around 137.4◦C, which is close to the eutectic melting temperature. The microstructure of the bulk solder and morphology of the intermetallic compound (IMC) changes with reflow temperature. The shear strength and elastic energy release, U of the joints increases with increasing reflow temperature. Fractographs of the failed joints shows the variation of fracture mechanisms under different reflow conditions. At low reflow temperatures, solder joints mainly failed with dimpled fracture. However at high reflow temperatures, Bi-rich phase served as a boundary to block the deformation of Sn-rich phase, causing the fracture path to deviate from the shear direction and resulted in an increase in strength and U value. iv ABSTRAK Aloi eutectic Tin- bismuth (Sn -58 wt. % Bi) muncul sebagai alternatif pateri tanpa- Plumbum yang berpotensi. Suhu lebur rendah (138oC) aloi ini menjadikannya calon yang sesuai bagi sambungan elektronik bertahap tinggi dan untuk pematerian komponen yang sensitif kepada haba. Keperluan untuk sambungan elektronik yang berketumpatan tinggi membawa kepada pembangunan teknik penyaduran dalam pematerian. Dalam karya ini , larutan penyaduran berasaskan asid metana sulfonik (MSA) disediakan untuk pengelektroenapan Sn -Bi aloi pateri. Kawalan komposisi aloi terjejas oleh jurang potensi besar antara kedua-dua elemen. Bahan tambahan elektrolit telah dimasukkan bagi membantu pemendapan serentak Sn dan Bi . Kesan-kesan hydroquinone (HQ) dan gelatin kepada ciri-ciri penyaduran aloi Sn -Bi telah disiasat . Kesan ketumpatan arus ke atas kelakuan pemendapan dan komposisi aloi Sn -Bi telah disiasat oleh kajian galvanostatic . Mikrostruktur dan komposisi pencirian aloi Sn -Bi disadur dan reflow yang didapati pada ketumpatan arus yang berbeza telah dilakukan oleh field emission scanning electron microscopy (FESEM) berserta dengan energy dispersive X-ray spectroscopy (EDS). Suhu lebur aloi Sn -Bi hampir-eutektik itu didapati dengan differential scanning calorimetry (DSC). Penghabluran deposit Sn -Bi juga dicirikan dengan X-ray diffraction (XRD). Penggunaan aloi Sn -Bi sebagai bahan pateri telah dinilai melalui sifat-sifat ricih mereka. Kesan suhu reflow pada mikrostruktur dan kekuatan ricih sambungan pateri Sn-Bi/Cu dikaji. Dalam larutan penyaduran tanpa tambahan dan dengan salah satu daripada bahan tambahan , kajian polarisasi menunjukkan bahawa penyaduran Bi berlaku pada kira-kira -25 hingga -45mV ; manakala penyaduran Sn pada kira-kira -410 hingga -420mV . Penambahan HQ mempunyai kesan penjerapan sederhana di permukaan elektrod. Sebaliknya, gelatin mempunyai kesan pengkompleks sederhana pada penyaduran Bi. Kesan sinergi HQ dan v gelatin yang mempunyai kesan penjerapan dan pengkompleksan yang kuat untuk mengurangkan jurang potensi besar antara Bi dan Sn kepada hanya 243mV. Dengan kedua-dua HQ dan gelatin ditambah ke larutan, penyaduran bersama Sn-Bi telah dicapai. Pencirian saduran menunjukkan bahawa kandungan Bi dalam aloi berkurangan dengan peningkatan ketumpatan arus . Aloi hampir-eutektik Sn-60.75 wt.% Bi didepositkan dari larutan yang mengandungi kedua-dua HQ dan gelatin pada ketumpatan arus 18mAcm-2. Offset takat lebur daripada analisis DSC adalah sekitar 137.4oC, yang terletak berhampiran dengan suhu lebur eutektik . Mikrostruktur pateri dan morfologi kompaun antara logam (IMC) berubah dengan suhu reflow. Kekuatan ricih dan pelepasan tenaga kenyal, U sambungan pateri meningkat dengan peningkatan suhu reflow. Fractographs sambungan yang patah menunjukkan variasi mekanisme patah di bawah keadaan reflow berbeza. Pada suhu reflow rendah, sambungan pateri gagal terutamanya dengan patah lekuk. Walau bagaimanapun pada suhu reflow tinggi, fasa kaya-Bi berkhidmat sebagai sempadan untuk menghalang ubah bentuk fasa kaya-Sn, menyebabkan corak patah menyimpang dari arah ricih dan menyebabkan peningkatan dalam kekuatan dan nilai U. vi ACKNOWLEDGEMENTS First and foremost, I would like to express my utmost gratitude to my supervisor, Prof. Dr. A.S.M.A. Haseeb for his guidance and support throughout my graduate studies. It has been a great pleasure working under his research group. Special thanks to Dr. Mohd Faizul Mohd Sabri, my co-supervisor, who also provided lots of useful insights on this study. Many thanks to my fellow research group members for helping in miscellaneous research works. They were constantly offering assistance and valuable input into my research. I wish to thank science officers and lab technicians who helped to perform tests and analyses, particularly Mr. Mohd Nazarul Zaman for FESEM and EDX analysis, and Mr. Adhli Iskandar Putera for lap shear tests (Instron 5848 MicroTester). I would also like to thank the Malaysian Ministry of Higher Education and University of Malaya for providing the funding for this project. Finally, I wish to thank my family members and friends for their unconditional support and encouragement to complete my Ph.D studies. vii TABLE OF CONTENTS Page Title Page i Original Literary Work Declaration Form ii Abstract iii Abstrak v Acknowledgements vii Table of Contents viii List of Figures xiii List of Tables xix List of Symbols and Abbreviations xx CHAPTER 1 INTRODUCTION 1.1 Introduction 1 1.2 Objectives 3 1.3 Scope and outline of work 3 CHAPTER 2 LITERATURE REVIEW 2.1 Solder materials 5 2.1.1 Sn-Pb solder 6 2.1.2 Selection criteria of Pb-free solders 8 2.1.3 Pb-free solder alloys 9 2.2 Solder deposition technologies 13 viii 2.2.1 Evaporation 13 2.2.2 Solid phase deposition 14 2.2.3 Electrodeposition 17 2.3 Principles of electrodeposition of metal and metal alloys 19 2.3.1 Electrodeposition of metal 19 2.3.1.1 Electrode reactions 20 2.3.1.2 Current 21 2.3.1.3 Polarization 22 2.3.1.4 Mechanism of electrodeposition 26 2.3.2 Electrodeposition of metal alloys 28 2.4 Electrodeposition of Sn and Sn alloys 31 2.4.1 Composition control of deposits 34 2.4.2 Plating bath stability 40 2.4.2.1 Common plating baths for electrodeposition of Sn- 41 based alloys 2.4.2.2 Stability of acidic plating baths 44 2.4.3 Electrolyte additives 47 2.4.3.1 Chelating agent 48 2.4.3.2 Antioxidants 51 2.4.3.3 Surfactants 52 2.4.3.4 Grain refiners and brighteners 55 ix 2.5 Solder joint formation and its strength 58 2.5.1 Intermetallic compound (IMC) formation 58 2.5.2 Shear strength of Sn-based solder joints 60 2.6 Summary 62 CHAPTER 3 ELECTROCHEMICAL STUDIES ON SN-BI ALLOY DEPOSITION 3.1 Background 66 3.2 Experimental 68 3.3 Results and discussion 70 3.3.1 Screening tests for additive selection 70 3.3.2 Polarization studies in Sn-MSA solution 71 3.3.3 Polarization studies in Bi-MSA solution 76 3.3.4 Polarization studies in Sn-Bi-MSA solution 79 3.3.5 Effects of agitation 81 3.3.6 Effects of additive concentration 91 3.3.7 Composition estimation from galvanostatic and 94 polarization curves 3.3.8 Plating bath stability 97 3.4 Chapter conclusions 100 x