Electrodeposition of Cu-Sn Alloys from Methanesulfonate Electrolytes A Thesis submitted by Naray Pewnim For the Degree of Doctor of Philosophy School of Chemical Engineering and Advanced Materials Newcastle University December 2012 ABSTRACT The most commonly used alloy in the electronics industry has been the ubiquitous tin- lead alloy. As the demand for electronic devices continues to increase, there have been concerns about the continued use of lead and its long term environmental impact. In the last decade there has been a push to ban the use of lead in electronic products. Legislation from various governments around the world limiting the use of lead has given rise to the drive to find suitable lead-free alternatives. The aim of this research project was to establish a systematic approach for the selection of electrochemical parameters for the electrodeposition of tin-rich copper-tin alloys from a single electroplating bath. By studying and understanding a model system such as copper-tin, one can then use the information obtained as a basis to successfully deposit various other tin binary alloys in the future. Tin-rich deposits were enabled by employing various strategies such as maintaining a high Sn to Cu ratio in the electrolyte and by using surface active agents that have been known to facilitate alloy co-deposition. The effect of surfactants on the tin content in the deposit was initially examined with the aid of a rotating cylinder Hull cell. It was found that the presence of fluorosurfactant was crucial in eliminating metal oxide formation. Cyclic voltammetry at a rotating disk electrode showed that inclusion of surfactant in the electrolyte had no effect on the reduction potential of tin which remained at -0.45 V vs SCE. However, the reduction potential for copper shifted from approximately -0.13 to -0.18 V vs SCE, thereby facilitating alloy co-deposition. Chronoamperometry and anodic stripping voltammetry showed that current efficiency for copper-tin deposition ranged from 55-92% depending on the deposition time and deposit composition. Results from voltammetry experiments were used in the next galvanostatic electrodeposition experiments at vitreous carbon electrodes. Deposits containing up to 96 wt.% tin were obtained from both direct current and pulse plating modes. It was found that an optimal current density of 22 mA cm -2 was needed to obtain desirable deposits. For pulse plating the peak current density should be set to 100 mA cm -2 with a duty cycle of 0.2. Cu-Sn alloys obtained consisted of two phases, tetragonal tin and a hexagonal Cu Sn intermetallic compound. Deposit annealing showed that the Cu Sn 6 5 3 intermetallic was not formed. i ACKNOWLEDGEMENTS I would like to thank Prof. Sudipta Roy for her expert advice and encouragement. She is a great role model and I have learned much from her throughout the years. I would never have made it without her kind understanding support and occasional prodding when I was not producing results at an acceptable rate. I would also like to thank the Ministry of Science and Technology, Royal Thai government for funding my PhD research and giving me an opportunity of a lifetime. Newcastle University’s Advanced Chemical and Materials Analysis (ACMA) unit, especially Pauline Carrick and Maggie White, for their help and suggestions on sample preparation and analysis; SEM, EDX, XRD. Prof. Roy’s electro-nanomaterials group. Past, present, and temporary members. Peter, Jeet, Swati, Tri, Mosaad, Simon for interesting and offbeat conversations over the years; in and out of the lab. You have certainly made my PhD life more endurable. Nat, thank you for always being there for me these past three years; through good times and bad. Through snowstorms, floods, and periods of inactivity when we both know that I really should be moving on with life. Not to mention the greatest graduation present I could ever wish for, the best coffee I’ve ever had in my life! My parents, for without them I would not be standing where I am today. Thank you for laying down a great foundation for me at a very early age. The decisions you made had a profound impact on my childhood and I truly believe they have shaped me into who I am today. I will fondly remember our time in Hawaii until the day I leave this planet. Arguably the best years of my life, let’s hope there are many more to come… ii CONTENTS 1. INTRODUCTION ................................................................................................... 1 1.1 BACKGROUND .................................................................................................. 1 1.2 TIN BASED SOLDER ALLOYS IN ELECTRONICS...................................................... 3 1.2.1 Lead-tin solder alloys .................................................................................. 4 1.2.2 Lead-free solder alloys ................................................................................ 5 1.3 SOLDERING TECHNIQUES ............................................................................... 10 1.3.1 Hand soldering .......................................................................................... 10 1.3.2 Wave soldering ......................................................................................... 10 1.3.3 Reflow soldering ....................................................................................... 11 1.4 MICROFABRICATION OF SOLDERS .................................................................... 13 1.4.1 Solder Paste ............................................................................................. 13 1.4.2 Solder preforms ........................................................................................ 13 1.4.3 Physical vapour deposition ....................................................................... 14 1.4.4 Electrodeposition ...................................................................................... 15 1.5 ELECTRODEPOSITION OF TIN .......................................................................... 16 1.5.1 Electrolyte systems ................................................................................... 17 1.5.2 Bath additives ........................................................................................... 22 1.6 ELECTRODEPOSITION OF TIN ALLOYS .............................................................. 24 1.7 AIMS AND OBJECTIVES .................................................................................... 29 2. FUNDAMENTAL THEORY .................................................................................. 30 2.1 THERMODYNAMICS ......................................................................................... 33 2.1.1 Stability of Electrolyte ................................................................................ 33 2.1.2 Standard Electrode Potentials ................................................................... 35 2.1.3 Overpotential deposition ........................................................................... 36 2.2 KINETICS ....................................................................................................... 37 2.2.1 Charge Transfer Kinetics .......................................................................... 37 2.2.2 Metal Nucleation ....................................................................................... 38 2.3 MASS TRANSFER ........................................................................................... 39 2.3.1 Migration ................................................................................................... 39 2.3.2 Convection ................................................................................................ 39 2.3.3 Diffusion .................................................................................................... 40 2.4 ALLOY CO-DEPOSITION STRATEGY .................................................................. 41 iii 2.5 ALLOY PLATING – DIRECT CURRENT ................................................................ 44 2.6 ALLOY PLATING – PULSE CURRENT ................................................................. 45 3. EXPERIMENTAL ................................................................................................. 47 3.1 ELECTROLYTES .............................................................................................. 47 3.2 ELECTROLYTE OPTIMISATION .......................................................................... 49 3.2.1 Assessment of Electrolyte ......................................................................... 50 3.2.2 Current Efficiency...................................................................................... 56 3.2.3 Choice of Applied Current ......................................................................... 57 3.3 ELECTROCHEMICAL CHARACTERISATION ......................................................... 58 3.3.1 Cyclic Voltammetry ................................................................................... 59 3.3.2 Chronoamperometry ................................................................................. 63 3.3.3 Anodic Stripping Voltammetry ................................................................... 64 3.4 ELECTRODEPOSITION EXPERIMENTS ............................................................... 65 3.4.1 Direct Current Plating ................................................................................ 65 3.4.2 Pulse Current Plating ................................................................................ 67 3.5 MATERIALS ANALYSIS ..................................................................................... 68 3.5.1 Optical Microscopy .................................................................................... 68 3.5.2 Scanning Electron Microscopy .................................................................. 68 3.5.3 X-Ray Diffraction ....................................................................................... 70 3.5.4 Annealing Procedure ................................................................................ 72 4. RESULTS: ELECTROLYTE OPTIMISATION ...................................................... 73 4.1 EFFECT OF ADDITIVES ON DEPOSITS ................................................................ 74 4.1.1 Electrolytes without surfactant .................................................................. 75 4.1.2 Electrolytes with surfactant added ............................................................. 77 4.1.3 Nitrogen degassing experiments ............................................................... 78 4.2 DEPOSIT THICKNESS ...................................................................................... 79 4.3 CURRENT EFFICIENCY .................................................................................... 82 4.4 DISCUSSION .................................................................................................. 83 4.5 CONCLUSION ................................................................................................. 84 5. RESULTS: ELECTROCHEMICAL CHARACTERISATION .................................. 85 5.1 CYCLIC VOLTAMMETRY ................................................................................... 86 5.1.1 Copper ...................................................................................................... 87 5.1.2 Tin ............................................................................................................ 90 5.1.3 Copper-Tin ................................................................................................ 94 iv 5.1.4 Current Efficiency Measurements ............................................................. 95 5.2 CHRONOAMPEROMETRY ................................................................................. 98 5.2.1 Region a, -0.41 to -0.44 V ......................................................................... 98 5.2.2 Region b, -0.45 to -0.47 V ......................................................................... 99 5.2.3 Region c, -0.48 to -0.50 V ......................................................................... 99 5.3 ANODIC STRIPPING VOLTAMMETRY ............................................................... 101 6. RESULTS: ELECTRODEPOSITION ................................................................. 104 6.1 GALVANOSTATIC ELECTRODEPOSITION STRATEGY ......................................... 104 6.2 DIRECT CURRENT PLATING ........................................................................... 108 6.2.1 DC Deposit Thickness ............................................................................ 109 6.2.2 DC Crystalline Structure.......................................................................... 109 6.3 PULSE CURRENT PLATING ............................................................................ 114 6.3.1 Pulse Plating Parameters........................................................................ 114 6.3.2 PP Deposit Composition ......................................................................... 116 6.3.3 PP Deposit Morphology .......................................................................... 119 6.3.4 PP Crystalline Structure .......................................................................... 120 6.4 ANNEALED DEPOSITS ................................................................................... 123 6.4.1 Annealed Deposit Microstructure ............................................................ 123 6.4.2 Crystalline Structure of Annealed Deposit ............................................... 125 7. DISCUSSION: LESSONS LEARNT .................................................................. 129 7.1 ROTATING CYLINDER HULL CELL EXPERIMENTS ............................................. 129 7.2 VOLTAMMETRY EXPERIMENTS....................................................................... 130 7.3 ELECTRODEPOSITION EXPERIMENTS ............................................................. 131 8. CONCLUSIONS ................................................................................................ 134 9. FUTURE WORK ................................................................................................ 136 REFERENCE ........................................................................................................... 136 APPENDIX A: SUPPLEMENTAL THEORY AND CALCULATIONS ......................... 142 APPENDIX B: ADDITIONAL XRD PATTERNS ........................................................ 147 APPENDIX C: PUBLICATIONS ................................................................................ 152 v LIST OF FIGURES Figure 1-1 Change in tin consumption for various applications from 1988 to 2009 [5]. ............... 3 Figure 1-2 Pb-Sn phase diagram [10]. ......................................................................................... 4 Figure 1-3 Cu-Sn phase diagram showing eutectic point at Cu0.7-Sn99.3 wt% [20]. ................. 9 Figure 1-4 Attachment of an SMD on a PCB in preparation for reflow soldering. ...................... 11 Figure 1-5 The PVD evaporation process used for metal deposition [28]. ................................. 14 Figure 2-1 A schematic representation of an electrochemical cell ............................................. 30 Figure 2-2 Model of the electrochemical double layer showing the Helmholtz layer and the diffuse double layer beyond the outer Helmholtz plane (OHP) [80]. ................................... 31 Figure 2-3 Pourbaix diagram of the Cu-O-H and the Sn-O-H system [10]. ................................ 34 Figure 2-4 Nucleation mechanisms a) discharge of an ad-atom followed by surface diffusion and b) direct discharge at a vacant lattice site [32]. ............................................................ 38 Figure 2-5 Concentration profile showing the evolution of a concentration depletion layer as a function of distance from electrode surface [80]. ................................................................. 40 Figure 2-6 Voltammogram of Cu-Ni alloy deposition showing partial currents for Cu and Ni deposition as well as the total deposition current needed to deposit the alloy [82]. ........... 42 Figure 2-7 Partial current potential diagrams showing systems where (a) A and B are kinetically controlled with different Tafel slope, similar exchange current density and (b) A and B are kinetically controlled with different Tafel slopes and exchange current densities [82]. ....... 43 Figure 2-8 Simple pulse plating waveform. ................................................................................. 45 Figure 3-1 Trapezoidal shape of a classical Hull cell. ................................................................ 49 Figure 3-2 Side view schematic of the RCH. The dotted lines denotes varying distance the current lines have to travel between the electrodes. ........................................................... 51 Figure 3-3 Schematic of the cylinder electrode showing section numbers. ................................ 53 Figure 3-4 After electroplating is completed, markings due to lacquer removal can be used to determine the deposit thickness and mark positions or cylinder sectioning. ....................... 54 Figure 3-5 Rota-Hull measuring block used to mark positions where the cylinder electrode should be sectioned. The numbers denote i /i ratios. ................................................... 55 (x/h) ave Figure 3-6 Schematic of the three-electrode setup used in electrochemical characterization experiments involving rotating disc electrodes. ................................................................... 58 Figure 3-7 Potential-time waveform of a typical cyclic voltammetry experiment. a) is the forward sweep and b) the reverse sweep. ........................................................................................ 60 Figure 3-8 A typical cyclic voltammogram (current-potential plot) showing a) deposition peak and two stripping peaks b) and c). ....................................................................................... 61 Figure 3-9 A typical chronoamperometry plot (current-time) showing two different current responses. ........................................................................................................................... 63 vi Figure 3-10 A typical anodic stripping voltammetry plot (current-potential). .............................. 64 Figure 3-11 Schematic of the two electrode setup used in electrodeposition experiments. ....... 65 Figure 3-12 Schematic of a pulse current plating instrumental setup. ........................................ 67 Figure 3-13 Annealing temperature profile. The preheating stage was 25 min. and the reflow stage was approximately 2 min at 260°C. ........................................................................... 72 Figure 4-1 Cu-Sn deposit showing the effect of varying Cu:Sn composition along the rotating cylinder electrode surface. The RCH rotation speed was 400 rpm and the average applied current density was 4.7 mA cm-2. ........................................................................................ 76 Figure 4-2 Surface microstructure of Cu-Sn deposits a) without surfactant, 35 wt.% Sn and b) with the addition of surfactant, 85 wt.% Sn. ........................................................................ 77 Figure 4-3 Backscatter electron image showing issues preventing accurate deposition thickness measurements from cylinder electrode cross-sections a) rough deposits and b) smearing of the deposition layer into the brass substrate. ...................................................................... 80 Figure 4-4 Cross section elemental map analysis of Sn deposits on an RCE. a) original SEM image, b) Sn distribution and c) Cu distribution maps. ........................................................ 81 Figure 4-5 Pourbaix diagram of the Sn-O-H system [10]. .......................................................... 82 Figure 4-6 Hydroquinone antioxidant is a redox molecule. ........................................................ 83 Figure 5-1 Cyclic voltammetry of the background electrolyte shows that significant hydrogen evolution start to occur at a potential of -0.6 V vs SCE. ...................................................... 86 Figure 5-2 Cyclic voltammetry of 0.015 M CuSO in 2.0 M MSA and hydroquinone antioxidant 4 with and without surfactant. The scan rate was 50 mV s-1 and 100 rpm. ............................ 88 Figure 5-3 Cyclic voltammetry of 0.015 M CuSO with surfactant. The scan rate was 50 mV s-1 4 and the RDE rotation speed was between 100 to 2000 rpm. a) whole scan region. b) zoom of Cu deposition region and c) zoom of Cu stripping region. .............................................. 89 Figure 5-4 Cyclic voltammetry of 0.15 M SnSO in 2.0 M MSA and hydroquinone antioxidant 4 with and without surfactant. The scan rate was 50 mV s-1 and 100 rpm. ............................ 90 Figure 5-5 Cyclic voltammetry of 0.15 M SnSO without surfactant. Scan rate was 50 mV s-1 4 and the RDE rotation speed was 100, 500, 1000 rpm. a) whole scan region b) under potential deposition of Sn and c) corresponding stripping peak for Sn UPD ...................... 92 Figure 5-6 Cyclic voltammetry of 0.15 M SnSO with surfactant. The scan rate was 50 mV s-1 4 and the RDE rotation speed was 100, 500, 1000 rpm. a) whole scan region b) under potential deposition of Sn and c) corresponding stripping peak for Sn UPD. ..................... 93 Figure 5-7 Cyclic voltammetry of Cu-Sn alloy on a Au RDE from an electrolyte composed of 0.15 M SnSO , 0.015 M CuSO , 0.01 M hydroquinone, 2.0 M methanesulfonic acid, and 4 4 0.01% vol DuPont™ Zonyl® FSN surfactant. The scan rate was 50 mV s-1 and the effect of varying RDE rotational speed from 100 to 2000 rpm shown. .............................................. 94 vii Figure 5-8 Current efficiency of Cu-Sn alloy plating calculated by comparing the anodic charge (Q ) to the cathodic charge (Q ) vs. time. A maximum current efficiency of 92% could be ac cc obtained at the RDE rotation speed of 1000 rpm. The scan rate was 50 mV s-1 and the effect of varying RDE rotational speed is shown. ................................................................ 97 Figure 5-9 Chronoamperometry showing metal deposition at various fixed potentials from a) - 0.41 to -0.44 V b) -0.45 to -0.47 V and c) -0.48 to -0.50 V carried out for 300 s. RDE rotation speed 100 rpm. ..................................................................................................... 100 Figure 5-10 Anodic stripping voltammetry of the deposits obtained at fixed potentials from chronoamperometry experiments. The scan rate was 15 mV s-1 and the RDE rotation speed was 1000 rpm. ........................................................................................................ 101 Figure 6-1 SEM images of a) Cu, b) Sn, and c) Cu-Sn deposits on vitreous carbon. The plating time was 600s and the current densities were 3, 20, and 25 mA cm-2, respectively. ........ 106 Figure 6-2 Cross section of Cu-Sn deposited on vitreous carbon at 22 mA cm-2 for a) 600 s, 8 µm thick and b) 1800 s, 28 µm thick. ................................................................................. 109 Figure 6-3 XRD pattern of vitreous carbon. The broad wide peaks are indicative of the amorphous nature of the substrate. ................................................................................... 110 Figure 6-4 Cu XRD pattern from International Centre for Diffraction Data (ICDD) database. . 110 Figure 6-5 XRD pattern of DC plated a) Cu b) Sn on Cu c) Cu-Sn on vitreous carbon and d) Cu-Sn on Cu. Characteristic Cu Sn peaks are denoted by arrows. ................................ 111 6 5 Figure 6-6 Crystal structures of Cu (face-centered cubic), Sn (tetragonal), and Cu Sn 6 5 (hexagonal) intermetallic phase [19]. ................................................................................. 113 Figure 6-7 Comparison of the deposit microstructure obtained with a) ϴ = 0.2 and b) ϴ = 0.5. The peak current density was 100 mA cm-2 and deposition time was 50 min. ................. 119 Figure 6-8 XRD pattern of Cu-Sn deposits from PP plating. ϴ = 0.2, T = 10 ms, i = 100 mA total p cm-2. ................................................................................................................................... 121 Figure 6-9 XRD pattern showing reproducibility of Cu-Sn deposits obtained via PP plating. Average current density 22 mA cm-2, ϴ = 0.2. .................................................................. 122 Figure 6-10 Microstructure of annealed deposits from a) DC and b) PP methods. Solder balls c) are observed only in DC deposits. ..................................................................................... 124 Figure 6-11 XRD pattern of annealed DC deposit. ................................................................... 125 Figure 6-12 XRD pattern of annealed PP deposit. ................................................................... 126 Figure A-1 Example of a Williamson-Hall plot used in XRD analysis. ....................................... 146 Figure B-1 XRD pattern of DC plated Cu on vitreous carbon. .................................................. 148 Figure B-2 XRD pattern of DC plated Sn on vitreous carbon (Cu base layer). AlCl impurity 3 present. .............................................................................................................................. 149 Figure B-3 XRD pattern of DC plated Cu-Sn on vitreous carbon (Cu base layer). ................... 150 Figure B-4 XRD pattern of DC plated Cu-Sn on vitreous carbon. ............................................. 151 viii LIST OF TABLES Table 1-1 Global mine production and reserves of tin in 2009 and estimates for 2010 [4]. ......... 2 Table 1-2 Official reference price of selected metals in July 2011 [17]. ....................................... 6 Table 1-3 Global mine production and reserves for selected lead-free alternatives in 2009 and estimates for 2010 [4]. ........................................................................................................... 7 Table 1-4 Eutectic composition and melting point of selected tin-based solder alloys [13, 18]. .. 8 Table 1-5 Main advantages and disadvantages of various solder fabrication techniques. ......... 16 Table 1-6 Advantages and disadvantages of various electroplating baths. ................................ 21 Table 1-7 List of various antioxidants and the effect on Sn concentration in MSA electrolytes [2]. ............................................................................................................................................. 22 Table 2-1 Gibbs standard free energy or formation for various Cu and Sn species [81]. The phases are aqueous (aq) and solid or crystal (c). ............................................................... 33 Table 2-2 Standard electrode potentials of selected electrolyte processes [81]. ....................... 36 Table 3-1 Electrolyte compositions and operating conditions in this research............................ 48 Table 3-2 The relationship between the distance along the cylinder electrode and the local current density, i . The applied current density, i , was 4.7 mA cm-2. The solution (x/h) ave contained 0.02 M CuSO , 0.2 M SnSO , hydroquinone and fluorosurfactant. .................... 53 4 4 Table 3-3 The RDE limiting current density of copper and tin in MSA-based electrolytes. ......... 62 Table 3-4 Equivalent PC plating time based on duty cycle and DC plating time. ....................... 68 Table 3-5 List of Cu, Sn, and Cu-Sn phase and their crystal properties used for XRD analysis (ICDD database). ................................................................................................................. 72 Table 4-1 Preliminary electrolyte composition used for electrolyte optimisation experiments. .. 73 Table 4-2 Final electrolyte composition used for electrolyte optimisation experiments. ............ 74 Table 4-3 Elemental composition of the brass cylinder electrode substrate. ............................. 74 Table 4-4 Elemental composition of Cu-Sn deposits from electrolytes without surfactant. ....... 75 Table 4-5 Elemental composition of Cu-Sn deposit from electrolytes with surfactant added. ... 77 Table 5-1 Electrolyte composition used in electrochemical characterisation experiments. ....... 85 Table 5-2 Scan rate and corresponding time constant. ............................................................... 96 Table 5-3 Charge consumed for the deposition and stripping of Cu and Sn during chronoamperometry experiments with potentials in the range of -0.41 to -0.47 V. The RDE rotation speed was 100 rpm and the deposition time was 300 s....................................... 102 Table 6-1 Variation of deposit thickness and microstructure as a function of applied current density in the range of 3-28 mA cm-2. ................................................................................ 108 ix