- Email: [email protected]
Ni couples
Journal of the Taiwan Institute of Chemical Engineers 43 (2012) 295–300
Contents lists available at SciVerse ScienceDirect
Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice
Interfacial reactions in Sn–Sb/Ni couples Yue-Ting Chen, Chih-Chi Chen * R&D Center for Membrane Technology and Department of Chemical Engineering Chung Yuan Christian University, Chung Li 320, Taiwan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 31 May 2011 Received in revised form 6 August 2011 Accepted 16 August 2011 Available online 19 September 2011
Sn–Sb alloys are viable candidates of high-temperature Pb-free solders, and Ni is the common surface finish in electronic packaging. Sn–Sb/Ni is an important soldering joint in electronic products. This study examines the Sn–Sb/Ni liquid/solid interfacial reactions at 270, 320, 400 and 500 8C. The experimental results show that Sn–Sb/Ni interfacial reactions are similar to those of Sn/Ni. At 270 and 320 8C, the reaction path is Sn–Sb/Ni3Sn4/Ni. At 400 and 500 8C, the reaction path is Sn–Sb/Ni3Sn4/Ni3Sn2/Ni3Sn/Ni. The growth rate constants of the intermetallic compounds at each reaction temperature are determined. Sb addition in pure Sn slightly decreases the reaction rate. The determined activation energy are 23.0 and 27.6 kJ/mol for Sn–5 wt%Sb/Ni and Sn–10 wt%Sb/Ni couples, respectively. ß 2011 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Keywords: Sn–Sb Interfacial reactions Diffusion Kinetics
1. Introduction
2. Experimental method
Although very intensive researches have been devoted to the replacement of low-temperature Sn–Pb solders, not much effort has been made for that of high-temperature Sn–Pb solders. High-temperature solders are frequently applied to step soldering of circuit modules [1]. To avoid re-melting of soldered joints in the subsequent reflow process, high-temperature solders are needed. High-temperature solders are also used in packaging of optical components and modules including LEDs and laser devices [1]. Sn–Sb and Sn–Sb-based alloys are promising high-temperature Pb-free solders [1–9]. Ni is the commonly encountered surface finish in electronic packaging. Sn–Sb/Ni is thus an important joint in electronic packaging using high-temperature solders. Reaction phases formed at the soldering joints are crucial to the mechanical properties, and thus to the reliability, so interfacial reactions between Sn–Sb solders and Ni need to be examined carefully for the reliability of Sn–Sb/Ni soldering joints. Sn–Sb/Ag [10–13] and Sn–Sb/Cu [14–16] interfacial reactions have been explored. However, Sn–Sb/Ni interfacial reactions have not yet been examined. This study examines the Sn–Sb/Ni interfacial reactions at 270, 320, 400 and 500 8C using the reaction couple technique. Sn–Sb solders examined are Sn–5 wt%Sb and Sn–10 wt%Sb. The effect of Sb on the interfacial reactions is verified. The reaction paths and the reaction kinetics are determined.
Sn–5 wt%Sb and Sn–10 wt%Sb solders were prepared with pure Sn (99.9%, SHOWA, Tokyo, Japan) and Sb (99.999%, Sigma–Aldrich, USA). Proper amounts of Sn and Sb shots were weighed and encapsulated in the quartz tube at lower than 10 2 mbar. The sample alloys were placed into a furnace at 800 8C for homogenization for a week. The sample alloys were then quenched into cold water. The Sn–Sb solder was placed on a Ni foil (0.5 mm thickness, 99.98%, Sigma–Aldrich, USA), and the Sn–Sb/Ni systems were placed in a horizontal quartz tube (I.D. 50 mm) at lower than 10 2 mbar. The quartz tube was then heated to 270, 320, 400 and 500 8C for interfacial reactions. The accuracy of the furnaces temperature is 3 8C. Although the real soldering temperatures used hardly exceed 300 8C, the reaction temperature up to 500 8C is used in this study to have a complete investigation upon the Sn– Sb/Ni interfacial reactions. Because these temperatures are higher than the liquidus temperatures of Sn–5 wt%Sb and Sn–10 wt%Sb alloys, Sn–Sb alloys were melted. A liquid/solid Sn–Sb/Ni reaction couple was thus formed. In order to have a complete investigation of the reaction phases in Sn–Sb/Ni interfacial reactions, long reaction times, 1–25 h, were examined. Upon completion of predetermined reactions, the samples were cross-sectioned and metallographically treated for interface observations. To have a better observation of the interface, part of the solder was etched away. The composition of the etchant is 95 vol.% CH3OH, 3 vol.% HNO3 and 2 vol.% HCl. Scanning electron microscopy (SEM, Hitachi, S-3000, Tokyo, Japan) equipped with backscattered electron image (BEI) was used for microstructure observations. An electron probe microanalyzer (EPMA, JEOL, JXA-8200, Tokyo, Japan) with wavelength dispersive
* Corresponding author. Tel.: +886 3 2654121; fax: +886 3 2654199. E-mail addresses: [email protected], [email protected] (C.-C. Chen).
1876-1070/$ – see front matter ß 2011 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jtice.2011.08.003
296
Y.-T. Chen, C.-C. Chen / Journal of the Taiwan Institute of Chemical Engineers 43 (2012) 295–300
spectroscopy (WDS, Oxford, UK) was used for compositional determination of the reaction phases. Pure Sn, Sb and Ni were used as the measurement standards. The measurement precision is within 100 1 wt%. The standard deviation for each element among measurements is less than 1 at.%. The interface was divided into at least 20 segments to measure the total thickness of the reaction phases. The average thickness was taken. The thickness of the reaction phase was determined using a commercial image analysis software by dividing the area of the reaction phase by its linear length. For each reaction condition, at least three reaction couples were conducted. The deviation of the measured thicknesses was controlled within 10%. Finally, the average thickness was reported.
3. Results and discussion Fig. 1(a) is the SEI micrograph of the Sn–5 wt%Sb/Ni couple reacted at 270 8C for 1 h. Part of the Sn–5 wt%Sb solder was etched away for better observation of the interface. As shown in Fig. 1(a), only one reaction phase is observed. According to EPMA analysis, its composition is Sn–2.4 at.%Sb–42.8 at.%Ni, comprising 57.2 at.% (Sn + Sb) and 42.8 at.% Ni. From the constituent Sn–Ni [17,18], Sn– Sb [19–21], and Sb–Ni [22,23] binary phase diagrams, this reaction phase is found to be the Ni3Sn4 phase with 2.4 at.%Sb solubility. It is also designated as the Ni3(Sn, Sb)4 phase. Its thickness is approximately 3 mm. Fig. 1(b) is the BEI micrograph of the Sn– 5 wt%Sb/Ni couple reacted at 270 8C for 25 h. The reaction phase
Fig. 1. (a) SEI micrograph of the Sn–5 wt%Sb/Ni couple reacted at 270 8C for 1 h. (b) BEI micrograph of the Sn–5 wt%Sb/Ni couple reacted at 270 8C for 25 h. (c) Close-up of the solder side of (b). (d) BEI micrograph of the Sn–10 wt%Sb/Ni couple reacted at 320 8C for 25 h. (e) Close-up of the solder side of (d).
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Fig. 2. (a) BEI micrograph of the Sn–5 wt%Sb/Ni couple reacted at 400 8C for 1 h. (b) Close-up of the interface of the Sn–5 wt%Sb/Ni couple reacted at 400 8C for 25 h. (c) BEI micrograph of the Sn–10 wt%Sb/Ni couple reacted at 400 8C for 1 h. (d) Close-up of the interface of the Sn–10 wt%Sb/Ni couple reacted at 400 8C for 25 h.
remains the Ni3Sn4 phase after 25 h of reaction. Fig. 1(c) is the close-up of the solder side of Fig. 1(b). As shown in Fig. 1(c), some dispersive phases are found in the solder side. Their composition is Sn–41.7 at.%Sb–0.6 at.%Ni. Based on the Sn–Sb binary phase diagram [19–21], this phase is the Sn3Sb2 phase. Because the amount of the Sn3Sb2 phase is small, it is unlike to be formed from the interfacial reaction. It is from the solidification precipitation during removal from the furnace. Similar phenomenon is also found in the previous study [11,13–15,24]. As the reaction temperature is raised to 320 8C, the reaction phase also remains the Ni3Sn4 phase with limited Sb solubility, whose composition is Sn–2.9 at.%Sb–43.2 at.%Ni. It is concluded the reaction path of the Sn–5 wt%Sb/Ni couple reacted at 270 and 320 8C is Sn–5 wt%Sb/ Ni3Sn4/Ni. Sn–10 wt%Sb/Ni interfacial reactions are similar to those of Sn–5 wt%Sb/Ni. Fig. 1(d and e) is the BEI micrographs of the Sn–10 wt%Sb/Ni couple reacted at 320 8C for 25 h. At 400 8C, Sn–5 wt%Sb/Ni interfacial reactions differ from those at 270 and 320 8C. Fig. 2(a) is the BEI micrograph of the Sn–5 wt%Sb/ Ni couple reacted at 400 8C for 1 h. As shown in Fig. 2(a), the reaction phase layer is still the Ni3Sn4 phase, and the dispersive Sn3Sb2 phase is observed as well. After 4, 9 and 16 h of reaction, the interfacial reactions are similar. Fig. 2(b) is the close-up of the interface of the Sn–5 wt%Sb/Ni couple reacted at 400 8C for 25 h. Three reaction phases are observed. The thickest reaction phase is the Ni3Sn4 phase, whose composition is Sn–1.9 at.%Sb–44.5 at.%Ni. The composition of the darkest phase adjacent to the Ni substrate is Sn–0.8 at.%Sb– 75.9 at.%Ni. The Ni content is 75.9 at.%, while that of (Sn + Sb) is 24.1 at.%. As seen in the Sn–Ni binary phase diagram [17,18], this
reaction phase is the Ni3Sn phase with negligible Sb solubility. It can also be designated as Ni3(Sn, Sb). Between the Ni3Sn4 and Ni3Sn phase, a gray phase is formed. As determined by EPMA, its composition is Sn–1.0 at.%Sb–60.5 at.%Ni. By using similar identification methodology, this reaction phase is found to be the Ni3Sn2 phase with 1.0 at.%Sb solubility, which can also be designated as Ni3(Sn, Sb)2. Therefore, the reaction phases of the Sn–5 wt%Sb/Ni couple reacted at 400 8C are the Ni3Sn4, Ni3Sn2, and Ni3Sn phases. Sn–10 wt%Sb/Ni interfacial reactions are similar to those of Sn– 5 wt%Sb/Ni as shown in Fig. 2(c and d). Comparing Fig. 2(a and c), it can be found that the amount of the Sn3Sb2 precipitation in the Sn– 10 wt%Sb/Ni couple is higher than that in the Sn–5 wt%Sb/Ni couple. Fig. 3 is the BEI micrograph of the Sn–5 wt%Sb/Ni couple reacted at 500 8C for 1 h. Similar to the couple reacted at 400 8C for 25 h, the Ni3Sn4, Ni3Sn2, and Ni3Sn phases are observed. The interfacial reaction in the Sn–10 wt%Sb/Ni couple at 500 8C is similar. Therefore, the reaction path of the Sn–5 wt%Sb/Ni and Sn– 10 wt%Sb/Ni couples at 400 and 500 8C is Sn–Sb/Ni3Sn4/Ni3Sn2/ Ni3Sn/Ni. According to the above experimental results, Sn–Sb/Ni interfacial reactions are similar to those of Sn/Ni [25–32]. At lower temperatures, only the Ni3Sn4 phase is observed. Both Ni3Sn2 and Ni3Sn phases are observed at higher temperatures. Sb does not alter the reaction path. Fig. 4(a and b) shows the Ni3(Sn, Sb)4 and Ni3Sn4 grains in Sn–5 wt%Sb/Ni and Sn/Ni couples reacted at 270 8C for 1 h, respectively. The Ni3(Sn, Sb)4 grain is near hexahedron while the Ni3Sn4 grain is faceted. Of all the Sn–Sb/Ni couples, the Ni3(Sn, Sb)4 phase is the first formed
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Y.-T. Chen, C.-C. Chen / Journal of the Taiwan Institute of Chemical Engineers 43 (2012) 295–300 Table 1 Sn–Sb/Ni kinetics data determined in this study. Sn–5 wt%Sb/Ni 1/2
k270 (m/s ) k320 (m/s1/2) k400 (m/s1/2) k500 (m/s1/2) Ea (kJ/mol)
Fig. 3. BEI micrograph of the Sn–5 wt%Sb/Ni couple reacted at 500 8C for 1 h.
reaction phase. Because Sn is the highest content element in the Ni3(Sn, Sb)4 phase, Sn is most likely the dominant diffusion species in the system [33]. The thermodynamically stable phases are Ni3Sn4, Ni3Sn2, and Ni3Sn phases at 270, 320, 400 and 500 8C. However, only the Ni3Sn4 phase is observed at 270 and 320 8C, and with the reaction time shorter than 25 h at 400 8C. Because the interfacial reactions are dominated by the kinetics, neither Ni3Sn2 nor Ni3Sn phase is observed due to the kinetics constraint. The Ni3Sn4 phase is the intermetallic compound
Fig. 4. (a) The Ni3(Sn, Sb)4 grain in the Sn–5 wt%Sb/Ni couple reacted at 270 8C for 1 h. (b) The Ni3Sn4 grain in the Sn/Ni couple reacted at 270 8C for 1 h.
3.8 10 6.2 10 1.3 10 1.6 10 23.0
8 8 7 7
Sn–10 wt%Sb/Ni 3.3 10 5.0 10 1.3 10 1.8 10 27.6
8 8 7 7
Sn/Ni NA NA NA NA 27.6 1.7 [26]
consuming the least Ni among the three intermetallic compounds. Similar phenomena are found in Mg–Ni and Al–Ti systems [34,35]. At lower temperatures or with shorter reaction time, nucleation is likely a problem in an interfacial reaction. Accordingly, some thermodynamically stable phases are likely to be missed in the reaction couples. At higher temperatures or with longer reaction time, the interfacial reactions tend to be dominated by mass transport. The missing phases are thus observed. The reaction phases grow thicker with longer reaction time. Fig. 5(a) shows the thickness data of Sn–5 wt%Sb/Ni couples. The reaction phase is thicker at higher temperature. The thickness of the reaction phase is approximately linear dependent to the square root of the reaction time, which indicates the growth of the reaction phases is diffusion controlled [36]. The slopes of the thickness curves are the rate constants k at each temperature. Based on the Arrhenius equation, k = k0exp( Ea/ RT), where k is the rate constant, k0 is the pre-exponential factor, Ea is the activation energy, R is the ideal gas constant (8.314 J/ mol K) and T is the absolute temperature, the activation energy of the reaction couple can be obtained. Fig. 5(b) is the Arrhenius plot of the Sn–5 wt%Sb/Ni couple, which slope is Ea/R. According to Fig. 5(b), the activation energy of the Sn– 5 wt%Sb/Ni couple is determined to be 23.0 kJ/mol. The kinetics data of the Sn–Sb/Ni interfacial reactions determined in this study are tabulated as Table 1. Similarly, the kinetics data of the Sn–10 wt%Sb/Ni couple can also be determined. Fig. 5(c and d) is the thickness data and Arrhenius plot of Sn–10 wt%Sb/Ni couples. The activation energy of the Sn–10 wt%Sb/Ni couple is determined to be 27.6 kJ/mol. Since the uncertainty of the thickness measurement is approximately 20%, the error bar range as shown in Fig. 5(a and c) is 20% of the measured data. Because the reaction phase thickness at 500 8C is largest among all reaction temperatures, the error bar range at 500 8C is largest among all reaction temperatures, and becomes smaller with decreased reaction temperatures. The activation energy of the Ni3Sn4 phase formed from the Sn/Ni interfacial reaction reported by Gur and Bamberger [26] and Kay and Mackay [27] are 27.6 1.7 and 37.23 kJ/mol, respectively. The activation energy of Sn–Sb/Ni systems obtained in this study is close to that by Gur and Bamberger [26]. As can be seen from Table 1, the reaction rate of the Sn– 5 wt%Sb/Ni couple is slightly higher than that of Sn–10 wt%Sb/ Ni. In Sn–Sb/Ni systems, Sn is the dominant reactive species, and Sb is inactive. The higher the Sn flux, the higher the reaction rate is. The Sn–5 wt%Sb solder has higher Sn content than the Sn– 10 wt%Sb solder. Accordingly, the reaction rate of the Sn– 5 wt%Sb/Ni couple is higher than that of the Sn–10 wt%Sb/Ni couple. According to the Sn–Sb binary phase diagram [19,20], the liquidus temperatures of Sn–5 wt%Sb and Sn–10 wt%Sb alloys are approximately 240 and 270 8C, respectively. The excess reaction temperatures above the liquidus temperature of the Sn–5 wt%Sb solder are greater than those of the Sn– 10 wt%Sb solder. Again, this also contributes to the higher reaction rate of the Sn–5 wt%Sb/Ni couple. The thicknesses of the Ni3Sn4 phase formed in the Sn/Ni, Sn–5 wt%Sb/Ni and Sn–
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Fig. 5. (a) Thicknesses of reaction phases in Sn–5 wt%Sb/Ni couples. (b) Arrhenius plot of the Sn–5 wt%Sb/Ni couples. (c) Thicknesses of reaction phases in Sn–10 wt%Sb/Ni couples. (d) Arrhenius plot of the Sn–10 wt%Sb/Ni couples.
10 wt%Sb/Ni couples at 270 8C for 1 h is 4, 3 and 2.5 mm, respectively. Sb addition in pure Sn decreases slightly the reaction rate. 4. Conclusion Sn–Sb/Ni interfacial reactions are similar to those of Sn/Ni. At 270 and 320 8C, the reaction path is Sn–Sb/Ni3Sn4/Ni. At 400 and 500 8C, the reaction path is Sn–Sb/Ni3Sn4/Ni3Sn2/Ni3Sn/Ni. The growth rate constants of the intermetallic compounds at each temperature are determined. Sb addition in pure Sn decreases slightly the reaction rate. The calculated activation energy of the Sn–5 wt%Sb/Ni and Sn–10 wt%Sb/Ni interfacial reactions are 23.0 and 27.6 kJ/mol, respectively. Acknowledgement The authors acknowledge the financial support of the National Science Council of Taiwan (Grant # NSC99-2221-E-033-051). References [1] Suganuma K, Kim S-J, Kim K-S. High-temperature lead-free solders: properties and possibilities. JOM 2009;61(1):64–71. [2] Chen S-W, Wang C-H, Lin S-K, Chiu C-N. Phase diagrams of Pb-free solders and their related materials systems. J Mater Sci Mater Electron 2007;18:19–37. [3] Corbin SF. High-temperature variable melting point Sn–Sb lead-free solder pastes using transient liquid-phase powder processing. J Electron Mater 2005;34:1016–25.
[4] Kim JH, Jeong SW, Lee HM. A thermodynamic study of phase equilibria in the Au–Sb–Sn solder system. J Electron Mater 2002;31:557–63. [5] Vassiliev V, Feutelais Y, Sghaier M, Legendre B. Thermodynamic investigation in In–Sb, Sb–Sn and In–Sb–Sn liquid systems. J Alloys Compd 2001;314: 198–205. [6] Ohtani H, Ishida K. A thermodynamic study of the phase equilibria in the Bi–Sn– Sb system. J Electron Mater 1994;23:747–55. [7] El-Daly AA, Swilern Y, Hammad AE. Creep properties of Sn–Sb based lead-free solder alloys. J Alloys Compd 2009;471:98–104. [8] Novakovic R, Giuranno D, Ricci E, Delsante S, Li D, Borzone G. Bulk and surface properties of liquid Sb–Sn alloys. Surf Sci 2011;605:248–55. [9] Plevachuk Y, Hoyer W, Kaban I, Kohler M, Novakovic R. Indentation creep study of lead-free Sn–5%Sb solder alloy. J Mater Sci 2010;45:2051–6. [10] Chen S-W, Chen P-Y, Wang C-H. Melting point lowering of the Sn–Sb alloys caused by substrate dissolution. J Electron Mater 2006;35:1982–5. [11] Lin C-Y, Lee C, Liu X, Yen Y-W. Phase equilibria of the Sn–Sb–Ag ternary system and interfacial reactions at the Sn–Sb/Ag joints at 400 8C and 150 8C. Intermetallics 2008;16:230–8. [12] Lee C, Lin C-Y, Yen Y-W. The 260 (C phase equilibria of the Sn–Sb–Ag ternary system and interfacial reactions at the Sn–Sb/Ag joints. J Alloys Compd 2008;458:436–45. [13] Chen S-W, Zi A-R, Chen P-Y, Wu H-J, Chen Y-K, Wang C-H. Interfacial reactions in the Sn–Sb/Ag and Sn–Sb/Cu couples. Mater Chem Phys 2008;111:17–9. [14] Lee C, Lin C-Y, Yen Y-W. The 260 8C phase equilibria of the Sn–Sb–Cu ternary system and interfacial reactions at the Sn–Sb/Cu joints. Intermetallics 2007;15:1027–37. [15] Jang JW, Kim PG, Tu KN, Lee M. High-temperature lead-free Sn–Sb solders: wetting reactions on Cu foils and phased-in Cu–Cr thin films. J Mater Res 1999;14:3895–900. [16] Vianco PT, Erickson KL, Hopkins PL. Solid state intermetallic compound growth between copper and high temperature, tin-rich solders—part I: experimental analysis. J Electron Mater 1994;23:721–7. [17] Nash P, Nash A. The Ni–Sn (nickel-tin) system. Bull Alloy Phase Diagram 1985;6:350–9. [18] Schmetterer C, Flandorfer H, Richter KW, Saeed U, Kauffman M, Roussel P, et al. A new investigation of the system Ni–Sn. Intermetallics 2007;15:869–84.
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[19] Chen S-W, Chen C-C, Zi A-R, Chen P-Y, Gierlotka W, Wu H-J. Phase equilibria of the Sn–Sb binary system. J Electron Mater 2008;37:992–1002. [20] Predel B, Schwermann W. Constitution and thermodynamics of the antimony– tin system. J Inst Met 1971;99:169–73. [21] Lidin S, Christensen J, Jansson K, Fredrickson D, Withers R, Nor’en L, et al. Incommensurate stistaite-order made to order. Inorg Chem 2009; 48:5497–503. [22] Cha GH, Lee SY, Nash P. The Sb–Ni system. In: Nash P, editor. Phase diagrams of binary nickel alloys. Materials Park, OH: ASM International; 1991. p. 284– 90. [23] Zhanmin C, Yoshikazu T, Ikuo O, Ryosuke K, Hongmin Z, Kiyohito I. Thermodynamic assessment of the Ni–Sb binary system. Rare Met 2008;27:384–92. [24] Lee HT, Hu SY, Hong TF, Chen YF. The shear strength and fracture behavior of Sn–Ag–xSb solder joints with Au/Ni–P/Cu UBM. J Electron Mater 2008; 37:867–73. [25] Shen J, Chan YC, Liu SY. Growth mechanism of Ni3Sn4 in a Sn/Ni liquid/solid interfacial reaction. Acta Mater 2009;57:5196–206. [26] Gur D, Bamberger M. Reactive isothermal solidification in the Ni–Sn system. Acta Mater 1998;46:4917–23. [27] Kay PJ, Mackay CA. The growth of intermetallic compounds on common basis materials coated with tin and tin–lead alloys. Trans Inst Met Finish 1976;54:68–74.
[28] Tomlinson WJ, Rhodes HG. Kinetics of intermetallic compound growth between nickel, electroless Ni–P, electroless Ni–B and tin at 453 to 493 K. J Mater Sci 1987;22:1769–72. [29] Kang SK, Ramachandran V. Growth kinetics of intermetallic phases at the liquid Sn and solid Ni interface. Scripta Metall 1980;14:421–4. [30] Haimovich J. Intermetallic compound growth in tin and tin–lead platings over nickel and its effects on solderability. Weld Res Suppl 1989;(March): 102s–11s. [31] van Beek JA, Stolk SA, van Loo FJJ. Multiphase diffusion in the systems Fe–Sn and Ni–Sn. Z Metallk 1982;73:439–44. [32] Bader S, Gust W, Hieber H. Rapid formation of intermetallic compounds by interdiffusion in the Cu–Sn and Ni–Sn systems. Acta Metall Mater 1994;43: 329–37. [33] d’Heurle FM, Gas P. Kinetics of formation of silicides: a review. J Mater Res 1986;1:205–21. [34] van Loo FJJ, Rieck GD. Diffusion in the titanium–aluminum system. I. Interdiffusion between solid Al and Ti or Ti–Al alloys. Acta Metall 1973;21:61–71. [35] Tsai MY, Chou MH, Kao CR. Interfacial reaction and the dominant diffusing species in Mg–Ni system. J Alloys Compd 2009;471:90–2. [36] Dybkov VI. Reaction diffusion and solid state chemical kinetics, 2nd rev. ed., Zurich: Trans Tech Publications Ltd.; 2010.
Contents lists available at SciVerse ScienceDirect
Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice
Interfacial reactions in Sn–Sb/Ni couples Yue-Ting Chen, Chih-Chi Chen * R&D Center for Membrane Technology and Department of Chemical Engineering Chung Yuan Christian University, Chung Li 320, Taiwan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 31 May 2011 Received in revised form 6 August 2011 Accepted 16 August 2011 Available online 19 September 2011
Sn–Sb alloys are viable candidates of high-temperature Pb-free solders, and Ni is the common surface finish in electronic packaging. Sn–Sb/Ni is an important soldering joint in electronic products. This study examines the Sn–Sb/Ni liquid/solid interfacial reactions at 270, 320, 400 and 500 8C. The experimental results show that Sn–Sb/Ni interfacial reactions are similar to those of Sn/Ni. At 270 and 320 8C, the reaction path is Sn–Sb/Ni3Sn4/Ni. At 400 and 500 8C, the reaction path is Sn–Sb/Ni3Sn4/Ni3Sn2/Ni3Sn/Ni. The growth rate constants of the intermetallic compounds at each reaction temperature are determined. Sb addition in pure Sn slightly decreases the reaction rate. The determined activation energy are 23.0 and 27.6 kJ/mol for Sn–5 wt%Sb/Ni and Sn–10 wt%Sb/Ni couples, respectively. ß 2011 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Keywords: Sn–Sb Interfacial reactions Diffusion Kinetics
1. Introduction
2. Experimental method
Although very intensive researches have been devoted to the replacement of low-temperature Sn–Pb solders, not much effort has been made for that of high-temperature Sn–Pb solders. High-temperature solders are frequently applied to step soldering of circuit modules [1]. To avoid re-melting of soldered joints in the subsequent reflow process, high-temperature solders are needed. High-temperature solders are also used in packaging of optical components and modules including LEDs and laser devices [1]. Sn–Sb and Sn–Sb-based alloys are promising high-temperature Pb-free solders [1–9]. Ni is the commonly encountered surface finish in electronic packaging. Sn–Sb/Ni is thus an important joint in electronic packaging using high-temperature solders. Reaction phases formed at the soldering joints are crucial to the mechanical properties, and thus to the reliability, so interfacial reactions between Sn–Sb solders and Ni need to be examined carefully for the reliability of Sn–Sb/Ni soldering joints. Sn–Sb/Ag [10–13] and Sn–Sb/Cu [14–16] interfacial reactions have been explored. However, Sn–Sb/Ni interfacial reactions have not yet been examined. This study examines the Sn–Sb/Ni interfacial reactions at 270, 320, 400 and 500 8C using the reaction couple technique. Sn–Sb solders examined are Sn–5 wt%Sb and Sn–10 wt%Sb. The effect of Sb on the interfacial reactions is verified. The reaction paths and the reaction kinetics are determined.
Sn–5 wt%Sb and Sn–10 wt%Sb solders were prepared with pure Sn (99.9%, SHOWA, Tokyo, Japan) and Sb (99.999%, Sigma–Aldrich, USA). Proper amounts of Sn and Sb shots were weighed and encapsulated in the quartz tube at lower than 10 2 mbar. The sample alloys were placed into a furnace at 800 8C for homogenization for a week. The sample alloys were then quenched into cold water. The Sn–Sb solder was placed on a Ni foil (0.5 mm thickness, 99.98%, Sigma–Aldrich, USA), and the Sn–Sb/Ni systems were placed in a horizontal quartz tube (I.D. 50 mm) at lower than 10 2 mbar. The quartz tube was then heated to 270, 320, 400 and 500 8C for interfacial reactions. The accuracy of the furnaces temperature is 3 8C. Although the real soldering temperatures used hardly exceed 300 8C, the reaction temperature up to 500 8C is used in this study to have a complete investigation upon the Sn– Sb/Ni interfacial reactions. Because these temperatures are higher than the liquidus temperatures of Sn–5 wt%Sb and Sn–10 wt%Sb alloys, Sn–Sb alloys were melted. A liquid/solid Sn–Sb/Ni reaction couple was thus formed. In order to have a complete investigation of the reaction phases in Sn–Sb/Ni interfacial reactions, long reaction times, 1–25 h, were examined. Upon completion of predetermined reactions, the samples were cross-sectioned and metallographically treated for interface observations. To have a better observation of the interface, part of the solder was etched away. The composition of the etchant is 95 vol.% CH3OH, 3 vol.% HNO3 and 2 vol.% HCl. Scanning electron microscopy (SEM, Hitachi, S-3000, Tokyo, Japan) equipped with backscattered electron image (BEI) was used for microstructure observations. An electron probe microanalyzer (EPMA, JEOL, JXA-8200, Tokyo, Japan) with wavelength dispersive
* Corresponding author. Tel.: +886 3 2654121; fax: +886 3 2654199. E-mail addresses: [email protected], [email protected] (C.-C. Chen).
1876-1070/$ – see front matter ß 2011 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jtice.2011.08.003
296
Y.-T. Chen, C.-C. Chen / Journal of the Taiwan Institute of Chemical Engineers 43 (2012) 295–300
spectroscopy (WDS, Oxford, UK) was used for compositional determination of the reaction phases. Pure Sn, Sb and Ni were used as the measurement standards. The measurement precision is within 100 1 wt%. The standard deviation for each element among measurements is less than 1 at.%. The interface was divided into at least 20 segments to measure the total thickness of the reaction phases. The average thickness was taken. The thickness of the reaction phase was determined using a commercial image analysis software by dividing the area of the reaction phase by its linear length. For each reaction condition, at least three reaction couples were conducted. The deviation of the measured thicknesses was controlled within 10%. Finally, the average thickness was reported.
3. Results and discussion Fig. 1(a) is the SEI micrograph of the Sn–5 wt%Sb/Ni couple reacted at 270 8C for 1 h. Part of the Sn–5 wt%Sb solder was etched away for better observation of the interface. As shown in Fig. 1(a), only one reaction phase is observed. According to EPMA analysis, its composition is Sn–2.4 at.%Sb–42.8 at.%Ni, comprising 57.2 at.% (Sn + Sb) and 42.8 at.% Ni. From the constituent Sn–Ni [17,18], Sn– Sb [19–21], and Sb–Ni [22,23] binary phase diagrams, this reaction phase is found to be the Ni3Sn4 phase with 2.4 at.%Sb solubility. It is also designated as the Ni3(Sn, Sb)4 phase. Its thickness is approximately 3 mm. Fig. 1(b) is the BEI micrograph of the Sn– 5 wt%Sb/Ni couple reacted at 270 8C for 25 h. The reaction phase
Fig. 1. (a) SEI micrograph of the Sn–5 wt%Sb/Ni couple reacted at 270 8C for 1 h. (b) BEI micrograph of the Sn–5 wt%Sb/Ni couple reacted at 270 8C for 25 h. (c) Close-up of the solder side of (b). (d) BEI micrograph of the Sn–10 wt%Sb/Ni couple reacted at 320 8C for 25 h. (e) Close-up of the solder side of (d).
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Fig. 2. (a) BEI micrograph of the Sn–5 wt%Sb/Ni couple reacted at 400 8C for 1 h. (b) Close-up of the interface of the Sn–5 wt%Sb/Ni couple reacted at 400 8C for 25 h. (c) BEI micrograph of the Sn–10 wt%Sb/Ni couple reacted at 400 8C for 1 h. (d) Close-up of the interface of the Sn–10 wt%Sb/Ni couple reacted at 400 8C for 25 h.
remains the Ni3Sn4 phase after 25 h of reaction. Fig. 1(c) is the close-up of the solder side of Fig. 1(b). As shown in Fig. 1(c), some dispersive phases are found in the solder side. Their composition is Sn–41.7 at.%Sb–0.6 at.%Ni. Based on the Sn–Sb binary phase diagram [19–21], this phase is the Sn3Sb2 phase. Because the amount of the Sn3Sb2 phase is small, it is unlike to be formed from the interfacial reaction. It is from the solidification precipitation during removal from the furnace. Similar phenomenon is also found in the previous study [11,13–15,24]. As the reaction temperature is raised to 320 8C, the reaction phase also remains the Ni3Sn4 phase with limited Sb solubility, whose composition is Sn–2.9 at.%Sb–43.2 at.%Ni. It is concluded the reaction path of the Sn–5 wt%Sb/Ni couple reacted at 270 and 320 8C is Sn–5 wt%Sb/ Ni3Sn4/Ni. Sn–10 wt%Sb/Ni interfacial reactions are similar to those of Sn–5 wt%Sb/Ni. Fig. 1(d and e) is the BEI micrographs of the Sn–10 wt%Sb/Ni couple reacted at 320 8C for 25 h. At 400 8C, Sn–5 wt%Sb/Ni interfacial reactions differ from those at 270 and 320 8C. Fig. 2(a) is the BEI micrograph of the Sn–5 wt%Sb/ Ni couple reacted at 400 8C for 1 h. As shown in Fig. 2(a), the reaction phase layer is still the Ni3Sn4 phase, and the dispersive Sn3Sb2 phase is observed as well. After 4, 9 and 16 h of reaction, the interfacial reactions are similar. Fig. 2(b) is the close-up of the interface of the Sn–5 wt%Sb/Ni couple reacted at 400 8C for 25 h. Three reaction phases are observed. The thickest reaction phase is the Ni3Sn4 phase, whose composition is Sn–1.9 at.%Sb–44.5 at.%Ni. The composition of the darkest phase adjacent to the Ni substrate is Sn–0.8 at.%Sb– 75.9 at.%Ni. The Ni content is 75.9 at.%, while that of (Sn + Sb) is 24.1 at.%. As seen in the Sn–Ni binary phase diagram [17,18], this
reaction phase is the Ni3Sn phase with negligible Sb solubility. It can also be designated as Ni3(Sn, Sb). Between the Ni3Sn4 and Ni3Sn phase, a gray phase is formed. As determined by EPMA, its composition is Sn–1.0 at.%Sb–60.5 at.%Ni. By using similar identification methodology, this reaction phase is found to be the Ni3Sn2 phase with 1.0 at.%Sb solubility, which can also be designated as Ni3(Sn, Sb)2. Therefore, the reaction phases of the Sn–5 wt%Sb/Ni couple reacted at 400 8C are the Ni3Sn4, Ni3Sn2, and Ni3Sn phases. Sn–10 wt%Sb/Ni interfacial reactions are similar to those of Sn– 5 wt%Sb/Ni as shown in Fig. 2(c and d). Comparing Fig. 2(a and c), it can be found that the amount of the Sn3Sb2 precipitation in the Sn– 10 wt%Sb/Ni couple is higher than that in the Sn–5 wt%Sb/Ni couple. Fig. 3 is the BEI micrograph of the Sn–5 wt%Sb/Ni couple reacted at 500 8C for 1 h. Similar to the couple reacted at 400 8C for 25 h, the Ni3Sn4, Ni3Sn2, and Ni3Sn phases are observed. The interfacial reaction in the Sn–10 wt%Sb/Ni couple at 500 8C is similar. Therefore, the reaction path of the Sn–5 wt%Sb/Ni and Sn– 10 wt%Sb/Ni couples at 400 and 500 8C is Sn–Sb/Ni3Sn4/Ni3Sn2/ Ni3Sn/Ni. According to the above experimental results, Sn–Sb/Ni interfacial reactions are similar to those of Sn/Ni [25–32]. At lower temperatures, only the Ni3Sn4 phase is observed. Both Ni3Sn2 and Ni3Sn phases are observed at higher temperatures. Sb does not alter the reaction path. Fig. 4(a and b) shows the Ni3(Sn, Sb)4 and Ni3Sn4 grains in Sn–5 wt%Sb/Ni and Sn/Ni couples reacted at 270 8C for 1 h, respectively. The Ni3(Sn, Sb)4 grain is near hexahedron while the Ni3Sn4 grain is faceted. Of all the Sn–Sb/Ni couples, the Ni3(Sn, Sb)4 phase is the first formed
298
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k270 (m/s ) k320 (m/s1/2) k400 (m/s1/2) k500 (m/s1/2) Ea (kJ/mol)
Fig. 3. BEI micrograph of the Sn–5 wt%Sb/Ni couple reacted at 500 8C for 1 h.
reaction phase. Because Sn is the highest content element in the Ni3(Sn, Sb)4 phase, Sn is most likely the dominant diffusion species in the system [33]. The thermodynamically stable phases are Ni3Sn4, Ni3Sn2, and Ni3Sn phases at 270, 320, 400 and 500 8C. However, only the Ni3Sn4 phase is observed at 270 and 320 8C, and with the reaction time shorter than 25 h at 400 8C. Because the interfacial reactions are dominated by the kinetics, neither Ni3Sn2 nor Ni3Sn phase is observed due to the kinetics constraint. The Ni3Sn4 phase is the intermetallic compound
Fig. 4. (a) The Ni3(Sn, Sb)4 grain in the Sn–5 wt%Sb/Ni couple reacted at 270 8C for 1 h. (b) The Ni3Sn4 grain in the Sn/Ni couple reacted at 270 8C for 1 h.
3.8 10 6.2 10 1.3 10 1.6 10 23.0
8 8 7 7
Sn–10 wt%Sb/Ni 3.3 10 5.0 10 1.3 10 1.8 10 27.6
8 8 7 7
Sn/Ni NA NA NA NA 27.6 1.7 [26]
consuming the least Ni among the three intermetallic compounds. Similar phenomena are found in Mg–Ni and Al–Ti systems [34,35]. At lower temperatures or with shorter reaction time, nucleation is likely a problem in an interfacial reaction. Accordingly, some thermodynamically stable phases are likely to be missed in the reaction couples. At higher temperatures or with longer reaction time, the interfacial reactions tend to be dominated by mass transport. The missing phases are thus observed. The reaction phases grow thicker with longer reaction time. Fig. 5(a) shows the thickness data of Sn–5 wt%Sb/Ni couples. The reaction phase is thicker at higher temperature. The thickness of the reaction phase is approximately linear dependent to the square root of the reaction time, which indicates the growth of the reaction phases is diffusion controlled [36]. The slopes of the thickness curves are the rate constants k at each temperature. Based on the Arrhenius equation, k = k0exp( Ea/ RT), where k is the rate constant, k0 is the pre-exponential factor, Ea is the activation energy, R is the ideal gas constant (8.314 J/ mol K) and T is the absolute temperature, the activation energy of the reaction couple can be obtained. Fig. 5(b) is the Arrhenius plot of the Sn–5 wt%Sb/Ni couple, which slope is Ea/R. According to Fig. 5(b), the activation energy of the Sn– 5 wt%Sb/Ni couple is determined to be 23.0 kJ/mol. The kinetics data of the Sn–Sb/Ni interfacial reactions determined in this study are tabulated as Table 1. Similarly, the kinetics data of the Sn–10 wt%Sb/Ni couple can also be determined. Fig. 5(c and d) is the thickness data and Arrhenius plot of Sn–10 wt%Sb/Ni couples. The activation energy of the Sn–10 wt%Sb/Ni couple is determined to be 27.6 kJ/mol. Since the uncertainty of the thickness measurement is approximately 20%, the error bar range as shown in Fig. 5(a and c) is 20% of the measured data. Because the reaction phase thickness at 500 8C is largest among all reaction temperatures, the error bar range at 500 8C is largest among all reaction temperatures, and becomes smaller with decreased reaction temperatures. The activation energy of the Ni3Sn4 phase formed from the Sn/Ni interfacial reaction reported by Gur and Bamberger [26] and Kay and Mackay [27] are 27.6 1.7 and 37.23 kJ/mol, respectively. The activation energy of Sn–Sb/Ni systems obtained in this study is close to that by Gur and Bamberger [26]. As can be seen from Table 1, the reaction rate of the Sn– 5 wt%Sb/Ni couple is slightly higher than that of Sn–10 wt%Sb/ Ni. In Sn–Sb/Ni systems, Sn is the dominant reactive species, and Sb is inactive. The higher the Sn flux, the higher the reaction rate is. The Sn–5 wt%Sb solder has higher Sn content than the Sn– 10 wt%Sb solder. Accordingly, the reaction rate of the Sn– 5 wt%Sb/Ni couple is higher than that of the Sn–10 wt%Sb/Ni couple. According to the Sn–Sb binary phase diagram [19,20], the liquidus temperatures of Sn–5 wt%Sb and Sn–10 wt%Sb alloys are approximately 240 and 270 8C, respectively. The excess reaction temperatures above the liquidus temperature of the Sn–5 wt%Sb solder are greater than those of the Sn– 10 wt%Sb solder. Again, this also contributes to the higher reaction rate of the Sn–5 wt%Sb/Ni couple. The thicknesses of the Ni3Sn4 phase formed in the Sn/Ni, Sn–5 wt%Sb/Ni and Sn–
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Fig. 5. (a) Thicknesses of reaction phases in Sn–5 wt%Sb/Ni couples. (b) Arrhenius plot of the Sn–5 wt%Sb/Ni couples. (c) Thicknesses of reaction phases in Sn–10 wt%Sb/Ni couples. (d) Arrhenius plot of the Sn–10 wt%Sb/Ni couples.
10 wt%Sb/Ni couples at 270 8C for 1 h is 4, 3 and 2.5 mm, respectively. Sb addition in pure Sn decreases slightly the reaction rate. 4. Conclusion Sn–Sb/Ni interfacial reactions are similar to those of Sn/Ni. At 270 and 320 8C, the reaction path is Sn–Sb/Ni3Sn4/Ni. At 400 and 500 8C, the reaction path is Sn–Sb/Ni3Sn4/Ni3Sn2/Ni3Sn/Ni. The growth rate constants of the intermetallic compounds at each temperature are determined. Sb addition in pure Sn decreases slightly the reaction rate. The calculated activation energy of the Sn–5 wt%Sb/Ni and Sn–10 wt%Sb/Ni interfacial reactions are 23.0 and 27.6 kJ/mol, respectively. Acknowledgement The authors acknowledge the financial support of the National Science Council of Taiwan (Grant # NSC99-2221-E-033-051). References [1] Suganuma K, Kim S-J, Kim K-S. High-temperature lead-free solders: properties and possibilities. JOM 2009;61(1):64–71. [2] Chen S-W, Wang C-H, Lin S-K, Chiu C-N. Phase diagrams of Pb-free solders and their related materials systems. J Mater Sci Mater Electron 2007;18:19–37. [3] Corbin SF. High-temperature variable melting point Sn–Sb lead-free solder pastes using transient liquid-phase powder processing. J Electron Mater 2005;34:1016–25.
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