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Anomalous growth of whisker-like bismuth–tin extrusions from tin-enriched tin–Bi deposits
Journal of Alloys and Compounds 472 (2009) 121–126
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Anomalous growth of whisker-like bismuth–tin extrusions from tin-enriched tin–Bi deposits Chi-Chang Hu a,∗ , Yi-Da Tsai b , Chi-Cheng Lin b , Gen-Lan Lee b , Sinn-Wen Chen a , Tai-Chou Lee b , Ten-Chin Wen c a Department of Chemical Engineering, National Tsing Hua University, 101, Section 2, Kuang-Fu Road, Hsin-Chu 30013, Taiwan b Department of Chemical Engineering, National Chung Cheng University, Chia-Yi 621, Taiwan c Department of Chemical Engineering, National Cheng Kung University, Tainan 701, Taiwan
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Article history: Received 22 April 2008 Accepted 29 April 2008 Available online 10 June 2008 Keywords: Metal extrusion Sn–xBi deposit Post-plating annealing Lead-free solder Electroplating
a b s t r a c t This article shows the first finding that the anomalous growth of Bi–Sn extrusions from tin-enriched alloys (Sn–xBi with x between 20 and 10 wt.%) can be induced by post-plating annealing in N2 between 145 and 260 ◦ C for 10 min although metal whiskers were commonly formed on the surface of pure metals or alloys of the enriched component. From SEM observations, very similar to Sn whiskers, Bi–Sn extrusions vary in size, shape, length, and diameter with changing the annealing temperature, which are highly important in regarding the potential for failure of electronic products. Annealing resulting in thermal expansion of Sn grains is believed to squeeze the Bi–Sn alloys with relatively low melting points to form whisker-like extrusions although the exact mechanism is unclear. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Metal whiskers are hair-like crystal structures commonly grown from the surfaces finished with pure metals or alloys of the enriched component [1–3]. The dimensions of whiskers are usually of some micrometers in the diameter but up to a length of hundreds of micrometers [1–3]. Due to the metallic and crystalline nature, metal whiskers have been revealed to cause shorts resulting in the failure of electronic systems. Actually, the formation of Sn whiskers from the high Sn content surface finish was found to cause damages of electronic systems in 1940s [2]. After this finding, many studies, reviews, and books tried to clarify the formation mechanism of Sn, Zn or other metal whiskers on the reliability problems of electronic systems [1,2,4–6]. In addition, the formation of metal whiskers is only found on a limited number of metals such as Sn, Zn, Cd, Bi, etc. [1–3]. Among them, Sn whiskers have been widely studied since tin was widely used as surface finishing or binding materials for Sn-plated terminals of all types of electronic components. Recently, renewed interest in the formation of Sn whiskers is due to the wide application of lead-free solder in electronic manufacturing [6]. Moreover, the importance of other metallic whiskers
∗ Corresponding author. Tel.: +886 3 5736027; fax: +886 3 5715408. E-mail address: [email protected] (C.-C. Hu). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.04.094
on the failures of high-reliability electronic systems significantly increases because of the worldwide transition to lead-free electronics. Unfortunately, the formation mechanism(s) of metal whiskers is still unclear and there is no reliable theory for predicting the dependence of whisker density or length on time or thermal history although several mechanisms of whisker growth have been proposed [4–6]. Furthermore, Sn whiskers have been found to be effectively depressed by adding suppressants such as Pb, Bi, Ag, etc. [7–9]. Since whisker formation is dependent on stress in the electroplated deposit, the influences of the electroplating variables such as current density, temperature, under bump metallization (UBM), and bath components [4,10–12] on the formation of Sn whiskers have been investigated. Moreover, the effects of metal/alloy thickness, post-plating annealing, and alloying agents [4,6c,7–9,12–14] on the formation of these whiskers have also been studied. For other metal whiskers with the exception of Zn [4b,4c,15], on the other hand, systematic studies on the formation mechanism are very limited [1,2]. Based on the above viewpoints, the continuous miniaturization of electronics has resulted in spacing between interconnects of components and a conductive whisker or extrusion will easily bridge the gap and increase the possibility of failures. The difference between a whisker and an extrusion is mainly attributed to the single-crystalline nature of the former metal while both metallic
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Table 1 Composition and variables for the electroplating of Sn–xBi deposits Compounds/variables
Sn–13Bi
Sn–19Bi
SnCl4 ·nH2 O Bi(NO3 )3 ·5H2 O Citric acid NH4 OH pHa Temperature Current density
0.18 M 0.02 M 0.4 M ≥0.2 M 3.0 25 ◦ C 40 mA cm−2
0.18 M 0.02 M 0.4 M ≥0.2 M 5.0 25 ◦ C 30 mA cm−2
a
pH was adjusted by 1 M HCl or 2 M NH4 OH.
materials show the same impact on the reliability of electronics. In this work, we demonstrate the first finding that the growth of Bi–Sn whisker-like extrusions on the Sn-enriched Sn–xBi deposits (with x between 10 and 20 wt.%) can be induced by post-plating annealing in N2 between 145 and 260 ◦ C although aside from Pb, Bi is the most effective element in mitigating the formation of tin whiskers. Accordingly, a probable rout for the growth of Bi–Sn extrusions is proposed in this work even though the exact mechanism(s) is unclear. 2. Experimental details The lead-free tin–bismuth deposits with variable compositions were electroplated from a simple CN− -free solution onto copper (99.5%, 1 cm × 2 cm) plates deposited with a nickel film (ca. 2 m) used as the UBM (denoted as Cu/Ni). The pretreatment procedures of Cu/Ni substrates completely followed our previous work [16–18]. These Cu/Ni substrates, rinsed with pure water, were vertically placed in a 500-ml jacket cell surrounded with an anode of platinum-coated stainless steel mesh and electroplated with Sn–xBi deposits, about 2 m, at various plating conditions with a constant passed charge density (72.0 C cm−2 ). After the Sn–xBi deposition, these electrodes were rinsed with pure water, vibrated in an ultrasonic bath for 5 min, and dried in a vacuum oven at room temperature.
The composition control of Sn–xBi deposits completely followed the process described in our previous work [18]. Typical compositions of the lead-free Sn–xBi plating bath and electroplating conditions used in this work are shown in Table 1, which mainly consisted of SnCl4 ·nH2 O, Bi(NO3 )3 ·5H2 O, and citric acid (0.2 M, complex agent) with pH being adjusted with 3 M HCl and 3 M NH4 OH. All solutions were prepared with pure water produced by a reagent water system (Milli-Q SP, Japan) at 18 M cm and all reagents were Merck, GR. Solution temperature was maintained at the specified temperatures with an accuracy of 0.1 ◦ C by means of a water thermostat (Haake DC3 and K20). All whisker-like extrusions were examined (ex situ) by means of a field-emission scanning electron microscope (FE-SEM, Hitachi S-4700) after the deposits have been annealed at the specified temperature for 10 min. There was no extrusion when the annealing temperature was ≤139 ◦ C, meanwhile no extrusion was found when freshly prepared deposits were kept in air at room temperature for several weeks. The composition of extrusions was measured using an energy-dispersive X-ray (EDX) spectroscope coupled with the above FE-SEM. The mean error of this EDX analysis is ca. ±1.5 wt.%. X-ray diffraction patterns were obtained from an X-ray diffractometer (Rigaku MiniFlex) using a Cu target.
3. Results and discussion Because of the miniscule dimensions, Bi–Sn extrusions cannot be visualized without magnification (typically through the scanning electron microscopic, SEM, observations). The SEM images shown in Fig. 1 reveal the formation of metal extrusions on the Sn–xBi (x = 10–20 in wt.%) finished surfaces with annealing at 260 ◦ C for 10 min although minor amount of Bi dendrites are sometimes found on the surface of the as-plated and annealed deposits. The grain size of as-plated Sn–Bi deposits is uniform, which is gradually decreased with increasing the tin content from a comparison of Fig. 1A and B (in average ∼0.8 and 2.2 m for Sn–19Bi and Sn–13Bi deposits, respectively). After annealing at 260 ◦ C for 10 min, whisker-like Bi–Sn extrusions in various dimensions (typically 50–70 m in length and 1–3 m in width) are clearly found on the surface of the annealed deposits. Note that most metal whiskers were found to grow from pure-metal thin films or the alloys of
Fig. 1. SEM morphologies of (A and B) as-plated and (C and D) annealed Sn–xBi deposits (260 ◦ C) with x = 19 (A and C), 13 (B and D) (in wt.%).
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Fig. 2. SEM morphologies of annealed Sn–19Bi deposits; (A, 260 ◦ C) needle-like; (B, 240 ◦ C) nodule-like; (C, 210 ◦ C) hook-like; (D, 180 ◦ C) plank-like; (E, 160 ◦ C) rod-shaped; and (F, 145 ◦ C) irregular-shaped extrusions.
enriched components [6c] with the exception of the eutectic Sn–Pb alloy found recently [13]. However, in this work, the growth of whisker-like Bi–Sn extrusions is effectively induced on the tinenriched deposits through a post-plating annealing process in N2 between 145 and 260 ◦ C for 10 min even though the Bi content is only as low as 20–10 wt.%. Figs. 1–3 illustrate the growth of Bi–Sn extrusions of various sizes, shapes, and dimensions on the Sn–xBi deposits (with x varying from 20 to 10 wt.%), which will be an important issue in employing the lead-free Sn–xBi surface finish. For example, typical needle-like extrusions or filaments show lengths of 50–70 m and diameters of approximately 0.8–2.5 m (see Figs. 1C, D, 2A and 3B). The needle-like extrusions seem to become shorter with decreasing the annealing temperature (see Fig. 2E). In addition, nodules, planks, celeries, hooks, or irregular eruptions in various sizes are more easily found at relatively lower annealing temperatures. Moreover, roll-like, grass-root-like, irregularly shaped, or toothpaste-like extrusions of more than 20 m in length are also observed on certain parts (see Figs. 2 and 3). The lowest tem-
perature of post-plating annealing inducing the growth of these whisker-like Bi–Sn extrusions has been confirmed to be 145 ◦ C. As mentioned previously, the actual reasons responsible for the growth of most kinds of metal whiskers/extrusions are unclear. The similarities in the shape, size, and diameter between Sn whiskers and Bi–Sn extrusions, however, may imply a common formation mechanism, i.e., the stress-inducing mechanism. For eutectic Sn–Cu, precipitates of Cu6 Sn5 in the grain boundaries of the finishes have been attributed to the driving force for the rapid growth of Sn whiskers [6c]. For Sn–Mn deposits, the formation of Sn whiskers and the phase transformation of MnSn2 occur simultaneously [19]. On the other hand, the growth of Bi–Sn extrusions has been confirmed to be visible only when the Bi content is between 10 and 20 wt.%, which is different from the formation of Sn whiskers from the pure Sn or Sn-enriched finish. Note that the formation of Sn whiskers has been found to depress by introducing minor Bi into the Sn matrix. In addition, the lowest annealing temperature (145 ◦ C) inducing the growth of Bi–Sn extrusions is much lower than the temperatures of liquidus of Sn–xBi with
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Fig. 3. SEM morphologies of annealed Sn–13Bi deposits; (A, 260 ◦ C) toothpaste-like; (B, 240 ◦ C) needle-like; (C, 240 ◦ C) roll-like; (D, 210 ◦ C) stick bank-like; (E, 160 ◦ C) nodulelike; (F, 160 ◦ C) irregular-shaped extrusions.
x between 10 and 20. Thus, the presence of Bi-enriched extrusions with minor amount of Sn (see Fig. 4B–D) as the post-plating annealing temperatures ≥145 ◦ C indicates that Bi is not completely dissolved into the Sn matrix although the maximum solubility of Bi in Sn is 21 wt.% from the Sn–Bi phase diagram [20]. This statement is supported by the weight ratio of Bi/Sn corresponding to Fig. 4B–D equal to 0.296, 0.301, and 0.234, respectively. Furthermore, phase separation of Sn and Bi without the presence of other phases is clearly found on both as-plated and annealed deposits (see Fig. 5). This phenomenon suggests that the growth of Bi–Sn extrusions seems to be not due to the formation of intermetallic compounds (as found in the Sn–Cu and Sn–Mn systems) because the 2 m nickel UBM film is used to reduce the interfacial reaction rate between Sn and Cu. On the other hand, the stress state in these 2 m Sn–xBi deposits might be influenced by the interfacial expansion (or minor interfacial reaction) between the Sn–xBi deposit and the Cu/Ni substrate, inducing the growth of Bi–Sn extrusions.
Based on the above results and discussion, the growth mechanism of whisker-like Bi–Sn extrusions is presumably similar to that of Sn–Pb whisker formation on the annealed eutectic Sn–Pb solder die attach material [13]. Since there is no biased potential or current applied on these deposits during the annealing process, the growth of Bi–Sn extrusions is not attributable to metal electromigration under high current densities [13]. On the other hand, thermal expansion of Sn grains during annealing will squeeze the Bi–Sn alloys with relatively low melting points (i.e., partial melting) to form extrusions (see Fig. 6). This phenomenon is likely accelerated by the presence of oxygen, probably due to the codeposition of OH− with Sn (e.g., Sn(OH)+ ) in weakly acidic media [21]. Moreover, an oxygen-containing Sn–Bi sub-monolayer is favorably formed on the surface of Sn–xBi deposits due to an increase in pH at the vicinity of the deposit–electrolyte interface when hydrogen is extensively evolved in the end of Sn–xBi deposition. The less expansion of this hydroxyl Sn–Bi sub-monolayer will drive the growth of metal extrusions due to the presence of inner stress/pressure
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Fig. 4. (A) A SEM photograph and the EDS spectra measured at the (B) top; (C) middle; and (D) bottom position of a whisker-like Bi extrusion on a Sn–19Bi deposit with annealing at 210 ◦ C for 10 min.
within the deposits upon annealing although the exact mechanism for the formation of whisker-like Bi–Sn extrusions is still not clear. 4. Conclusions The anomalous growth of whisker-like, Bi-enriched extrusions from Sn–xBi deposits (with x between 20 and 10 wt.%) induced by post-plating annealing is clearly demonstrated in this work. Bismuth–tin extrusions in different sizes, shapes, and dimensions are clearly found on these Sn–xBi deposits with annealing between 145 and 260 ◦ C. Upon annealing, thermal expansion of Sn grains may squeeze the Bi–Sn alloys with relatively low melting points to form extrusions, which should be similar to that of Sn–Pb whisker formation on the reflowed eutectic Sn–Pb solder although the exact mechanism is still unclear. Finally, product designers, part engineers, failure analysts, and quality assurance specialists should be aware of the possibility of formation of Bi–Sn extrusions (but not Sn whiskers) on Sn–xBi deposits with post-plating annealing in N2 between 145 and 260 ◦ C for 10 min only. Acknowledgment
Fig. 5. XRD patterns of an (A) as-plated Sn–19Bi deposit with annealing at (B) 145, (C) 210, and (D) 260 ◦ C in N2 for 10 min; where (o) Sn, (z) Bi, () Cu, and (x) Ni.
The financial support of this work, by the National Science Council of the Republic of China under contract no. NSC 96-2628E-007-146-MY2, is gratefully acknowledged.
Fig. 6. The formation scheme of Bi–Sn extrusions on Sn–xBi deposits upon annealing.
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References [1] (a) http://nepp.nasa.gov/whiskers/.; (b) http://www.calce.umd.edu/lead-free/tin-whisk-ers/. [2] H. Leidecker, J. Brusse, Tin Whiskers: A History of Documented Electrical System Failures, Technical Presentation to Space Shuttle Program Office, April 2006. [3] (a) US Patent 4,325,795; (b) L. Mayer, R. Rickett, H. Stenemann, J. Appl. Phys. 33 (1962) 982; (c) D.R. Overcash, B.A. Ratnam, M.J. Skove, E.P. Stillwell, Phys. Rev. Lett. 44 (1998) 1348. [4] (a) K.L. Tu, Acta Metall. 21 (1973) 347; (b) U. Lindborg, Metall. Trans. A: Phys. Metall. Mater. Sci. 6 (1975) 1581; (c) B.D. Dunn, ESA Sci. Tech. Rev. 2 (1976) 1. [5] A.P. Levitt, Whisker Technology, Wiley-Interscience, NY, 1970. [6] (a) M.W. Barsoum, E.N. Hoffman, R.D. Doherty, S. Gupta, A. Zavaliangos, Phys. Rev. Lett. 93 (2004) 206104; (b) J. LeBret, M. Norton, J. Mater. Res. 18 (2003) 585; (c) G.T.T. Sheng, C.F. Hu, W.J. Choi, K.N. Tu, Y.Y. Bong, L. Nguyen, J. Appl. Phys. 92 (2002) 64. [7] K.S. Kim, C.H. Yu, J.M. Yang, Thin Solid Films 504 (2006) 350; K.S. Kim, C.H. Yu, J.M. Yang, Microelectron. Reliab. 46 (2006) 1080. [8] G.T. Galyon, R. Gedney, Circuit Assembly 15 (8) (2004) 26.
[9] G.T. Galyon, A History of Tin Whisker Theory: 1946 to 2004, SMTAI International Conference, Chicago, IL, September 26–30, 2004. [10] L. Zakraysek, Plat. Surf. Finish. 64 (1977) 38. [11] V.K. Glazunova, N.T. Kudryavtsev, J. Appl. Chem. USRR (translated form Zhurnal Prikladnoi Khimii) 36 (1963) 543. [12] T.A. Woodrow, E.A. Ledbury, Evaluation of Conformal Coatings as a Tin Whisker Mitigation Strategy, IPC/JEDEC 8th International Conference on Pb-Free Electronic Components and Assemblies, San Jose, CA, April 18–20, 2005. [13] H. Leidecker, Whiskers of Sn–Pb on Reflowed Die Attach Solder Used in the Manufacture of Laser Diode Arrays, December 2003 (see http://nepp.nasa.gov/whisker/reference/reference.html). [14] H.P. Kehrer, H.G. Kadereit, Appl. Phys. Lett. 16 (1970) 411. [15] (a) J. Downs, Met. Finish. 92 (8) (1994) 23; (b) J. Brusse, M. Sampson, IEEE IT Prof. Mag. 6 (2004) 43. [16] C.C. Hu, C.Y. Lin, T.C. Wen, Mater. Chem. Phys. 44 (1996) 233. [17] C.C. Hu, C.K. Wang, G.L. Lee, Electrochim. Acta 51 (2006) 3692. [18] C.C. Hu, J.L. Lee, J. Chin. Inst. Chem. Eng. 37 (2006) 589. [19] K. Chen, G.D. Wilcox, Phys. Rev. Lett. 94 (2005) 066104-1. [20] M. Hansen, K. Anderko, Constitution of Binary Alloys, 2nd ed., McGraw-Hill, NY, 1958. [21] Y. Zhang, J.A. Abys, in: M. Schlesinger, M. Paunovic (Eds.), Modern Electroplating, 4th ed., John Wiley & Sons, NY, 2000.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Anomalous growth of whisker-like bismuth–tin extrusions from tin-enriched tin–Bi deposits Chi-Chang Hu a,∗ , Yi-Da Tsai b , Chi-Cheng Lin b , Gen-Lan Lee b , Sinn-Wen Chen a , Tai-Chou Lee b , Ten-Chin Wen c a Department of Chemical Engineering, National Tsing Hua University, 101, Section 2, Kuang-Fu Road, Hsin-Chu 30013, Taiwan b Department of Chemical Engineering, National Chung Cheng University, Chia-Yi 621, Taiwan c Department of Chemical Engineering, National Cheng Kung University, Tainan 701, Taiwan
a r t i c l e
i n f o
Article history: Received 22 April 2008 Accepted 29 April 2008 Available online 10 June 2008 Keywords: Metal extrusion Sn–xBi deposit Post-plating annealing Lead-free solder Electroplating
a b s t r a c t This article shows the first finding that the anomalous growth of Bi–Sn extrusions from tin-enriched alloys (Sn–xBi with x between 20 and 10 wt.%) can be induced by post-plating annealing in N2 between 145 and 260 ◦ C for 10 min although metal whiskers were commonly formed on the surface of pure metals or alloys of the enriched component. From SEM observations, very similar to Sn whiskers, Bi–Sn extrusions vary in size, shape, length, and diameter with changing the annealing temperature, which are highly important in regarding the potential for failure of electronic products. Annealing resulting in thermal expansion of Sn grains is believed to squeeze the Bi–Sn alloys with relatively low melting points to form whisker-like extrusions although the exact mechanism is unclear. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Metal whiskers are hair-like crystal structures commonly grown from the surfaces finished with pure metals or alloys of the enriched component [1–3]. The dimensions of whiskers are usually of some micrometers in the diameter but up to a length of hundreds of micrometers [1–3]. Due to the metallic and crystalline nature, metal whiskers have been revealed to cause shorts resulting in the failure of electronic systems. Actually, the formation of Sn whiskers from the high Sn content surface finish was found to cause damages of electronic systems in 1940s [2]. After this finding, many studies, reviews, and books tried to clarify the formation mechanism of Sn, Zn or other metal whiskers on the reliability problems of electronic systems [1,2,4–6]. In addition, the formation of metal whiskers is only found on a limited number of metals such as Sn, Zn, Cd, Bi, etc. [1–3]. Among them, Sn whiskers have been widely studied since tin was widely used as surface finishing or binding materials for Sn-plated terminals of all types of electronic components. Recently, renewed interest in the formation of Sn whiskers is due to the wide application of lead-free solder in electronic manufacturing [6]. Moreover, the importance of other metallic whiskers
∗ Corresponding author. Tel.: +886 3 5736027; fax: +886 3 5715408. E-mail address: [email protected] (C.-C. Hu). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.04.094
on the failures of high-reliability electronic systems significantly increases because of the worldwide transition to lead-free electronics. Unfortunately, the formation mechanism(s) of metal whiskers is still unclear and there is no reliable theory for predicting the dependence of whisker density or length on time or thermal history although several mechanisms of whisker growth have been proposed [4–6]. Furthermore, Sn whiskers have been found to be effectively depressed by adding suppressants such as Pb, Bi, Ag, etc. [7–9]. Since whisker formation is dependent on stress in the electroplated deposit, the influences of the electroplating variables such as current density, temperature, under bump metallization (UBM), and bath components [4,10–12] on the formation of Sn whiskers have been investigated. Moreover, the effects of metal/alloy thickness, post-plating annealing, and alloying agents [4,6c,7–9,12–14] on the formation of these whiskers have also been studied. For other metal whiskers with the exception of Zn [4b,4c,15], on the other hand, systematic studies on the formation mechanism are very limited [1,2]. Based on the above viewpoints, the continuous miniaturization of electronics has resulted in spacing between interconnects of components and a conductive whisker or extrusion will easily bridge the gap and increase the possibility of failures. The difference between a whisker and an extrusion is mainly attributed to the single-crystalline nature of the former metal while both metallic
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Table 1 Composition and variables for the electroplating of Sn–xBi deposits Compounds/variables
Sn–13Bi
Sn–19Bi
SnCl4 ·nH2 O Bi(NO3 )3 ·5H2 O Citric acid NH4 OH pHa Temperature Current density
0.18 M 0.02 M 0.4 M ≥0.2 M 3.0 25 ◦ C 40 mA cm−2
0.18 M 0.02 M 0.4 M ≥0.2 M 5.0 25 ◦ C 30 mA cm−2
a
pH was adjusted by 1 M HCl or 2 M NH4 OH.
materials show the same impact on the reliability of electronics. In this work, we demonstrate the first finding that the growth of Bi–Sn whisker-like extrusions on the Sn-enriched Sn–xBi deposits (with x between 10 and 20 wt.%) can be induced by post-plating annealing in N2 between 145 and 260 ◦ C although aside from Pb, Bi is the most effective element in mitigating the formation of tin whiskers. Accordingly, a probable rout for the growth of Bi–Sn extrusions is proposed in this work even though the exact mechanism(s) is unclear. 2. Experimental details The lead-free tin–bismuth deposits with variable compositions were electroplated from a simple CN− -free solution onto copper (99.5%, 1 cm × 2 cm) plates deposited with a nickel film (ca. 2 m) used as the UBM (denoted as Cu/Ni). The pretreatment procedures of Cu/Ni substrates completely followed our previous work [16–18]. These Cu/Ni substrates, rinsed with pure water, were vertically placed in a 500-ml jacket cell surrounded with an anode of platinum-coated stainless steel mesh and electroplated with Sn–xBi deposits, about 2 m, at various plating conditions with a constant passed charge density (72.0 C cm−2 ). After the Sn–xBi deposition, these electrodes were rinsed with pure water, vibrated in an ultrasonic bath for 5 min, and dried in a vacuum oven at room temperature.
The composition control of Sn–xBi deposits completely followed the process described in our previous work [18]. Typical compositions of the lead-free Sn–xBi plating bath and electroplating conditions used in this work are shown in Table 1, which mainly consisted of SnCl4 ·nH2 O, Bi(NO3 )3 ·5H2 O, and citric acid (0.2 M, complex agent) with pH being adjusted with 3 M HCl and 3 M NH4 OH. All solutions were prepared with pure water produced by a reagent water system (Milli-Q SP, Japan) at 18 M cm and all reagents were Merck, GR. Solution temperature was maintained at the specified temperatures with an accuracy of 0.1 ◦ C by means of a water thermostat (Haake DC3 and K20). All whisker-like extrusions were examined (ex situ) by means of a field-emission scanning electron microscope (FE-SEM, Hitachi S-4700) after the deposits have been annealed at the specified temperature for 10 min. There was no extrusion when the annealing temperature was ≤139 ◦ C, meanwhile no extrusion was found when freshly prepared deposits were kept in air at room temperature for several weeks. The composition of extrusions was measured using an energy-dispersive X-ray (EDX) spectroscope coupled with the above FE-SEM. The mean error of this EDX analysis is ca. ±1.5 wt.%. X-ray diffraction patterns were obtained from an X-ray diffractometer (Rigaku MiniFlex) using a Cu target.
3. Results and discussion Because of the miniscule dimensions, Bi–Sn extrusions cannot be visualized without magnification (typically through the scanning electron microscopic, SEM, observations). The SEM images shown in Fig. 1 reveal the formation of metal extrusions on the Sn–xBi (x = 10–20 in wt.%) finished surfaces with annealing at 260 ◦ C for 10 min although minor amount of Bi dendrites are sometimes found on the surface of the as-plated and annealed deposits. The grain size of as-plated Sn–Bi deposits is uniform, which is gradually decreased with increasing the tin content from a comparison of Fig. 1A and B (in average ∼0.8 and 2.2 m for Sn–19Bi and Sn–13Bi deposits, respectively). After annealing at 260 ◦ C for 10 min, whisker-like Bi–Sn extrusions in various dimensions (typically 50–70 m in length and 1–3 m in width) are clearly found on the surface of the annealed deposits. Note that most metal whiskers were found to grow from pure-metal thin films or the alloys of
Fig. 1. SEM morphologies of (A and B) as-plated and (C and D) annealed Sn–xBi deposits (260 ◦ C) with x = 19 (A and C), 13 (B and D) (in wt.%).
C.-C. Hu et al. / Journal of Alloys and Compounds 472 (2009) 121–126
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Fig. 2. SEM morphologies of annealed Sn–19Bi deposits; (A, 260 ◦ C) needle-like; (B, 240 ◦ C) nodule-like; (C, 210 ◦ C) hook-like; (D, 180 ◦ C) plank-like; (E, 160 ◦ C) rod-shaped; and (F, 145 ◦ C) irregular-shaped extrusions.
enriched components [6c] with the exception of the eutectic Sn–Pb alloy found recently [13]. However, in this work, the growth of whisker-like Bi–Sn extrusions is effectively induced on the tinenriched deposits through a post-plating annealing process in N2 between 145 and 260 ◦ C for 10 min even though the Bi content is only as low as 20–10 wt.%. Figs. 1–3 illustrate the growth of Bi–Sn extrusions of various sizes, shapes, and dimensions on the Sn–xBi deposits (with x varying from 20 to 10 wt.%), which will be an important issue in employing the lead-free Sn–xBi surface finish. For example, typical needle-like extrusions or filaments show lengths of 50–70 m and diameters of approximately 0.8–2.5 m (see Figs. 1C, D, 2A and 3B). The needle-like extrusions seem to become shorter with decreasing the annealing temperature (see Fig. 2E). In addition, nodules, planks, celeries, hooks, or irregular eruptions in various sizes are more easily found at relatively lower annealing temperatures. Moreover, roll-like, grass-root-like, irregularly shaped, or toothpaste-like extrusions of more than 20 m in length are also observed on certain parts (see Figs. 2 and 3). The lowest tem-
perature of post-plating annealing inducing the growth of these whisker-like Bi–Sn extrusions has been confirmed to be 145 ◦ C. As mentioned previously, the actual reasons responsible for the growth of most kinds of metal whiskers/extrusions are unclear. The similarities in the shape, size, and diameter between Sn whiskers and Bi–Sn extrusions, however, may imply a common formation mechanism, i.e., the stress-inducing mechanism. For eutectic Sn–Cu, precipitates of Cu6 Sn5 in the grain boundaries of the finishes have been attributed to the driving force for the rapid growth of Sn whiskers [6c]. For Sn–Mn deposits, the formation of Sn whiskers and the phase transformation of MnSn2 occur simultaneously [19]. On the other hand, the growth of Bi–Sn extrusions has been confirmed to be visible only when the Bi content is between 10 and 20 wt.%, which is different from the formation of Sn whiskers from the pure Sn or Sn-enriched finish. Note that the formation of Sn whiskers has been found to depress by introducing minor Bi into the Sn matrix. In addition, the lowest annealing temperature (145 ◦ C) inducing the growth of Bi–Sn extrusions is much lower than the temperatures of liquidus of Sn–xBi with
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Fig. 3. SEM morphologies of annealed Sn–13Bi deposits; (A, 260 ◦ C) toothpaste-like; (B, 240 ◦ C) needle-like; (C, 240 ◦ C) roll-like; (D, 210 ◦ C) stick bank-like; (E, 160 ◦ C) nodulelike; (F, 160 ◦ C) irregular-shaped extrusions.
x between 10 and 20. Thus, the presence of Bi-enriched extrusions with minor amount of Sn (see Fig. 4B–D) as the post-plating annealing temperatures ≥145 ◦ C indicates that Bi is not completely dissolved into the Sn matrix although the maximum solubility of Bi in Sn is 21 wt.% from the Sn–Bi phase diagram [20]. This statement is supported by the weight ratio of Bi/Sn corresponding to Fig. 4B–D equal to 0.296, 0.301, and 0.234, respectively. Furthermore, phase separation of Sn and Bi without the presence of other phases is clearly found on both as-plated and annealed deposits (see Fig. 5). This phenomenon suggests that the growth of Bi–Sn extrusions seems to be not due to the formation of intermetallic compounds (as found in the Sn–Cu and Sn–Mn systems) because the 2 m nickel UBM film is used to reduce the interfacial reaction rate between Sn and Cu. On the other hand, the stress state in these 2 m Sn–xBi deposits might be influenced by the interfacial expansion (or minor interfacial reaction) between the Sn–xBi deposit and the Cu/Ni substrate, inducing the growth of Bi–Sn extrusions.
Based on the above results and discussion, the growth mechanism of whisker-like Bi–Sn extrusions is presumably similar to that of Sn–Pb whisker formation on the annealed eutectic Sn–Pb solder die attach material [13]. Since there is no biased potential or current applied on these deposits during the annealing process, the growth of Bi–Sn extrusions is not attributable to metal electromigration under high current densities [13]. On the other hand, thermal expansion of Sn grains during annealing will squeeze the Bi–Sn alloys with relatively low melting points (i.e., partial melting) to form extrusions (see Fig. 6). This phenomenon is likely accelerated by the presence of oxygen, probably due to the codeposition of OH− with Sn (e.g., Sn(OH)+ ) in weakly acidic media [21]. Moreover, an oxygen-containing Sn–Bi sub-monolayer is favorably formed on the surface of Sn–xBi deposits due to an increase in pH at the vicinity of the deposit–electrolyte interface when hydrogen is extensively evolved in the end of Sn–xBi deposition. The less expansion of this hydroxyl Sn–Bi sub-monolayer will drive the growth of metal extrusions due to the presence of inner stress/pressure
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Fig. 4. (A) A SEM photograph and the EDS spectra measured at the (B) top; (C) middle; and (D) bottom position of a whisker-like Bi extrusion on a Sn–19Bi deposit with annealing at 210 ◦ C for 10 min.
within the deposits upon annealing although the exact mechanism for the formation of whisker-like Bi–Sn extrusions is still not clear. 4. Conclusions The anomalous growth of whisker-like, Bi-enriched extrusions from Sn–xBi deposits (with x between 20 and 10 wt.%) induced by post-plating annealing is clearly demonstrated in this work. Bismuth–tin extrusions in different sizes, shapes, and dimensions are clearly found on these Sn–xBi deposits with annealing between 145 and 260 ◦ C. Upon annealing, thermal expansion of Sn grains may squeeze the Bi–Sn alloys with relatively low melting points to form extrusions, which should be similar to that of Sn–Pb whisker formation on the reflowed eutectic Sn–Pb solder although the exact mechanism is still unclear. Finally, product designers, part engineers, failure analysts, and quality assurance specialists should be aware of the possibility of formation of Bi–Sn extrusions (but not Sn whiskers) on Sn–xBi deposits with post-plating annealing in N2 between 145 and 260 ◦ C for 10 min only. Acknowledgment
Fig. 5. XRD patterns of an (A) as-plated Sn–19Bi deposit with annealing at (B) 145, (C) 210, and (D) 260 ◦ C in N2 for 10 min; where (o) Sn, (z) Bi, () Cu, and (x) Ni.
The financial support of this work, by the National Science Council of the Republic of China under contract no. NSC 96-2628E-007-146-MY2, is gratefully acknowledged.
Fig. 6. The formation scheme of Bi–Sn extrusions on Sn–xBi deposits upon annealing.
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