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Effect of composition and cooling rate on microstructure and tensile properties of Sn–Zn–Bi alloys
Journal of Alloys and Compounds 352 (2003) 237–245
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Effect of composition and cooling rate on microstructure and tensile properties of Sn–Zn–Bi alloys a, a a b Young-Sun Kim *, Keun-Soo Kim , Chi-Won Hwang , Katsuaki Suganuma a
Department of Adaptive Machine Systems, Graduate School of Engineering, Osaka University, 2 -1 Yamadaoka, Suita, Osaka 565 -0871, Japan b Institute of Scientific and Industrial Research, Osaka University, 8 -1 Mihogaoka, Ibaraki, Osaka 567 -0047, Japan Received 23 August 2002; accepted 12 September 2002
Abstract The effects of the addition of Bi to Sn–Zn near eutectic alloy and cooling rate on the thermal and mechanical properties were investigated. With the addition of up to 8 mass% Bi to Sn–8 mass% Zn, the melting temperature decreases from 198.4 to 186.1 8C, but the pasty range increases from 2.36 to 6.72 8C. Sn–9 mass% Zn and Sn–8 mass% Zn–(1,2,3,6,8) mass% Bi alloys have far lower undercooling than the other Sn-based lead-free alloys, and the temperature decreases with Bi content. It was also confirmed that the difference of cooling rates significantly affects the microstructure and mechanical properties. In the case of fast cooling, alloys have fine Sn, Zn, and Bi phases with smooth surfaces. But in the case of slow cooling, Sn–Zn alloys exhibit coarse dendrite structure and large needle or rod-like Zn-rich precipitates with rough surface and voids, and extensive segregation of massive Bi toward a surface. 2002 Elsevier Science B.V. All rights reserved. Keywords: Sn–Zn; Bi segregation; Lead-free; Solder; Eutectic alloy; Cooling rate
1. Introduction Many countries are going to ban the use of lead and its compounds, which are known to be toxic to the human body and to cause serious environmental problems [1,2]. Thus, the development of lead-free solders has emerged as one of the key issues in the electronics packaging industries. For developing some new solder alloys, one tends to think of various properties such as liquidus temperature, pasty range, wettability, microstructure, mechanical properties of solder as well as the reliability of soldered joints [3]. Among them, the first step in finding a suitable alloy candidate is to consider a low melting temperature alloy that can replace Sn–37Pb alloy, which has been widely used as interconnection material in electronic packaging due to its low melting temperature [2,4,5]. The pasty range is one of the most important characteristics of solder. The alloy begins to melt at a temperature called the solidus, but it is not completely liquid until a higher temperature called the liquidus. The range between the solidus and the *Corresponding author. Tel.: 181-6-6879-8521; fax: 181-6-68798522. E-mail address: [email protected] (Y.S. Kim).
liquidus is referred to as the melting temperature range, pasty range or mushy zone [3,5]. At temperatures between solid and liquid phases, it contains different proportions of solid and liquid phase and leads to segregation during solidification of alloy and lift-off developing during soldering [3,5]. In order to deal with this problem, it is much more preferable to use an eutectic solder or a solder with a narrow pasty range [3]. It is also important to correlate the microstructure of the alloy with its mechanical properties in order to optimize the service performance of the interconnection found by the solders [7]. Sn–Zn eutectic alloy has been considered as one of the lead-free solder alloys that can replace Sn–37 mass% Pb eutectic solder without increasing soldering temperature. Sn–Zn binary alloy has excellent mechanical properties but is susceptible to oxidation and corrosion [3,6]. Therefore, alloying Bi to Sn–Zn near eutectic alloy could improve the soldering property in electronic packaging by lowering melting temperature [7,8]. But alloys with a high Bi concentration need to be controlled because of the brittle nature of Bi and the strong tendency for segregation [9–15]. This work aimed to investigate the effect of Bi addition on melting behavior, phase formation and mechanical
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0925-8388(02)01168-4
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Fig. 1. Schematic illustration of tensile test for specimens.
properties of Sn–Zn alloys, and also the effects of cooling rate on precipitation behavior of Bi and mechanical properties.
2. Experimental procedure Alloys of Sn–9 mass% Zn eutectic and of Sn–8 mass% Zn–(1,2,3,6,8 mass%) Bi were used in the present work. Hereafter, composition unit ‘mass%’ is omitted. The raw material alloys were provided by Senju Metals. The alloy ingots were remelted at 300 8C and cast into steel mold. They were cooled at two different rates, 12 8C s 21 (designated as AC, air cooled) and 18310 23 8C s 21 (FC, furnace
Fig. 3. Transformation point as a function of Bi content (a) on heating and (b) on cooling.
cooled). The AC is equivalent to that adopted in the practical reflow process in industries. The ingots were sliced at 1 mm and were polished with 0.3 mm Al 2 O 3 powder. The tensile specimens had a gauge length of 25 mm, width of 8 mm, and thickness of 1 mm as shown in Fig. 1. The tensile tests were carried out at room temperature at
Fig. 2. Differential scanning calorimetry (DSC) curves for Sn–9Zn–0Bi and Sn–8Zn–(1,3,6)Bi alloys (a) on heating and (b) on cooling.
Fig. 4. Undercooling temperature of solder alloys.
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Fig. 5. Optical microscopy of tensile specimens prepared by FC: (a) Sn–9Zn, (b) Sn–8Zn–1Bi, and (c) Sn–8Zn–6Bi.
strain rates from 3.33310 24 to 3.33310 22 s 21 for the AC samples and only 3.33310 24 s 21 for the FC samples. The thermal reaction of the alloys was evaluated by differential scanning calorimetry (DSC). Specimens for DSC were prepared and about 20 mg cut from the ingot. Heating and cooling were carried out at a constant heating and cooling rate of 0.5 8C min 21 from 25 to 300 8C in Ar
gas flow with a Shimadzu DSC-50 thermal analysis machine. Microstructure was observed by OM (optical microscopy), SEM (scanning electron microscopy, Hitachi, S-
Fig. 6. Phase diagrams of (a) Sn–Zn binary [16] and (b) Sn–8Zn–(0– 8)Bi ternary conducted by Thermo-Calc.
Fig. 7. SEM of Sn–Zn–Bi alloys prepared by AC: (a) Sn–9Zn, (b) Sn–8Zn–3Bi, and (c) Sn–8Zn–6Bi.
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Fig. 8. SEM of Sn–8Zn–6Bi alloy (enlarged image of Fig. 7c).
was observed on heating for each alloy and the large exothermic peak was observed with proceeding small one on cooling. The onset point of endothermic peak, indicated as ‘A’, is the starting temperature of a reaction and the offset point, indicated as ‘B’, is the end temperature. On heating, the onset point is called the solidus temperature of the alloys and the offset point is called the liquidus. The plots of the transformation points both on heating and on cooling as a function of Bi content are shown in Fig. 3. The transformation temperatures of the alloys decrease with increasing of Bi content. The onset point and the offset point decreases with the increase of Bi content
2150). The elemental distribution was evaluated by using EPMA (electron probe microanalysis, Jeol JXA-8800R). XRD (X-ray diffraction analysis, Rigaku) was also evaluated. The phase diagrams were calculated by Thermo-Calc with the Adamis7 data base.
3. Result and discussion
3.1. Thermal properties as Pb-free solder Fig. 2 shows the typical DSC profiles of Sn–9Zn, Sn–8Zn–(1,2,3,6,8)Bi alloys. The large endothermic peak
Fig. 9. XRD of Sn–Zn–Bi alloy (unmarked: b-Sn): (a) Sn–9Zn, (b) Sn–8Zn–1Bi, (c) Sn–8Zn–3Bi, (d) Sn–8Zn–6Bi.
Fig. 10. Influence of strain rate and of addition amount of Bi on UTS, 0.2% proof strength, and elongation.
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from 198.4 into 186.1 8C and from 200.8 into 193.4 8C, respectively, on heating. Therefore, Sn–8Zn–8Bi alloy melts at the temperature of only about 3 8C greater than that of 37Pb–63Sn solder, i.e. 183 8C. The pasty ranges of alloys slightly increase from 2.36 to 6.72 8C. It implies that the Sn–9Zn and alloying Bi to Sn–8Zn alloys become hyper- or hypo-eutectic alloy near Sn–Zn binary eutectic alloy. The undercooling is the deviation between the heating and cooling curves, which is usually experienced in the low melting temperature alloys. Undercooling of Sn–Zn system alloys are far lower than those of the other Sn base lead-free solder alloys as shown in Fig. 4. With increasing Bi content, undercooling decreases from 4.06 to 1.19 8C.
3.2. Solidification conduct of alloys The solidification properties of Sn–Zn–Bi alloys can be observed through the cooling curve with macrostructure and phase diagram. Fig. 5 shows macro-photographs of alloys to observe primary phase, which were prepared by FC because their structures are similar to those of equilibrium state. Sn–9Zn alloy of Fig. 5a displayed large needle-like structure in the eutectic matrix. Sn–8Zn–1Bi alloy of Fig. 5b showed a display of coarse dendrite structure in the eutectic matrix. Fig. 5c showed mixed macro-structure of Fig. 5a and b in the eutectic matrix as in the cases of adding more than 3Bi to Sn–8Zn alloy. And, in order to understand the formation of primary phase, thermodynamic simulation was conducted by Thermo-Calc (Fig. 6b). Sn–9Zn alloy has as much as 0.2% more Zn than
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eutectic contents as shown in binary phase diagram of Fig. 6a [17]; the order of its cooling reaction is L→L1primary Zn→primary Zn1eutectic (Sn1Zn). The microstructure of the alloy has large and long Zn phases that are displayed with the eutectic matrix, as shown in Fig. 5a. Also, a small peak before the large peak of variation of caloric on the cooling curve of DSC (Fig. 2b) is the peak of L→L1 primary Zn reaction, that is to say, the large Zn phase (Fig. 5a) is formed as the primary phase and the primary Zn first appears on solidification. Fig. 6b shows that in the case of Sn–8Zn–1Bi and Sn–8Zn–2Bi alloys, the reaction order of cooling is L→L1primary Sn→primary Sn1eutectic (Sn1Zn) and in the case of Sn–8Zn–3Bi, Sn–8Zn–6Bi, and Sn–8Zn–8Bi alloys, the reaction orders of cooling are L→L1primary Zn→L1secondary Sn1primary Zn→primary Zn1 secondary Sn1eutectic (Sn1Zn). Therefore, in the case of Sn–8Zn–1Bi and Sn–8Zn–2Bi the alloys have primary Sn phase (Fig. 5b). In the case of adding more than 3Bi to Sn–8Zn, the first displayed phase is the Zn phase (Fig. 5c), like the Sn–9Zn reaction, and the next large phase is the secondary Sn phase (Fig. 5c). The very small signal obtained at the liquidus temperature corresponding to the very small change in the slope of the enthalpy vs. temperature curve is evident. This is consistent with the small or undetectable signal in the experiments [16]. There is possible solubility of the Zn atom in the Sn lattice by 0.6 at% but seldom that of the Sn atom in the Zn lattice, as shown in Fig. 6a. Therefore, in the case that the Zn phase is formed as the primary phase (Fig. 5a and c), DSC profiles (0Bi, 3Bi and 6Bi of Fig. 2b) on cooling clearly appear as a small peak
Fig. 11. SEM of the fracture surface of alloys prepared by AC with varying strain rate and Bi contents.
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before the large peak of variation of caloric, than in the case of primary Sn (1Bi of Figs. 2b and 5b).
3.3. Microstructure of alloys Fig. 7 is the SEM image of Sn–Zn–Bi alloys and Fig. 8 is the magnification photograph of Sn–8Zn–6Bi alloy (Fig. 7c). Sn–9Zn alloy shows simple structure that Zn-rich phase is homogenously dispersed in the b-Sn matrix.
Fig. 12. Optical microscopy of Sn–Zn–Bi alloys with varying cooling rate and Bi contents.
Sn–8Zn–3Bi alloy has much more of the Zn-rich phase in the b-Sn matrix than that of Sn–9Zn alloy and there is Bi phase not hardly extraction but solution in the b-Sn matrix. And in the case of Sn–8Zn–6Bi alloy, much Zn and Bi phases are precipitated in the b-Sn matrix. The matrix phase with the gray color is b-Sn, the dark needle like and platelet phase is the Zn-rich phase and the white platelet is the rhombohedral Bi phase. Fig. 9 shows the profiles of XRD patterns for the Sn–9Zn and Sn–8Zn–(1–8)Bi alloys. The alloys of Sn– 9Zn and Sn–8Zn–(1,2)Bi consist of two phases; a body centered tetragonal b-Sn matrix phase and a secondary phase of hexagonal Zn, respectively. In the case of adding more than 3 mass% Bi to Sn–8Zn alloy, the peak of the rhombohedral Bi phase begins to appear, because the Sn–Bi phase diagram shows that solubility of Bi in Sn is only about 2 mass% at room temperature [17].
Fig. 13. Influence of cooling rate and of Bi contents on UTS, 0.2% proof strength, and elongation.
Y.S. Kim et al. / Journal of Alloys and Compounds 352 (2003) 237–245
3.4. Mechanical properties of alloys Fig. 10 shows the results of tensile test for Sn–Zn system alloys as a function of strain rate and of adding Bi to Sn–8Zn alloy at room temperature. With increasing Bi content, the ultimate tensile strength (UTS) and the 0.2% proof stress of the alloys increased and the elongation of the alloys decreased. The values of UTS and 0.2% proof stress of the alloys appeared to increase while elongation of the alloys decreased with the strain rate. At the fracture surfaces, as shown in Fig. 11, was observed the effect of the Bi addition to Sn–Zn alloy at a strain rate of 3.333 10 24 , 3.33310 23 , and 3.33310 22 s 21 , respectively. A ductile dimple pattern can be seen on the fracture surface of Sn–9Zn alloys at all strain rates. The dimple size decreases with increasing strain rate for Sn–9Zn alloy.
Fig. 14. SEM of fracture surface of alloys with varying cooling rate and Bi contents.
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With the increase of Bi content, alloys exhibited cleavage fracture with little or no ductility and this was indicative of a lower toughness. The alloys showed brittle fracture up to the strain rate. The microstructures of the alloys are affected by the difference of cooling rate as shown in Fig. 12. In the case of fast cooling, Sn–9Zn alloy exhibited a much finer and more uniform microstructure within which was dispersed a rod or needle-like Zn phase in the b-Sn matrix. With increasing Bi content, much Zn and Bi precipitations are extracted from the b-Sn matrix. On the other hand, Sn–Zn alloys manufactured by slow cooling rate exhibit coarse dendrite structure and large and long needle or rod-like Zn-rich precipitates. And Bi phases are hardly precipitated in the matrix in spite of alloying a lot of Bi to Sn–8Zn alloy. There is extensive segregation of massive Bi along the growth direction of dendrite into the surface [11,12,14,15]. Fig. 13 are results of tensile test for Sn–Zn–Bi alloys as a function of cast cooling rate and of Bi content to Sn–8Zn at room temperature. In the case of AC, the UTS, the 0.2% proof stress and the elongation of the alloys are better than those of FC alloys. Because in the case of AC, alloys have the structure with which are dispersed fine precipitations in the matrix, but coarse dendrite structure and long Zn precipitations in the case of FC as shown in Fig. 12.
Fig. 15. Schematic of fracture pattern: (a) fracture types and (b) stress– strain curves.
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Fig. 14 shows the fracture morphology of alloys. In the case of AC, the fracture surface of the Sn–9Zn alloy shows a huge dimple as the typical ductility–fracture. With the increase of Bi content, the dimple size becomes smaller and the fracture type changes into the brittle-fracture pattern. In the case of FC, the fracture surface of Sn–9Zn alloy has a small dimple pattern. With increasing Bi content, fracture type changes into the ductile fracture pattern. Thus, the fractures of Sn–8Zn–6Bi and Sn–8Zn– 8Bi alloys show the typical brittle-fracture pattern and fractures occur along the coarse dendrite stem or long Zn precipitations. Fig. 15 illustrate fracture patterns of Sn–Zn–Bi alloys; there are basically three types: • Typical ductility (type I): Sn–9Zn alloys fractured like the fracture behavior of common metals as shown in Fig. 13b. • Complex fracture pattern (type II): alloys by AC were toughened by dispersing fine Zn and Bi precipitations in the matrix. The fracture surfaces of the alloys show the partially dimpled and the partially cleavage fracture. • Faceted fracture pattern (type III): in this case, the alloys were prepared by FC; they have coarse dendrite, very large Zn precipitations and segregation of massive Bi particles into the surface. This pattern shows a straight faceted fracture by following the long Zn precipitation or coarse dendrite stem.
3.5. Defects of FC alloys In the case of FC, the solder alloy has several defects as
shown in Fig. 16; coarse dendrite structure and Zn precipitation, rough surface, voids and extensive macrosegregation of massive Bi particles to surface. Therefore, Bi-bearing Sn–Zn alloys are important to control the cooling rate.
4. Conclusion An experimental work was conducted in order to investigate the effects of the addition of Bi to Sn–Zn near-eutectic alloy on the thermal and mechanical properties for the development of lead-free solder. Also the microstructures and mechanical properties of alloys manufactured under different cooling rates were compared. The results are summarized as follows: • With the increase of Bi addition content, the melting point decreases from 198.4 to 186.1 8C, but the pasty range increases from 2.36 to 6.72 8C. Sn–9Zn and Sn–8Zn–(1,2,3,6,8) Bi solder alloys have far lower undercooling than those of other Sn-based alloys and the undercooling decreases with increasing Bi content. • It was also confirmed that the difference in cooling rates of cast alloy significantly affected the microstructure and mechanical properties. In the case of AC, alloys exhibited the fine microstructure within which were dispersed Zn and Bi precipitations in the b-Sn matrix. The alloys have good mechanical properties. • On the other hand, Sn–Zn alloys manufactured by FC have coarse dendrite structure and long needle- or rod-like Zn-rich phase with rough surface and voids, and extensive segregation of massive Bi into the surface. In this case, the alloys have bad mechanical properties.
Acknowledgements The present work was supported by a Grant-in-Aid for Scientific Research (A) from The Japan Ministry of Education, Culture, Sports, Science and Technology in 2001. The authors acknowledge the Senju Metal Industry Co. for providing the solder alloys.
References
Fig. 16. Defects observed in the tensile specimen of FC.
[1] M. McCormack, S. Jin, H.S. Chen, J. Electron. Mater. 23 (1994) 687. [2] K. Suganuma, Curr. Opin. Solid State Mater. Sci. 5 (2001) 55–64. [3] NCMS, Lead-free Solder Project Final Report, NCMS Report 0401 RE 96, National Center for Manufacturing Sciences, Michigan, 1997. [4] H.-W. Miao, J.G. Duh, Mater. Chem. Phys. 71 (2001) 255–271. [5] M. Rettenmayr, P. Lambracht, B. Kempf, C. Tschudin, J. Electron. Mater. 31 (2002) 278–285.
Y.S. Kim et al. / Journal of Alloys and Compounds 352 (2003) 237–245 [6] P.J. Wood, K.L. Nimmo, J. Electron. Mater. 23 (1994) 709–713. [7] M. McCormack, S. Jin, J. Electron. Mater. 23 (1994) 715–720. [8] S.W. Yoon, J.R. Soh, H.M. Lee, B.J. Lee, Acta Mater. 45 (1997) 951–960. [9] H. Ohtani, K. Ishida, J. Electron. Mater. 23 (1994) 747–755. [10] Y. Cui, X.J. Liu, I. Ohnuma, R. Kainuma, H. Ohtani, K. Ishida, J. Alloys Comp. 320 (2001) 234–241. [11] K. Suganuma, S.H. Hur, K.S. Kim, H. Nakase, Y. Nakamura, Mater. Trans. JIM 42 (2001) 286–291. [12] K. Suganuma, Scripta Mater. 38 (1998) 1333.
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[13] K. Suganuma, Technology of Pb-free Soldering, Vol. 159, Kogyo Chosakai Publishing, 2000, pp. 127–145. [14] C.M. Chuang, T.S. Lui, L.H. Chen, J. Mater. Sci. 37 (2002) 191–195. [15] S. Sengupta, H. Soda, A. McLean, J. Mater. Sci. 37 (2002) 1747– 1758. [16] K.W. Moon, W.J. Boettinger, U.R. Kattner, F.S. Biancaniello, C.A. Handwerker, J. Electron. Mater. 29 (10) (2000) 1122–1136. [17] M. Hansen, Constitution of Binary Alloys, McGraw-Hill, New York, 1958.
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Effect of composition and cooling rate on microstructure and tensile properties of Sn–Zn–Bi alloys a, a a b Young-Sun Kim *, Keun-Soo Kim , Chi-Won Hwang , Katsuaki Suganuma a
Department of Adaptive Machine Systems, Graduate School of Engineering, Osaka University, 2 -1 Yamadaoka, Suita, Osaka 565 -0871, Japan b Institute of Scientific and Industrial Research, Osaka University, 8 -1 Mihogaoka, Ibaraki, Osaka 567 -0047, Japan Received 23 August 2002; accepted 12 September 2002
Abstract The effects of the addition of Bi to Sn–Zn near eutectic alloy and cooling rate on the thermal and mechanical properties were investigated. With the addition of up to 8 mass% Bi to Sn–8 mass% Zn, the melting temperature decreases from 198.4 to 186.1 8C, but the pasty range increases from 2.36 to 6.72 8C. Sn–9 mass% Zn and Sn–8 mass% Zn–(1,2,3,6,8) mass% Bi alloys have far lower undercooling than the other Sn-based lead-free alloys, and the temperature decreases with Bi content. It was also confirmed that the difference of cooling rates significantly affects the microstructure and mechanical properties. In the case of fast cooling, alloys have fine Sn, Zn, and Bi phases with smooth surfaces. But in the case of slow cooling, Sn–Zn alloys exhibit coarse dendrite structure and large needle or rod-like Zn-rich precipitates with rough surface and voids, and extensive segregation of massive Bi toward a surface. 2002 Elsevier Science B.V. All rights reserved. Keywords: Sn–Zn; Bi segregation; Lead-free; Solder; Eutectic alloy; Cooling rate
1. Introduction Many countries are going to ban the use of lead and its compounds, which are known to be toxic to the human body and to cause serious environmental problems [1,2]. Thus, the development of lead-free solders has emerged as one of the key issues in the electronics packaging industries. For developing some new solder alloys, one tends to think of various properties such as liquidus temperature, pasty range, wettability, microstructure, mechanical properties of solder as well as the reliability of soldered joints [3]. Among them, the first step in finding a suitable alloy candidate is to consider a low melting temperature alloy that can replace Sn–37Pb alloy, which has been widely used as interconnection material in electronic packaging due to its low melting temperature [2,4,5]. The pasty range is one of the most important characteristics of solder. The alloy begins to melt at a temperature called the solidus, but it is not completely liquid until a higher temperature called the liquidus. The range between the solidus and the *Corresponding author. Tel.: 181-6-6879-8521; fax: 181-6-68798522. E-mail address: [email protected] (Y.S. Kim).
liquidus is referred to as the melting temperature range, pasty range or mushy zone [3,5]. At temperatures between solid and liquid phases, it contains different proportions of solid and liquid phase and leads to segregation during solidification of alloy and lift-off developing during soldering [3,5]. In order to deal with this problem, it is much more preferable to use an eutectic solder or a solder with a narrow pasty range [3]. It is also important to correlate the microstructure of the alloy with its mechanical properties in order to optimize the service performance of the interconnection found by the solders [7]. Sn–Zn eutectic alloy has been considered as one of the lead-free solder alloys that can replace Sn–37 mass% Pb eutectic solder without increasing soldering temperature. Sn–Zn binary alloy has excellent mechanical properties but is susceptible to oxidation and corrosion [3,6]. Therefore, alloying Bi to Sn–Zn near eutectic alloy could improve the soldering property in electronic packaging by lowering melting temperature [7,8]. But alloys with a high Bi concentration need to be controlled because of the brittle nature of Bi and the strong tendency for segregation [9–15]. This work aimed to investigate the effect of Bi addition on melting behavior, phase formation and mechanical
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0925-8388(02)01168-4
Y.S. Kim et al. / Journal of Alloys and Compounds 352 (2003) 237–245
238
Fig. 1. Schematic illustration of tensile test for specimens.
properties of Sn–Zn alloys, and also the effects of cooling rate on precipitation behavior of Bi and mechanical properties.
2. Experimental procedure Alloys of Sn–9 mass% Zn eutectic and of Sn–8 mass% Zn–(1,2,3,6,8 mass%) Bi were used in the present work. Hereafter, composition unit ‘mass%’ is omitted. The raw material alloys were provided by Senju Metals. The alloy ingots were remelted at 300 8C and cast into steel mold. They were cooled at two different rates, 12 8C s 21 (designated as AC, air cooled) and 18310 23 8C s 21 (FC, furnace
Fig. 3. Transformation point as a function of Bi content (a) on heating and (b) on cooling.
cooled). The AC is equivalent to that adopted in the practical reflow process in industries. The ingots were sliced at 1 mm and were polished with 0.3 mm Al 2 O 3 powder. The tensile specimens had a gauge length of 25 mm, width of 8 mm, and thickness of 1 mm as shown in Fig. 1. The tensile tests were carried out at room temperature at
Fig. 2. Differential scanning calorimetry (DSC) curves for Sn–9Zn–0Bi and Sn–8Zn–(1,3,6)Bi alloys (a) on heating and (b) on cooling.
Fig. 4. Undercooling temperature of solder alloys.
Y.S. Kim et al. / Journal of Alloys and Compounds 352 (2003) 237–245
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Fig. 5. Optical microscopy of tensile specimens prepared by FC: (a) Sn–9Zn, (b) Sn–8Zn–1Bi, and (c) Sn–8Zn–6Bi.
strain rates from 3.33310 24 to 3.33310 22 s 21 for the AC samples and only 3.33310 24 s 21 for the FC samples. The thermal reaction of the alloys was evaluated by differential scanning calorimetry (DSC). Specimens for DSC were prepared and about 20 mg cut from the ingot. Heating and cooling were carried out at a constant heating and cooling rate of 0.5 8C min 21 from 25 to 300 8C in Ar
gas flow with a Shimadzu DSC-50 thermal analysis machine. Microstructure was observed by OM (optical microscopy), SEM (scanning electron microscopy, Hitachi, S-
Fig. 6. Phase diagrams of (a) Sn–Zn binary [16] and (b) Sn–8Zn–(0– 8)Bi ternary conducted by Thermo-Calc.
Fig. 7. SEM of Sn–Zn–Bi alloys prepared by AC: (a) Sn–9Zn, (b) Sn–8Zn–3Bi, and (c) Sn–8Zn–6Bi.
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Fig. 8. SEM of Sn–8Zn–6Bi alloy (enlarged image of Fig. 7c).
was observed on heating for each alloy and the large exothermic peak was observed with proceeding small one on cooling. The onset point of endothermic peak, indicated as ‘A’, is the starting temperature of a reaction and the offset point, indicated as ‘B’, is the end temperature. On heating, the onset point is called the solidus temperature of the alloys and the offset point is called the liquidus. The plots of the transformation points both on heating and on cooling as a function of Bi content are shown in Fig. 3. The transformation temperatures of the alloys decrease with increasing of Bi content. The onset point and the offset point decreases with the increase of Bi content
2150). The elemental distribution was evaluated by using EPMA (electron probe microanalysis, Jeol JXA-8800R). XRD (X-ray diffraction analysis, Rigaku) was also evaluated. The phase diagrams were calculated by Thermo-Calc with the Adamis7 data base.
3. Result and discussion
3.1. Thermal properties as Pb-free solder Fig. 2 shows the typical DSC profiles of Sn–9Zn, Sn–8Zn–(1,2,3,6,8)Bi alloys. The large endothermic peak
Fig. 9. XRD of Sn–Zn–Bi alloy (unmarked: b-Sn): (a) Sn–9Zn, (b) Sn–8Zn–1Bi, (c) Sn–8Zn–3Bi, (d) Sn–8Zn–6Bi.
Fig. 10. Influence of strain rate and of addition amount of Bi on UTS, 0.2% proof strength, and elongation.
Y.S. Kim et al. / Journal of Alloys and Compounds 352 (2003) 237–245
from 198.4 into 186.1 8C and from 200.8 into 193.4 8C, respectively, on heating. Therefore, Sn–8Zn–8Bi alloy melts at the temperature of only about 3 8C greater than that of 37Pb–63Sn solder, i.e. 183 8C. The pasty ranges of alloys slightly increase from 2.36 to 6.72 8C. It implies that the Sn–9Zn and alloying Bi to Sn–8Zn alloys become hyper- or hypo-eutectic alloy near Sn–Zn binary eutectic alloy. The undercooling is the deviation between the heating and cooling curves, which is usually experienced in the low melting temperature alloys. Undercooling of Sn–Zn system alloys are far lower than those of the other Sn base lead-free solder alloys as shown in Fig. 4. With increasing Bi content, undercooling decreases from 4.06 to 1.19 8C.
3.2. Solidification conduct of alloys The solidification properties of Sn–Zn–Bi alloys can be observed through the cooling curve with macrostructure and phase diagram. Fig. 5 shows macro-photographs of alloys to observe primary phase, which were prepared by FC because their structures are similar to those of equilibrium state. Sn–9Zn alloy of Fig. 5a displayed large needle-like structure in the eutectic matrix. Sn–8Zn–1Bi alloy of Fig. 5b showed a display of coarse dendrite structure in the eutectic matrix. Fig. 5c showed mixed macro-structure of Fig. 5a and b in the eutectic matrix as in the cases of adding more than 3Bi to Sn–8Zn alloy. And, in order to understand the formation of primary phase, thermodynamic simulation was conducted by Thermo-Calc (Fig. 6b). Sn–9Zn alloy has as much as 0.2% more Zn than
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eutectic contents as shown in binary phase diagram of Fig. 6a [17]; the order of its cooling reaction is L→L1primary Zn→primary Zn1eutectic (Sn1Zn). The microstructure of the alloy has large and long Zn phases that are displayed with the eutectic matrix, as shown in Fig. 5a. Also, a small peak before the large peak of variation of caloric on the cooling curve of DSC (Fig. 2b) is the peak of L→L1 primary Zn reaction, that is to say, the large Zn phase (Fig. 5a) is formed as the primary phase and the primary Zn first appears on solidification. Fig. 6b shows that in the case of Sn–8Zn–1Bi and Sn–8Zn–2Bi alloys, the reaction order of cooling is L→L1primary Sn→primary Sn1eutectic (Sn1Zn) and in the case of Sn–8Zn–3Bi, Sn–8Zn–6Bi, and Sn–8Zn–8Bi alloys, the reaction orders of cooling are L→L1primary Zn→L1secondary Sn1primary Zn→primary Zn1 secondary Sn1eutectic (Sn1Zn). Therefore, in the case of Sn–8Zn–1Bi and Sn–8Zn–2Bi the alloys have primary Sn phase (Fig. 5b). In the case of adding more than 3Bi to Sn–8Zn, the first displayed phase is the Zn phase (Fig. 5c), like the Sn–9Zn reaction, and the next large phase is the secondary Sn phase (Fig. 5c). The very small signal obtained at the liquidus temperature corresponding to the very small change in the slope of the enthalpy vs. temperature curve is evident. This is consistent with the small or undetectable signal in the experiments [16]. There is possible solubility of the Zn atom in the Sn lattice by 0.6 at% but seldom that of the Sn atom in the Zn lattice, as shown in Fig. 6a. Therefore, in the case that the Zn phase is formed as the primary phase (Fig. 5a and c), DSC profiles (0Bi, 3Bi and 6Bi of Fig. 2b) on cooling clearly appear as a small peak
Fig. 11. SEM of the fracture surface of alloys prepared by AC with varying strain rate and Bi contents.
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before the large peak of variation of caloric, than in the case of primary Sn (1Bi of Figs. 2b and 5b).
3.3. Microstructure of alloys Fig. 7 is the SEM image of Sn–Zn–Bi alloys and Fig. 8 is the magnification photograph of Sn–8Zn–6Bi alloy (Fig. 7c). Sn–9Zn alloy shows simple structure that Zn-rich phase is homogenously dispersed in the b-Sn matrix.
Fig. 12. Optical microscopy of Sn–Zn–Bi alloys with varying cooling rate and Bi contents.
Sn–8Zn–3Bi alloy has much more of the Zn-rich phase in the b-Sn matrix than that of Sn–9Zn alloy and there is Bi phase not hardly extraction but solution in the b-Sn matrix. And in the case of Sn–8Zn–6Bi alloy, much Zn and Bi phases are precipitated in the b-Sn matrix. The matrix phase with the gray color is b-Sn, the dark needle like and platelet phase is the Zn-rich phase and the white platelet is the rhombohedral Bi phase. Fig. 9 shows the profiles of XRD patterns for the Sn–9Zn and Sn–8Zn–(1–8)Bi alloys. The alloys of Sn– 9Zn and Sn–8Zn–(1,2)Bi consist of two phases; a body centered tetragonal b-Sn matrix phase and a secondary phase of hexagonal Zn, respectively. In the case of adding more than 3 mass% Bi to Sn–8Zn alloy, the peak of the rhombohedral Bi phase begins to appear, because the Sn–Bi phase diagram shows that solubility of Bi in Sn is only about 2 mass% at room temperature [17].
Fig. 13. Influence of cooling rate and of Bi contents on UTS, 0.2% proof strength, and elongation.
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3.4. Mechanical properties of alloys Fig. 10 shows the results of tensile test for Sn–Zn system alloys as a function of strain rate and of adding Bi to Sn–8Zn alloy at room temperature. With increasing Bi content, the ultimate tensile strength (UTS) and the 0.2% proof stress of the alloys increased and the elongation of the alloys decreased. The values of UTS and 0.2% proof stress of the alloys appeared to increase while elongation of the alloys decreased with the strain rate. At the fracture surfaces, as shown in Fig. 11, was observed the effect of the Bi addition to Sn–Zn alloy at a strain rate of 3.333 10 24 , 3.33310 23 , and 3.33310 22 s 21 , respectively. A ductile dimple pattern can be seen on the fracture surface of Sn–9Zn alloys at all strain rates. The dimple size decreases with increasing strain rate for Sn–9Zn alloy.
Fig. 14. SEM of fracture surface of alloys with varying cooling rate and Bi contents.
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With the increase of Bi content, alloys exhibited cleavage fracture with little or no ductility and this was indicative of a lower toughness. The alloys showed brittle fracture up to the strain rate. The microstructures of the alloys are affected by the difference of cooling rate as shown in Fig. 12. In the case of fast cooling, Sn–9Zn alloy exhibited a much finer and more uniform microstructure within which was dispersed a rod or needle-like Zn phase in the b-Sn matrix. With increasing Bi content, much Zn and Bi precipitations are extracted from the b-Sn matrix. On the other hand, Sn–Zn alloys manufactured by slow cooling rate exhibit coarse dendrite structure and large and long needle or rod-like Zn-rich precipitates. And Bi phases are hardly precipitated in the matrix in spite of alloying a lot of Bi to Sn–8Zn alloy. There is extensive segregation of massive Bi along the growth direction of dendrite into the surface [11,12,14,15]. Fig. 13 are results of tensile test for Sn–Zn–Bi alloys as a function of cast cooling rate and of Bi content to Sn–8Zn at room temperature. In the case of AC, the UTS, the 0.2% proof stress and the elongation of the alloys are better than those of FC alloys. Because in the case of AC, alloys have the structure with which are dispersed fine precipitations in the matrix, but coarse dendrite structure and long Zn precipitations in the case of FC as shown in Fig. 12.
Fig. 15. Schematic of fracture pattern: (a) fracture types and (b) stress– strain curves.
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Fig. 14 shows the fracture morphology of alloys. In the case of AC, the fracture surface of the Sn–9Zn alloy shows a huge dimple as the typical ductility–fracture. With the increase of Bi content, the dimple size becomes smaller and the fracture type changes into the brittle-fracture pattern. In the case of FC, the fracture surface of Sn–9Zn alloy has a small dimple pattern. With increasing Bi content, fracture type changes into the ductile fracture pattern. Thus, the fractures of Sn–8Zn–6Bi and Sn–8Zn– 8Bi alloys show the typical brittle-fracture pattern and fractures occur along the coarse dendrite stem or long Zn precipitations. Fig. 15 illustrate fracture patterns of Sn–Zn–Bi alloys; there are basically three types: • Typical ductility (type I): Sn–9Zn alloys fractured like the fracture behavior of common metals as shown in Fig. 13b. • Complex fracture pattern (type II): alloys by AC were toughened by dispersing fine Zn and Bi precipitations in the matrix. The fracture surfaces of the alloys show the partially dimpled and the partially cleavage fracture. • Faceted fracture pattern (type III): in this case, the alloys were prepared by FC; they have coarse dendrite, very large Zn precipitations and segregation of massive Bi particles into the surface. This pattern shows a straight faceted fracture by following the long Zn precipitation or coarse dendrite stem.
3.5. Defects of FC alloys In the case of FC, the solder alloy has several defects as
shown in Fig. 16; coarse dendrite structure and Zn precipitation, rough surface, voids and extensive macrosegregation of massive Bi particles to surface. Therefore, Bi-bearing Sn–Zn alloys are important to control the cooling rate.
4. Conclusion An experimental work was conducted in order to investigate the effects of the addition of Bi to Sn–Zn near-eutectic alloy on the thermal and mechanical properties for the development of lead-free solder. Also the microstructures and mechanical properties of alloys manufactured under different cooling rates were compared. The results are summarized as follows: • With the increase of Bi addition content, the melting point decreases from 198.4 to 186.1 8C, but the pasty range increases from 2.36 to 6.72 8C. Sn–9Zn and Sn–8Zn–(1,2,3,6,8) Bi solder alloys have far lower undercooling than those of other Sn-based alloys and the undercooling decreases with increasing Bi content. • It was also confirmed that the difference in cooling rates of cast alloy significantly affected the microstructure and mechanical properties. In the case of AC, alloys exhibited the fine microstructure within which were dispersed Zn and Bi precipitations in the b-Sn matrix. The alloys have good mechanical properties. • On the other hand, Sn–Zn alloys manufactured by FC have coarse dendrite structure and long needle- or rod-like Zn-rich phase with rough surface and voids, and extensive segregation of massive Bi into the surface. In this case, the alloys have bad mechanical properties.
Acknowledgements The present work was supported by a Grant-in-Aid for Scientific Research (A) from The Japan Ministry of Education, Culture, Sports, Science and Technology in 2001. The authors acknowledge the Senju Metal Industry Co. for providing the solder alloys.
References
Fig. 16. Defects observed in the tensile specimen of FC.
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