Properties of low melting point Sn Zn Bi solders

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1 Journal of Alloys and Compounds 397 (2005) Properties of low melting point Sn Zn Bi solders Jian Zhou, Yangshan Sun, Feng Xue Department of Material Science and Engineering, Southeast University, 2 Sipailou, Nanjin , PR China Received 10 June 2004; accepted 10 December 2004 Available online 8 April 2005 Abstract Because of low melting point, it is possible to substitute Sn Zn base alloys as a lead-free solder for toxic Sn Pb solder. In the present paper, Sn Zn Bi alloys were investigated by, respectively, changing Bi or Zn concentration, to display their melting points, wettabilities, mechanical properties and reliabilities of joints with Cu. The results show that addition of Bi could obviously improve wettability of Sn Zn base alloys with decrease melting point. Tensile strength of solders and shear strength of joint with Cu could be enhanced by small Bi dissolving in the matrix of the alloys, but mass addition of Bi would cause decrease of tensile strength. Reducing Zn concentration can amend mechanical properties of the solders without obviously changing the melting points and paste ranges of them Elsevier B.V. All rights reserved. Keywords: Lead-free solders; Sn alloys; Melting point; Wetting 1. Introduction The harmful effects of lead on environment and human health have prompted a series of searches on lead-free solders for electronics industry. At present, several lead-free solder alloys, such as Sn Ag alloys and Sn Zn alloys, have showed potential for replacing traditional Sn Pb solder [1 4]. One of the important requirements for electronics solders is processing temperature as low as Sn Pb eutectic alloy. As a substitution candidate, the melting point of Sn Zn eutectic alloy is closest to the Sn Pb eutectic alloy. To improve other properties of this alloy, new researches for Sn Zn base solder alloys are being carried out [5 9]. Poor wettability is the most crucial problem with Sn Zn binary alloy. Previous works have indicated that high surface tension of liquid phase and ZnO float on the liquid surface have restrained this solder from wetting copper [10 13]. In the present research, Sn Zn alloys have been affiliated with active element Bi in order to improve the wettability of Corresponding author. Tel.: addresses: Jethro0313@hotmail.com (J. Zhou), yangshan@public1.ptt.js.cn (Y. Sun) /$ see front matter 2005 Elsevier B.V. All rights reserved. doi: /j.jallcom the alloys. Thermal properties, wettabilities and mechanical properties of the solders have been investigated and shear strength of solder/cu joints has been displayed. 2. Experimental 99.9% Sn, 99.9% Zn and 99.9% Bi were melted in N 2 under temperature of 450 C and cast into a steel mold. The 12 mm diameter rod ingot then was left to cool in air. Chemical compositions of specimens are given in Table 1. Solders and copper sheets for wetting experiment were ultrasonically cleaned in acetone. Thermal analyses of solders were performed by Perkin-Elmer Pyris 6 differential scanning calorimetry (DSC). Specimens for DSC were about 20 mg. Heating was carried out at rate of 5 C/min from 170 to 220 C in Ar flow. Wetting experiment was conducted in N 2 and complied with GB The dimensions of the gauge section of specimens for tensile test are shown in Fig. 1(a). The tensile specimens were polished with No. 500 Al 2 O 3 sandpaper. Tensile tests were performed at room temperature at a strain rate of 2 mm/min. Joint strength was evaluated by a joint shear strength test. As shown in Fig. 1(b), the Cu/solder/Cu joints were used as test specimen. 转载

2 J. Zhou et al. / Journal of Alloys and Compounds 397 (2005) Table 1 Chemical compositions of specimens Alloys Zn Bi Sn Alloys Zn Bi Sn 1 9 / Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Microstructure of the alloys and the joints was observed by optical microscopy (OM) and scanning electron microscopy (SEM), and was analyzed by X-ray diffraction (XRD) and electron probe microanalyzer (EPMA). Specimens for observation were etched in a solution of 5 vol% HCl and 95 vol% C 2 H 5 OH after polished. 3. Results and discussion 3.1. Thermal properties of Sn Zn Bi alloys Fig. 2 shows DSC profiles of Sn Zn eutectic alloy and a series of Sn Zn Bi alloys. The melting points as a function of Bi concentration are shown in Fig. 2(a). The melting points of the alloys decrease with increase of Bi concentration, but the pasty ranges of the alloys are enlarged simultaneously. The effect of Zn concentration on transformation points of the alloys can be described from Fig. 2(b and c). There is no obvious change both in transformation temperatures and in the pasty ranges of the two groups of the alloys when Zn concentration rises from 6 to 12 wt%. Two notable peaks appear in the curves of Sn 3Zn 4Bi and Sn 3Zn 6Bi, which could result from two transformations taking place on heating. Generally, those two transformation processes carry through a large temperature range. This indicates Fig. 1. Schematic illusion of mechanical properties tests: (a) tensile test for solders and (b) shear test for joints. that those alloys would exist as part liquid for a long time on solidification. This character of the alloys disables themselves from forming reliable joints in the process of welding Wettability of Sn Zn Bi alloys The addition of Bi could improve wettability of Sn Zn based solders on Cu substrate. As shown in Fig. 3(a), wetting area increases with Bi concentration from 2 to 10 wt%. Bi is one of the surface-active elements [1,14]. Surface tension of liquid Sn Bi eutectic alloy is far lower than that of Sn Pb or Sn Zn eutectic alloy in the same state. For this reason, the addition of Bi could decrease surface tension of the liquid solders, and accelerate their spreading out on Cu substrates. Wetting area as a function of Zn concentration is shown in Fig. 3(b). The addition of Zn slightly enhances the wetting on Cu by Sn XZn (4,6)Bi. Compared with Sn or Bi, Zn is inclined to react with Cu, although it is not a surface-active element. In other words, Zn has greater diffusivity than Sn or Bi when coupling with Cu. As shown in Fig. 4, the increase of Zn in Sn Zn Bi solders causes more Zn atoms to diffuse into Cu substrate. The depth of the interaction layers is about 10 m. This kind of diffusivity usually breaks the statics balance on solder/cu interface, which helps the edge of solders stretch out Mechanical properties and microstructure Fig. 5 shows the results of tensile test for Sn Zn Bi solder alloys as a function of Bi and Zn concentration. On condition that Bi concentration is not beyond 4 wt%, the ultimate tensile strength (UTS) sharply increases with Bi concentration increasing. When it exceeds 4 wt%, UTS slowly decreases with Bi concentration rising. Elongation of the alloys decreases with Bi concentration increasing. A peak also appears in the curve of tensile strength versus Zn concentration. It is indicated that small addition of Bi could be dissolved in Sn matrix to strengthen the matrix. As shown in Fig. 6(a and b), there is no Bi-rich phase in the matrix of Sn 9Zn 2Bi. Comparing with that, polyhedral Birich phase and needle like Zn-rich phase simultaneously precipitate in the matrix of the Sn 9Zn 6Bi solder, as shown in Fig. 6(c). Fig. 7 is the XRD profile of Sn 9Zn 6Bi. Those massive precipitations always induce crack in the matrix, which result in the weakness of strength of the alloys. However, with increase of Zn concentration, massive Znrich phases precipitate in the matrix, as shown in Fig. 6(f). For the same reason, with addition of Bi, it weakens the strength of the solders. Stress focuses or flaws usually occur at the interface between massive Zn-rich phase or Bi-rich phase and matrix. Therefore, elongation of Sn Zn Bi alloys would decrease with Bi or Zn concentration increasing.

3 262 J. Zhou et al. / Journal of Alloys and Compounds 397 (2005) Fig. 2. DSC curves for Sn 9Zn and different Sn Zn Bi solders (on heating): (a) Sn 9Zn XBi, (b) Sn XZn 6Bi and (c) Sn XZn 6Bi. Fig. 3. Wetting areas as a function of Bi or Zn concentration: (a) Sn 9Zn XBi, (b) Sn XZn 6Bi Joint properties and microstructure Joint shear strength as a function of Bi concentration is shown in Fig. 8. Small addition of Bi can improve shear strength of Sn Zn Bi solder/cu joints. When Bi concentration exceeds 4 wt%, the result is reversed. Shear strength of Sn Zn Bi solder/cu joints is below Sn 40Pb/Cu joints. Shear strength of Sn 40Pb/Cu joint in the present research is adjacent to Sn 40Pb alloy with reference to the previous works [1], but those of Sn Zn Bi/Cu joints is far below that of Sn 9Zn solder. Fig. 9 shows microstructures of the joints and fracture surfaces of Sn 40Pb/Cu and Sn 9Zn 2Bi/Cu joints. The fractures occur in solder regions along the interfaces of Fig. 4. EMPA line analysis across solder/cu interface: (a) Sn 3Zn 6Bi/Cu, (b) Sn 9XZn 6Bi/Cu and (c) Sn 12Zn 6Bi/Cu.

4 J. Zhou et al. / Journal of Alloys and Compounds 397 (2005) Fig. 5. UTS and elongation as a function of Bi or Zn concentration: (a) Sn 9Zn XBi and (b) Sn XZn 6Bi. Fig. 6. SEM micrograph of: (a) Sn 9Zn 2Bi, (b) Sn 9Zn 2Bi, (c) Sn 9Zn 6Bi, (d) Sn 6Zn 6Bi, (e) Sn 6Zn 6Bi and (f) Sn 12Zn 6Bi. Sn 40Pb/Cu and Sn 9Zn 2Bi/Cu. The difference between them is that a lot of voids form along the interface of Sn 9Zn 2Bi/Cu, as shown in Fig. 9(c and d). Voids near the interface have reduced the area bearing stress in test. This implies the weakness of Sn Zn Bi/Cu joints. As shown in Fig. 10, Zn-rich phase obviously exists above the void, but near the reaction region there is no Zn-rich phase because Zn atoms have diffused into reaction region. In previous research, voids were attributed to the non-wetting region left in the solder after the passage of the reaction front growing Fig. 7. XRD profiles of Sn 9Zn 6Bi. Fig. 8. Shear strength as a function of Bi concentration.

5 264 J. Zhou et al. / Journal of Alloys and Compounds 397 (2005) amount of the voids, that void should result from shrinkage in the progress solidification. 4. Conclusions Fig. 9. OM micrograph of: (a) Sn 40Pb/Cu interface, (b) Sn 40Pb/Cu fracture surface, (c) Sn 9Zn 2Bi/Cu interface and (d) Sn 9Zn 2Bi/Cu fracture surface. The addition of Bi could obviously improve wettability of Sn Zn base alloys with decrease in melting point. Paste ranges could be enlarged with Bi concentration increase, which indicates that Bi concentration of Sn Zn Bi alloy as a solder must be controlled. Wettability of Sn Zn Bi alloys can be enhanced by adjusting Zn concentration in a given range, without influence on melting point. Tensile strength of solders and shear strength of joint with Cu could be improved by small Bi dissolving in the matrix of the alloys, but mass addition of Bi would cause decrease of them. To reach favorable mechanical properties of solder, mass Zn-rich phase and Bi-rich phase must be controlled. Voids often occur along the interface of Sn Zn Bi/Cu joints. It likely resulted from shrinkage of liquid solders in solidification. Compared with Sn 40Pb/Cu joints, those voids along the interface lead to the weakness of Sn Zn Bi/Cu joints. References Fig. 10. SEM micrograph of Sn 9Zn 6Bi. into Cu [11]. But the voids observed in the present joints are not non-wetting area, because a thin solder layer exists at the voids bottom, as shown in Fig. 9(d). Those are not the Kirkendall voids, which are formed by a preferential diffusion of elements, and must be formed along or near the Cu/reaction middle region of the reaction layer or at the interface between the reaction layer and the solder. According to the size and the [1] A. Mulugeta, S. Guna, Mater. Sci. Eng. 27 (2000) [2] S. Katsuki, Curr. Opin. Solid State Mater. Sci. 5 (2001) [3] M. Mccormack, G.W. Kammlot, H.S. Chen, et al., Appl. Phys. Lett. 9 (1994) [4] K.K. Sung, K.S. Amit, J. Electron. Mater. 8 (1994) [5] R.K. Shiue, L.W. Tsay, C.L. Lin, et al., Microelectron. Reliability 43 (2003) [6] S. Katsuki, M. Toshikazu, N. Hiroji, et al., J. Mater. Res. 4 (2000) [7] Y.S. Kim, K.S. Kim, C.W. Hwang, et al., J. Alloys Compd. 352 (2003) [8] J.M. Song, G.F. Lan, T.S. Lui, et al., Scripta Mater. 48 (2003) [9] C.M.L. Wu, D.Q. Yu, C.M.T. Law, et al., J. Electron. Mater. 9 (2000) [10] K.L. Lin, C.L. Shih, J. Electron. Mater. 2 (2003) [11] S. Katsuki, N. Koiichi, S. Takeshi, et al., J. Mater. Res. 10 (1998) [12] K.L. Lin, L.H. Wen, J. Mater. Sci.: Mater. Electron. 1 (1998) 5. [13] D.Q. Yu, J. Zhao, L. Wang, Chin. J. Nonferrous Met. 8 (2003) 102. [14] L.C. Prasad, A. Mikula, J. Alloys Compd. 282 (1999) 279.