In recent years, increasing environmental and health concerns

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1 CHINA FOUNDRY Vol.9 No.4 Research on lamellar structure and micro-hardness of directionally solidified Sn-58Bi eutectic alloy *Hu Xiaowu, Li Ke and Ai Fanrong (School of Mechanical & Electrical Engineering, Nanchang University, Nanchang , Jiangxi, China) Abstract: In this work, the Sn-58Bi (weight percent) eutectic alloy was directionally solidified at a constant temperature gradient (G = 12 K mm -1 ) with different growth rates using a Bridgman type directional solidification furnace. A lamellar microstructure was observed in the Sn-58Bi samples. The lamellar spacing and micro-hardness of longitudinal and transversal sections were measured. The values of lamellar spacing of both longitudinal and transversal sections decrease with an increase in growth rate. The microhardness increases with an increase in the growth rate and decreases with an increase in the lamellar spacing. The dependence of lamellar spacing on growth rate, and micro-hardness on both growth rate and lamellar spacing were obtained by linear regression analysis. The relationships between the lamellar spacing and growth rate, microhardness and growth rate, and micro-hardness and lamellar spacing for transversal and longitudinal sections of Sn-58Bi eutectic alloy were given. The fitted exponent values obtained in this work were compared with the previous similar experimental results and a good agreement was obtained. Key words: directional solidification; eutectic alloy; microstructure; micro-hardness CLC numbers: TG Document code: A Article ID: (2012) In recent years, increasing environmental and health concerns over the toxicity of lead combined with strict legislation to ban the use of lead-based solders have provided an inevitable driving force for the development of lead-free solder alloys [1-4]. The tin-based alloys are preferential candidate materials for electronics industry due to tin s attractive combination of economic advantages (wide availability, relatively low cost, environmental friendly, a long history of use in solders, and availability of fluxes) and physical properties (low melting temperature, high electrical conductivity, and good wettability for common transition metals and alloys) [5-8]. In general, lead-free solders can be classified into three groups according to temperature ranges. Nowadays, more attention has been paid to the solder alloys in the middle temperature range such as Sn-Cu, Sn-Ag-Cu, Sn-Zn and Sn- Ag alloys [9-12]. It should be mentioned that the low-temperature lead-free solders have also been a popular topic in recent years. Sn-Bi solder is an alternative to replace Pb-based solders in the low temperature ranges. When the parts to be soldered are susceptible to thermal damage, low-temperature soldering is necessary. Moreover, the low-temperature Sn-Bi solder alloy has many advantages in special soldering applications, such *Hu Xiaowu Male, born in 1982, Ph.D. His research and teaching interests mainly focus on eutectic solders. By now, he has published more than 15 technical papers. xwhmaterials@yahoo.cn Received: ; Accepted: as the solder joints of temperature-sensitive zones and the outer layers of classification packaging, which can reduce the influence of soldering temperature on the inner layer of microelectronic packaging. Furthermore, the tensile strength and creep resistance of the eutectic Sn-Bi solder is higher than that of the eutectic Sn-Pb solder. The Sn-Bi solder alloys have recently received much attention as possible lead-free alternatives [13]. In this paper, the dependences of the lamellar spacing (l) and microhardness (H V ) on the growth rate (V) in the Sn-58Bi solder alloy were experimentally investigated. Therefore, the effects of solidification parameters on microstructure development and micro-hardness were confirmed. 1 Experimental procedure 1.1 Sample production and microstructure observation The binary Sn-58Bi (by weight percent) eutectic alloy was used in the directional solidification (DS) process. The Sn- 58Bi master ingot was prepared in an electric resistance furnace under a high pure argon atmosphere. The samples for directional solidification were machined to rods 6.9 mm in diameter and 100 mm in length from the ingot by spark machining. The DS experiments were carried out in an improved Bridgman vertical vacuum furnace as described by Hu et al [14-16]. The temperature gradient close to the solid/liquid interface was measured to be approximately 12 K mm -1. During the directional solidification 360

2 November 2012 process, the samples in the furnace were heated to 300 in a vacuum of 10-1 Pa with high purity argon to prevent the evaporation of the components in the Sn-58Bi eutectic alloy. The samples were withdrawn at the selected growth rate and the solidification length of the samples was over 60 mm to ensure a steady solidification state. Then the samples were quenched by putting them rapidly into the liquid Ga-In-Sn reservoir. The directionally solidified samples were cut into pieces with a length of 10 mm. The longitudinal and transversal sections were chosen from the stable growth zone in which the microstructure is uniformly distributed. After polishing with SiC paper (180, 500, 1000 and 2500 grits in turn), the samples were etched with 5 ml HNO 3 and 95 ml C 2 H 5 OH solution for 5 to 10 s. The microstructures of the samples were observed for both longitudinal and transversal sections of samples using a scanning electron microscope (SEM) equipped with an energy dispersive X-ray (EDX) spectrometer as well as a computer controlled image system. 1.2 Measurements of lamellar spacing and micro-hardness The lamellar spacings (λ long and λ tr ) were measured from the photographs of longitudinal and transversal sections by using a linear intercept method [15, 17], combining with the SISC IASV 8.0 image analysis software. The value of the lamellar spacing for each sample was obtained from the mean value of at least 50 Research & Development readings. Micro-hardness was measured using a Fujitech FM-700 model hardness measuring device with a 25 g load and a dwell time of 10 s. The micro-hardness was taken from the average of at least 15 measurements on the transversal section (H V(tr) ) and the longitudinal section (H V(long) ). 2 Results and discussion 2.1 Microstructure observation The SEM microstructures of samples in both longitudinal and transversal sections are shown in Fig. 1. The EDX results in Fig. 2 show the solubility of components in each phase. According to the EDX results and the solubility of each element, the grey phase is the pure Bi phase and the dark phase is the Sn solid solution phase. 2.2 Dependency of lamellar spacing on growth rate As expected, the microstructures varied with the growth rate at a constant temperature gradient (G = 12 K mm -1 ). As the growth rate increases, the lamellar spacing decreases. As shown in Figs. 1(a1) and 1(a2), the largest inter-lamellar spacing was obtained at a minimum value of growth rate (V = 1 μm s -1 ). A low growth rate allows more intense atomic diffusion. Thus, the largest inter-lamellar spacing is attributed to the (a1) Longitudinal section (a2) Transversal section V = 5 μm s -1 V = 1 μm s -1 (b1) (b2) Growth direction 361

3 CHINA FOUNDRY Vol.9 No.4 Longitudinal section Transversal section (c2) (d1) (d2) (e1) (e2) (f1) (f2) V = 100 μm s-1 V = 50 μm s-1 Fig. 1: SEM images of the directionally solidified Sn-58Bi eutectic solder with different growth rates (1 μm s-1 to 100 μm s-1) in a constant temperature gradient (G = 12 K mm-1) 362 Growth direction V = 30 μm s-1 V = 10 μm s-1 (c1)

4 November 2012 Research & Development (a) (b) (c) Fig. 2: Composition analysis of the Sn-58Bi eutectic solder alloy using SEM EDX: (a) Lamellar eutectic structure, (b) Pure Bi (grey phase, spectrum 1), (c) Sn-rich phase (dark phase, spectrum 2) more intense cooperative growth and to the longer diffusion length. On the other hand, the smallest inter-lamellar spacing was obtained at the maximum value of growth rate (V = 100 μm s -1 ) as shown in Figs. 1(f1) and 1(f2). The measured values of lamellar spacings (λ) for the directionally solidified Sn-58Bi eutectic solder alloy with different growth rates at a constant temperature gradient (G = 12 K mm -1 ) are given in Table 1. Table 1: Values of lamellar spacings (λ) and microhardnesses (H V ) for the directionally solidified Sn-58Bi eutectic solder with different growth rates at a constant temperature gradient (G = 12 K mm -1 ) Growth rate Lamellar spacings Microhardnesses V (μm s -1 ) λ long (μm) λ tr (μm) H V(long) (kg mm -2 ) H V(tr) (kg mm -2 ) The variation of lamellar spacing (λ) versus growth rate (V) is essentially linear on the logarithmic scale as can be seen in Fig. 3, and the linear regression analysis gives the equation as: λ = k l V -n (for a constant growth rate) where k 1 is a constant and n is an exponent value of growth rate. The relationships between the lamellar spacing and growth rate for the longitudinal and transversal sections were obtained by binary regression analysis as λ long = 10.4V -0.5 and λ tr = 9.3V The values of the exponents (0.5 and 0.54) are close to the 0.5 predicted by Jackson-Hunt eutectic theory [17], the 0.51 and 0.50 obtained by Böyük and Maraşlı [18] for Sn-Ag-Cu eutectic alloy and Böyük et al. [19] for Al-Cu-Ag eutectic alloy. However, the exponent values of 0.5 and 0.54 are smaller than the value of 0.58 obtained by Grugel et al. [20] for Sn-Cu eutectic solder. λ λ long = 10.4V -0.5 λ tr = 9.3V Fig. 3: Variation of lamellar spacings (λ) as a function of growth rate (V) at a constant temperature gradient (G = 12 K mm -1 ) 363

5 CHINA FOUNDRY H V(long) = 19.87V 0.05 H V(tr) = 20.6V 0.06 Fig. 4: Variation of micro-hardness (H V ) as a function of growth rate (V) for directionally solidified Sn-58Bi eutectic solder with different growth rates (1 μm s -1 to 100 μm s -1 ) at a constant temperature gradient (G = 12 K mm -1 ) Vol.9 No Dependency of microhardness on growth rate Table 1 and Fig. 4 show the variations of microhardness (H V ) as a function of growth rate (V) at a constant temperature gradient (G = 12 K mm -1 ). The error bars in Fig. 4 indicate the errors in the H V values, which are in the range of ±5%. The value of microhardness (H V ) increases with an increase in growth rate (V). The relationships between H V and V for longitudinal and transverse sections were obtained by using of linear regression analysis, they are H V(long) = 19.87V 0.05 and H V(tr) = 20.6V The exponent values obtained in the present work, i.e., 0.05 and 0.06 for the longitudinal and transversal sections, respectively, are close to the values of 0.04 and 0.07 obtained for the Al-Si eutectic alloy by Yllmaz et al. [21] and Vnuk et al. [22] ; but rather lower than the values of 0.11 and 0.12 obtained for the Sn-Bi eutectic alloy by Çadırlı et al. [23]. 2.3 Dependency of micro-hardness on lamellar spacing As can be seen in Table 1 and Fig. 5, at a constant temperature gradient (G = 12 K mm -1 ) the value of micro-hardness (H V ) decreases with an increasing value of lamellar spacings (λ). The dependences of micro-hardnesses on the lamellar spacings of the transversal and longitudinal sections were obtained by using of curve fitting as: and tr tr tr long long long as shown in Figs. 5(a) and 5(b), respectively. H V = exp(-0.52λ tr ) exp(-4.55λ tr ) (a) H V = exp(-5λ long ) +7.73exp(-0.37λ long ) (b) Lamellar spacing, λ tr (mm) Lamellar spacing, λ long (mm) Fig. 5: Variation of micro-hardness (H V ) as a function of lamellar spacing (λ) for (a) transversal section and (b) longitudinal section 3 Conclusions (1) In the present work, Sn-58Bi eutectic alloy was directionally solidified using a Bridgman type directional solidification furnace and the microstructures were observed to be the lamellar structure consisting of Sn and Bi phases. (2) The eutectic lamellar spacing (l ) for directionally solidified Sn-58Bi eutectic alloy at a constant temperature gradient (G = 12 K mm -1 ) decreases with increasing the growth rate (V). The relationships between the lamellar spacing and the growth rate for the longitudinal and transversal sections were obtained by binary regression analysis as l long = 10.4V -0.5 and l tr = 9.3V (3) The value of the micro-hardness (H V ) for directionally solidified Sn-58Bi eutectic alloy at a constant temperature gradient (G = 12 K mm -1 ) increases with increasing the growth rate (V). The relationships between the micro-hardness and growth rate for the longitudinal and transversal sections were obtained by linear regression as H V(long) = 19.87V 0.05 and H V(tr) = 20.6V (4) The value of the microhardness (H V ) decreases with increasing lamellar spacing (l). The relationships between the micro-hardness and lamellar spacing for the longitudinal and transversal sections were obtained by using of curve fittinig method; they are H V(tr) = exp(-0.52l tr ) exp(- 4.55l tr ) and H V(long) = (-5l long )+7.73exp(-0.37l long ). References [1] Shen J, Liu Y C, Gao H X, Wei C and Yang Y Q. Formation of bulk Ag 3 Sn inter-metallic compounds in Sn-Ag lead-free solders in solidification. J. Elect. Mater., 2005, 34: [2] Shen J, Liu Y C, Han Y J, Zhang P Z, and Gao H X. Formation of Bulk Inter-metallic Compound Ag 3 Sn in Slowly-Cooled Lead- Free Sn-4.0wt.%Ag Solders. J. Mater. Sci. Technol., 2005, 21: 364

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