M A TE RI A L S C H A RAC TE RI ZA T ION 6 1 ( 2 01 0 ) 1 2 6 0–1 2 6 7
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Effect of solidification parameters on the microstructure of Sn-3.7Ag-0.9Zn solder U. Böyük a,⁎, S. Engin b , H. Kaya a , N. Maraşlı c a
Department of Science Education, Education Faculty, Erciyes University, Kayseri, Turkey Institute of Science and Technology, Department of Physics, Erciyes University, Kayseri, Turkey c Department of Physics, Faculty Arts & Sciences, Erciyes University, Kayseri, Turkey b
AR TIC LE D ATA
ABSTR ACT
Article history:
In this work, Sn–Ag–Zn alloy of eutectic composition (Sn-3.7wt.%Ag-0.9wt.%Zn) was
Received 30 March 2010
directionally solidified upward at a constant temperature gradient (G = 4.33 K/mm) in a
Received in revised form 8 July 2010
wide range of growth rates (V = 3.38–220.12 μm/s) and a constant growth rate (V = 11.52 μm/s)
Accepted 27 August 2010
with different temperature gradients (G = 4.33–12.41 K/mm) using a Bridgman type directional solidification furnace. The microstructure was observed to be a rod Ag3Sn structure in the matrix of β–Sn from the directionally solidified Sn-3.7wt.%Ag-0.9wt.%Zn
Keywords:
samples. The values of eutectic spacing (λ) were measured from transverse section of
Directional solidification
samples. The dependency of eutectic spacing on the growth rate (V) and temperature
Eutectics
gradient (G) were determined with linear regression analysis. The dependency of λ on the
Growth from melt
values of V and G were found to be λ = 10.42V − 0.53 and λ = 0.27G − 0.48, respectively. The values
Ternary alloys
of bulk growth were also determined to be λ2V = 86.39 μm3/s by using the measured values of λ and V. The results obtained in present work were compared with the previous similar experimental results obtained for binary and ternary alloys. © 2010 Published by Elsevier Inc.
1.
Introduction
In the electronics industry's ongoing search for a standard lead-free solder formulation, tin-based alloys stand at the forefront of all candidates. This position is attributable to the tin's attractive combination of economic advantages (wide availability, relatively low cost, environmental benignity, a long history of use in solders, and availability of fluxes) and physical properties (low melting temperature, high electrical conductivity, and good wettability of common transition metals and alloys) [1–5]. Most Sn-base lead-free solders contain only minor amounts of alloying additions. For example, in the widely studied Sn–Ag–Cu (SAC) alloy, more than 95% of the solder is Sn, as measured by weight [6]. However, in these solders, the thick Cu6Sn5 IMC (intermetallic compound) layer and large
Ag3Sn primary phase are often reported to influence the integrity and reliability of solder joint [7–10]. In order to improve the reliability of the Pb-free solder joint, recently it is proposed to reduce Ag and Cu content as well as to add minor alloying elements such as Zn, In, Bi, Co, Ni in Sn-based solder [11–16]. Though Sn–Ag solder has great properties of strength, resistance to creep and thermal fatigue [17–19], the small addition of Zn can improve the mechanical performance at no cost of the ductility and wettability. And, the combination of Zn and Ag dramatically reduces the corrosion potential [20]. Thus, aims of present work were to experimentally investigate the dependence of the eutectic spacing (λ) on the growth rate (V) and temperature gradient (G) in the Sn–Ag–Zn alloy of eutectic composition (Sn-3.7wt.%Ag-0.9wt.%Zn) and compare the results with the previous experimental results for binary and ternary alloys.
⁎ Corresponding author. Tel.: +90 352 437 4901x37096; fax: + 90 352 437 88 34. E-mail address:
[email protected] (U. Böyük). 1044-5803/$ – see front matter © 2010 Published by Elsevier Inc. doi:10.1016/j.matchar.2010.08.007
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2.
Experimental Procedure
2.1.
Sample Production and Microstructure Observation
The composition of Sn–Ag–Zn alloy was chosen to be Sn-3.7wt. %Ag-0.9wt.%Zn to growth eutectic phases from ternary liquid. Thus, the Sn–Ag–Zn alloy was melted under the vacuum using 99.99% pure tin, silver and zinc and poured into 13 graphite crucibles (200 mm in length 4 mm ID and 6.35 mm OD) held in a specially constructed casting furnace (Hot Filling Furnace) at approximately 50 °C above the melting point of alloy. The molten alloy was directionally solidified from bottom to top to ensure that the crucible was completely full. Then, each sample was positioned into a graphite cylinder (300 mm in length 10 mm ID and 40 mm OD) and placed together in the Bridgman type furnace. Solidification of samples was carried out with different temperature gradients (G = 4.33–12.41 K/mm) at a constant growth rate (V = 11.52 μm/ s) and with different growth rates (V = 3.38–220.12 μm/s) at a constant temperature gradient (G = 4.33 K/mm) by different speeded synchronous motors. The details of Bridgman type directional solidification furnace and experimental procedures are also given in Refs. [21–23]. The quenched sample was removed from the graphite crucible and cut into lengths typically 8 mm. The longitudinal and transverse sections were ground flat with SiC paper (180, 500, 1000, 2500 grit) and the longitudinal and transverse sections of ground samples were cold mounted with epoxy-resin. After polishing, the samples were etched with 80 ml glycerin [C3H5(OH)3], 10 ml acetic acid [CH3COOH], and 10 ml nitric acid [HNO3] for 30 s. After metallographic process, the microstructures of the samples were revealed. The microstructures of samples were photographed from both transverse and longitudinal sections with an Olympus DP12 digital camera placed in conjunction with an Olympus BHX type light optical microscope and a LEO scanning electron microscopy (SEM). Typical SEM images and optical microscopy images of directionally solidified Sn–Ag–Zn eutectic sample from longitudinal and transverse sections for different growth rates and temperature gradients are shown in Figs. 1 and 2, respectively. The microstructures of samples were characterized with an energy dispersive X-ray (EDX) spectrometers as well as a computer controlled image system (see Fig. 3). Solid solubility of Ag in solid Sn, solid solubility of Sn in solid Ag, solid solubility of Zn in solid Ag and solid solubility of Ag in solid Zn are 0.1wt.%Ag, 10.2wt.%Sn, 25.6wt.%Zn and 1.6wt.%Ag at 210, 218, 200 and 150 °C, respectively [24]. According to the EDX results as shown in Fig. 3, microstructure of directionally solidified Sn-3.7wt.%Ag-0.9wt.%Zn alloy and the solubility of components in each phases, the microstructure consists of only a regular rod eutectic (Ag3Sn) and does not consist of AgZn phases in the Sn-rich matrix (β–Sn phase). Thus, the white phase is the β–Sn phase, the dark phase is Ag3Sn phase and the composition of the quenched liquid is 95.17wt.%Sn, 3.78wt.%Ag and 1.05wt.%Zn. The EDX results for the quenched liquid phase is close the eutectic composition. Different solidification parameters were applied to explore the possible formation of intermetallics phases in β–Sn
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Fig. 1 – Typical SEM images of directionally solidified Sn–Ag–Zn eutectic alloy (a) longitudinal section (b) transverse section.
matrix. While cooling rate is so slow (about 0.16 K/s) that it could be considered as an equilibrium solidification process, which corresponds to a eutectic reaction: L → ζ − AgZn + Ag3Sn + β–Sn [25,26]. Phase diagram of Sn–Ag–Zn ternary alloy is given in Fig. 4 [26,27]. It follows that β–Sn, AgZn and Ag3Sn phases will separate out in the slowly cooled solder and eutectic microstructure consist of the mixture of ζ-AgZn and Ag3Sn IMCs in the matrix of β–Sn [25]. Although the cooling rate was in the range of 0.05–0.95 K/s in present work, the microstructure of solidified Sn–Ag–Zn ternary eutectic alloy was observed to be only a rod Ag3Sn in the matrix of β–Sn as shown in Figs. 1–3. AgZn phase in the matrix of β–Sn could not be observed with an optical light microscope and SEM. There is no any evidence to show the microstructure of directionally solidified Sn-3.7wt.% Ag–0.9wt.%Zn alloy also consists of AgZn phase in the Sn-rich matrix (β–Sn phase) in present work. Thus, this microstructural characteristic is similar as the Sn–Ag solder cooled at the different solidification condition [28]. Morphology of the solidified Sn–Ag–Zn solder even at such a high growth rate (V = 220.12 μm/s) was observed cellular eutectic colonies (Fig. 2c). The microstructure of solidification sample quite different from that of solidified eutectic Sn-3.5wt.Ag-0.9wtCu solder, which clearly exhibits gray primary β–Sn dendrites at high growth rates (higher than 44.33 μm/s) [29]. Thus, the small addition of Zn in Sn–Ag solder can suppress the formation of β–Sn even at such a growth rate (V = 220.12 μm/s) as shown in
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Fig. 2 – Typical optical images of the growth morphologies of directionally solidified Sn-3.5wt.%Ag-0.9wt.%Zn eutectic alloy with different growth rate (V = 3.38–220.12 μm/s) at a constant temperature gradient (G = 4.33 K/mm) and different temperature gradient (G = 4.33–12.41 K/mm) at a constant growth rate (V = 11.52 μm/s), (a) longitudinal section (b) transverse section (V = 3.38 μm/s, G = 4.33 K/mm), (c) longitudinal section (d) transverse section (V = 220.12 μm/s, G = 4.33 K/mm), (e) longitudinal section (f) transverse section (V = 11.52 μm/s, G = 4.33 K/mm) and (g) longitudinal section (h) transverse section (V = 11.52 μm/s, G = 12.41 K/mm).
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Fig. 3 – The chemical composition analysis of Sn-3.7wt.%Ag-0.9wt.%Zn eutectic alloy by using SEM EDX, the matrix phase is β–Sn and rod phase is Ag3Sn.
Fig. 2c–d. Besides, typical images of optical microscopy for different temperature gradients are shown in Fig. 2e–h. As can be seen from Fig. 2, the dependency of microstructure on the growth rate is stronger than on the temperature gradient.
2.2. Measurement of Solidification Parameters and Eutectic Spacing The temperature in the specimen was measured with a K-type 0.25 mm in diameter insulated three thermocouples, which
were fixed within the sample with spacing of 10–20 mm. The cooling rates were recorded with a data-logger via computer during the growth. The temperature gradient (G = ΔT/ΔX) in the liquid phase and the value of growth rate (V = ΔX / Δt) for each sample were determined using the measured values of ΔT, ΔX and Δt. Details of the measurement of ΔT, ΔX and Δt are given in Refs. [21–23]. Schematic illustration of eutectic spacing measurements for the Sn-3.7wt.%Ag-0.9wt.%Zn alloy is defined in Fig. 5. As defined in Fig. 5, the eutectic rod spacing is a distance from the
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Fig. 4 – Phase diagram of Sn–Ag–Zn ternary alloy (a) at the Sn-rich corner [26] (b) at the vertical section [27].
center of a rod to center of nearest neighbor rod. Thus, the eutectic rod spacing was measured on transverse and longitudinal sections. In a longitudinal view, the eutectic spacing seems to be different in each grain because they were cut under different angles θ, to the polished surface. For that reason, longitudinal sections are inadequate for evaluation of the eutectic spacing without the geometrical correction. It was observed that the values of λtr measured on the transverse section are more reliable than the values of λL measured on longitudinal section. There might be λtr is perpendicular to the growth direction whereas λL is parallel to the growth direction and it depends on the polished plane. So, λ values measured from transverse section were more preferred than longitudinal section. The rod spacings (λL) were measured with a linear intersection method on the longitudinal section as shown in Fig. 5b and d [30]. Two different methods were used to measure the eutectic spacings on the transverse sections [31,32]. The first method is the triangle method. The triangle occurred by joining the centers of rod the three neighbor (Fig. 5e). In this method at least 100–120 rod spacings, λtr, values were measured for each specimen at least on ten different regions for each specimen. The second method is the area counting method (Fig. 5f). In this method at least 100–120 rod spacings, λa, values were measured for each specimen for statistical reliability. λ is average values of λa and λtr and also they are given in Figs. 6 and 7.
3.
Result and Discussion
3.1.
Effect of Growth Rate on the Eutectic Spacing
As expected, the formations of the microstructures have varied with the growth rates and temperature gradient. As the growth rate and temperature gradient are increased, the eutectic spacing decreases. The highest eutectic spacing was obtained at the minimum values of the growth rate and temperature gradient (V = 3.38 μm/s, G = 4.33 K/mm) as shown in Fig. 2a and b. On the other hand, the smallest eutectic
spacing was obtained at the maximum values of growth rate and a constant temperature gradient (V = 220.12 μm/s, G = 4.33 K/mm) as shown in Fig. 2c and d. Variation of λ versus V is essentially linear on the logarithmic scale. It can be seen from Fig. 6, the data form straight lines, the linear regression analysis gives the proportionality equation as, λ = k1 Vn ðfor the constant GÞ
ð1Þ
where k is a constant and n is an exponent value of growth rate. The relationships between the eutectic spacing and growth rates were determined as λ = 10.42V − 0.53 by using linear regression analysis. It is apparent that the exponent value of growth rate (0.53) is close to 0.50 predicted by Jackson–Hunt eutectic theory [33]. The values of the exponent relating to the growth rates (0.53) obtained in this work are in good agreement with the values of 0.51, 0.46, 0.51, 0.49 and 0.50 obtained by Böyük and Maraşlı [29] for Sn–Ag–Cu eutectic alloy, Wilde et al. [34] for Al–Cu–Ag eutectic alloy, Çadırlı et al. [22], Witusiewicz et al. [35] for In–Bi–Sn eutectic alloy, Böyük et al. [23] for Al–Cu–Ag eutectic alloy, respectively. Besides, the exponent value of 0.53 is some smaller than the value of 0.58 obtained by Grugel et al. [36] for Sn–Cu eutectic alloy. Bulk growth rates (λ2V) obtained in the present work is given in Fig. 6. The value of λ2V = 86.39 μm3/s obtained in present work is close to (62, 59.6, 72, 64, 61.43, 93.43) μm3/s obtained by Wilde et al. [37] for Al–Cu–Ag, El Mahallawy et al. [38] for Al–Al3Ni eutectic alloy, Moore and Elliott [39] for Sn–Cd eutectic alloy, Lee et al. [40] for Al2O3/Y3Al5O12/ZrO2 alloys, Çadırlı et al. [41] for Sn–Cu eutectic alloy and Böyük and Maraşlı [29] for Sn–Ag–Cu eutectic alloy, respectively.
3.2.
Effect of Temperature Gradient on the Eutectic Spacing
The influence of the temperature gradient on eutectic spacing has not been considered in theoretical studies. However, the influence of the temperature gradient cannot be ignored for regular and irregular eutectic systems. So, the influence of the
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Fig. 5 – The shematic illustration of rod spacings measurements from the longitudinal and transverse sections (a) transverse section, (b) longitudinal section, (c) Shematic view, (d) intersection method, (e) triangle method, (f) area counting method.
temperature gradient on the eutectic spacing was investigated by several authors [42–46]. The variations of rod spacing with temperature gradient at a constant growth rate (V = 11.52 μm/s) are shown in Fig. 7. The variation of the eutectic spacing versus temperature gradient is essentially linear on the logarithmic scale. It can see Fig. 7, the data form straight lines, the linear regression analysis gives the proportionality equation for the constant growth rate as, λ = k2 Gm ðfor the constant VÞ
ð2Þ
where k is a constant and m is an exponent value of the temperature gradient. The relationships between the eu-
tectic spacing and temperature gradient were determined as λ = 0.27G − 0.48 by using linear regression analysis. The values of the exponent relating to the temperature gradient for eutectic spacing were found to be 0.48. The value of the exponent relating to the temperature gradient obtained in this work is in good agreement with the value of 0.40, 0.53, 0.46 and 0.49 obtained by Kaya et al. [42], for In–Bi–Sn, Böyük et al. [23] for Al–Cu–Ag and Böyük and Maraşlı [43] for Sn–Ag–Cu ternary eutectic alloys, respectively. Besides, this exponent value of 0.48 is bigger than the values of 0.33, 0.30 and 0.28 obtained by Toloui and Hellawell [44] for Al–Si, Çadırlı et al. [45] for Al–Cu and Kaya et al. [46] for Sn–Zn binary eutectic alloys, respectively.
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4. The variation of eutectic spacing with the temperature gradient for directionally solidified Sn–Ag–Zn eutectic alloy was investigated and relationships between the eutectic spacing and the temperature gradient were obtained by binary regression analysis as, λ = 0.27G − 0.48.
Acknowledgement This project was supported by the Erciyes University Scientific Research Project Unit under Contract No: FBA 10-2891. The authors are grateful for Erciyes University Scientific Research Project Unit for their financial support.
Fig. 6 – Variation of eutectic spacing as a function of growth rate at a constant temperature gradient (G = 4.33 K/mm).
4.
Conclusions
1. In the present work, Sn–Ag–Zn eutectic alloy was solidified unidirectional using a Bridgman type directional solidification furnace and the microstructures were observed to be the eutectic matrix β–Sn and rod Ag3Sn intermetallic. 2. Eutectic spacing decrease inversely as the square root of the growth rate for directionally solidified Sn–Ag–Zn eutectic alloy at a constant temperature gradient. The relationships between the eutectic spacing and growth rate were obtained by binary regression analysis as, λ = 10.42V − 0.53. 3. The value of bulk growth rate was determined by using the experimental values of λ and V and found to be λ2V = 86.39, which is in good agreements with the bulk growth rate values obtained in previous works.
Fig. 7 – Variation of eutectic spacing as a function of temperature gradient at a constant growth rate (V = 11.52 μm/s).
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