Microhardness and tensile stress of directionally solidified the Bi-Sn-Ag ternary

1. 2nd International Advances in Applied Physics and Materials Science Congress Microhardness and tensile stress of directionally solidified the Bi-Sn-Ag ternary alloy under controlled conditions H. Kaya 1, S. Engin 2, E.Çadırlı 3, U. Böyük 1, N. Maraşlı 4 1)Department of Science Education, Faculty of Education, Erciyes University, Kayseri, Turkey 2)Department of Physics, Institute of Science and Technology, Erciyes University, Kayseri, Turkey 3)Department of Physics, Faculty of Science, Niğde University, Niğde, Turkey 4)Department of Physics, Faculty of Science, Erciyes University, Kayseri, Turkey Abstract The development of lead-free solders has emerged as one of the key issues in the electronics packaging industries. Bi-Sn-Ag ternary alloy has been considered as one of the lead-free solder materials that can replace the toxic Pb-Sn eutectic solder without increasing soldering temperature. This study investigates the effect of temperature gradient and growth rate on the mechanical of the Bi-Sn-Ag alloy.Bi-43.47 wt.% Sn-0.68 wt.% Ag alloy was directionally solidified upward with different growth rates (V=6.6-132.8 mm/s) at a constant temperature gradient (G=2.3 K/mm) and with different temperature gradients (G=2.3-5.7 K/mm) at a constant growth rate (V=13.2 mm/s) in the Bridgman-type growth apparatus. The microhardness (HV) and tensile stress (s) were measured from directionally solidified samples. The dependency of the HV and s for directionally solidified Bi-Sn-Ag alloy on the solidification parameters (G, V) were investigated and the relationships between them were obtained by using regression analysis. According to present results, HV andsof directionally solidified Bi-Sn-Ag alloy increase with increasing G and V. The microhardness and the tensile stress were found to be G and V dependent, varying from 199 to 274 MPa and 86 to 135 MPa, respectively. The establishment of the relationships among HV, s, G, and V can be given as HV=kG0.14, HV =kV0.11, s =kG0.50, s =kV0.14 1. Introduction In recent years, increasing environmental and health concerns over the toxicity of lead combined with the strict legislation to ban the use of lead-based solders have provided an inevitable driving force for the development of lead-free solder alloys [1-3]. Among those lead-free solder alloys the Bi-Sn-Ag alloy has received more attention. Bi-43.47 wt.% Sn-0.68 wt.% Ag solders possess several fascinating features such as low cost as well as low reflow temperature of 415 K. In addition, Bi-43.47 wt.% Sn-0.68 wt.% Ag alloy offers better mechanical properties (high joining strength, good wettability) than the conventional Pb-Sn solders [4]. Directionally solidified Bi-43.47 wt.% Sn-0.68 wt.% Ag ternary alloy can be a suitable candidate for replacement of Pb-Sn solder due to its convenient mechanical and thermo-physical properties [5] and relatively low cost, however it needs more study. Thus the aims of present work were to study the dependency of microhardness and tensile stress on the solidification processing parameters (V and G) for directionally solidified Bi-43.47 wt.% Sn-0.68 wt.% Ag alloy. 2. Experimental Procedure Bi-43.47 wt.% Sn-0.68 wt.% Ag alloys were prepared by melting weighed quantities of Bi, Sn and Ag (high purity >99.99 %) in a graphite crucible placed into a vacuum melting furnace[6]. After allowing time for the melt to become homogeneous, the molten alloy was poured intographite crucibles in a hot filling furnace. Each sample was than positioned in a Bridgman–type furnacein a graphite cylinder[6]. Solidification of the samples was carried out with different temperature gradients (G=2.3-5.7 K/mm) at a constant growth rate (V=13.2mm/s) with different growth rate ranges (V=6.6–132.8 μm/s) at a constant temperature gradient(G=2.3 K/mm)in the Bridgmantype growth apparatus (Fig. 1). Fig. 1.Experimental setup 2.1. Measurement of growth rate (V) and temperature gradient (G)and eutectic spacing (l) The temperature in the sample was measured with K-type 0.25 mm in diameter insulated three thermocouples which were fixed within the sample with spacing of 10mm. In the present work, a 1.2 mm OD × 0.8 mm ID alumina tube was used to insulate the thermocouples from the melt. All thethermocouple’s ends were then connected the measurement unit consists of data-logger and computer. The cooling rates were recorded with a data-logger via computer during the growth. When the solid/liquid interface was at the second thermocouple, the temperature difference between the first and second thermocouples (T) was read from data-logger record. The time taken for the solid-liquid interface phases the thermocouples separated by known distances was read from data-logger record. Thus, the value of growth rate (V= X / t) for each sample was determined using the measured value of t and known value of X. The temperature gradient (G =T/X) in the liquid phase for each sample was determined using the measured values of T and X. The eutectic spacing (lL) were measured with a linear intersection method [7] on the longitudinal section. Two different methods were used to measure the rod spacing (lT) on the transverse sections. The first method is the triangle method [8]. The second method is the area counting method [9]. Typical Optical image is shown in Fig.2. The microstructure consists of complex regulareutectic in the Sn-rich matrix phase. 2.2.The measurement of microhardness (HV) Microhardness measurements in this work were made with a Future-Tech FM-700model hardness measuring test device using a (10-50) g load and a dwell time of 10 s giving a typical indentation depth about 40-60 mm, which is significantly smaller than the original solidified samples. The microhardness was the average of at least 30 measurements. The variations of microhardness with the solidification processing parameters are plotted and given in Fig.3. Fig.3Microhardness of the Bi-Sn-Ag eutectic alloy as a function of G and V It can be seen from Fig.3,G and Vvalues are increasing with the HVvalues increased. It is found that the HV increases from 21.4 kg/mm2 to 23.8kg/mm2, when temperature gradient increasing from 2.3 K/mm to 5.7 K/mmand also HV increases from 19.9 kg/mm2 to 27.4kg/mm2, when growth rate increasing from 6.6 mm/s to 132.8 mm/s.The relationshipsamong the HV , G and Vare given in Table 1. As can be seen from Table 1,the exponent values of G and Vare equal to 0.14 and 0.11, respectively. 2.3.The measurement of tensile stress (s) The measurements of tensile stress were made at room temperature at a strain rate of 10-3 s-1 with a Shimadzu AG-IS universal testing machine. The round rod tensile samples with diameter of 4 mm and gauge length of 30 mm were prepared from directionally solidified rod samples with different temperature gradients (G=2.3-5.7 K/mm) at a constant growth rate (V=13.2mm/s)anddifferent growth rates (6.6–132.8 μm/s) at a constant temperature gradient (G=2.3 K/mm). The tensile axis was chosen parallel to the growth direction of the sample. The tensile tests were repeated three times and the average value was taken. It has been found that a standard deviation is approximately 5 %. Typical stress–strain curves of Sn-Bi-Ag eutectic alloy are shown in Fig.4. As can bee seen from Fig.4 values are increased with increasing G and V, but strain (%) values are decreased. Fig.4a Typical stress-strain curvesat a constant V for unidirectional solidifiedSn-Bi-Ag eutectic alloy • Fig.4b Typical stress-strain curves at a constant G for unidirectional solidified Sn-Bi-Ag eutectic alloy • Figure 5 show the results of the s as a function of Gand V for Sn-Bi-Ag eutectic alloy . It can be seen from Fig.5, G and V values are increasing with the svalues increased. It is found that the s increases from 86.5 MPa to 135.8 MPa, when temperature gradient increasing from 2.3K/mm to 5.7K/mm and also s increases from 75.6 MPa to 116.5 MPa, when growth rate increasing from 6.6mm/s to 132.8mm/s The relationshipsamong the s,Gand V are given in Table 1. The exponent value of G and V is equal to 0.50 and 0.14, respectively. • Fig.5Variation of the s with G and V • Table 1 The regression relatioships among G, V , HV and s • 3. Result and Conclusions • In this work, the influence of the G and V on the HV and sof unidirectional solidified Sn-Bi-Ag eutectic alloys were investigated. Obtained results are summarized as follows; • By increasing G and V, HV increases. The establishment of the relationships among G, V and HVcan be given as • HV=kG0.14 , HV=kV0.11 • 2. Also, by increasing G and V, tensile stress increase and strain decreases. The establishment of the relationships among G, V andscan be given as • s=kG0.50, s =kV0.14 • 3. As can be seen from exponent values in the obtained relationships, the effect of G at a constant V is more effective on microhardness (HV) and tensile stress (s) of studied alloys than do growth rate (V) at a constant temperature gradient (G). 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B26, (1995) 1049 Fig.2The optical image of directionally solidified Bi-43.47 wt.% Sn-0.68 wt.% Ag eutectic alloy (G=2.3 K/mm, V=132.8 mm/s)