WETTABILITY AND ELECTRICAL PROPERTIES OF Bi-Sn BASED LEAD FREE SOLDER ALLOYS

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1 International Journal of Physics and Research (IJPR) ISSN Vol. 3, Issue 3, Aug 2013, 1-6 TJPRC Pvt. Ltd. WETTABILITY AND ELECTRICAL PROPERTIES OF Bi-Sn BASED LEAD FREE SOLDER ALLOYS RIZK MOSTAFA SHALABY Metal Physics Laboratory, Physics Department, Faculty of Science, Mansoura University, Mansoura, Egypt ABSTRACT Wettability, electrical resistivity and thermo-elasticity of five systems namely, Bi-42wt. % Sn, Bi-40 wt. %Sn-2 wt. % In, Bi-40 wt.% Sn- 2 wt.% Ag, Bi-38 wt.% Sn-2 wt.% In- 2 wt. % Ag and Sn-wt.%37Pb have been investigated and analyzed. The examined physical properties are improved by addition Ag and In at small level, 2 wt. % to the Bi-42Sn based alloy. These improved properties indicate that these alloys are adequate for low temperature soldering applications. Contact angle measurements demonstrated that In and Ag additions improved wetting/spreading performance on Cu; a contact angle of was observed for Bi-38Sn-2In-2Ag. In this study, the addition of In and Ag decreases the contact angle and hence remarkably improve the wettability. Also the electro-resistivity of Bi-38Sn-2In-2Ag solder alloy is about ρ = Ω.cm is much lower than that of Sn-37Pb solder. It is recommended that the quaternary Bi-38Sn-2In-2Ag alloy consider as a suitable substitutes for Sn-37Pb traditional solder alloy. KEYWORDS: Small Addition, Electrical Resistivity, Thermo-Elasticity, Longitudinal Strain, Wettability INTRODUCTION The Bi-Sn has recently been considered as one of the lead free solder materials that can replace the Sn-37Pb solder without increasing soldering temperature. Bismuth has also been used as the alloying element in the binary Sn-Bi systems to provide suitable substitutes for Sn-Pb solder alloys. Sn-Bi based alloys are an alternative to replace Pb-based solders in the low temperature ranges [1]. Moreover, the low-temperature Bi-Sn solder alloy has many advantages in special soldering applications, such 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 classification packaging. In addition Bi-42Sn solder is suitable for hermetic packaging of electronic device and microelectromechanical systems (MEMS) movable parts [2]. Wettability and mechanical properties of Sn-Ag-Bi are described in [3]. It is found that contact angle measurements demonstrated that Bi additions improved wetting/spreading performance on Cu; a minimum contact angle of 31±4 was observed with 4.83 wt. % Bi addition. The measurement of contact angle is used to determine the extent of wetting, which, in turn, is an issue in the reliability of electronic packaging. With the advent of the new Pb-free solder alloys, there is a renewed interest to evaluate the contact angles of the proposed alloys as a measure of their wettability. The experimental assessment of the solder wettability can be used to predict the performance of the new solder alloys. The wetting-balance [4] and sessile-drop techniques [5-7] have been extensively used to measure the contact angles in order to determine wettability. However, there are still few wettability data for the Pb-free solders. As one of the most promising candidates of the Pb-free solder alloys, Sn-Bi-Xternary systems have been studied in mechanical properties and interfacial reactions between solders and substrates. The purpose of this study is to investigate the wetting behavior of the Bi-Sn Pb-free solder by measuring the contact angles on the copper substrate. The wetting behavior of a new Sn-Bi-Cu Pbfree solder on Cu substrate was investigated by sessile drop method an Ar-H2 flow in the temperature range from 493 K to 623 K was studied [8]. The results show that 69.5Sn-30Bi-0.5Cu exhibits good wettability on Cu substrate. Variation of

2 2 Rizk Mostafa Shalaby electrical resistivity for Sn-Bi-Zn ternary alloys with the temperature in the range of K was also measured by using a slandered dc four-point probe technique [9]. Indium and silver additions at 2 wt. % were made to the Bi-Sn alloy in a study to develop a Bi-Sn binary composition. Effect of silver and indium addition on mechanical properties and indentation creep behavior of rapidly solidified Bi-Sn based lead free solder alloy was studied by [10]. It is concluded that the In and Ag containing solder alloy exhibited a good combination of higher creep resistance, good mechanical properties and lower melting temperature as compared with Pb-Sn eutectic conventional solder alloy. Despite all of previous investigations the literature lacks any data for the wettability or electrical resistivity of the dilute Sn-Bi alloys containing small addition of Ag and In at 2 wt.% introduced as the third component. It is necessary to overcome some lacks in literature such as wettability, electrical resistivity and thermo-elasticity to obtain improved Bi-Sn solder. We should be combination between low cost, good wettability and electrical resistivity for electronics assembling. It is therefore the aim of this paper to study the effect of small addition of Ag and In content on wettability, electrical resistivity and thermoelasticity of the Bi-Sn alloys to be used for electronic packaging applications. EXPERIMENTAL PROCEDURES Alloy Design and Preparation In the present study Bi-42wt.% Sn, Bi-40 wt.%sn-2 wt.% In, Bi-40 wt.% Sn- 2 wt.% Ag and Bi-38 wt.% Sn-2 wt.5in- 2 wt.% Ag and Sn-37 wt.% Pb were prepared from high-purity Bi, Sn, In, Ag and Pb (99.99 %) by a single cooper roller melt spinning technique. Required quantities of the used metals were weighed out and melted in a porcelain crucible. After the alloys were molten, the melt was done in air at a melt temperature 500 C. The speed of cooper wheel was fixed at 2900 r.p.m., which corresponds to a linear speed of 30.4 ms -1 to produce ribbons of 4mm wide and 60 µm thick. In situ resistivity measurements have been carried out using the so-called double bridge circuit [11]. The contact angle measurement were performed on Bi-42wt. % Sn, Bi-40 wt. %Sn-2 wt.% In, Bi-40 wt.% Sn- 2 wt.% Ag and Bi-38 wt.% Sn-2 wt.5in- 2 wt.% Ag and Sn-37 wt.% Pb on Cu substrate at 277 C for 60 s elapsed time. The contact angle obtained photographically as a function of solder alloys. Theoretical Considerations Electro-Resistivity and Thermo-Elasticity In order to determine the longitudinal strain the experimental sample is thermally distorted for its electrical resistivity to change. On the basis of the study carried out by Aidun et al [12] and Kamal et al [13], if a piece of a metal is thermally distorted, its electrical resistance will change. The electrical resistivity ρ is a fundamental property, it is of interest to determine from R versus T data the relative electro-resistivity Δρ/ρ where Δρ represents the change in the electrical resistivity due to the temperature T and ρ is the value of the resistivity before the application of the temperature. For a sample of cross sectional area A and length L the resistance is R= ρl/a (1) It follows from this equation that the temperature will produce (2) Where Δρ/ρ is the relative thermo-elasticity, Δρ represents the change in the electrical resistivity due to heating and ρ is the value of the resistivity before the application of this heating. In according with Takayama and Maddin [14] the change of the resistivity, Δρ, as a function of the longitudinal strain ε xx can be expressed as

3 Wettability and Electrical Properties of Bi-Sn Based Lead Free Solder Alloys 3 (3) Where, ρ and R 0 are the resistivity and resistance in an unheated sample respectively. The quantity ΔL/L can be calculated from Young's modulus Y, i.e. ΔL/L=F/YA (4) Where F is the force applied to the sample, on the assumption of isotropic behavior, from the poisson's ratio it follows that ΔA/A= - 2 F/YA (5) Thus (6) The experimental values of ΔR/R and the calculated values of Δρ/ρ for the solder material are shown in Figure 3. Wettability Measurement Solder spreading on a substrate is fundamental for the development of new solders. It is a complex phenomenon involving joint configuration, flux selection, substrate finishing and many other factors [15]. When the solder melts, the increase in contact area indicates the wetting behavior of solder [16]. The extent of wetting is measured by the contact angle that is formed at the juncture of a solid and liquid (molten solder), as shown in Figure 1. Wetting is essential to solder joint reliability. Rewetting is one of the most common failure mechanisms in solder joint. Spreading test was used here to determine the solder joint wettability. In a spreading test, the spreading diameter and height of a solder bump were measured. Assuming a bump takes an ideal shape of a spherical cap, the solder volume can be calculated by Equation 7 [17]. The wetting angle, α, can be calculated by Equation 9, which does not dependent on the quantity of the solder. (7) (8) (9) Figure 1: Wetting Angle Calculation

4 4 Rizk Mostafa Shalaby Spreading tests were conducted with either solder paste or solder performs. The results of the spread test using solder pastes are shown in Figure 1. The test was conducted on copper substrate. The peak reflow temperature for eutectic Sn-42Bi is 170 C. The results show that Sn-37Pb control solder pastes wetted better (with smallest α) than Bi-42Sn. However, the wetting of Bi-42Sn on copper substrate is still acceptable (α is smaller than 45 ). RESULTS AND DISCUSSIONS Electro-Resistivity and Thermo-Elasticity Electrical resistivity is usually used in measuring the electrical conductivity of the alloys, and as a physical property of an electronic packaging material, it also attracted more and more researcher s attention. The electrical resistivity behavior of the melt spun Bi-42Sn and Sn-37Pb solder alloys is well summarized by the curves shown in Figure 2. It is found that the electrical resistivity of the studied melt spun lead free solder alloys increase with rise in temperature. Thermal vibration, foreign atoms in solid solution and plastic deformation of the lattice increases the resistivity. At higher temperatures the thermal disturbance of the lattice can be described in terms of quantized elastic waves or phonons, and the increase in electrical resistivity can be visualized as resulting from collisions between electrons and phonons. Moreover, by addition of bismuth to Sn, solid solution formation causes a pronounced increase of the electrical resistivity. It is found that the resistivity of Bi-Sn rapidly solidified alloy is about 13 x 10-6 ohm.cm due to high concentration of vacancies in the matrix, which hinder the motion of conduction electrons from one site to another in the matrix. Also, it is found experimentally that bismuth atoms, as a semimetal, act as scattering centers, and increasing the concentration of bismuth results in an increase resistivity (or lowers the conductivity). Generally speaking, the semimetals have properties somewhere between those of semiconductors. Because of their crystal structure, this is usually anisotropic in electrical properties and because of the small number of charge carriers. In the case of melt-spun ribbons of Sn-Bi-Ag, the resistivity falls down to the value 12.92x10-6 ohm.cm. It is clarified that when the silver content added to Sn-Bi a decrease in resistivity occurs because of the disruption of the orderly atomic arrangement is removed during the solidification of Sn-Bi -Ag, so a net decrease in the resistivity occurs. Bi-Sn-2Ag-2In has the minimum electrical resistivity value about 7.8x10-6 ohm.cm whose value is lower than Sn-37Pb and Bi-42Sn. However, the resistivity enhancement of Ag of 2 wt. % addition to the basic alloy Bi-Sn is also obvious, i.e. the addition of the Ag as an alloying element enhances the electrical conductivity of the type of alloy due to the fact that the action of this alloying element reduces the scattering centers and decrease of cracks propagation due to thermal effects. Typical relative electro-resistivity strain curves are shown in Figure 3 for Bi-42Sn and Sn-37Pb solder alloys. It should be noted that the relative electro-resistivity strain curve has a positive deviation from a straight line. It is suggested that internal atomic arrangements occurs below the elastic limit. Figure 2: Electrical Resistivity as a Function of Temperature of all Prepared Solders

5 Wettability and Electrical Properties of Bi-Sn Based Lead Free Solder Alloys 5 Wettability Figure 3: Relative Electro-Resistivity as a Function of Longitudinal Strain of Solder Alloys Wettability plays a key role in evaluating the soldering technology of lead-free solders. The wettability represents one of the major properties for critical development of the soldering process. In this work, the wettability of Bi-42Sn and Sn-37pb based solder alloys were evaluated by the photographically method which can reflect on the wettability of the solder on a copper substrate under a certain environment [18]. The contact angle measurement were performed on rapidly solidified Bi-42wt.% Sn, Bi-40 wt.%sn-2 wt.% In, Bi-40 wt.% Sn- 2 wt.% Ag and Bi-38 wt.% Sn-2 wt.5in- 2 wt.% Ag and Sn-37 wt.% Pb on Cu substrate at (530 K) for 60 s. elapsed time. The contact angle obtained photographically as a function of solder as shown in Figure 4. Sn-37 Pb solder has a very low contact angle of 8 compared to Bi-42Sn solder which have The contact angle of the Bi-42Sn solder alloys is relatively high, because Bi is a surface active element and the surface tension of the liquid Bi-Sn eutectic alloy is far lower than that of Sn-Pb or Sn-Zn eutectic alloys in the same state [19]. The contact angle data indicated that Ag and In additions to Bi-Sn solder alloy resulted in a decrease in the contact angle values from 40.5 for Bi-Sn alloy to for Bi-38Sn-2Ag-2In alloy. After addition Ag and In decrease the contact angle hence improved the wettability. Comparing the Bi-42Sn solders between the In-doped solder and Ag-doped solder, it was found that the addition of trace amounts of In or Ag improves the wettability of the Bi-42Sn solder. These improvements obtained by adding trace amounts of In or Ag elements can be explained through the effect of small elements on the interfacial tensions. As rare small elements have surface-active properties, they can more easily agglomerate at the solder/flux interface in the molten state, which can decrease the interfacial surface tension between the liquid solder and substrate and accelerate the wetting of the solder alloys on the copper substrate. CONCLUSIONS Figure 4: Contact Angle on Cu Substrate In present work, the influence of small addition of Ag and In on the wettability, electro-resistivity and thermoelasticity of Bi-42Sn binary alloy were investigated. The results are summarized as follows: The wettability, electrical resistivity and longitudinal strain of Bi-42Sn based solder alloy can be significantly improved by adding an appropriate amounts of Ag and In at 2 wt. %. The addition of small amounts of Ag or In elements has high influence on the contact

6 6 Rizk Mostafa Shalaby angle of Bi-42Sn lead free solder alloy. Experimental results show that the best contact angle and electrical resistivity for quaternary alloy Bi-38Sn-2In-2Ag are and ohm.cm respectively. Consequently, adding small amounts of Ag or In elements can significantly improves the wettability of Bi-42Sn based solder joints. Moreover, adding small amount of Ag element as a ternary element was superior to adding small amount of In for improving the properties (electro-resistivity and longitudinal strain) of the Bi-42Sn solder. Small addition of Ag and In at 2 wt.% can improve the reliability of Bi-42Sn solder joints in practical electronic applications. Hence, it is confirmed that the quaternary alloy Bi- 38Sn-2In-2Ag is the best solder for all studied physical properties and considered as lead free solder material that can replace the Sn-37Pb solder. ACKNOWLEDGEMENTS This work is carried out at metal physics laboratory, Faculty of Science, Mansoura University under supervisor Prof. Dr. Mustafa Kamal chairman of metal physics group at Physics Department, Faculty of Science, Mansoura University. Thanks to Prof. Dr. Mustafa Kamal for his co-operation. REFERENCES 1. C.H.Raeder, L.E.Felton,D.B.Knorr, G.B.Schmeelk, and D.Lee, IEE/CHMT Int l Electron Manufacturing Technology Symposium (1993),p D.M.Zhang, G.F.Ding, H.Wang, Zh.Jiang, and J.Y.Yao, J.Funct.Mater.Dev.12, (2006) P.T.Vianco, J.A.Rejent, Journal of Electronic Materials, 28, 10 (1999) Wei XQ, Huang HZ, Zhou L, Zhang M, Liu XD. Mater Lett 61 (2007) Kozlova O, Voytovych R, Devismes MF, Eustathopoulos N. Mater Sci Eng A 495(2008) Ma GF, Liu N, Zhang HF, Li H, Hu ZQ, J Alloys Compd 456(2008) Arenas MF, Acoff VL, J Electron Mater 33(2004) Likun Zang, Zhangfu Yuan, Hongxin Zhao, Xiaorui Zhang, Materials Letters 63 (2009) Emin Cadirli, Ugur Boyuk, Hasan Kaya, Necmettin Marasli, Journal of Nin-Crystalline Solids, 357, (2011) R.M.Shalaby, Materials Science and Engineering A, 560 (2013) Y.A.Geller and A.G.Rakhshtadt." Science of Materials" Mir publishers, Moscow, R.Aidun, Sigurds Arajs and M.C.Martin, Materials science and engineering, 50 (1981) M.Kamal and A.B.El-Bediwi, Radiation effects and Defects in solids, 147, (1999) S.Takayama and R.Maddin, Scripta Metallurgica, 9, (1975) Saiz, E.,C.W.Hwang, K.Suganuma, and A.P.Tomsia.Acta Materialia, 51: (2003) Wu,C.M.L., D.Q.Yu, C.M.T.Law, and L.Wang. J. Electronic Materials. 31(9): (2002) F, Hua, Z. Mei, J. Glazer 48th Electronic Components and Technology Conference, 1998, pp Z.G.Chen, Y.W.Shi, Z.D.Xia, and Y.F.Yan, Electron Process Technol.2, (2003) B.Huang and N.C.Lee, Int.Symp.Microelectron.Proc.3906, (1999) 711.