FABRICATION AND CHARACTERIZATION OF Zn-xSb ALLOYS AS CANDIDATE TO REPLACE Pb BASED SOLDER ALLOYS

Transcription

1 Available online at Proceedings of the 1 st International Conference on Engineering Materials and Metallurgical Engineering December, 2016 Bangladesh Council of Scientific and Industrial Research (BCSIR) Dhaka, Bangladesh FABRICATION AND CHARACTERIZATION OF Zn-xSb ALLOYS AS CANDIDATE TO REPLACE Pb BASED SOLDER ALLOYS Md Khairul Islam a, *, Md Arifur Rahman Khan a, Toufiq Ahmed a, Aninda Nafis Ahmed b, M. A. Gafur b ABSTRACT a Institute of Mining, Mineralogy and Metallurgy, BCSIR, Joypurhat-5900, Bangladesh b Pilot Plant and Process Development Centre, BCSIR, Dhaka-1205, Bangladesh The sustenance and further upgradation of modern electronics largely depends on the suitable replacement of Pbcontaining electronic solder alloys due mainly to their toxicity and health hazardous nature. Zinc based solder alloys can be very promising candidate to substitute Pb- based solder alloys especially in electronic applications. In this research, zinc based alloys containing 1.5, 2.5, 3.5 and 4.5 wt. % antimony (Sb) have been fabricated through casting method in permanent metal mold and several properties of these alloys were analyzed thoroughly with a view to exploring their suitability to be used instead of Pb containing solders. Fine grains comprising with eutectic phase of Zn-Sb and Zn phase were revealed in the microstructure of these alloys. Microhardness of the alloys showed an incremental trend with increasing Sb content in the alloys upto 3.5wt% Sb while hardness decreased in Zn-4.5%Sb alloy. TG-DTA results indicate that melting of these alloys starts at around 410 C and finishes at around 430 C, whereas, literature shows melting of the eutectic alloy (Zn-2.5Sb) occurs at 411 C. Thermal and Mechanical properties of the alloys have been correlated with composition and microstructure. Finally, conclusion has been drawn that, proposed Zn-Sb alloys might be a suitable replacement for Pb based solder alloys. Keywords: Pb- free, Solder, Electronic,toxicity, Mechanical properties 1. INTRODUCTION With the increasing demand of electronic products the need for manufacturing reliable solder alloys has been increasing rapidly. Solder alloys are widely used to join electronic components. Due to mainly their good wettability, high ductility and low shear modulus, Pb containing alloys such as the Pb 5Sn and Pb 10Sn have been extensively used in die attaching of power devices. Not only this, these alloys still remain the exemptions in Restriction of Hazardous Substances (RoHS) directive [1]. To date there is no reliable high-temperature solder alloy which can entirely replace these high-pb solders [2]. Therefore it is essential to search for a suitable high-temperature solder having the similar malting characteristic as Pb 5Sn or Pb 10Sn, as well as the other thermal, mechanical and electrical properties. Numerous candidates including 80Au 20Sn [3, 4], Bi based alloys [5, 6],Sn Sb based alloys [7], and Zn based alloys have been reported [8 11] in the existing literature. One of the most important characteristics is the high-temperature performance, which requires maintaining high-temperature strength at the reflow temperature of the soldering process. Other typical requirements of high temperature solders are melting range in ºC, good electrical conductivity, good mechanical properties, and fluxlessness[12]. Among the high temperature Pb-free solder candidates gold-based alloys are highly expensive, while Au Sn alloy form brittle intermetallic compounds and thus limits the use as high temperature solder[13-14]. Bismuth-based alloys, especially Bi Ag-based alloys also become brittle due to the similar reason and exhibit relatively low electrical/thermal conductivities [15-16]. In contrast Zn-based alloys, especially Zn Sn and Zn Al, are more ductile compared to others as there found no intermetallic compounds in these alloys, have proper melting range, and good thermal/electrical conductivities [17-18]. Moreover, these alloys are comparatively inexpensive since Zn is relatively cheaper. Although a lot of study have been performed in Zn based alloys, with the authors concern there are very few studies in Zn-Sb binary alloys as a replacement of Pb based high temperature solder alloys. In this study, binary Zn-xSb (x= wt% Sb) alloys have been studied in order to explore their potential in replacement of the conventional Pb containing alloys. 2. EXPERIMENTAL

2 Zn- Sb solder alloys were developed from Zn (99.9%) and Sb (99.9%) available in local market. First of all, Zn and Sb were cut into small pieces from the respective ingot. The calculated amount of Zn as well as Sb was taken in a cleaned graphite crucible. The graphite crucible containing the Zn and Sb was put into a muffle furnace for melting. The metals were heated at 600 C and held there for 10 minutes. By this time, the metals became molten. The crucible containing molten metal was taken out of the furnace, stirred mechanically for a short while to homogenize the molten alloy and finally poured into a metal mold. Prior to stirring and pouring a degasser (NH 4 Cl salt) was used to remove entrapped gases present in the molten alloy. The same procedure was followed for all the compositions of the selected alloy. Several pieces of specific dimensions were cut from the as cast alloys for different characterization. To perform microstructural analysis rectangular specimens were prepared. After progressive grinding in several SiC grit papers, the specimens were polished in a polishing cloth fixed on the rotating polishing wheel. Fine gamma alumina powder was used as polishing media. After final polishing stage a smooth mirror polished surface was obtained. The specimens were rinsed with distilled water, dried and finally dried with acetone. The prepared surface of the specimen was etched with an etching solution containing 95 ml ethanol (96%) and 5 ml hydrochloric acid (32%) to reveal the microstructure in metallurgical microscope. Hardness test was performed in a Vickers microhardness tester (MATSUZAWA Hardness tester, Japan). For every specimen load of 10Kg was applied during 20 seconds at a speed of 300µm/sec. Five indentations were taken in every specimen and the average is reported with standard deviation. Powdered sample was collected from cutting of the as cast alloy ingots separately. Small amount of powdered sample (15-20 mg) was taken in a platinum pan and then put into the chamber. These were heated at a rate of 20 ºC /min from 25 to 600ºC along with a reference of Alumina in SII EXSTAR TG/DTA DTA curves were obtained and analyzed the change of phase. XRD analysis was carried out using a Bruker D8 advance (Germany). Cylindrical samples of 5mm diameter and 8mm height were used for XRD. Cu-Kα radiation with wavelength of nm was used for the inspection. Line counts was plotted against the diffraction angle 2θ starting from 10ºto 90ºwith an increment of 0.02º. The obtained peaks in different 2θ position were analyzed with the standard available for the same experimental condition. 3. RESULTS AND DISCUSSION 3.1 DIFFERENTIAL THERMAL ANALYSIS (DTA) Melting characteristics of the proposed solder alloys were determined from DTA results. The melting range of the alloys are shown in table 1 as well as in fig. 2. The temperature range of melting differs only slightly with the changing of composition in the range of wt% Sb in Zn. Since, in this study Zn is alloyed with different amount of Sb, the melting curves do not show a single melting temperature indicating the melting point of the pure Zn, rather the curves show a range of temperature within which the complete melting of the alloys accomplishes. However, the melting temperature of the alloyed zinc has decreased to some extent due to alloying with Sb. The melting point of virgin zinc is C, after alloying with Sb the onset of melting is found in around 409 C significantly lowering the melting temperature. Zn-Sb alloys have two eutectic compositions which is evident from the binary phase diagram of Zn-Sb (Fig.1). One eutectic is at 1.7 wt.% antimony with a melting point of 412 C and another eutectic at 80 wt.% antimony showing a melting point of 505 C. In this study the lower melting region, otherwise said higher Zn containing compositions, was chosen with a view to getting a lower melting temperature of the solder alloys. Phase diagram shows that solidification range increases with left or right shifting of compositions from the eutectics. This is also evident from the experimental findings of this study. 148

3 FIG. 1: BINARY PHASE DIAGRAM OF ZN-SB [19] DTA (uv) Sb-Zn 3.5 Sb-Zn 2.5 Sb-Zn 1.5 Sb-Zn Temperature ( C) FIG. 2: DTA CURVES OF THE Zn-xSb SOLDER ALLOYS 149

4 st Proceedings of the 1 ICEMME, 22-24, Dec, 2016, Dhaka, Bangladesh Table 1: Melting Characteristics of Zn-xSb solder alloys Sample ID Melting Start ( C) Zn-1.5 Sb Zn-2.5 Sb Zn-3.5 Sb Zn-4.5 Sb Melting Finish ( C) Melting/Solidification Range ( C) MICROSTRUCTURAL ANALYSIS a c b d FIG. 3:MICROSTRUCTURE of (a) Zn-1.5 Sb (b) Zn-2.5 Sb (c) Zn-3.5 Sb and (d) Zn-4.5 Sb in 500X MAGNIFICATION. Fig. 3(a-d) shows the optical micrographs of the Zn-xSb solder alloys. Zn-1.5 Sb alloy contains at least two phases where the phase which seems to be black is actually alternate layer of Zn and epsilon (ε) otherwise known as eutectic phase. There are two eutectic phase in the Zn-Sb system. Here the eutectic might be the right

5 eutectic shown in the phase diagram with a composition of Zn-1.7wt% Sb since the selected composition ranges from wt% Sb. The other phase which looks white is Zn matrix. Fig. 3 (b, c and d) also shows the similar structure comprising eutectic and Zn phase. The difference in phase contrast may be due to the etching effect. 3.2 X RAY DIFFRACTION ANALYSIS (XRD) Figure 4 shows the XRD pattern of the selected Zn-Sb alloys along with standard pattern of pure zinc. The XRD pattern of the alloys are compared with the standard data of pure zinc in the range of 10 to 90º of Two- Theta value (2θ=10-90º). The sharp peaks of all the alloys in the lower as well as higher Two-Theta value very well match with the standard Zn pattern. From fig.2 it is observed that there is a very little or no shift in the peak position of the Zn-Sb alloys with respect to the position of standard Zn. The analysis with PDF-2 revealed no evidences of forming any phase containing Zn and Sb although there is presence of some phases such as eutectic phase are present in the microstructure of the alloys. This is due to the fact that, the amount of Sb was less than 5wt% and XRD could hardly detect if the constituent is less than 5%. However, there was difference in the peak intensity and hence in full width at half maxima (FWHM) values which is related to the crystallite size of the respective alloys. The average crystallite size of all these alloys was calculated using Scherrer equation. τ = kλ β cos θ Where, τ is the mean size of the ordered (crystalline) domains, which may be smaller or equal to the grain size; K is a dimensionless shape factor, with a value close to unity. The shape factor has a typical value of about 0.9, but varies with the actual shape of the crystallite; λ is the X-ray wavelength; here used Cu k-alpha, λ= nm β is the line broadening at half the maximum intensity (FWHM), after subtracting the instrumental line broadening, in radians. This quantity is also sometimes denoted as Δ(2θ); θ is the Bragg angle. Standard Zn Zn-1.5 Sb Zn-2.5 Sb Zn-3.5 Sb Zn-4.5 Sb FIG. 4: XRD PATTERN OF Zn-Sb SOLDERS ALONG WITH STANDARD ZINC 151

6 Table 2: Calculation of Crystallite size by Scherrar equation Sample ID FWHM (degree) 2 Theta Crystallite size (nm) Average (nm) Zn-1.5Sb Zn-2.5Sb Zn-3.5Sb Zn-4.5Sb The calculation of crystallite size using Scherrer equation is shown in table 2. In this calculation the FWHM of the most left two prominent peaks were taken in consideration and the average crystallite size is reported since the peaks lying in the lower 2θ region (usually in between 30-50º 2θ value) yield best results in crystallite size calculation [20]. It is found that the crystallite size of the alloys varies from around 29nm to 32nm in case of Zn-xSb (x= wt %) solder alloys. In the selected range of composition there was found no trend in the crystallite size with increasing Sb contents in the solder alloys. 3.4 HARDNESS Vickers hardness of the as cast Zn-xSb solder alloys is presented in table 3 as well as in fig. 5. A slight variation of hardness in these alloys was found with the increasing Sb content. Hardness increases from 30.6 HV to 34.5 HV while composition changes from Zn-1.5 wt%sb to Zn-3.5wt% Sb. Interestingly, hardness fall in case of further increase in Sb content towards 4.5wt%. Further analysis of the next nearby compositions may provide interesting results with a trend in the change of hardness with increasing Sb content. Table 3: Vickers Hardness of Zn-xSb Alloys Sample ID Average HV Standard deviation Zn-1.5 Sb Zn-2.5 Sb Zn-3.5 Sb Zn-4.5 Sb Hardness (HV) Zn-1.5 Sb Zn-2.5 Sb Zn-3.5 Sb Zn-4.5 Sb FIG.5: COLUMN CHART SHOWING VICKERS HARDNESS OF Zn-xSb ALLOYS 152

7 4. CONCLUSION A study of Zn-Sb has been conducted to fabricate and characterize pb free Zinc based solder alloy. Microstructuaral analysis revealed eutectic structure of Zn-Sb alloys along with a zinc phase. Hardness increases with increasing Sb content upto 3.5wt% but decreases in Zn-4.5wt% Sb alloy. Melting temperature decreases from pure Zn with the addition of Sb. However, the melting temperature is in the range of high temperature solder alloys. Crystallite size of the alloys lies in between nm. Finally, it can be concluded that Zn-Sb alloys might be used as an alternative of pb based solder alloys in high temperature applications. However, further study is a must to understand the reliability issues. 5. REFERENCES [1] Tu, K. N., A. M. Gusak, and M. Li. "Physics and materials challenges for lead-free solders." Journal of Applied Physics 93, no. 3 (2003): [2] MK Islam, A Sharif, Zn-Based Solders for High Temperature Electronic Application, Reference Module in Materials Science and Materials Engineering (2016), 1 [3] Liu, Y. C., J. W. R. Teo, S. K. Tung, and K. H. Lam. "High-temperature creep and hardness of eutectic 80Au/20Sn solder." Journal of Alloys and Compounds 448, no. 1 (2008): [4] Kim, Seongjun, Keun-Soo Kim, Katsuaki Suganuma, and Goro Izuta. "Interfacial reactions of Si die attachment with Zn-Sn and Au-20Sn high temperature lead-free solders on Cu substrates." Journal of electronic materials 38, no. 6 (2009): [5] Takaku, Yoshikazu, Ikuo Ohnuma, Ryosuke Kainuma, Yasushi Yamada, Yuji Yagi, Yuji Nishibe, and Kiyohito Ishida. "Development of Bi-base high-temperature Pb-free solders with second-phase dispersion: Thermodynamic calculation, microstructure, and interfacial reaction." Journal of electronic materials 35, no. 11 (2006): [6] Song, Jenn-Ming, Hsin-Yi Chuang, and Zong-Mou Wu. "Interfacial reactions between Bi-Ag hightemperature solders and metallic substrates." Journal of Electronic Materials 35, no. 5 (2006): [7] Jang, J. W., P. G. Kim, K. N. Tu, and Michael Lee. "High-temperature lead-free SnSb solders: Wetting reactions on Cu foils and phased-in Cu Cr thin films." Journal of materials research 14, no. 10 (1999): [8] Takaku, Yoshikazu, Komei Makino, Keita Watanabe, Ikuo Ohnuma, Ryosuke Kainuma, Yasushi Yamada, Yuji Yagi, Ikuo Nakagawa, Takashi Atsumi, and Kiyohito Ishida. "Interfacial reaction between Zn-Albased high-temperature solders and Ni substrate." Journal of Electronic Materials 38, no. 1 (2009): [9] Rettenmayr, M., P. Lambracht, B. Kempf, and C. Tschudin. "Zn-Al based alloys as Pb-free solders for die attach." Journal of Electronic Materials 31, no. 4 (2002): [10] Takahashi, Toshihide, Shuichi Komatsu, Hiroshi Nishikawa, and Tadashi Takemoto. "Improvement of high-temperature performance of Zn-Sn solder joint." Journal of electronic materials 39, no. 8 (2010): [11] Manikam, Vemal Raja, and Kuan Yew Cheong. "Die attach materials for high temperature applications: a review." IEEE Transactions on Components, Packaging and Manufacturing Technology 1, no. 4 (2011): [12] Chidambaram, Vivek, Jesper Hattel, and John Hald. "Design of lead-free candidate alloys for hightemperature soldering based on the Au Sn system." Materials & Design 31, no. 10 (2010): [13] Tsai, J. Y., C. W. Chang, Y. C. Shieh, Y. C. Hu, and C. R. Kao. "Controlling the microstructure from the gold-tin reaction." Journal of electronic materials 34, no. 2 (2005): [14] Song, H. G., J. P. Ahn, and J. W. Morris. "The microstructure of eutectic Au-Sn solder bumps on Cu/electroless Ni/Au." Journal of electronic materials 30, no. 9 (2001): [15] Song, Jenn-Ming, Hsin-Yi Chuang, and Zong-Mou Wu. "Interfacial reactions between Bi-Ag hightemperature solders and metallic substrates." Journal of Electronic Materials 35, no. 5 (2006): [16] Lalena, John N., Nancy F. Dean, and Martin W. Weiser. "Experimental investigation of Ge-doped Bi-11Ag as a new Pb-free solder alloy for power die attachment." Journal of electronic materials 31, no. 11 (2002): [17] Kim, Seong-Jun, Keun-Soo Kim, Sun-Sik Kim, Chung-Yun Kang, and Katsuaki Suganuma. "Characteristics of Zn-Al-Cu alloys for high temperature solder application." Materials transactions 49, no. 7 (2008): [18] Shimizu, T., H. Ishikawa, I. Ohnuma, and K. Ishida. "Zn-Al-Mg-Ga alloys as Pb-free solder for dieattaching use." Journal of electronic materials 28, no. 11 (1999): [19] of access: 27/10/2016) [20] (Date of access: 27/10/2016) 153