Effects of Ga addition on the wetting properties and tensile properties of Sn-Zn-Ag solder alloys

Transcription

1 Indian Journal of Engineering & Materials Sciences Vol. 21, December 2014, pp Effects of Ga addition on the wetting properties and tensile properties of Sn-Zn-Ag solder alloys Kang-I Chen a *, Chin-Hsiang Cheng b, Sean Wu a, Yeu-Long Jiang c & Tsung-Chie Cheng d a Department of Electronics Engineering and Computer Science, Tung Fang Design Institute, 110 Dongfang Rd., Hunei Dist., Kaohsiung City 82941, Taiwan b Department of Aeronautics and Astronautics, Research Center for Energy Technology and Strategy, National Cheng Kung University, 1, University Road, Tainan City 701, Taiwan c Graduate Institute of Optoelectronic Engineering and Department of Electrical Engineering, National Chung Hsing University, 250 KuoKuang Rd., Taichung 402, Taiwan d Department of Mechanical Engineering, National Kaohsiung University of Applied Sciences, 415 Chien Kung Road, Sanmin District, Kaohsiung 80778, Taiwan Received 9 September 2013; accepted 15 May 2014 The effects of Ga addition on the wetting properties and tensile properties of Sn-8.55Zn-0.5Ag-xGa lead-free solder alloys are investigated. The x content of the solders investigated is 0~3 wt%. The results indicate that Ga exhibits a prominent influence on the wetting behavior as well as the mechanical properties of the solders. The wetting properties are improved remarkably with the increase of the Ga content in the Sn-8.55Zn-0.5Ag lead-free solder. The tensile test shows that 1~2 % Ga alloys have significant improvement in UTS, when compared with that of the binary Sn-Zn and Sn-Zn-0.5Ag alloys. As for Sn-Zn-0.5Ag, the addition of Ga elements has provided a good wetting force, wetting time and tensile strength. Keywords: Lead-free solder, Sn-Zn-Ag, Ga, Tensile properties, Wetting properties Due to the harmful effects of lead (Pb) on the environment and human health, a variety of new leadfree solders have been developed to eliminate the usage of Sn-Pb solders in electronic assemblies 1,2. The development of lead-free solders has become an important issue. At present, several lead-free solder alloys, such as Sn-Ag based alloys and Sn-Zn based alloys, have shown potential in the replacement of the conventional Sn-Pb alloy 2-4. However, the Sn-Ag alloy systems have higher melting points (216~221 C) than that of the eutectic Sn-Pb alloy. A higher melting point requires higher soldering temperatures. The soldering temperature is generally 30 C (reflow soldering) above the melting temperature of the specified solder, which may cause some damage to the other electronic components. A low processing temperature is desirable for preventing heat damage to electronic devices during soldering. Hence, the Sn-Zn solders systems are expected to be another alternative to Sn-Pb because of its low melting temperature. *Corresponding author ( kic0205@gmail.com) Sn-9Zn (wt%) solder has a low eutectic temperature (198 C), close to that of the Sn-Pb alloy, along with its low cost and desirable mechanical properties 4-7. Nevertheless, the tendency of oxidation and poor wetting ability of this alloy system limits the scope of its application 7,8. Recently, many Sn-based ternary and quaternary systems have been investigated in the attempt to overcome the shortfalls of Sn-9Zn. Some authors have tried to add a third element, such as Ag 9-14, or Ga 15-18, to the Sn-Zn system, in order to improve the melting temperature, wettability, and mechanical properties of the alloys. It has been reported 19,20 that small additions of Ag can improve the ductility of Sn-Zn base solders. It was found 21 that the addition of 0.5 wt% Ag increases the tensile strength and ductility of the solder and also lowers the melting temperature. Increasing the content of Ga not only improves the mechanical properties but also decreases the melting temperature of Sn-Zn base solder 15,22. The tensile strength of the Sn-Zn base solder increases as the Ga content increases up to 2.0 wt%, while the ductility decreases 22.

2 622 INDIAN J. ENG. MATER. SCI., DECEMBER 2014 The present work investigates the microstructure, wetting properties and tensile properties of Sn-8.55Zn- 0.5Ag-xGa solder alloys at various Ga contents. Experimental Procedure The Sn-8.55Zn-0.5Ag-xGa (wt%) lead-free solders are made of pure tin, zinc, silver, and gallium (purity of 99.99%). The Ga content of the solders investigated was 0 ~ 3 wt%. The molten alloys were homogenized at 800 C for one hour before they were cast in a stainless steel mold at a cooling rate of 95 C/min. The as-cast alloys were then machined with a wire cutting machine into tensile specimens, as shown in Fig. 1. Tensile tests were performed at a crosshead speed of 1 mm/min according to ASTM- A370 standard. The wetting properties of the Sn-8.55Zn-0.5AgxGa solders were investigated with a wetting balance method using a pure Cu bar (φ1 mm) as the substrate at 230 C. The wetting experiment was performed at a dipping time of 15 s, in a dipping depth of 9.9 mm, and at a dipping speed of 15 mm/s. The wettability of Fig. 1 Schematic diagram of a tensile specimen this solder on Cu was investigated with a wetting balance applying a newly developed flux. A typical wetting curve is shown in Fig. 2, where the wetting time (t w ) indicates the time at which the molten solder contact angle to the Cu wire was 90, as shown at point B. The maximum wetting force (F w ) can be obtained as the curve shows a stable state after complete immersion, when the measured force remains constant. Phase identification of the various solders was carried out by employing an X-ray diffractometry at 30 kev using Cu-Kα radiation, with diffraction angles (2θ) ranging from 34 to 48 at a constant scanning speed of 1 /min. The microstructures of the solders were investigated with a scanning electron microscope (SEM) and electron probe microanalysis (EPMA). The composition of the precipitates and compounds was analyzed with an energy dispersive spectrometer (EDS). Results and Discussion Figure 3 shows the Sn-Zn phase diagram 23. Elemental Sn melts at 232 C. Sn-9 wt% Zn is the eutectic composition for the Sn-Zn system, and the microstructure is lamellar, consisting of alternating Sn-rich and Zn-rich phases, with a melting temperature of C. The scanning electron micrographs of the Sn- 8.55Zn-0.5Ag-xGa alloys presented in Fig. 4 exhibit a two-phase colony structure. The matrix phase with light contrast is β-sn, and the darker needle-like phase is the Zn-rich phase. The SEM image of the Sn-Zn- 0.5Ag alloy exhibits a fine, uniform, two-phase, eutectic-colony structure. The 1 wt% Ga alloy possesses a less uniform more complicated Fig. 2 The variation of the wetting force over time Fig. 3 The Sn-Zn phase diagram

3 CHEN et al.: Sn-Zn-Ag SOLDER ALLOYS 623 Fig. 4 The scanning electron micrographs of the Sn-8.55Zn-0.5Ag-xGa solders containing 0.0Ga, 1.0Ga, 2.0Ga, and 3.0Ga microstructure, which is hypoeutectic with the primary β-sn matrix (white) and Sn-Zn eutectic phases. The solders containing 2 wt% Ga and 3 wt% Ga exhibit a coarser and non uniform microstructure. With the increase of Ga, the fraction of the Sn/Zn eutectic region decreases while the Sn-matrix region increases, as well as the Zn-rich phase precipitates growing and dispersing in the Sn-matrix. The X-ray diffraction (XRD) patterns of the Sn- 8.55Zn-0.5Ag-xGa solder alloys are shown in Fig. 5. Bragg peaks corresponding to Sn(211), Sn(220), Zn(002), Zn(100), Zn(002), AgZn 3 (101) and AgZn 3 (002) are observed in these solders. The precipitates ε-agzn 3 along with the β-sn matrix and Zn-rich phases are identified in the solders. It is known that there is almost no solid solubility of Ag in Sn. The addition of Ag to the Sn-Zn binary eutectic can result in the formation of Ag 3 Sn precipitates. However, the Ag selectively combines with Zn, instead of Sn, to form Ag-Zn. This compound is identified to be ε-agzn 3 phase. These phases have wide compositional ranges of 67.4 ~ 87.4 at% Zn (55.6 ~ 80.8 wt%zn) for AgZn The Ga-Zn 24, Ga-Sn 25, and Ga-Ag 26 binary phase diagrams indicate that Ga may form a solid solution with Sn, Zn, and Ag. The Ga-Zn phase diagram 24 shows that hexagonal (Zn) solid solution possesses a maximum solubility of 2.36 at% Ga at 260 C. Meanwhile, Sn exhibits a maximum solubility of 7.1 at% Ga 25. Thus, most of the Ga is dissolved in the Snrich and Zn-rich phases. The Sn-Zn exhibits a binary eutectic behavior with no intermetallic compounds and a limited solubility of the two elements in each phase. Thus, AgZn 3 is the only intermetallic compound detected in the XRD investigation. It was found that an increase in the gallium content makes the Sn (220) peak of the Sn-matrix phase shift slightly toward a higher angle. The atomic radii of tin and gallium are nm and nm, respectively 23. The dissolution of gallium atoms in the Sn matrix will

4 624 INDIAN J. ENG. MATER. SCI., DECEMBER 2014 Fig. 5 The XRD patterns of the Sn-8.55Zn-0.5Ag-xGa solders containing 0.0Ga, 1.0Ga, 2.0Ga, and 3.0Ga Fig. 6 Tensile stress-strain cures of the Sn-8.55Zn-0.5Ag-xGa solders thus reduce the lattice parameter of the Sn structure. An increase in the Ga content does not lead to any variation in the XRD intensity of the Ag-Zn compound. The cooling reaction for the Sn-8.55Zn- 0.5Ag-xGa alloy at about 198 C results in L (liquid) Sn matrix + Sn/Zn eutectic + Ag-Zn compound. The effect of the Ga alloying additives on the mechanical properties can be seen from the strainstress curves shown in Fig. 6. The results of the test for the ultimate tensile strengths (UTS) and the elongation values of the solders are given in Table 1. The tensile strength of the Sn-8.55Zn-0.5Ag, Sn- 8.55Zn-0.5Ag-1.0Ga, Sn-8.55Zn-0.5Ag-2.0Ga, and Sn-8.55Zn-0.5Ag-3.0Ga were 50, 78, 108, and 76 MPa, respectively. The elongations of the Sn-9Zn- 0.5Ag, Sn-8.55Zn-0.5Ag-1.0Ga, Sn-8.55Zn-0.5Ag- 2.0Ga, and Sn-8.55Zn-0.5Ag-3.0Ga were 50%, 37%, 30% and 7%, respectively. The UTS of the solder

5 CHEN et al.: Sn-Zn-Ag SOLDER ALLOYS 625 increases to 108 MPa with the increase in the Ga content. However, it decreases to 76 MPa when the Ga addition increases to 3 wt%. The tensile test shows that 1~2 % Ga alloys have significant improvement in their UTS, when compared to that of the binary Sn-Zn and Sn-Zn-0.5Ag alloys (Table 1). The UTS is improved by 55% in a 1 wt% Ga addition and by more than 110% with a 2 wt% Ga addition. The 2 wt% Ga solder has the highest UTS, while 3 wt% Ga had the lowest UTS. The Gacontaining alloys exhibit greater tensile strength than the plain Sn-8.55Zn-0.5Ag alloy because Ga atoms that go into solid solution ordinarily impose lattice strain on the surrounding host atoms. The greatly increased strength of the alloys was probably due to the solid solution hardening effects of Ga in the Sn matrix. However, the 3 wt% Ga alloy exhibited the lowest ductility, which is due to the segregation of the Ga atom at the grain boundaries. The increasing addition of Ga from 1 to 2 wt% for the Sn-Zn-Ag alloy tends to increase the tensile strength of the alloys, while lowering the elongation of the Ga-containing alloys. The results show that the increase in Ga content from 0 to 2 wt% for Sn-Zn-0.5Ag solders results in an increase in UTS from 50 to 108 MPa, as well as a decrease in elongation from 50% to 30%. Alloys (wt%) Table 1 The mechanical properties of the solders. Tensile strength (MPa) Failure elongation (%) Reference Sn-8.55Zn-0.5Ag Sn-8.55Zn-0.5Ag-1.0Ga Sn-8.55Zn-0.5Ag-2.0Ga Sn-8.55Zn-0.5Ag-3.0Ga 76 7 Sn-37Pb Sn-9Zn Sn-8.55Zn-1.0Ag Sn-8.55Zn-1.0Ag-0.45Al Table 2 The wetting force and wetting times of the lead-free solders. Alloys (wt%) Wetting force (mn) Wetting times (s) Reference Sn-8.55Zn-0.5Ag Sn-8.55Zn-0.5Ag-1.0Ga Sn-8.55Zn-0.5Ag-2.0Ga Sn-8.55Zn-0.5Ag-3.0Ga Sn-9Zn * Sn-8.55Zn-1.0Ag * Sn-8.55Zn-1.0Ag-0.45Al * *The wetting experiment was carried out with the solders at 250 C. In the wetting balance test, the molten solder climbs up the Cu bar due to the wetting force exerted on it when it is dipped into the solder bath. This is similar to the condition in wave soldering. The wetting force, F w, can be expressed as 27 : F w = p γ cosθ ρgv Where p is the perimeter of Cu bar, γ the surface tension of the molten solder in contact with the flux, θ the contact angle, ρ the density of the solder, g the gravity acceleration, and V is an immersed volume. Wettings occurring in short wetting times is considered to be good. The Ga addition has a prominent effect on wetting forces and the wetting times of lead-free solders, as shown by the test results of Table 2. The wetting force of Sn-9Zn-0.5Ag, Sn-8.55Zn-0.5Ag- 1.0Ga, Sn-8.55Zn-0.5Ag-2.0Ga, and Sn-8.55Zn- 0.5Ag-3.0Ga were 0.24, 0.52, 0.96 and 1.03 mn, respectively. The wetting times of the Sn-9Zn-0.5Ag, Sn-8.55Zn-0.5Ag-1.0Ga, Sn-8.55Zn-0.5Ag-2.0Ga, and Sn-8.55Zn-0.5Ag-3.0Ga were 2.37, 1.58, 1.00 and 0.48 sec, respectively. Cheng and Lin 28 observed that a wetting force exists for the Sn-8.55Zn-1.0Ag-0.45Al solder with a value of 0.75 mn, while the wetting time is about 1.10 s. The wetting test shows that 2~3% Ga alloys have significant improvement in their wetting force and wetting times, when compared to that of the binary Sn-Zn and Sn-Zn-1.0Ag-0.45Al alloys. The wetting force is improved by 130% with a 2.0 wt% Ga addition and by more than 150% with 3.0 wt% Ga addition. The wetting force and wetting times of the Sn-8.55Zn-0.5Ag-xGa solders on the Cu substrate at 230 C are shown in Figs 7 and 8, respectively. The Fig. 7 The wetting force of the Sn-8.55Zn-0.5Ag-xGa solders

6 626 INDIAN J. ENG. MATER. SCI., DECEMBER 2014 results show that the increase in the Ga content from 0 to 3 wt% for the Sn-Zn-0.5Ag solders could result in an increase in the wetting force from 0.24 to 1.03 mn, as well as a decrease in the wetting time from 2.37 to 0.48 s. The wetting forces increase as the content of Ga increases from 0 to 3 wt%, while the wetting times Fig. 8 The wetting time of the Sn-8.55Zn-0.5Ag-xGa solders decrease. These results indicate that the addition of Ga into the Sn-Zn-Ag solder increases the wettability of the solders on Cu. Figure 9 shows the backscattered-electron imaging micrograph and elemental analysis of the interface between Sn-8.55Zn-0.5Ag-1.0Ga and the Cu substrate after the dipping test. Two compound layers are observed at the solder/cu interface. The inner compound layer of about 0.5 µm thickness is the Cu 5 Zn 8 phase. The outer compound layer of about 1.2 µm thickness is the AgZn 3 compound. The elemental analysis in the present study shows that Sn is established in the bulk solder and that Cu exists not only on the substrate side but also on the interface side of the bulk solder. Cu is present at the interface between the solder and the substrate in the layers of Cu 5 Zn 8, as observed by the EDS analysis. Ga is found in the Zn-rich phases. In addition, Ag is also found at the interface. The coexistence of Ag and Zn at the interface is believed to be AgZn 3 compounds. It has been reported 2,6 that Cu 5 Zn 8 is formed at the interface between the Sn-Zn solder and the Cu substrate. Meanwhile, Ag-Zn compounds are formed in the solder as Ag was added to the Sn-Zn-Al solder 10,29 and the Sn-Zn-Al-Ga solder 21,30. For the Fig. 9 The backscattered-electron imaging micrograph and elemental analysis of the interface between Sn-8.55Zn-0.5Ag-1.0Ga and Cu

7 CHEN et al.: Sn-Zn-Ag SOLDER ALLOYS 627 view of the Gibbs energy 31, the G values of the compound formations with Cu-Zn and Ag-Zn at 250 C are kj/mol and -6.4 kj/mol, respectively. The G values show that it is easier to form the Cu- Zn compound than the Ag-Zn compound. The Zn-Cu phase diagram 23 shows that Cu and Zn exhibit mutual solubility with each other. Thus, Zn tends to diffuse toward Cu during dipping. Ag may compete with Cu for Zn to form an Ag-Zn compound during wetting. This competition is believed to reduce the interfacial concentration of Zn. The addition of the Ga increases the wetting forces while decreasing the wetting time. These results indicate that the addition of Ga to the Sn-Zn-Ag solder increases the wettability of the solders on Cu. Conclusions The microstructure of the Sn-8.55Zn-0.5Ag-x0.5Ga alloys shows a needle-like precipitate of the Zn-rich phase, the Ag-Zn compound, and the hypoeutectic Snmatrix. With the addition of a Ga element, the Snmatrix region increases, and the Zn-rich phase becomes coarser. The wetting properties are improved remarkably with the increase in the Ga content in the Sn-8.55Zn-0.5Ag lead-free solder. The wetting force increases as the content of the Ga increases from 0 to 3 wt%, while the wetting time decreases. The tensile test shows that the 1~2% Ga alloys show significant improvement in UTS, when compared to that of the binary Sn-Zn and the Sn-Zn-05Ag alloys. As for the Sn-Zn-0.5Ag, the addition of the Ga elements provided better wetting force, wetting time and ultimate tensile strength (UTS). However, adding an excessive amount of Ga has the opposite effect of lowering tensile elongation. The Sn-8.55Zn-0.5Ag-2.0Ga solders exhibit greater tensile strength, wetting force and wetting time than the Sn-9Zn and Sn-Zn-0.5Ag solders. The experimental results show that the wettability of the Sn-8.55Zn-0.5Ag-xGa solder alloys is improved by the addition of the Ga. Acknowledgements Financial support received for this study from the National Science Council of the Republic of China under NSC E is gratefully acknowledged. References 1 McCormack M & Jin S, JOM, 45 (1993) Abtew M & Selvaduray G, Mater Sci Eng, 27 (2000) Glazer J, Int Mater Rev, 405 (1995) Miller C M, Anderson I E & Smith J F, J Electron Mater, 23 (1994) Mavoori H, Chin J, Vaynman S, Moran B, Keer L & Fine M, J Electron Mater, 26 (1997) Suganuma K, Murata T, Noguchi H & Toyoda Y, J Mater Res, 15 (2000) Wood E P & Nimmo K L, J Electron Mater, 23 (1994) Suganuma K, Niihara K, Shoutoku T & Nakamura Y, J Mater Res, 13 (1998) Chang T C, Hsu Y T, Hon M H & Wang M C, J Alloys Compd, 360 (2003) Huang C W & Lin K L, J Mater Res, 18 (2003) Huang C W & Lin K L, Mater Trans, 45 (2004) Lin K L & Shih C L, J Electron Mater, 32 (2003) Song J M, Lui T S, Lan G F & Chen L H, J Alloys Compd, 379 (2004) Wang H, Xue S, Zhao F & Chen W, J Mater Sci:Mater Electron, 21 (2010) Lin K L, Chen K I & Shi P C, J Electron. Mater, 32 (2003) Chen K I, Cheng S C, Wu Sean & Lin K L, J Alloys Compd, 416 (2006) Chen W X, Xue S B & Wang H, Mater Des, 31 (2010) Liu N S & Lin K L, Scr Mater, 52 (2005) McCormack M & Jin S, J Electron Mater, 23 (1994) Song J M, Lan G F, Lui T S & Chen L H, Scr Mater, 48 (2003) Chen K I & Lin K L, J Electron Mater, 31 (2002) Chen K I & Lin K L, J Electron Mater, 32 (2003) Hansen M, Constitution of Binary Alloys, 2 nd ed, (McGraw- HillBook Co., New York), Dutkiewicz J, Moser Z, Zabdyr L, Gohil D D, Chart T G, Ansara I & Girard C, Bull Alloy Phase Diagrams, 11 (1990) Anderson T J & Ansara I, J Phase Equilibria, 13 (1992) Baren R, Bull Alloy Phase Diagrams, 11 (1990) Wu C M L, Law C M T, Yu D Q & Wang L, J Electron Mater,32(2003) Cheng S C & Lin K L, J Electron Mater, 31 (2002) Lai R S, Lin K L & Salam B, J Electron Mater, 38 (2009) Huang C W & Lin K L, J Mater Res, 19 (2004) Hultgren R, Desai P D, Hawkins D T, Gleiser M & Kelley K K, Selected Values of the Thermodynamic Properties of Binary Alloys (Metals Park, OH:ASM), 1973.