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Isothermal Fatigue Properties of Sn--Ag--Cu Alloy Evaluated By Materials Transactions, Vol. 46, No. 11 (2005) pp. 2309 to 2315 Special Issue on Lead-Free Soldering in Electronics III #2005 The Japan Institute of Metals Isothermal Fatigue Properties of Sn–Ag–Cu Alloy Evaluated by Micro Size Specimen Yoshiharu Kariya1, Tomokazu Niimi2;*, Tadatomo Suga3 and Masahisa Otsuka4 1Ecomaterials Center, National Institute for Materials Science, Tsukuba 305-0044, Japan 2Graduate School of Shibaura Institute of Technology, Tokyo 108-8548, Japan 3Department of Precision Engineering, School of Engineering, The University of Tokyo, Tokyo 113-8656, Japan 4Materials Science and Engineering Department, Shibaura Institute of Technology, Tokyo 108-8548, Japan Micro-bulk fatigue testing developed to investigate the fatigue lives and damage mechanisms of Sn–3.0Ag–0.5Cu and Sn–37Pb solder alloys. The fatigue life of micro-bulk solder obeyed Manson–Coffin’s empirical law, and the fatigue ductility exponents were about 0.5 for both Sn–Ag–Cu and Sn–Pb alloys. The fatigue life of Sn–3.0Ag–0.5Cu alloy was 10 times longer than that of Sn–37Pb alloy under symmetrical cycling at 298 K, although fatigue resistance of Sn–3.0Ag–0.5Cu alloy was not very superior under asymmetrical wave and elevated temperature condition. The fatigue crack was developed from extrusion and intrusion of slip band in Sn–3.0Ag–0.5Cu alloy, while the crack was observed at colony boundary and grain boundary in Sn–37Pb alloy. The difference in damage mechanism may affect the susceptibility to fatigue life test condition of reversibility. (Received May 23, 2005; Accepted October 11, 2005; Published November 15, 2005) Keywords: solder, lead-free solder, fatigue life, miniature testing, tin–silver–copper, tin-lead, damage mechanism, crack initiation, micro-bulk 1. Introduction method for solder alloys, and the isothermal fatigue proper- ties of lead-free solder alloy were tested to understand fatigue Understanding fatigue properties of solder alloys is damage of lead-free solder alloy in small volume. important to evaluate reliability of solder joints, since cyclic mechanical deformation can occur at solder joints due to 2. Experimental thermomechanical fatigue (e.g., environmental-temperature cycles on joined materials with different coefficients of 2.1 Specimen preparation thermal expansion). In addition to thermomechanical fatigue, Sn–3.0 mass%Ag–0.5 mass%Cu (Senju Metal Industry the solder joint can also be subjected to mechanical fatigue, Co., Ltd., hereinafter Sn–3.0Ag–0.5Cu) and Sn–37 mass%Pb typically when the board or substrate is bent during (Nihon handa Co., Ltd., hereinafter Sn–37Pb) were used in mechanical handling or from vibration forces. Therefore, a this study. A very small size as-solidified specimen that had number of studies on mechanical fatigue of solder alloys to 0.5 mm diameter and 2 mm of gauge length was employed to understand fatigue failure of solder joints have been measure mechanical properties.6) Figure 1 shows appearance performed to date. In general, mechanical properties of of the miniature size fatigue specimen used in this study. The solder alloys such as creep and fatigue are measured using specimen was fabricated by following procedures. Solder larger volume bulk specimens than real solder joints1–5) as wire was pressed by a divided metal mold to form specimen there are many difficulties in sample preparation and shape at near melting point of each alloy. Then, the mold mechanical testing. However, the microstructure of the real containing solder alloy was heated up to 30 K above the joint is totally different to that of the large size mechanical melting temperature of the solder, and the solder was molten testing specimen, since solidification conditions of real joints in the mold. After the solder was completely molten, the cannot be duplicated in large size. We proposed a tensile mold containing the melt was cooled down on the copper testing using micro size specimen to obtain the mechanical plate to achieve rapid cooling. Cooling rate was about 4 K/s behavior of Sn–Ag–Cu lead free solder alloy, and the testing in this study, which is similar to the rate in a conventional resulted in that the mechanical behaviors of the micro size reflow process. The fabrication procedure is shown schemati- and the large specimens are not similar.6) In addition to such differences in the mechanical properties, specimen size also affects crack initiation and propagation behaviors in low cycle fatigue. Therefore, the low cycle fatigue test should also be performed using micro size specimen. A number of studies on low cycle fatigue of solder alloys were reported to date,7–14) whereas there are no studies on that using small volume specimen. In this study, we developed micro-bulk fatigue testing 2mm *Graduate Student, Shibaura Institute of Technology. Present address: Dowa Mining Co., Ltd. 1-8-2 Marunouchi, Chiyoda-ku, Tokyo 100-8282, Japan Fig. 1 Appearance of the micro bulk specimen. 2310 Y. Kariya, T. Niimi, T. Suga and M. Otsuka Cooling & remove Pressed by mold at near melting point Heated up to Tm+30K Fig. 2 Fabrication procedure of the micro bulk specimen. cally in Fig. 2. The specimens were heat-treated at 423 K were used in asymmetrical cycling. Test temperature was for 2 h to stabilize the microstructure at high temperature controlled by an infrared image furnace, and was maintained fatigue test. within 0.2 K throughout the test. 2.2 Isothermal fatigue tests 2.3 Microstructural observation Total strain controlled isothermal fatigue tests were carried The specimens were ground with two grades of SiC papers out in air at 298 and 398 K with the application of a precise (#500, and 1200) and then mechanically polished with displacement controlled mechanical testing machine diamond paste (15 mm). Finally, the specimens were polished (Saginomiya MFT-01). Figure 3 shows schematic illustration with colloidal silica suspension. After polishing, they were of the fatigue testing machine. A linear DC motor was observed using an optical microscope without any chemical employed for an actuator of the machine, and the displace- etching. ment resolution of the actuator and maximum load capacity are 20 nm and 200 N, respectively. The total strain range 3. Result and Discussion chosen was between 0.3 and 1.3 percent. Fatigue life was defined as the number of cycles at which the load decreases to 3.1 Microstructure 20 percent from initial value. The total strain was obtained by Figure 5 shows optical micrographs for the Sn–3.0Ag– measuring a displacement between specimen fixing jigs with 0.5Cu (polarized image) and Sn–37Pb. The microstructure of a capacitance displacement sensor that has a resolution of Sn–3.0Ag–0.5Cu alloy consists of Sn dendrites surrounded 0.04 mm. Two types of symmetrical (Fast–Fast) and asym- by a eutectic of Sn, Ag3Sn and Cu6Sn5. Very fine spherical metrical (Slow–Fast) continuous strain cycling as shown in intermetallic compounds were formed in the eutectic region. Fig. 4 were employed in this study to investigate an effect of Morphology of the Sn dendrite is columnar and equiaxial irreversibility of loading profile on the fatigue life. A fast dendrites were not observed in the specimen as can be seen in ramp rate of 10À2 sÀ1 and a slow ramp rate of 3 Â 10À4 sÀ1 Figs. 5(a) and (b). The Sn dendrites with similar crystal orientation are clustered (same contrast region), making large grains in the miniature specimen. Only a few grains were observed in the gauge section in the micro bulk specimen, Halogen lamp Specimen whereas a large volume cast specimen seems to be in polycrystalline state with equiaxial Sn dendrites.6) Load cell Linear motor Diaphragm Figures 5(c) and (d) show optical micrograph of as solidified Sn–37Pb alloy. The microstructure consists of fine Fixing jigs Air bearing lead and tin solid solutions. The classic eutectic structure Linear scale :20nm (two paralleled lamellar structure) was suppressed by Capacitance sensor Cooling water substantial undercooling, and fine two phase mixed structure Control unit and relatively small size colonies were observed in the micro Air bulk specimen. The size of Pb phases was about 1–5 mm in Fig. 3 Schematic illustration of fatigue testing machine for the micro bulk the microstructure. specimen. 3.2 Fatigue life of micro bulk specimen Figure 6 shows hysteresis loop observed at total strain ·Fast-Fast ·Slow-Fast . -2 -1 range of 1.2% at three cycles for the micro bulk specimen. ε = 1× 10 s . -4 -1 ε = 3 × 10 s . ε = 1 × 10 -2 s-1 Typical hysteresis loop of low cycle fatigue was observed as shown in Fig. 6. Young’s modulus obtained from the strain strain hysteresis loop were about 42 GPa that well agrees with a 8,9) cycles cycles typical value measured in a bulk specimen. The modulus means that strain measurement method adopted here is Fig. 4 Strain profiles for low cycle fatigue test. appropriate, and that the testing method enables to precisely Isothermal Fatigue Properties of Sn–Ag–Cu Alloy Evaluated by Micro Size Specimen 2311 200µm 20µm (a) Sn-3.0Ag-0.5Cu (b) Sn-3.0Ag-0.5Cu (higher magnification) 50µm 20µm (c) Sn-37Pb (d) Sn-37Pb (higher magnification) Fig. 5 Optical micrographs for the micro bulk specimens of Sn–3.0Ag–0.5Cu and Sn–37Pb. Stress, σ / MPa -1 50 10 Sn-3.0Ag-0.5Cu 40 in ε ∆ 30 -2 298K,Slow-Fast 20 10 298K,Fast-Fast 10 -0.80 -0.60 -0.40 -0.20 0.20 0.40 0.60 0.80 -3 Strain(%) 10 398K,Slow-Fast -10 398K,Fast-Fast -20 Inelastic Strain Range, -30 -4 10 1 2 3 4 5 6 -40 10 10 10 10 10 10 -50 Number of Cycles to Failure, N f Fig.
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