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 102 s1 and a slow ramp rate of 3 104 s1 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. 6 Hysteresis loop for the miniature specimen at strain range of 1.2% at Fig. 7 Manson–Coffin plots for Sn–3.0Ag–0.5Cu alloy. 298 K for Sn–3.0Ag–0.5Cu.

" N ¼ C ð1Þ measure mechanical response of micro bulk solder specimen. p f Figure 7 shows relationship between inelastic strain range where "p is the inelastic strain range, Nf is the fatigue life, and fatigue life of Sn–3.0Ag–0.5Cu alloy at 289 and 398 K. is the exponent and C is the fatigue ductility. The exponent The data on strain range versus fatigue life are plotted on log– for all condition is about 0.5, which is similar to that log scales against the inelastic strain range. All data obeyed measured in large bulk specimen of Sn–Ag solders.6–12) As Manson–Coffin’s law as follows. can be seen in Fig. 7, the fatigue life of the alloy is sensitive 2312 Y. Kariya, T. Niimi, T. Suga and M. Otsuka

-1 -1 10 10 Sn-37Pb in in ε ε

∆ ∆ Sn-Pb (398K,Fast-Fast) -2 -2 Sn-Ag-Cu 10 298K,Slow-Fast 398K,Fast-Fast 10 (298K,Fast-Fast)

Sn-Pb (Slow-Fast)

398K,Slow-Fast -3 -3 Sn-Ag-Cu 10 10 (398K,Slow-Fast) Sn-Ag-Cu 298K,Fast-Fast (398K,Fast-Fast) Sn-Pb (298K,Fast-Fast) Inelastic Strain Range, Inelastic Strain Range,

-4 -4 10 10 1 2 3 4 5 6 1 2 3 4 5 6 10 10 10 10 10 10 10 10 10 10 10 10

Number of Cycles to Failure, N f Number of Cycles to Failure, N f

Fig. 8 Manson–Coffin plots for Sn–37Pb alloy. Fig. 9 Comparison between Sn–Ag–Cu and Sn–Pb.

to symmetry of strain cycling and test temperature. The Sn–3.0Ag–0.5Cu is much longer than that of Pb–Sn eutectic fatigue life in asymmetrical cycling is approximately one- in symmetrical cycling at 298 K and the life of Sn–3.0Ag– tenth of that in symmetrical strain cycling, and increase in 0.5Cu is about seven times of that of Pb–Sn eutectic. test temperature reduces the life to one-third of that at lower However, the life of Sn–3.0Ag–0.5Cu is inferior to the life of temperature. Thus, asymmetrical cycling dramatically re- Sn–37Pb at 398 K in symmetrical cycling. In addition, the duces the life of Sn–3.0Ag–0.5Cu alloy, whereas a delete- fatigue life of Sn–3.0Ag–0.5Cu in slow–fast cycling is very rious effect of temperature upon fatigue life is less than that similar to that of Pb–Sn eutectic as is evident in Fig. 9. Thus, of reversibility in strain profile. superior fatigue resistance of Sn–3.0Ag–0.5Cu alloy was Figure 8 shows relationship between inelastic strain range obtained only in symmetrical cycling at 298 K. and fatigue life of Sn–37Pb alloy at 289 and 398 K. All data for Sn–37Pb also obeyed Manson–Coffin’s law, and the 3.3 Fatigue damage formation mechanism exponent is about 0.5 for each condition similar to Sn– 3.3.1 Damage mechanism of Sn–3.0Ag–0.5Cu alloy 3.0Ag–0.5Cu alloy. The life of Sn–37Pb is also very sensitive Figure 10 shows surface morphology and cross-sectional to symmetry of strain cycling as shown in Fig. 8, while the optical micrograph near crack initiation site for Sn–3.0Ag– deleterious effect of asymmetrical cycling upon fatigue 0.5Cu alloy subjected to the total strain range of 1.0% at damage is more moderate than that in Sn–3.0Ag–0.5Cu alloy. 298 K in fast–fast cycling. The formation of a pair of On the other hand, the test temperature does not have intrusion and extrusion was observed on fatigued surface of deleterious effect on the fatigue life of the alloy. The fatigue Sn–3.0Ag–0.5Cu alloy. The displacements within the slip life at 398 K in symmetrical cycling is obviously longer than band were not fully reversed, which leads to the fatigue crack that at 298 K, and the life at 398 K in asymmetrical cycling is nucleation. The fatigue crack initiation along the slip band almost equivalent to that at 298 K. The result shows that test was revealed by the cross-sectional observation as shown in temperature enhances the life in symmetrical mode, which Fig. 10(b). Thus, the fatigue crack started from the slip bands does not have deleterious effect on fatigue life in asym- and propagated within grain interior in the symmetrical metrical mode for Sn–37Pn alloy, whereas an increase in test cycling at 298 K. However, the crack propagated along a high temperature has deleterious one for Sn–3.0Ag–0.5Cu. The angle grain boundary, when the crack arrives at the boundary Sn–37Pb is widely known as one of the superplasticity alloys. as shown in Fig. 11. Thus, the mixed mode of transgranular The alloy exhibits exceptional ductility (over a thousand and intergranular failure is predominant for Sn–3.0Ag–0.5Cu percents of the failure elongation) when pulled under tension alloy at 298 K in the symmetrical cycling. at high temperature.15) The exceptional ductility is induced Figure 12 shows fatigue crack initiation and propagation by the grain boundary sliding with fine microstructure. The in Sn–3.0Ag–0.5Cu alloy at 398 K. The formation of an low cycle fatigue life strongly depends on the fracture intrusion and extrusion pair was also observed, and the ductility, and high ductility leads superior low cycle fatigue fatigue crack initiation along the slip band was visible at endurance.7) In this study, the grain boundary sliding may 398 K in fast–fast cycling. However, the fatigue crack prefers govern deformation of the alloy at 398 K, since very fine to propagate along a high angle grain boundary rather than to microstructure was obtained as shown in Fig. 5. The propagate within grain interior as can be seen in Fig. 12(a). mechanism leads high ductility which enhances the fatigue Especially, the crack nucleation prefers the high angle grain life of Sn–37Pb at 398 K. boundary, which makes a large angle with the loading axis. Figure 9 shows comparison between Sn–3.0Ag–0.5Cu and Any significant recrystallization was not observed in the Sn–37Pb for low cycle fatigue endurance. The fatigue life of symmetrical cycling at 398 K, even though the test temper- Isothermal Fatigue Properties of Sn–Ag–Cu Alloy Evaluated by Micro Size Specimen 2313

25µm

(a) Surface 50µm

(a) Fast-Fast

25µm

(b) Cross-section

Fig. 10 Surface morphology (a) and cross-sectional optical micrograph (b) for Sn–3.0Ag–0.5Cu alloy subjected to fatigue at total strain range of 0.5% 50µm at 298 K. (b) Slow-Fast

Fig. 12 Cross-sectional optical micrographs showing fatigue crack prop- agation in Sn–3.0Ag–0.5Cu alloy at total strain range of 0.4% at 398 K.

as shown in Fig. 12(b), even though that was not observed in the symmetrical cycling. As the recrystallization requires a high strain energy accumulation, the result suggests that the strain energy due to cyclic deformation in the asymmetrical cycling is much higher than that in the symmetrical cycling. The slip deformation in the asymmetrical cycling may be irreversible, so that plastic strain is highly accumulated around crack nucleation site by irreversible slip deformation, 100µm which leads to the recrystallization of tin phase. The crack propagation rate at the grain boundary is faster than that in grain interior at elevated temperature condition, which Fig. 11 Cross-sectional optical micrograph showing fatigue crack prop- agation in Sn–3.0Ag–0.5Cu alloy at total strain range of 0.5% at 298 K. contributes to the reduction in fatigue life as shown in Fig. 7. Thus, the asymmetrical cycling induces the recrystal- lization of tin phase, and the recrystallization promotes the crack propagation due to intergranular failure for Sn–3.0Ag– ature corresponds to 0.8Tm of the alloy. Thus, the intergra- 0.5Cu alloy. However, the deleterious effect of asymmetrical nular failure is predominant at 398 K in the symmetrical cycling upon fatigue life of Sn–Ag–Cu alloy is yet to be cycling. An increase in the test temperature may promote the elucidated. crack propagation due to intergranular failure, and contribu- 3.3.2 Damage mechanism of Sn–37Pb alloy ting to the reduction in fatigue life as shown in Fig. 7. Figure 13 shows surface morphology and cross-sectional On the other hand, the recrystallization of tin was observed optical micrograph of Sn–37Pb alloy subjected to fatigue at around the fatigue crack in the asymmetrical cycling at 398 K the total strain range of 1.0% at 298 K in symmetrical 2314 Y. Kariya, T. Niimi, T. Suga and M. Otsuka

1.0% at 398 K in symmetrical cycling. The fatigue crack propagated at the boundary between Pb and Sn phases, which is evidence of grain boundary sliding. The grain boundary sliding during cyclic deformation often induces cavities at the triple point, which accelerates fatigue damage. However, it is difficult to generate the cavities, if the microstructure is fine enough to causes high ductility. The microstructure tested at 398 K was still kept fine, although the microstructure was coarsened by the fatigue test at the elevated temperature. Therefore, the grain boundary sliding enhanced fatigue life of Sn–37Pb, since it induces exceptional ductility. 10µm The difference in damage mechanism between Sn–Ag–Cu and Pb–Sn may affect the sensitivity of fatigue life to test (a) Surface temperature and reversibility of loading profile as shown in Fig. 9. However, further investigation will be required to elucidate the deleterious effect of reversibility of loading profile on the fatigue life for Sn–37Pb alloys, and further crystallographic investigations are now under way.

4. Summary

Micro-bulk fatigue testing was developed to investigate the fatigue lives and damage mechanisms of Sn–3.0Ag– 0.5Cu and Sn–37Pb solder alloys. The fatigue life of Sn– 3.0Ag–0.5Cu was much longer than that of Pb–Sn eutectic in symmetrical cycling at 298 K, and the life of Sn–3.0Ag– 25µm 0.5Cu was about seven times of that of Pb–Sn eutectic. However, the fatigue life of Sn–3.0Ag–0.5Cu at 398 K was (b) Cross-section somewhat shorter than that of Pb–Sn eutectic, since the life of Sn–37Pb was increased with increasing temperature. In Fig. 13 Surface morphology (a) and cross-sectional optical micrograph (b) addition, the fatigue life of Sn–3.0Ag–0.5Cu in asymmetrical for Sn–37Pb alloy subjected to fatigue at total strain range of 0.25% at cycling was very similar to that of Pb–Sn eutectic. Thus, Sn– 298 K. 3.0Ag–0.5Cu alloy exhibits superior fatigue resistance compared with Pb–Sn eutectic in symmetrical cycling at 298 K, whereas the lives were almost equivalent in asym- metrical cycling or at the elevated temperature. The fatigue crack started from the slip bands and propagated within grain interior in the symmetrical cycling at 298 K. However, an increase in the test temperature promoted the crack propagation due to intergranular failure, and contributed to the reduction in fatigue life. In addition, the asymmetrical cycling induced the recrystallization of tin phase, and the recrystallization promoted the crack propaga- tion due to intergranular failure for Sn–3.0Ag–0.5Cu alloy. On the other hand, the crack was observed at colony boundary in Sn–37Pb alloy at 298 K. The fatigue crack 25µm propagated at the boundary between Pb and Sn phases, which is induced by the grain boundary sliding. The grain boundary sliding enhanced fatigue resistance of Sn–37Pb, since the Fig. 14 Cross-sectional optical micrograph for Sn–37Pb alloy subjected to grain boundary sliding induces exceptional ductility. The fatigue at total strain range of 0.5% at 398 K. difference in damage mechanism between Sn–Ag–Cu and Pb–Sn may affect the susceptibility of test condition to fatigue endurance. cycling. As can be seen in the figure, the fatigue crack was initiated and propagated along colony boundary, which is Acknowledgments totally different to the failure mechanism in Sn–3.0Ag–0.5Cu alloy. Similar failure mode is often observed in actual solder The authors gratefully acknowledge support for this joints made using Sn–37Pb alloy.16) research by a grant from METI (Ministry of Economy. Figure 14 shows cross-sectional optical micrograph of Sn– Trade and Industry, Japan). 37Pb alloy subjected to fatigue at the total strain range of Isothermal Fatigue Properties of Sn–Ag–Cu Alloy Evaluated by Micro Size Specimen 2315

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