Low-cycle fatigue behavior and mechanisms of a lead-free solder 96.5Sn/3.5Ag |
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Authors: | Kanchanomai Chaosuan Miyashita Yukio Mutoh Yoshiharu |
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Affiliation: | (1) Department of Mechanical Engineering, Nagaoka University of Technology, 1603-1 Kamitomioka, 940-2188 Nagaoka, Japan |
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Abstract: | Low-cycle fatigue tests of as-cast Sn-Ag eutectic solder (96.5Sn/3.5Ag) were performed using a noncontact strain controlled system at 20°C. The fatigue behavior followed the Coffin-Manson equation with a fatigue-ductility exponent of 0.76. Without local deformation and stress concentration at contact points between the extensometer and the specimen surface in strain-controlled fatigue tests, crack initiation and propagation behavior was observed on the specimen surface using a replication technique. After failure, the longitudinal cross sections were also examined using scanning electron microscopy (SEM). Microcracks initiated from steps at the boundary between the Sn-dendrite and the Sn-Ag eutectic structure and cavities along the boundaries especially around the Ag3Sn particles. Stage II crack propagated in mixed manner with intergranular cracks along the Sn-dendrite boundaries and transgranular cracks through the Sn-dendrites and the Sn-Ag eutectic structure. Propagation of stage II cracks could be expressed by the relation of dac/dN = 4.7 × 10−11[ΔJ]1.5, where ac is the average crack length and ΔJ is the J-integral range. After fatigue tests, small grains were observed in Sn-dendrites near the fracture surface. |
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Keywords: | Low-cycle fatigue crack initiation crack propagation small grain formation lead-free solder material 96.5Sn/3.5Ag |
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