Influence of frequency on low cycle fatigue behavior of Pb-free solder 96.5Sn–3.5Ag

Low cycle fatigue (LCF) tests on as-cast Sn–Ag eutectic solder (96.5Sn–3.5Ag) were carried out using the non-contact strain controlled system at 20 °C with different frequencies (10−3–1 Hz). The LCF behavior in this frequency range followed the Coffin–Manson equation. The fatigue ductility coefficient was found to be influenced by frequency. Frequency-modified Coffin–Manson model successfully described frequency effects on the fatigue behavior. Steps at the boundaries of Sn-dendrites were the initiation sites for microcracks in the case of low frequency fatigue tests, while for high frequency tests, cracks predominantly initiated at the boundaries of subgrains formed in Sn-dendrites. The link up of these cracks and the propagation of cracks inside the specimen occurred both transgranularly through Sn–Ag eutectic phases, and intergranularly along Sn-dendrite boundaries and/or subgrain boundaries. The dislocation rearrangement (polygonization) is considered to be the cause of this small grain formation.