Anomalous multiple pop-in behavior in Cu-Sn-based intermetallic compounds during nanoindentation

Mechanical properties of binary intermetallic compounds (IMCs) on Sn-based solder (Cu6Sn5 and Cu3Sn) on the interface between the solder alloy and its bonding pads by nanoindentation are examined. Cu6Sn5 and Cu3Sn binary IMCs are intentionally formed in between Sn and Cu plates. A Cu-Sn diffusion couple was prepared and annealed at 325 °C for 48 h in an Ar atmosphere. While the elastic modulus of Cu3Sn (~140 GPa) was about 12% higher than that of Cu6Sn5 (~125 GPa) at 0.05 s−1, the hardness of Cu3Sn (~6 GPa) was about 15% lower than that of Cu6Sn5 (~7 GPa). Cracks were only observed on Cu6Sn5, which had a fracture toughness around 0.71 MPam. Anomalous multiple pop-in events were mainly observed on Cu6Sn5 at strain rates up to 0.1s s−1 during the loading period of the nanoindentation test. It is presumed that multiple pop-in events in Cu6Sn5 correlated with the crystallographic characteristics of specific intermetallic compounds.