The polymorphic transformation in interfacial intermetallic compounds in three lead-free solder joints was investigated using in situ heating/isothermal/cooling observation techniques in high-voltage transmission electron microscopy (HVTEM). Here, the hexagonal η- to monoclinic η′-Cu₆Sn₅ polymorphic transformation was visualised systematically through zone-axis electron diffraction patterns and real-space imaging. Cu₆Sn₅ grains obtained in the as-reflowed solder joints that did not contain Ni (Sn–0.7Cu/Cu and Sn–3.0Ag–0.5Cu/Cu) show weak reflections in the diffraction patterns from the η′-Cu₆Sn₅. In Sn–0.7Cu–0.05Ni/Cu joints, no weak reflections were present in the diffraction patterns of the (Cu,Ni)₆Sn₅ grains indicating the presence of Ni prevented the η- to η′-Cu6Sn5 transformation. The movement of bend contours was also observed at around 186 °C in adjacent the grain boundaries in Cu₆Sn₅ but not in the (Cu,Ni)₆Sn₅. We also uncover the origin of the weak reflections and confirm the superstructure of monoclinic η′-Cu₆Sn₅ by obtaining atomic-resolution images within the areas from which the selected area diffraction patterns were obtained.
