Research Output
Direct observation of the Ni stabilising effect in interfacial (Cu,Ni)₆Sn₅ intermetallic compounds
Imaging the polymorphic transformation in a single Cu6Sn5 grain in a solder joint
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Publication
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.
Publication
In-situ observations of the polymorphic transformation in a single targeted Cu6Sn5 grain constrained between Sn-0.7 wt % Cu solder and Cu-Cu3Sn phases and the associated structural evolution during a solid-state thermal cycle were achieved via a high-voltage transmission electron microscope (HV-TEM) technique. Here, we show that the monoclinic η′-Cu6Sn5 superlattice reflections appear in the hexagonal η-Cu6Sn5 diffraction pattern upon cooling to isothermal 140 °C from 210 °C. The in-situ real space imaging shows that the η′-Cu6Sn5 contrast pattern is initiated at the grain boundary. This method demonstrates a new approach for further understanding the polymorphic transformation behavior on a real solder joint.