Microstructural evolution of Cu–Sn–Ni compounds in full intermetallic micro-joint and in situ micro-bending test

This study focuses on the microstructural evolution process of Cu–Sn–Ni intermetallic compounds (IMCs) interlayer in the micro-joints, formed from the initial Ni/Sn (1.5 µm)/Cu structure through transient liquid phase (TLP) soldering. Under the bonding temperature of 240 °C, the micro-joints evolve into Ni/(Cu, Ni)₆Sn₅/(Cu, Ni)₃Sn/Cu structure, where the interfacial reactions on Cu/Sn and Sn/Ni are suppressed by the atoms diffusing from the opposite side. The thickness of (Cu, Ni)₃Sn layer on plated Cu layer still increases with the prolonged dwell time. When the bonding temperature was elevated to 290 °C, the phase transformation of (Cu, Ni)₆Sn₅ into (Cu, Ni)₃Sn has been accelerated, thus the majority of IMCs interlayer is constituted with (Cu, Ni)₃Sn. However, a small amount of Ni-rich (Cu, Ni)₆Sn₅ phases still remain near the Ni substrate and some of them close to the center-line of IMCs interlayer. The state between (Cu, Ni)₆Sn₅ and the adjacent (Cu, Ni)₃Sn tends to reach equilibrium in Ni content based on the observation from Transmission Electron Microscope (TEM). In addition, the Cu–Sn–Ni IMCs micro-cantilevers were fabricated from these micro-joints using Focus Ion Beam (FIB) for the in situ micro-bending test, the results indicate a high ultimate tensile strength as well as the brittle fracture in the inter- and trans-granular modes.