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 transgranular modes.