Microstructure and properties of electroless Ni–P/Sn2.5Ag0.7Cu0.1RE micro-joints during thermomigration

The Ni-P stratum was fabricated upon the Cu substrate via an electroless plating technique, and the microstructure and properties of electroless Ni-P/Sn2.5Ag0.7Cu0.1RE micro-joints under temperature gradient was studied. Research indicates that in the initial stage of thermomigration in micro-solder joints, the intermetallic compound (IMC) in the Ni-P/soldering seam transition area appears as both "needle-shaped" and "block-shaped" (Ni, Cu)₃Sn₄, with an average thickness of 1.1-1.5 μm. Additionally, between Cu and (Ni, Cu)₃Sn₄ IMC, there exists a 0.8 μm thick "layered" Ni₃P within the Ni-P layer. The temperature gradient causes the asymmetric growth of (Ni, Cu)₃Sn₄ IMC and the asymmetric evolution of the Ni-P layer at the hot and cold ends of the micro-solder joint. The Ni-P layer evolution is divided into two stages: Ni-P → Ni₃P + Ni and Ni₃P + Sn → Ni-Sn-P, and the cold end structure evolves faster than the hot end. After 60 h under the temperature gradient condition of 550 °C/cm, the shear fracture position of the micro-solder joint shifts from the soldering seam to the Ni-Sn-P/IMC layer junction, and fracture mode changes from ductile fracture dominated by dimples to brittle fracture dominated by cleavage and slip steps, corresponding to a decrease of 21.8% in micro-solder joint pushing shear force from 16N.