Evolution mechanism of interconnect interface and shear properties of 64.8Sn35.2Pb microbump during flip chip bonding

Effect of flip chip bonding parameters on microstructure at the interconnect interface and shear properties of 64.8Sn35.2Pb microbumps were investigated in this work. Results show that the main intermetallic compound (IMC) at the interconnect interface is (Ni, Cu)₃Sn₄ phase, and meanwhile a small amount of (Cu, Ni)₆Sn₅ phase with a size of 50–100 nm is formed around (Ni, Cu)₃Sn₄ phase. The orientation relationship of [1¯5¯6](Ni, Cu)₃Sn₄//[152](Cu, Ni)₆Sn₅ and (601)(Ni, Cu)₃Sn₄//(2¯01)(Cu, Ni)₆Sn₅ is found between these two phases, and the atomic matching at the interface of the two phases is low. The highest shear force of 77.3 gf is achieved in the 64.8Sn35.2Pb microbump at the peak temperature of 250 °C and parameter V₁ because dense IMCs and no cracks form at the interconnect interface. Two typical fracture modes of microbumps are determined as solder fracture and mixed fracture. The high thermal stress presenting in the thick IMCs layer induces crack initiation, and cracks propagate along the α/β phase boundaries in the Sn-Pb solder under shear force, leading to a mixed fracture mode in the microbumps.