A superior interfacial reliability of Fe–Ni UBM during high temperature storage

Ball shear test was conducted on the SnAgCu/Fe–Ni solder joints, as well as SnAgCu/Cu for comparison after reflow and 150 °C high temperature storage following the industrial JEDEC standards. According to microstructural observation, Fe–Ni UBMs show better diffusion barrier effect than Cu UBM, which form very thin FeSn₂ or FeSn₂+(Cu,Ni)₆Sn₅ layers at the interface without any Kirkendall voids. With such thin IMC layers, the shear strengths of Fe–Ni solder joints were comparable to Cu solder joints, which are in the range of 6.3–6.4 mg/μm² after reflow and then fall into 4.7–5.1 mg/μm² after 1000 h storage. Statistic observations revealed four kinds of fracture mode among failed solder joints, which are ductile fracture, brittle fracture, UBM fracture and pad lift respectively. The UBM fracture is the most common failure mode for Cu solder joint due to the fast consumption and low mechanical strength of Cu UBM, while the ductile failure takes the majority for Fe–Ni UBMs although it gradually turned into brittle fracture later as the aging time increased. The related mechanism of fracture behavior was also discussed concerning the physical properties of interfacial IMCs. Combining the low IMC growth rate, the comparable shear strength and the ductile fracture mode, Fe–45Ni UBM is superior to Cu UBM on the interfacial reliability of high temperature storage.