Effect of electromigration on the microstructural evolution and shear fracture behavior of mixed assembled Cu/Sn–3.0Ag–0.5Cu-ball/Sn–58Bi-paste/Cu joints in board-level packaging

To reduce thermal shock on thinner devices and substrates during reflow process, low melting-point Sn-58Bi (SnBi) solder paste was studied to replace high melting-point Sn-3.0Ag-0.5Cu (SAC305) solder paste. In this study, SAC305 solder ball as integrated circuit (IC) pin was reflowed using SnBi solder paste on substrate board to form mixed assembled Cu/SAC305-ball/SnBi-paste/Cu BGA structure joints, then the microstructural evolution and shear properties of mixed assembled joints after thermal aging and current stressing were investigated. Results show that current stressing induces a polarity effect on growth of interfacial Cu
₆
Sn
₅
layer of the mixed assembled joints during electromigration, and promotes the growth of Cu
₆
Sn
₅
layer at the anodic Cu/SAC305-ball interface significantly, while reversely on the cathode side. Bi atoms migrate to the anode side to form the Bi-rich phase layer at anodic SnBi-paste/Cu interface. In addition, regardless of change in electron flow direction, difference in migration rate between Bi and Sn atoms leads to formation of voids near the SnBi-paste/Cu interface, and shear strength of mixed assembled joints decreases with prolonging current stressing time, the fracture occurs mainly at SnBi-paste/Cu interface. Fracture mode is mixed ductile-brittle fracture and brittle fracture when SnBi-paste/Cu interface is on cathode side and anode side, respectively.