Impact of Co on Thermal Aging in Sn58Bi/Cu Solder Joints: IMC Growth and Transformation in Mechanical Properties

The microstructure, evolution of intermetallic compound (IMC) layers, and mechanical property changes in Sn58Bi/Cu and Sn58Bi–0.3Co/Cu solder joints were studied during thermal aging at 393 K. During aging, the nucleation rate of the Cu₆Sn₅ phase was significantly enhanced by introducing Co particles, forming a substitution solid solution (Cu, Co)₆Sn₅ with a small block-like structure that was freely distributed within the matrix. The IMC layer was transformed from Cu₆Sn₅ to (Cu, Co)₆Sn₅, forming a more stable structure that effectively suppressed the Cu₃Sn layer growth. During the aging process, the size of (Cu, Co)₆Sn₅ grains was significantly smaller than Cu₆Sn₅ grains. Furthermore, gradual growth into prismatic shapes was observed in (Cu, Co)₆Sn₅ grains, with a relatively wide grain size distribution. The introduction of Co effectively inhibited the expansion of cracks during the aging process, and the probability of fracture occurring at the matrix/IMC interface was significantly reduced. Adding Co increased the shear strength of Sn58Bi/Cu solder joints within the identical aging period.

Graphical Abstract