Investigating the impact of cobalt incorporation on the transformation of Cu6Sn5 layer to (Cu, Co)6Sn5: Microstructural and mechanical insights

Abstract

In this study, cobalt (Co) particles were incorporated as reinforcing agents into the Sn58Bi solder. The solderability of the solder and the strength of the solder joints were investigated. The results indicated that adding Co particles did not alter the low melting point characteristics of the Sn58Bi solder, yet its undercooling was reduced, and the wettability was enhanced. In the Sn58Bi/Cu solder joint, adding Co led to forming free (Cu, Co)6Sn5 intermetallic compound (IMC) and generating the CoSn2 phase through microalloying. The interfacial IMC was gradually transformed from a fan-shaped Cu6Sn5 to a flatter layer-like (Cu, Co)6Sn5 IMC, eventually exhibiting a coral-like structure with increased Co content. Adding Co particles increased the thickness of the IMC layer, but at low levels, the Co could refine the IMC grains. Simultaneously, implementing grain refinement and Orowan strengthening resulted in heightened microhardness and shear strength. In the shear test, the fracture of solder joints predominantly transpired within the solder matrix. Excessive Co particles were prone to aggregating within the solder, forming irregular block-shaped (Cu, Co)6Sn5. Concurrently, the coral-like IMC at the interface led to a decline in the mechanical performance of the solder joints, with some interfacial IMC being exposed in the fracture region.