Interfacial behavior and joint strength of Sn–Bi solder with solid solution compositions

Abstract

To obtain the interfacial behaviors and the joint strength of Sn–Bi solid solution solder, Sn–2.5Bi and Sn–5Bi solders were selected to be investigated in this work with pure Sn/Cu solder joints as a reference. The results showed that the growth behaviors of intermetallic compound (IMC) at Sn–Bi solder joints were different under reflow soldering and isothermal aging, which was related with the presence of Bi atoms and the diffusion rate of Cu atoms. Furthermore, the Cu6Sn5 particles in solder matrix and the Bi precipitation could change the diffusion path of Cu atoms, which caused the change on the distribution and size of Cu6Sn5 in pure Sn, Sn–2.5Bi and Sn–5Bi solder matrix after isothermal aging. The size and distribution of Cu6Sn5 in solder matrix finally affected the shear strength of solder joints. The shear strength of Sn–2.5Bi/Cu solder joints increased compared with pure Sn/Cu solder joints, and there was no obvious change with the aging times for Sn–2.5Bi/Cu solder joints due to the effect of the reinforcement of Bi atoms. However, the shear strength of Sn–5Bi/Cu solder joints increased firstly and then decreased with the prolongation of aging times. This phenomenon was attributed to the segregation of Bi and the large amounts of Cu6Sn5 particles distributed in solder matrix during the aging condition. Besides, the IMC thickness in Sn–5Bi/Cu solder joint played the important role on the shear strength due to the brittle nature of IMC. In pure Sn solder matrix, the size of Cu6Sn5 particles were larger than that in Sn–Bi solder matrix, which deteriorated the shear strength on pure Sn solder. From the observation on the fracture morphologies, Sn/Cu and Sn–2.5Bi/Cu solder joints showed the ductile fracture occurred in the solder matrix, while Sn–5Bi/Cu solder joints exhibited a mixed fracture.