Formation and evolution of intermetallic layer structures at SAC305/Ag/Cu and SAC0705-Bi-Ni/Ag/Cu solder joint interfaces after reflow and aging

In this paper, the formation and evolution characteristics of the intermetallic compounds (IMCs) in SAC305/Ag/Cu and SAC0705-3.5Bi-0.05Ni/Ag/Cu solder during reflow and 150 °C isothermal aging are investigated. Experimental results indicate that Ag3Sn forms as soon as the SAC305/Ag/Cu solder spheres wetted to the substrates. With increased soldering time, the Ag layer on a Cu substrate dissolved into the molten SAC305 solder and the interfacial IMC consisted of Cu6Sn5. The Ag layers show a faster dissolution rate in SAC0705-3.5Bi-0.05Ni/Ag/Cu than in SAC305/Ag/Cu, which is attributed to a larger concentration gradient of Ag for SAC0705-3.5Bi-0.05Ni/Ag/Cu. The formation and coarsening of a Cu3Sn layer between Cu6Sn5 and the Cu substrate caused the formation of Kirkendall voids and delamination during aging in the SAC305/Ag/Cu. A small addition of Ni in the solder significantly suppressed the formation of a Cu3Sn layer in the SAC0705-3.5Bi-0.05Ni/Ag/Cu, resulting in fewer voids in the soldering interface.