Microstructure evolution and growth kinetics of intermetallic compound in SAC305/Ag and SAC305/Cu solder joints during solid-state aging

Microstructure evolution and growth kinetics of intermetallic compound (IMC) in Sn-3Ag-0.5Cu (SAC305) /Ag and SAC305/Cu solder joints were investigated during isothermal aging at temperatures of 140, 160, and 180 °C for up to 528 h. Top-view and cross-sectional images were analyzed to elucidate the transformation processes of Cu₆Sn₅ and Ag₃Sn grains on Cu and Ag substrates, respectively. In the top-view images, the Cu₆Sn₅ grains on the Cu substrate, initially exhibiting a scallop-type morphology after reflow, transformed into hexagonal-type grains during isothermal aging. In contrast, the Ag₃Sn grains formed on Ag substrate, characterized by a prism-type morphology after reflow, underwent a process of transformation from scallop-type to hexagonal-type grains. The cross-sectional images revealed the formation of a planar-type IMC layer on both the Ag and Cu substrates. In the SAC305/Cu joint, after the aging time, a double-layer structure was seen in the way of bottom Cu₃Sn layer and top Cu₆Sn₅ layer outside with many Ag₃Sn tiny particles. However, in the condition of SAC305/Ag joint, only a few Cu₆Sn₅ particles were observed around the surface of the Ag₃Sn IMC with a single layer. Kinetic analyses showed that the control mechanism for IMC growth on both substrates was volume diffusion and the apparent activation energies for the IMC growth were calculated as 89.641 kJ/mol for the SAC305/Ag joint and 97.082 kJ/mol for the SAC305/Cu joint.