Effect of Ag addition on the microstructure and distribution of second-phase particles in tin-based solder alloys

In this study, the effect of Ag content (x = 0, 1, 2, 3 wt%) on the microstructural evolution and the distribution of second phase particles in Sn-0.7Cu-xAg were analyzed utilizing scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Morphology of second-phase particles in low-temperature fracture surfaces of Sn-3.5Ag and Sn-0.7Cu solder alloys was characterized using TEM and focused ion beam (FIB) techniques. The results demonstrated that Ag within the solder matrix significantly inhibited the formation of Cu₆Sn₅ intermetallic compounds (IMCs). With increasing Ag content, Cu₆Sn₅ particles evolved from extensive, strip-like structures to a finer, more uniform distribution. Ag₃Sn particles predominantly accumulated at grain boundaries, with minimal Ag detected within the solder matrix. In contrast, Cu₆Sn₅ particles, along with a significant dispersion of copper (Cu) throughout the matrix, were also observed at grain boundaries. Quantitative analysis demonstrated that the suppression of Cu₆Sn₅ formation (assessed via Gibbs free energy calculations) reached its maximum efficacy at a critical Ag concentration of 0.17 at.%, with diminishing effects observed when Ag content exceeded 0.35 at. %.