Influence of minor Ag nano-particles additions on the microstructure of Sn30Bi0.5Cu solder reacted with a Cu substrate

The objective of this paper is to study the influence of minor Ag nano-particles additions on the microstructural evolution of Sn30Bi0.5Cu lead-free solder matrices and the interfacial reactions. Ag nano-particle reinforced SnBiCu–xAg (x = 1, 2, 5) composite solder pastes were prepared by a mechanical mixing method and then were reflowed on Cu substrates. Three cooling methods (furnace cooling, air cooling and water cooling) were adopted in this study. Microstructural evolution of composite solder matrices and the intermetallic compound (IMC) layers formed in solder joints were investigated by microstructural observation and phase analysis. The addition of Ag nano-particles did not change the microstructure of solder matrices apparently both in the furnace cooled and water cooled samples. In the air cooled samples, Bi-rich grains were refined in the composite solder matrices due to the adsorption of Ag₃Sn micro-particles on them. The growth of Cu₆Sn₅ IMC layers in air cooled SnBiCu–xAg/Cu solder joints was influenced by the adsorption of Ag₃Sn micro-particles both on the surface of Cu₆Sn₅ layers and on the surface of the Bi-rich grains.