Effect of Ag solutes on the solid-state Cu dissolution in the Sn3.5Ag

Abstract

The effect of Ag solutes on the solid-state Cu dissolution in the Sn3.5Ag solder was studied. The solid-state Cu dissolution depends on the dissolution of the interfacial Cu6Sn5 compound layer, which is constant with time. In a low annealing temperature range (150, 165, and 180 °C), the Ag solutes forming Ag3Sn phase attaching (near) the interfacial Cu-Sn compound layer, which retards dissolution of the interfacial Cu6Sn5 compound layer. The activation energy of Cu dissolution into Sn3.5Ag solder is 0.37 eV, which is less than Cu in pure Sn solder (1.48 eV). Interestingly, the consumption mechanism of the Cu wire changes at a high annealing temperature of 200 °C, where the consumption of the Cu wire in the Sn3.5Ag/Cu-wire/Sn3.5Ag system is faster than in the Sn/Cu-wire/Sn system. Therefore, the higher Ag solubility in the Sn3.5Ag solder matrix causes fewer Ag3Sn precipitates in the Sn lattice than in pure Sn solder, which is the main factor promoting Cu6Sn5 dissolution at the high annealing temperature of 200 °C. Additionally, the extensive dissolution of the interfacial Cu6Sn5 compound layer in the Sn3.5Ag/Cu-wire/Sn3.5Ag system resulted in an irregular decomposition with prolonged annealing (greater than 10 h) at the high annealing temperature of 200 °C. After 35 h of annealing, the interfacial Cu6Sn5 compound layer broke off and dissolved into the Sn3.5Ag solder matrix.