Influence of Au addition on the phase equilibria of near-eutectic Sn-3.8Ag-0.7Cu Pb-free solder alloy

This paper illustrates the influence of Au addition on the phase equilibria of Sn-Ag-Cu (SAC) near-eutectic alloys and on the interface reaction with the Cu substrate. From the thermal and microstructural characterization of Sn-3.8Ag-0.7Cu alloys containing various amounts of Au, it is found that the Au promotes the formation of a quaternary-eutectic reaction at 204.5°C ± 0.3°C. The equilibrium phases in the quaternary-eutectic microstructure are found to be AuSn4, Ag3Sn, βSn, and Cu6Sn5. While the addition of Au to Sn-3.8Ag-0.7Cu alloys is also found to increase liquidus temperature and the temperature ranges of the phase equilibria field for primary phases, such influences from Au are found to be less pronounced when the alloys were reacted with the Cu substrate. Because of the formation of the Au-Cu-Sn-ternary interface intermetallic, it is found that a majority of Au added to the solder is drained from the melt. The drainage of Au reduces the impact of Au on the phase equilibria of the solder alloys in the joint. It is further found that the involvement of Au in the interface reaction results in a change of the interface phase morphology from the conventional scallop structure to a compositelike structure consisting of (AuCu)6Sn5 grains and finely dispersed, βSn islands.