The role of eutectic colonies in the tensile properties of a Sn–Zn eutectic solder alloy

Abstract

Listen

Translate article

The growth of eutectic colonies in Sn–Cu, Sn–Zn and Sn–Ag–Cu eutectic alloys has already been reported in the literature. However, relationships between this kind of microstructure and mechanical properties remain undetermined for solders. The use of water-cooled copper (Cu) and AISI 1020 low-C steel molds and the eutectic Sn-9 wt.%Zn alloy make it possible to address this matter. The samples grown in the Cu mold demonstrated higher solidification rates than those developed in the low-C steel mold. Overall, the microstructure is constituted by Zn-lamellae embedded in a Sn-rich matrix. The Zn lamellae are not only uneven in thickness but also irregularly perforated. Due to Cu dissolution into the alloy, a small fraction of Cu5Zn8 intermetallic particles formed during solidification of the Sn-9 wt.%Zn alloy in the Cu mold. The contamination with Cu appears to be responsible for the improvement in the distribution of Zn-lamellae. The decrease in spacing between broken lamellae measured from SEM images, as well as a higher number of Zn particles per area, explain such occurrence. Ductility and tensile strength of different samples could allow the establishment of relationships among properties vs. eutectic colony spacing. For the Cu mold, the motion of Cu towards the alloy as well as higher solidification rates, allowed microstructures to be formed combining 60% of strain to fracture and 52 MPa of ultimate tensile strength. These achievements are mainly due to the finest spacings of both the eutectic colony (λc = 36 μm) and the Zn lamellae (λL=0.9 μm), besides homogeneous distribution of Cu across the resulting microstructure.