Evolution of Structure Unidirectionally Solidified Sn–Ag<SUB>3</SUB>Sn Eutectic Alloy

Abstract

Sn-Ag 3 Sn eutectic alloy is known as an attractive candidate to replace Sn-Pb soldering alloy. Since the solidified structure directly relates to the mechanical properties of joints, understanding of solidified structure of this alloy system is very important. Therefore, in order to understand the revolution of solidified structure, unidirectional solidification experiments have been carried out using eutectic Sn-Ag 3 Sn alloy. When the growth velocity is larger than the critical value, Sn-dendrites solidifies as a primary phase. This is attributed to the skewed coupled zone in this alloy system. In this case. the volume fraction of the primary Sn increases with increasing growth velocity and approaches a constant value. Interdendritic eutectic is composed with fibrous Ag 3 Sn and Sn matrix. On the other hand, when the growth velocity becomes lower, a eutectic structure appears without any primary phase. In this case, a piece of eutectic Ag 3 Sn becomes large and changes from fibrous to plate-like with decreasing growth velocity. This morphological change may be due to the anisotropy of surface tension of eutectic Ag 3 Sn.