Microstructure and tensile properties of Sn–1Cu lead-free solder alloy produced by directional solidification

Abstract

Cu6Sn5 fiber reinforced Sn in situ composites with a nominal composition Sn–1Cu (wt%) were produced by specially controlled directional solidification using a laboratory-scale Bridgman furnace equipped with a liquid metal cooling (LMC) device. The microstructure of as-produced composites was characterized by using optical microscopy (OM), electron microscopy (SEM) and microanalysis (EDX). The tensile strength and plasticity at room temperature were examined by tensile tests. The microstructure observation showed that the microstructure consisted of β-Sn matrix and fiber-like Cu6Sn5 intermetallics compounds (IMCs). For a constant temperature gradient (12Kmm−1), it was found that the spacing between Cu6Sn5 fibers and diameter of single crystalline Cu6Sn5 fiber were mainly controlled by the solidification rate (V), and both of them decreased with increasing solidification rate. The strength was dominated by the Cu6Sn5 fiber alignment, such as the spacing and diameter. Thus the tensile tests results have been correlated to fiber spacing (λ) and diameter (d), since fiber growth has prevailed along all obtained Sn–1Cu samples. It was found that the ultimate tensile strength (UTS) and yield tensile stress (YS) initially increased with increasing solidification rate which ranged from 5 to 60μms−1, and decreased with further increasing solidification rate, such as 100μms−1. In contrast, the elongation (EL) decreased with increasing solidification rate due to the increased amount Cu6Sn5 IMC quantity.