Length Scale of the Cellular Microstructure Tailoring Tensile Properties of Zn–20 wt%Sn–2 wt%Cu Solder Alloy

Abstract

High temperature solder alloys are characterized by melting temperatures between 230 and 400 °C. They are commonly used as component joining material in electronics and automotive industries. Characteristics such as excellent wettability, suitable mechanical properties and oxidation resistance made lead-based Pb–Sn alloys widely used for such applications. However, the toxicity of lead, associated with environmental risks and human health caused by the undue disposal of these materials restricted the employment of these alloys. Under such circumstances, new high temperature alloys have been recently developed. Zn–Sn alloys are considered promising for the substitution. However, improvements in the mechanical properties, wettability and some other application properties are required. These are expected to be achieved with the addition of third alloying elements such as Ag or Cu. The present work aims to evaluate the addition of 2.0 wt% of Cu in the resultant microstructure constituting the Zn–20 wt%Sn alloy. Also, effects of the Cu alloying over solidification thermal parameters must be pondered. For this purpose, data from a directionally solidified (DS) Zn–20 wt%Sn–2 wt%Cu alloy will be compared with results found in literature for the DS Zn–20 wt%Sn alloy. Microstructural analyzes were performed using SEM, optical microscopies and X-ray diffraction. The microstructures consist of Zn-rich phase in the form of plate-like cells; intercellular regions containing eutectic mixture (Zn-α + Sn-β) and the CuZn5-e intermetallic phase. Experimental analysis resulted in mapping the growth variations of cell spacing against cooling rate and growth velocity. The cell spacing was shown to be vital to maximize the tensile properties of Zn–Sn–Cu solders.