Sn-Mg lead-free solder alloy: Effect of solidification thermal parameters on microstructural features and microhardness

Abstract

In the last decade, several studies have been developed on lead-free alloys as potential candidates to replace Sn-Pb alloys in soldering processes. Sn-Mg alloys arise as promising alternatives due to characteristics such as low melting point, low coefficient of thermal expansion, suitable mechanical properties, electrical resistivity and low cost of Mg as an alloying element. However, the literature is scarce on studies relating the microstructure features of unsteady state cooling conditions during solidification to the resulting properties of Sn-Mg alloys. It is of the utmost importance to know the influence of the solidification cooling rate on the representative length scale of the alloy microstructure, since it varies from about 0.4 to 8 °C s-1 in the soldering process. In the present study the Sn 2.1 wt%Mg eutectic alloy is solidified under unsteady state conditions over a nickel substrate for a range of solidification cooling rates from 0.5 to 12 °C s-1. The microstructure is shown to be formed by a mixture of β-Sn and fibrous Mg2Sn intermetallics (IMC) and an experimental growth law is proposed relating the interphase spacing between Mg2Sn fibers (λ F) and the cooling rate. With the decrease in the cooling rate a pronounced decrease in the Mg2Sn IMC fraction is shown to occur; from about 46% to 23%. Consequently, hardness, that is shown to depend on both λ F and Mg2Sn fraction, decreases significantly with the decrease in the solidification cooling rate.