Effect of solidification parameters on the microstructure of Sn-3.7Ag-0.9Zn solder

In this work, Sn–Ag–Zn alloy of eutectic composition (Sn-3.7wt.%Ag-0.9wt.%Zn) was directionally solidified upward at a constant temperature gradient (G = 4.33 K/mm) in a wide range of growth rates (V = 3.38–220.12 μm/s) and a constant growth rate (V = 11.52 μm/s) with different temperature gradients (G = 4.33–12.41 K/mm) using a Bridgman type directional solidification furnace. The microstructure was observed to be a rod Ag3Sn structure in the matrix of β–Sn from the directionally solidified Sn-3.7wt.%Ag-0.9wt.%Zn samples. The values of eutectic spacing (λ) were measured from transverse section of samples. The dependency of eutectic spacing on the growth rate (V) and temperature gradient (G) were determined with linear regression analysis. The dependency of λ on the values of V and G were found to be λ = 10.42V − 0.53 and λ = 0.27G − 0.48, respectively. The values of bulk growth were also determined to be λ2V = 86.39 μm3/s by using the measured values of λ and V. The results obtained in present work were compared with the previous similar experimental results obtained for binary and ternary alloys.

Research Highlights
► Sn–Ag–Zn eutectic alloy was directionally solidified upward under the different solidification conditions.
► The microstructure was observed to be a rod Ag3Sn structure in the matrix of β–Sn.
► The dependency of λ on the values of V and G were found to be λ = 10.42V − 0.53 and λ = 0.27G − 0.48, respectively.
► The values of bulk growth were determined to be λ2V = 86.39 μm3/s by using the measured values of λ and V.