The effects of temperature gradient and growth rate on the microstructure of directionally solidified Sn–3.5Ag eutectic solder

The mechanical properties (microhardness, tensile strength) of alloys are controlled by their microstructure, which depends strongly on temperature gradient (G) and growth rate (V). Thus, it is important to understand the relationships among G, V and microstructure (rod eutectic) of Sn–Ag solders. The Sn–3.5 wt% Ag eutectic alloy was directionally solidified upward with a constant growth rate, V (16.5 μm/s) at different temperature gradients, G (1.43–4.28 K/mm) and with a constant temperature gradient, G (3.93 K/mm) at different growth rates, V (8.3–500 μm/s) in a Bridgman–type directional solidification furnace. The rod spacings (λ) have been measured from both longitudinal section (parallel to the growth direction, λL) and transverse section (perpendicular to the growth direction, λT) of the samples. The undercooling values (ΔT) were calculated by using V, λ and system parameters (K1 and K2). It was found that the values of λ (λT, λL) decrease while V and G are increasing. The relationships between rod spacing and solidification parameters (G and V) were obtained by linear regression analysis. The dependences of eutectic spacings λ on undercooling (ΔT) are also analyzed. λ2V, λΔT, ΔTV−0.5 and ΔTG−0.5 values were determined by using λ, ΔT, V and G values. The results obtained in this work are compared with the Jackson–Hunt eutectic theory and the similar experimental works. The experimental $$ \lambda_{\text{T}}^{ 2} {\text{V}} $$ value (159.3 μm3/s) is slightly lower than the result 174.6 μm3/s calculated from Jackson–Hunt eutectic theory.