Microstructures of eutectic Sn–Ag–Zn solder solidified with different cooling rates

Microstructures of eutectic Sn–3.7 wt.% Ag–0.9 wt.% Zn solder were investigated under different cooling rates (0.16 K/s, about 102 K/s and about 104 K/s). As learned from microstructural analyses, two kinds of IMCs (Ag3Sn and AgZn) were found. At rates of 0.16 K/s and about 102 K/s, the solidified microstructures were so similar to each other that both Ag3Sn and AgZn IMCs particles were found homogeneously distributing in the formed eutectic microstructures. And the amount of nano-sized Ag3Sn IMCs in the solder increases with the cooling rate increasing. Regarding to AgZn IMCs, there are two kinds of crystalline structures, hexagonal close-packed structure (ζ-AgZn) and simple cubic structure (β′-AgZn) existing in each of them. Instead of the above mentioned IMCs, primary β-Sn dendrites and gray eutectic colonies were found in the lead-free solder cooled with a rate about 104 K/s, which could be attributed to the shift of eutectic point under rapid solidification condition. The microstructural stability of the rapidly solidified solder was explored by high-temperature annealing. After annealing the β-Sn dendrites grew and ternary Ag–Zn IMCs separated on the grain boundaries.