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 10(2) 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 (zeta-AgZn) and simple Cubic structure (beta'-AgZn) existing in each of them. Instead of the above mentioned IMCs, primary beta-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 In microstructural stability of the rapidly solidified solder was explored by high-temperature annealing. After annealing the beta-Sn dendrites grew and ternary Ag-Zn IMCs separated on the grain boundaries. (C) 2007 Published by Elsevier B.V.