Microstructure evolution and mechanical properties of Sn0.7Cu0.7Bi lead-free solders produced by directional solidification

Sn-0.7 wt.%Cu-0.7 wt.%Bi (Sn0.7Cu0.7Bi) lead-free solder alloy was directionally solidified undergoing temperature gradient of 12 K/mm and growth rate (V) varying from 5 to 100 μm/s. The microstructure was characterized along the growth direction and the present experimental results included the inter-fiber spacing (λ), fiber diameter (d), ultimate tensile strength (UTS), yield tensile strength (YS) and elongation to fracture (EL) at room temperature. The aim is to examine the effects of growth rate on both microstructure and mechanical properties. The microstructure observation shows that the microstructure consists of β-Sn matrix and rod-like Cu6Sn5 intermetallics compounds (IMCs). For a constant temperature gradient (12 K/mm), it is found that the inter-Cu6Sn5 fiber spacing and fiber diameter are mainly controlled by the growth rate (V), and both of them decrease with increased growth rate. The tensile results show that the UTS and YS initially increase with increased growth rate which ranges from 5 to 60 μm/s, then decrease with further increased growth rate, such as 100 μm/s. In addition, the EL is between 25% and 32.5% as the growth rate is ranging from 5 to 60 μm/s, and achieves to the highest value (41%) with the growth rate of 100 μm/s. That is due to the formation of finer Cu6Sn5 in the solders. Finally, the fracture surfaces of the solders are examined.