Improved creep resistance and thermal behavior of Ni-doped Sn-3.0Ag-0.5Cu lead-free solder

To develop lead-free solders for advance electrical components, a series of Sn-3.0Ag-0.5Cu (SAC 305) solders containing small amounts of Ni have been investigated. Results showed that the addition of Ni not only decreased the amount of undercooling by about 7.9-8.5 degrees C, but also reduced the solidus temperature of SAC(305) solder from 219.9 to 216.2 degrees C. Microstructure analysis revealed that Ni could replace the Cu atoms in the Cu6Sn5 phase and generates a new eta-(Cu, Ni)(6)Sn-5 IMC phase containing large amount of Ni after 0.5% Ni addition. The high solubility of Ni in Cu6Sn5 increased the substitutional defects and generated inter-atomic stress around Ni atoms, which in turn impeded the dislocation movements in different crystal directions, resulting in an increase in the creep resistance and total fracture time of 0.5% Ni-doped SAC(305) solder. In addition, the formation of fine fiber-like Ag3Sn and finer dot-shaped precipitates at the surface of beta-Sn matrix could provide more obstacles for dislocation pile up in the adjacent grains and enhanced the creep resistance. However, when the concentration of Ni exceeded 0.5 wt%, the benefits of creep behavior and fracture time were reduced due to the formation of small amount of abrasive Ag3Sn and coarsening of (Cu, Ni)(6)Sn-5 IMCs in the eutectic colony. (C) 2013 Elsevier B.V. All rights reserved.