Controlling Ag3Sn Plate Formation and Its Effect on the Creep Resistance of Sn–3.0Ag–0.7Cu Lead-Free Solder by Adding Minor Alloying Elements Fe, Co, Te and Bi

Abstract

Preventing the formation of large platelets of Ag3Sn intermetallic compounds (IMCs) during solidification of solder joints has become a significant challenge in the design of Sn–Ag–Cu lead-free solder alloys. Large platelets of Ag3Sn are generally considered as undesirable as their presence can create solidification defects and causes mechanical property anisotropy. In the present work, the synergetic effects of adding 0.1 wt% of Fe, Co, Te and 2 wt% Bi to Sn–3.0Ag–0.7Cu (SAC 307) solder are studied in terms of the growth of large platelets Ag3Sn IMCs and the resulting alloy’ creep resistance as well as their thermal behavior. Although minor Fe, Co, Te and Bi alloying elements addition causes large increase in the degree of undercooling from 3.4 to 22.3 °C with maintaining the pasty range and melting temperature at the same levels, the modified SAC307–FeTeCoBi alloy exhibits considerable increase in creep resistance (~ 10 times) and large fracture life-time than SAC (307) solder at same stress levels and testing temperatures. This is attributed to the transition of Ag3Sn IMCs from large platelets into fine needle-like morphology and formation of new (Cu,Co)6Sn5, FeSn2, SnTe IMCs and Bi particles, which could provide more obstacles for dislocation movement at the interphase boundaries.