Composition design and mechanical properties of low-temperature Sn-Bi-In-(Ag) solders with low Bi content

Three kinds of Sn-Bi-In-(Ag) solder alloys were calculated by the cluster-plus-glue-atom (CPGA) theory. The main purpose of adding In and Ag elements to Sn-Bi solders is to reduce the brittleness of Sn-Bi solders and to promote the mechanical reliability. The addition of In element decreased the melting point and expanded the melting range of the solders. However, the excellent wettability of Sn-Bi-In counteracted the adverse effects of the increase in melting range. Moreover, In promoted the interfacial reaction and transformed the interfacial intermetallic compound (IMC) from scalloped Cu
6
Sn
5
to prismatic Cu
6
(Sn,In)
5
. The shear displacement of Sn-46Bi-3.3In solder joint increased by about 16.4 % from 0.85 mm to 0.99 mm. The addition of Ag further enhanced the toughness of the solders. Considering the performance and cost, Sn-46Bi-3.3In-0.5Ag was the optimal low-temperature solder.