Bismuth Improvements on the Mechanical Performance of an As-Cast Eutectic Sn-Cu Alloy

The eutectic Sn-Cu alloy considered one of the potential alternatives to the toxic Sn-Pb solder alloys. This work aims to improve the mechanical performance of the eutectic Sn-Cu alloy in terms of rupture time and creep properties through studying the effects of bismuth (Bi) and silver (Ag) contents with x = 0.3 and 0.5 wt.% for each on the mechanical properties of as-cast eutectic Sn-Cu alloy. Ternary as-cast Sn-Cu-X (X= Bi or Ag) alloys were investigated using x-ray diffractions (XRD) and Creep testing machine. The results revealed that the 0.3 and 0.5 wt.% of Bi additions to the as-cast eutectic Sn-Cu alloy do not promote the Cu6Sn5 IMC formation but just shifted it from 102 to the 202 orientations. The above-mentioned Bi additions have refined the β-Sn particle size and enlarged Cu6Sn5 IMC, thus reduced the lattice distortion that is directly enhanced the mechanical performance and reliability for these as-cast alloys through the tensile tests with different loads at room temperature (RT). Adding 0.3 and 0.5 wt.% of Bi to the as-cast eutectic alloy formed other IMC (Ag3Sn) alongside the Cu6Sn5 phase which is mismatched with it due to their different crystal structures (Ag3Sn (orthorhombic) and Cu6Sn5 (Hex)). For that, the structural stability decreased, which leads to low resistance for the external forces and low mechanical reliability. The mechanical improvements (high rupture time (5498.85 s), low strain rate, and stress exponent (9.48)) that have been reported with the 0.5 wt.% of Bi addition to the as-cast eutectic alloy which is strongly related to its high structural stability compared to that for the other additions. From the mechanical point of view, the Sn-0.7Cu-0.5Bi alloy is recommended to be the most mechanical reliable alloy for the large-scale production and processing soldering and electronic assembly.