Influence of cerium addition on microstructure and properties of Sn–Cu–(Ag) solder alloys

In the present work, the effect of Ce concentration on microstructure refinement, melting behavior and mechanical properties of a series of near-eutectic Sn–Ag–Cu alloys have been investigated. Twelve different alloys with Ce concentration between 0.005 and 0.171 wt% were prepared by melting of the pure elements in high purity argon, followed by quenching to room temperature. The samples microstructure and phase constitution were investigated by a combination of light microscopy, scanning electron microscopy, X-ray diffraction, and energy dispersive X-ray spectroscopy. In the alloys a total of five different phases have been identified: β-Sn, Cu6Sn5, Ag3Sn, CeO2, and CeSn3. The Ce addition is found to contribute to the refinement of mostly eutectic microstructures. The CeO2 and/or CeSn3 phases are observed in alloys with high Ce concentration. The absence of these particles in alloys with low Ce concentration (0.005–0.011 wt%) shows a complete dissolution of Ce in the β-Sn solid solution. The elongation and ultimate tensile strength of the alloys were determined under a constant strain rate of 2 mm min−1. Moreover, the toughness (i.e. specific fracture energy) was calculated from the area underneath the force–extension curve. Melting temperatures are found to change slightly with bulk Ce concentration. Solution strengthening by Ce atoms, grain refinement, and the stress concentration along both types of Ce-rich particles of undesirable shapes are considered to be the main factors influencing the mechanical properties of the investigated alloy systems.