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.