Effects of cobalt on the nucleation and grain refinement of Sn-3Ag-0.5Cu solders

We show that cobalt-microalloying causes significant grain refinement in large samples (60 g) of Sn-3Ag-0.5Cu solder. Nucleation occurs on the (100) facet of αCoSn₃ crystals with a reproducible orientation relationship of (100)_Sn || (100)_CoSn3 with [001]_Sn || [001]_CoSn3, which has a planar lattice mismatch of ∼4%. In 550 μm solder joints on Cu substrates, Co microalloying alters the βSn nucleation location to an αCoSn₃ particle in the bulk liquid and increases the number of βSn nucleation events when the cooling rate is higher than ∼5 K/s. However, Co-microalloying caused only weak grain refinement in sample volumes relevant to solder joints which is explained using nucleation and grain refinement theories.