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.