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 alpha CoSn3 crystals with a reproducible orientation relationship of (100)(Sn) parallel to (100)(CoSn3) with [001](Sn) parallel to [001](CoSn3), which has a planar lattice mismatch of similar to 4%. In 550 mu m solder joints on Cu substrates, Co microalloying alters the beta Sn nucleation location to an alpha CoSn3 particle in the bulk liquid and increases the number of beta Sn nucleation events when the cooling rate is higher than similar to 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. (C) 2016 Elsevier B.V. All rights reserved.