The potential of cobalt substrates to control the microstructure of 550 μm Sn-3Ag-0.5Cu ball grid array (BGA) joints is explored. It is shown that cobalt substrates give a small and reproducible nucleation undercooling for tin and prevent the formation of large primary Ag3Sn blades and Cu6Sn5 rods. βSn dendrites grew from the CoSn3 reaction layer and the βSn grains are shown to inherit their orientation from the CoSn3 layer due to heterogeneous nucleation with the following orientation relationship: (100)Sn||(100)CoSn3 with [001]Sn||[001]CoSn3. Changes in the dendrite and eutectic microstructure are shown to be caused by the reduced nucleation undercooling for βSn and the altered solidification path due to cobalt substrate dissolution.