Nucleation, grain orientations, and microstructure of Sn-3Ag-0.5Cu soldered on cobalt substrates

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.