Hypo-eutectic microstructure formation and nanomechanical response in Sn-3.0Ag-0.5Cu solder balls: Effects of undercooling

The microstructure control and refinement in length scale are pursued in the development of next generations of Pb-free solder alloys with high reliability in harsh environments. In the present work, the influences of nucleation undercooling, as brought by solder ball size, over the microstructure evolution and nanomechanical behavior in Sn-3.0Ag-0.5Cu (wt%) Pb-free solder alloys, have been quantitively investigated using Differential Scanning Calorimeter (DSC), Scanning Electron Microscopy (SEM) and nanoindentation. The results reveal that under -cooling increases with the decrease of solder ball size. The frequency of interlaced grain microstructure was significantly increased in 200 mu m balls, with a further increase of undercooling by-10 K. The volume fraction of primary beta-Sn increased with larger undercooling, leading to microstructure refinement with a finer spacing of both beta-Sn dendrite and eutectic Ag3Sn. The length scale of refinement is on the order of-2, as the solder ball size reduced from 650 mu m to 200 mu m. Nano-hardness and creep resistance increase with smaller ball size and larger undercooling. Nano-hardness values are relatively higher as more eutectic regions being measured due to the strengthening effect of finer and denser eutectic Ag3Sn, and also the refinement of interlaced beta-Sn grains. The creep exponent is insensitive to the loading force or solder ball size. The variation of nanomechanical defor-mation behavior was reduced in the 200 mu m balls, due to the substantial microstructure refinement, while the dominant creep mechanism stays as the dislocation climb for various solder ball sizes.