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

• Published in: Materials characterization 2023-04, Vol.198, p.112707, Article 112707
• Main Authors: Wang, B., Hu, X.J., Sun, W., Liao, J.L., Peng, H.L., Hou, N., Chen, B., Zeng, G.
• Format: Article
• Language: English
• Subjects: Microstructure; Nano indentation; Nanomechanical properties; Sn-3.0Ag-0.5Cu solder; Undercooling
• ISSN: 1044-5803; 1873-4189
• DOI: 10.1016/j.matchar.2023.112707

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 undercooling increases with the decrease of solder ball size. The frequency of interlaced grain microstructure was significantly increased in 200 μm balls, with a further increase of undercooling by ∼10 K. The volume fraction of primary β-Sn increased with larger undercooling, leading to microstructure refinement with a finer spacing of both β-Sn dendrite and eutectic Ag3Sn. The length scale of refinement is on the order of ∼2, as the solder ball size reduced from 650 μm to 200 μm. 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 β-Sn grains. The creep exponent is insensitive to the loading force or solder ball size. The variation of nanomechanical deformation behavior was reduced in the 200 μm balls, due to the substantial microstructure refinement, while the dominant creep mechanism stays as the dislocation climb for various solder ball sizes. •Sn3.0Ag0.5Cu ball solidification is nucleation controlled with size independency.•More hypo-eutectic and refined microstructure formed with larger undercooling.•Interlaced microstructure becomes more frequent via a deeper undercooling by 10 K.•The fine eutectic Ag3Sn enhances the nano-hardness and creep properties of solder.•Nanomechanical data variation was reduced with refined homogeneous microstructure.