Effects of size and cooling rate on solidification behavior of freestanding Sn-3.0Ag-0.5Cu solder balls

Abstract The diversity of electronic product types needs variable solder joint sizes and processing conditions, which can lead to the complexity and diversity of solidified microstructure in Sn-based solder joints. Due to the significant anisotropy of β-Sn in physical and mechanical properties, the solidified microstructure of the Sn-based solders after soldering should be carefully controlled. In this paper, Sn-3.0Ag-0.5Cu freestanding solder balls were used to understand the effects of size and cooling rate on the solidification behavior and microstructure. Larger undercooling, ∆T, was detected for smaller solder balls or under larger cooling rate. With the solder ball size increased from small level of 100 and 200 μm to medium level of 400 μm and then to large level of 700 μm and 1200 μm, the dominant solidified microstructure transformed from interlaced grain & beach ball to randomness and then to single grain. Correspondingly, three nucleation & growth models, i.e., single grain (∆T ≤ 20 K), cyclic twin (20 K ≤ ∆T ≤ 25 K) and interlaced grain & beach ball (25 K ≤ ∆T), were proposed to explain the solidification behavior of β-Sn. A mapping of the most likely formed β-Sn microstructure regarding solder ball size and cooling rate was established to not only predict the major microstructure of solder balls but also optimize the process parameters for solder ball fabrication.