Significance of phase formation in multicomponent lead-free solder alloys

This research focuses on the significance of alloying elements and phase formation in the microstructure, mechanical properties, wettability and interfacial behaviour of multicomponent lead-free solder alloys to address important parameters for achieving high-performance solder joints in electronic packages. For this purpose, five lead-free solder alloys including Sn-0.7Cu, Sn-1.0Ag-0.5Cu, Sn-3.0Ag-0.5Cu, Sn-3.0Ag-0.5Cu-0.8Bi and Sn-4.0Ag-0.5Cu-4.1In (all in wt%) were fabricated and studied in this investigation. Experiments and thermodynamic calculations confirm the formation of different phases including Cu₆Sn₅ in binary SnCu, Cu₆Sn₅ and Ag₃Sn in ternary Sn-Ag-Cu and, Cu₆(Sn,In)₅ and Ag₃(Sn,In) in quaternary Sn-Ag-Cu-In solder alloys. Nevertheless, the β-Sn matrix can dissolve a small amount of Bi, which effectively produces finer eutectic phases of β-Sn and Cu₆Sn₅ during solidification in the quaternary Sn-Ag-Cu-Bi solder alloy. In fact, there is a favorable bonding tendency between SnAg, SnCu and SnIn, which promotes the formation of intermetallic compounds. In addition, the significant difference between the atomic radius of Sn elements with Cu and In (>10 %) promotes the formation of secondary phases. The slightly positive mixing enthalpy of Sn and Bi, together with their similarity in atomic size, allows for slight solubility of Bi in Sn. The addition of 0.8 wt% Bi and 4.1 wt% In reduced the melting points and solidification range of the lead-free solder alloy and led to improved wettability. The quaternary solder alloys exhibit superior thermal and mechanical properties compared to conventional alloys, making them potential replacements for lead-containing solder alloys.