Identification of the role of zinc in Sn–Cu solder and interfacial intermetallic growth through experimental results and phase-field simulations

• Zn effect on Sn–Cu solder studied via experiments and phase-field modelling. • 0.2 wt% Zn alters microstructure but not intermetallic growth at 150 °C. • ≥0.8 wt% Zn suppresses intermetallic growth, voids, and crack formation. • Cu 5 Zn 8 layer forms at interface, affecting intermetallic nucleation and growth. Intermetallic compound (IMC) formation significantly impacts the reliability of lead-free solder joints, with trace elements like Ni and Zn offering potential to control IMC growth. However, the mechanisms by which trace Zn influences microstructural evolution and interfacial reactions remain underexplored. This study examines microstructural evolution and interfacial behaviour in five Sn–Cu–Zn solder alloys (0–1 wt% Zn). Zn destabilises the eutectic interface, producing cellular morphologies with mixed eutectic at cell centres and CuZn IMCs in interdendritic regions. Even trace Zn notably affects solidification by reducing undercooling and altering β-Sn growth. While trace Zn has limited effect on suppressing η-Cu 6 Sn 5 , ε-Cu 3 Sn, or Kirkendall voids on Cu, it reduces IMC thickness, indicating growth inhibition. Adding ≥ 0.8 wt% Zn reduces IMC thickness at 150 °C by 68.5 % after 1000 h, from ∼ 16.5 µm (Sn–Cu) to ∼ 5–5.5 µm, also lowering void growth. The presence of a thin γ-Cu 5 Zn 8 layer (<100 nm) at the interface substantially alters the nucleation, growth, and morphology of the η-phase. Phase-field simulation helped explain the experimental observations indicating thin γ-Cu 5 Zn 8 formation at the solder-substrate interface that enhanced nucleation of η-Cu 6 Sn 5 but reduced the growth kinetics of η and ε phases by creating a diffusion barrier for Cu atoms.