Study of the World’s finest 15 μm stainless-steel wire: Microstructure, electro-thermal annealing mechanisms, and wire-bond joint electrical performance

This study reports, for the first time, the successful fabrication and implementation of the world’s finest 15 μm 304L stainless-steel bonding wire for advanced electronic packaging applications. Unlike conventional Au, Cu, or Ag bonding wires, the proposed Fe-based wire offers a low-cost alternative while avoiding intermetallic compound (IMC) formation at bonding interfaces. The ultrafine wire, produced through severe cold drawing (∼98% reduction), exhibits an ultrahigh tensile strength approaching 3000 MPa due to strain-induced martensitic transformation. A controlled electro-thermal annealing process (0.08 A, 5 min) effectively induces recrystallization, grain growth, and martensite-to-austenite reversion, reducing excessive work hardening while stabilizing elongation at 8–10% and maintaining tensile strength near 1000 MPa. Electrically assisted tensile tests further reveal current-dependent softening mechanisms governed by Joule heating, dynamic strain effects, and surface oxide formation. Notably, even at 80% of the fusing current, the wire retains substantial tensile strength (∼800 MPa) together with enhanced ductility. Wire-bonding evaluations demonstrate reliable first and second bonds on both Cu and Al substrates, with stable resistance and fusing current behavior independent of current direction. The absence of IMC formation, combined with high mechanical robustness and electro-thermal stability, establishes a new Fe-based wire-bonding strategy and highlights the strong potential of stainless-steel ultrafine wires as next-generation interconnection materials.