Application Characteristics of Ultra-Fine 15 μm Stainless Steel Wires: Microstructures, Electrical Fatigue, and Ball Formation Mechanisms.

Stainless steel wires exhibit excellent mechanical properties and are widely used in engineering applications. This study fabricates 15 μm stainless steel wires for potential integration into wire bonding technology for electronic packaging. The research explores the microstructural characteristics, electrical conduction mechanisms, and ball formation behavior of ultra-fine stainless-steel wires to assess their feasibility for wire bonding applications. Results indicate that both 15 μm and 30 μm stainless steel wires exhibit elongated grains with outstanding tensile strength and hardness. Compared to the 30 μm wires, the 15 μm wires undergo more pronounced work hardening, leading to higher tensile strength and resistance. This study investigates the differences between vacuum and electrified annealing processes to address the work hardening and ductility issues in stainless steel wires. Results confirm that the hardness of the original wire significantly decreases after vacuum annealing at 780 °C for 15 min. Furthermore, using the derived equation, T=IV2.3085×10-3+25, the annealing temperature of 780 °C is converted into an equivalent current, and electrify annealing is conducted under a condition of 0.08 A for 15 min. The annealed wires exhibit a softening effect and enhance ductility. Furthermore, due to stored deformation energy and recrystallization effects, the electrical fatigue life of 15 μm stainless steel wires is approximately 300 cycles. After electrifying annealing, the base microstructure becomes more homogeneous due to thermal effects, reducing fatigue life to around 150 cycles. However, due to the softening effect, the annealed wires make the EFO process easier and minimize solidification segregation in the free air ball (FAB) microstructure, demonstrating their potential for electronic packaging applications.