Sputtered ultra-thin Au coating on Cu wire: enhancing ball formation and reliability for advanced wire bonding

Au-coated Cu wire mitigates Cu oxidation while preserving excellent electrical and thermal conductivity, thereby enhancing second-bond reliability. However, conventional Au coatings are often too thick, disrupting free air ball (FAB) formation through molten Au encapsulation during the electronic flame-off (EFO) process. This study employs sputtering to maintain the Au coating at ~ 20 nm, improving FAB morphology. Thermal stability, bonding strength, and interfacial properties were evaluated via heat treatment and high-temperature aging. Results show that Au-coated Cu wire retained coating integrity after 300 °C treatment yet degraded beyond 400 °C due to Au diffusion along grain boundaries, causing interfacial defects. Aging tests (175 °C, 24–72 h) confirmed severe oxidation in Cu wire, whereas Au-coated Cu wire remained stable under its protective Au layer. FAB morphology was similar to Cu wire, with no Au segregation. Bonding tests revealed strong first bond adhesion in both wires, but aged Cu wire exhibited failures that Au-coated Cu wire avoided. Compared to Cu and Pd-coated Cu wires, Au-coated Cu wire also showed the lowest electrical resistance. Overall, sputter-deposited Au-coated Cu wire effectively improves oxidation resistance, FAB formation, and bonding reliability, offering strong potential for semiconductor packaging.