Multilayered Sn/Ag3Sn electroplating on Cu alloys for high reliable electronic/electric materials

We report a multilayered Sn/Ag film electroplated on Cu alloys with high reliability as promising coating materials for LED lead-frames and for electric connectors. The multilayered Sn/Ag films consisted of an Ag-Sn alloy film with 30-80 nm-thick as Ag₃Sn phase covered on a 1 μm-thick Sn film on Cu alloy substrates. The as-deposited Sn/Ag₃Sn films exhibited a high specular reflectivity around 70% at the wavelengths of 480 nm, which was lower than a commercial bright Ag film with total reflectivity of 90% but much higher than a commercial bright Au film in 24% at the same wavelength. It was found through an accelerated sulfuring test that, the reflectivity of the Sn/Ag₃Sn films was almost unchanged even after immersing in a 0.2% (NH₄)₂S_x solution for 60 min, compared to the abrupt decrease to 15% for the conventional Ag film even after immersing for 10 min. The excellent sulfuring resistance and durability of the multilayered Sn/Ag₃Sn films can be attributed to the existence of a stable SnO₂ film existed on the surface of Ag₃Sn alloy, which was revealed by a UPS analysis in a vacuum chamber by heating at 473 - 673 K, thus protecting underneath Ag₃Sn alloy film and inhibiting the chemical reaction of Ag and S^2- ions.