Contact Resistance Stability and Mechanism of Multilayered Sn/Ag Electroplating on Cu Alloys under High Temperature Circumstance

The contact resistances of multilayered Sn/Ag electroplating with a 50 nm-thick Ag-Sn alloy film as Ag3Sn nano-flakes in 50-200 nm across on copper alloys were investigated after aging at ambient temperatures of 150, 175, and 200 °C for 120 h to 3000 h in air. The microstructures and surface characteristics of the Sn/Ag coatings after aging at different temperatures and periods were elaborated using SEM, FE-SEM, XRD, TEM, GDOES, and AES. The Sn/Ag coatings exhibited low and stable contact resistances equivalent to that of as-plated sample even after aging at 200 °C for 3000 h, compared to the ever-increasing contact resistances for conventional reflowed Sn coatings without Ag plating, especially at the low-load side. The excellent stability of the contact resistance of the multilayered Sn/Ag coatings can be ascribed mainly to Ag3Sn microparticles that are agglutinated on the coating surfaces, and to their good oxidizing resistance, irrespective of the oxidation of Cu-Sn alloys during aging. Moreover, the oxide films on the Sn/Ag coatings after aging were much thinner than those on the reflowed Sn coatings. In particular, a tiny amount of Ag was included the Cu-Sn intermetallic compounds and agglutinated at the Cu3Sn/Cu interface with aging. This phenomenon can be attributed to the inward Ag diffusion countered against the outward Cu diffusion from the base materials, which enhanced the anti-oxidation and adhesion characteristics of the coatings, thus leading to a high reliability under high-temperature circumstance for electrical devices in automotive applications.