Novel silver-enhanced hard gold electrodeposit applied for electrical contacts: comparison with conventional gold-cobalt alloy

An Au-Ag coating has been envisaged as an alternative to the conventional Au-Co electrodeposit. In this study, an Au-Ag coating was electrodeposited on Cu substrates pre-coated with a Ni layer. The contact resistance of the Au-Ag coating was observed to be lower than that of the Au-Co coating. Under extended aging, the Au-Ag coating demonstrated a slightly increased contact resistance, while the contact resistance of the Au-Co coating increased significantly. Additionally, the solderability of the Au-Ag coating surpassed that of the Au-Co coating. Qualitative analyses demonstrate that, in the case of Au-Co coating, a considerable amount of Ni diffused through the Au layer to react with O-2 in the air to form oxides during aging. The Co elements also oxidised due to aging. The formation of the oxide layer induced an increased contact resistance and diminished solderability. In addition, the Au-Co coating consisted of finer grains than the Au-Ag coating, promoting the Ni diffusion through the Au layer.