Temperature-Time Effects on Film Growth and Contact Resistance of a Plated Copper-Tin-Zinc Alloy used as a Surface Finish on Electronic Components

An electrodeposited copper-tin-zinc alloy having a nominal composition in the range by weight of 55-65 percent Cu, 20-30 percent Sn, and 10-20 percent Zn is commonly used as a surface finish on electronic components, particularly fuse end caps. The contact resistance of brass disks plated with this alloy (1µm thick) is 1-10 due to a film of the oxides of Sn, Zn, and some Cu. Heat treatment at 100°C causes the oxide thickness to increase, with the contact resistance simultaneously increasing to > 100 . At 75°C and below, oxide growth is not detectable. The growth rate of the oxide film at 100°C is parabolic with a rate constant of 1.9 x 10^-17cm²/s. The oxide film is very brittle and readily fractures due to tangential wipe, when contact resistance would fall to the millohm range from > 100 . This alloy as a surface finish on electronic components has the potential of causing overheating, transients in logic circuits, and adding a significant resistance to the circuit involved, particularly in the absence of any wiping action.