Friction, wear and electric properties of tin-coated tin bronze for separable electric connectors

The effect of repeated insertions and withdrawals on separable electric tin connectors are studied in a model contact tool. The friction, wear and electric properties of hot-dipped and electroplated tin, coated on a tin bronze substrate (CuSn4) was analysed for different coating thicknesses and normal loads. Three characteristic stages could be identified during wear: plowing of the rider into the soft tin layer, sliding of the rider on the hard intermetallic compound, and, finally, penetration of the intermetallic compound, with subsequent plowing of the rider into the substrate material. The differences between the stages regarding friction, wear and electric properties were significant. In this paper, friction, wear and electric properties when the rider is plowing into the soft tin layer are analysed and discussed. A simple semi-empiric model is proposed, valid as long as the pure tin coating is not completely removed. From this model, the contact resistance can be derived as a function of the insertion force. The model illustrates an inherent problem in using tin-coatings for connectors: a low insertion force achieved by applying a low normal force will result in a high contact resistance characterized by extreme sensitivity to variations of normal force.