The deterioration of tin-coated connectors caused by vibration was studied by using a new approach. In order to simulate the true contact situation during vibration a controlled oscillating tangential force was applied on a model tin-contact, while the displacement was free to vary. This study shows that the friction and the amplitude of the tangential force acting on the contact are critical parameters for a contact subjected to vibration, since they have a significant impact on the relative slip at the contact interface. Almost any slip (>1 µm) resulted in an increased contact resistance in the performed experiments. Initial slip caused an increase of the contact resistance, and also an increase of the contact area with a subsequent increase in friction force. The slip came to stop, as the friction force became equal to the applied tangential force.Provided the slip was stopped before the contact failed as described, a stable contact resistance was achieved despite an applied oscillating tangential force. In all other cases the slip continued and the contact inevitably failed.With a force controlled fretting bench test the critical tangential force for a specific contact can easily be determined, and the results can be used to predict the risk for fretting corrosion.
The effect of repeated irisertions and withdrawals on separable electric tin connectors are studied in iz model contact tool. The friction, wear and electric properties of hot-dipped aiid 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 soR 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 ithe 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.
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