High contact resistance readings on clean microwave mobile contacts

In a real-life environment, metallic surfaces are always covered with an insulating film of some type. This can be partly minimized by the use of noble metals for contacts, but even in case of gold, the best material in terms of reactivity with the environment, such films occur. The contact resistance consists therefore in constriction resistance and film (or tunneling) resistance. Gold is known to have little affinity in creating frictional polymers (5% when compared with platinum) but it is known to have some affinity regarding absorption of carbon atoms forming the so-called carbon-oxygen film. Analysis of contacts that exhibited high contact resistance revealed presence of carbon and oxygen imbedded in the reed and probe contact surfaces. These carbon/oxygen areas were not visually identifiable using standard optical techniques, including UV light, and standard inspection magnification of 20 to 40 x. X-ray photo-electron spectroscopy (XPS) analysis revealed that there were no other elements present in the contact and non contact areas of the corresponding paths except Au, C and O. At the same time, measured bonding energies for carbon and oxygen, confirmed the most probable composition of the carbonaceous monolayer. Non-contact areas taken as a background in the evaluation had slightly different composition, however with the same type of carbon structures i.e. bonding energies. XPS measurements conducted on various contacts showed that the thickness of such a film is usually less than 10 nm. The work presents an analysis that shows, based on RF data interpretation, that the mobile contact member and the fixed contact member were in intimate contact and the equivalent dielectric constant of the insulating layer is higher than one. In addition, using typical values for the tunnel resistivity presented in reference books by R. Holm and K. E. Pitney it is possible to obtain high contact resistance readings comparable with the experimental data. Paper also examines test methodology and limitations in terms of ability to detect tunnel conduction through certain thicknesses of surface films.