In this report, Focused Ion Beam (FIB) -- SEM technique was applied to observe the tin plated fretting contacts. Spatial distributions of tin, tin oxide and so on have been confirmed quantitatively in two plating thickness of 1 and 5 µm.

Tin and its alloys have been applied for the plating of electrical contacts for low electrical power conditions. In particular, tin-plated contacts are widely used as connector contacts in automotive applications and as make-break contacts in keyboard switches. In the relationship between contact resistance (R) and contact load (W) for both solid and plated tin, singularities have been found. Previously established and well known theories on the deformation of contact interfaces cannot explain these singularities. In this study, to clarify these singularities, and to obtain a contact model explaining this phenomenon, contact traces for contact load were examined by SEM and STM. The obtained microscopic images indicated piling-up at the periphery of the contact area for both solid and plated tin. In this case the contact configuration comprised a platinum probe with a hemispherical tip surface and a flat tin surface for both solid and plated. When the probe was loaded, this tip of the probe sank into the soft tin surface owing to its lower hardness. In case of solid tin, the sinking of the probe surface into the tin surface causes piling-up around the periphery of the contact trace. In this deformation process, since the periphery of the indentation of the indented contact area severely slid against the surface of the platinum probe while applying a contact load, the contact resistance rapidly decreased with load. In this case, the center portion of the true contact area was not affected mechanically; thus, the surface film on the bottom portion of the deformed of the flat surface did not break down mechanically. On the other hand, in the case of a tin plated surface, similar piling up occurred; however, it was accompanied by scattering and separation of tin crystal grains from the surface. As a result of this process, a decrease in contact resistance similar to that for the solid tin occurred. Since the piling-up of the contact surface is a very important process in the application of connectors, the above-mentioned unusual characteristics were clarified in this study.

A press-fit connection is a solderless electrical connection technology, which utilizes the mechanical contact force generated between through-holes on a printed circuit board (PCB) and terminals with a width slightly larger than the through-hole diameter. This technology has been widely noted recently as a measure against the "Lead Free Requirement" of materials comprising electric/electronic devices, especially in the area of automobile connector. For the application of this technology to automobile connectors, we have to take into account the severe requirement, such as (1) the adaptation to wider through-hole diameter tolerance range and (2) the establishment of connection reliability for the various PCB surface treatments. As a result, we have determined the minimum and maximum contact forces satisfying the long term connection reliability and designed the terminal shape, which has been refined the N-shape cross section developed before, by using three dimensional finite element methods (FEM). Furthermore, we have developed a new type of hard tin plating on terminals, thus preventing the scraping-off of tin during the insertion process, that could result in a short-circuit on the PCB, for the Organic Solderability Preservative (OSP) treated PCB. The press-fit connector for the automobile airbag Electronic Control Units (ECUs) we developed has been able to transfer to the mass-production phase successfully from August 2005.