Contact Resistance Characteristics of Elastomeric Complex Structure Contacts for Gold and Tin Printed Circuit Board
Characteristics of conductive elastomer which is composed of silicone rubber and dispersed carbon black particles shows conductive and elastic properties in one simple material. This material has been widely applied to contacts of switches and connectors of a liquid crystal panels. However, since surface state of the contact is very soft, it is difficult to remove contaminant films of contaminated opposite side contact surface and to obtain low contact resistance owing to break the film. This study has been conducted to clear structures of the elastomer which indicates removal characteristics for contaminant films and low contact resistance. As specimens, six different types of the elastomer contacts composed by different type of dispersed such materials as carbon and metal fibers, metal mesh, and plated surfaces were used. The opposite contacts were Au and Sn plated contact surface on a printed circuit board (PCB). The elastomeric contacts contained hard materials showed lower contact resistance than only dispersed carbon particles in the elastomer matrix for both contaminated PCB contact surfaces.
Field Studies of Contact Materials: Contact Resistance Behavior of Some Base and Noble Metals
The contact resistance of various metals was determined after exposure of from three months to four years in telephone central offices and in nonair-conditioned locations. The logarithm of median contact resistance was stable for some materials, but with most it increased on aging according to linear, parabolic, or logarithmic time relationships. The kinetics of contact resistance change depended on the intrinsic chemical reactivity of the material, the air pollutants which were present in the exposure site, and especially on the relative humidity. Contact resistance-force behaviors were affected also by the topographic and hardness characteristics of the metals. The overall ranking in central offices on the basis of stability of contact resistance from best to worst was 1) gold; 2)high gold-silver alloys, palladium, high palladium-silver alloys, 60Sn40Pb, and tin; 3) copper, high copper alloys, and nickel; and 4) silver. Where relative humidity was only slightly less well-contro11ed, as in a nonair-conditioned room in an urban central office building, except for gold the contact resistances of all materials were from one and two orders of magnitude greater after 40 months of exposure, and often displayed different kinetic behaviors. Palladium was exposed in a total of 18 sites and its contact resistance was found to be highly variable, even exceeding 1000 \Omega in less than 1 year when relative humidity was high and reactive chlorine-containing air pollutants were present. Pore corrosion of gold electrodeposits and creep of tarnish films from adjacent base metals did not occur in central offices except when the substrate was silver and the level of sulfiding pollutants was high.
Mechanism of Tarnishing of Precious-Metal Contact Alloys
Studies of the tarnishing of the Au-Ag, Au-Cu, Pd--Ag, and Pd-Cu systems revealed that film formation was controlled by diffusion either in the surface film or in the near-surface region of the alloy. Tarnishing of the silver-containing alloys is controlled by alloy diffusion, which can be described as a two-stage process. Initially, all alloys tarnish at nearly a linear rate characteristic of the pure reactive element. Beyond a transition thickness, the alloys tarnish according to a cubic-rate law. The rate-controUing mechanism associated with the simple cubic rate was identified as "short-circuit" diffusion via dislocations and/or'"grain boundaries. The two tarnish Charactefistics of an alloy such as Au-Ag were shown to be greatly affected by the surface metallurgical condition.