The alternate to gold plating in this area is tin (or tin alloys)–in terms of frequency and quantity of present-day usage, as well as in terms of cost and efficacy. Tin is not a noble metal. It and its alloys do form oxide films in normal air exposure. In moderately severe environments the tin surface can become heavily coated with oxides or other non-conductive corrosion products. Its usefulness as a contact material lies in the fact that tin is a very soft material and its oxides are comparatively hard and brittle. It is consequently relatively easy to break through the oxide film mechanically so as to establish metal-to-metal contact with the underlying tin base. But certain minimum values of mechanical force, motion, and geometry are required to accomplish this breakthrough of films, and some of the selection criteria are related to the availability of those minimum mechanical requirements.
I. Tin Plated Contacts Need at Least 100 grams Normal Contact Force
Higher forces are desirable whenever it is possible to provide them. Limitations on the high force side are usually determined by:
a.Total effort required to engage/disengage multiple-circuit connectors, due to high friction.
b.Wear on the plating, from large numbers of durability cycles. Durability cycling requirements of 50 cycles or more is considered large.
c.Physical size and strength of contact spring members.
d.Spring deflection requirements. Dimensional tolerances sometimes require resilient springs with large deflections so as to accommodate max and min. tolerance conditions between mating contacts. This may be incompatible with high contact force in the worst-case condition.
II. Contacts Must be Mechanically Stable in the Mated Condition
Motion of the contact interface during its service life is the single most important cause of failure of tin plated contacts. Relative motions of about 0.1 mil or greater are sufficient to break the gas tight seal at a tin-tin interface and allow formation of oxide film. Small cyclic motions of this kind, as might be caused by vibration or mechanical disturbance, or by thermal expansion/contraction, are called "fretting motions". Fretting motion results in fretting corrosion, which is simply an accelerated oxide film growth at the contact interface caused by the constant exposure and re-exposure of clean metal due to the relative motion.
Fretting Motions
Three types of fretting motion are illustrated. Rocking and Rotation are worse than Translation in some respects, since there is no self-cleaning action associated with these motions.
The severity and seriousness of fretting motion may be demonstrated on an apparatus which produces a ± 5° rotational motion. Clean tin plated contacts having an initial resistance of about 1 milliohm will develop over 1 ohm resistance in less than 30 minutes on this apparatus, which frets at 10 cycles per minute, (lubricated tin plated contacts and gold plated contacts show no change in the same test).
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