By Robert S. Mroczkowski, Sc.D.
In my contribution to the May 2000 issue of Connector Specifier, the topic was the contact interface — the point(s) where the two mating surfaces come together. The surface structure and how it affects contact resistance was the focus. Implicit in that discussion was the fact that the surfaces in contact must be metallic (i.e., free from surface films). Surface films, if present, must be disrupted during mating to create the required metallic interface. Clearly, the surface material is critical to achieving and maintaining this "film-free" condition. In most connectors, the spring members, usually copper alloys selected for their mechanical characteristics, are coated. The coating is often referred to as the surface "finish." Contact between the finishes on the plug and receptacle contacts is the beginning of acceptable connector performance.
There are two different approaches to a "film-free" interface. One is to use materials that are immune to film formation (i.e., materials that do not corrode). The second is to use materials on which the surface films that form are readily displaced. The classic contact finish examples of these two cases are gold and tin. Gold does not form surface films, and the surface oxides that are the dominant films on tin are easily displaced during mating of the connector.
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Figure 1. Schematic illustration of the kinetics of fretting corrosion. The top diagram shows the displaced tin oxide and extruded tin that provides the original low-resistance contact interface. Fretting motions, indicated in the other diagrams, result in the build-up of an oxide debris layer at the contact interface. As the oxide debris accumulates, the resistance increases both in magnitude and variability.
This is not to say that either gold or tin contact finishes are immune to degradation by corrosion. As would be expected, the degradation kinetics are different in the two cases. Gold finishes degrade by corrosion of other components within the finish system, primarily exposed copper alloy of the contact spring. Tin finishes degrade by reoxidation of the tin during the life of the product.
Tin Finishes
The utility of tin as a contact finish derives from the disparate hardness of the surface tin oxide and the underlying tin metal. Tin oxides are very hard and brittle. They are also very thin — on the order of a few hundred nanometers. Tin itself is very soft. The composite finish then is a thin, hard and brittle surface over a soft and ductile layer. When a load is applied to such a composite, the surface oxide cracks and the load transfers to the soft and ductile tin, which flows readily leading to widening of the cracks in the oxide. Tin extrudes through the cracks leading to the desired metallic contact interface. This process, of course, is greatly enhanced by the sliding/wiping action that occurs on connector mating. Therefore, it is easy to create the initial contact interface in tin-finished connectors.
The problem, however, is that the tin is thermodynamically driven to reoxidize in the event the wiped tin becomes exposed to the environment. Such exposure occurs if the contact interface moves. Such movement is referred to as fretting and the reoxidation of the tin that accompanies fretting is called fretting corrosion (see Figure 1). Small-scale motions, a few tens to a few hundreds of microns, are sufficient to drive the fretting corrosion mechanism. Fretting corrosion is, in fact, the major degradation mechanism and limiting factor in the use of tin as a contact finish.
There are two ways to avoid fretting corrosion. First is to prevent the fretting or the motion. This is why tin-finished connector systems use high values of contact force. High contact force produces high friction forces at the contact interface and improves the mechanical stability of the interface. A second approach is to prevent the oxidation by use of a contact lubricant, which provides a "sealant" around the contact interface to inhibit access of the environment to the interface. Both methods are successfully used in connectors.
Gold Finishes
As mentioned, gold does not corrode and is often referred to as a "noble" metal. Other materials used as contact finishes are corrosion-resistant, such as palladium and alloys of gold and palladium. None of these materials, however, have the nobility of gold. Palladium alloys, palladium(80)/nickel(20), palladium(80)/cobalt(20) and palladium(60)/silver(40), generally with a gold "flash," are the most commonly used noble alternatives to gold. Note the word "alternatives" rather than "substitutes." These alloys have somewhat different characteristics than gold and these differences should be considered.
Noble metal finish systems consist of the surface finish, usually gold or a gold-flashed palladium alloy over a nickel underplate over the copper alloy base metal. Thickness of the gold ranges from flash to 1.25 mm, with 0.40 and 0.80 mm being the most common specifications. The palladium alloy finishes are typically gold flash over 0.40 mm of alloy. Nickel underplate thicknesses are in the range of 1.25 to 2.50 mm. Given these thicknesses, it is easy to understand why the contact finish "system" includes the base metal. Mechanically, the contact forces used in connectors induce a stress that is felt through the finish to the base metal. Selection of an appropriate noble metal contact finish thickness depends on the corrosivity of the application environment and the number of mating cycles the connector is intended to support without degradation.
A brief digression on flash: Specifications on flash thickness vary from gold color, which can be realized at less than 0.10 to 0.25 mm, with several variants between. Flash thickness is important for two reasons — coverage and durability. Thickness correlates to surface coverage for thin coatings. A gold color does not imply complete surface coverage. Thickness also correlates to durability — the number of mating cycles that can be realized without wearthrough of the flash. In both cases, exposure of the underlying palladium alloy, or the nickel underplate in some cases, can lead to direct corrosion degradation at the contact interface.
As mentioned, corrosion degradation in noble finishes results from exposure of base metal, either the spring copper alloy or the palladium alloy. Exposed copper alloy can arise from selective plating practices where the finish is applied in a controlled manner only at the area of contact. It can also arise from plating defects or damage to the finish.
Once exposed, copper is susceptible to oxidation or sulfidation, among other processes. These corrosion products may be at the contact interface, if they arise from plating defects, or around the interface where they only come into play if the contact interface moves into such areas. Exposed palladium alloy falls primarily into the first category, corrosion products in the contact interface. Exposed palladium alloy is susceptible to fretting corrosion in the same fashion as tin finishes. The number of cycles to failure, however, is two to three orders of magnitude larger than for tin systems. Despite this, fretting should be taken into account when gold-flashed systems are being considered. This is particularly true for gold-flashed nickel systems where the fretting behavior is much more similar to tin in the number of cycles to failure.
Conclusion
Noble metal- and tin-finished contact systems provide reliable performance in a wide range of connector applications. Individual application requirements should be carefully considered before selecting the best finish for a given application. Noble metal finishes are more forgiving than tin over the range of connector applications.
ROBERT S. MROCZKOWSKI, Sc.D., is the founder, connNtext associates, 38 Cider Press Road, Mannheim, PA 17445; (717) 664-2246; Fax: (717) 664-1666; E-mail: connNtext@earthlink.net.
