Dr. Bob on Connector Wear Mechanisms
Connector wear mechanisms are important to connector performance because the wear process removes the contact finish that provides corrosion protection for the contact spring materials. The most commonly used contact spring materials are copper alloys, all of which are susceptible to corrosion in typical connector operating environments.
There are two primary causes for wear in connectors. The most obvious one is the wear that occurs every time the connector is mated, when the plug and receptacle contact surfaces slide against one another. The second is wear that occurs due to fretting motions during the application life of the connector. As a reminder, fretting motions are small-scale motions, a few to a few tens of microns, that result from mechanical disturbances or thermal expansion mismatch forces.
First, a few general comments on wear: Wear mechanisms can be very complex, so this discussion will be limited to some simplified observations on wear processes in connectors. Wear can be described by a simple equation, though the meaning of the parameters in the equation can be rather complex.
V = k F L/H
Where V is the wear volume (the volume of metal removed from the interface during a single wear event), k is the wear coefficient, F is the applied load (in the case of connectors, the contact normal force), L is the length of motion of the wear event, and H is the hardness of the metals in contact (the contact finish).
Interpretation of F, L, and H is relatively straightforward. The contact force, F, is a connector design parameter. The length of the wear event is the engagement length during mating, or the length of a fretting motion. Engagement length is a connector design parameter, but the length of fretting motions is a variable dependent on many parameters. In the equation, H is the hardness of the metals in contact. For like materials, it would be the hardness of the given material. For two different metals it would be a composite hardness. In connectors, the two contact springs generally have the same surface finish, such as gold, but the thickness of the nickel underplate and the hardness of the contact spring materials may be different so a composite hardness is appropriate. For a given connector, however, F, L, and H can be considered, sort of, as “knowns.”
The wear volume, V, requires some interpretation. The parameter of interest in connector wear is the loss in finish thickness during each wear event. Given V = At, where A is the area of the contact interface and t the thickness of the material removed. A depends on the contact geometry. This dependence is obvious when you think of the wear due to a needle’s point in contact with a surface in comparison to a one-centimeter diameter ball bearing. Again, for a given connector system this relationship can be considered, sort of, a “known.”
But k is a variable with many parameters. Among the most important parameters are the contact force, F; the hardness, H; the contact geometry; the surface roughness; and the state of lubrication of the surfaces in contact. Once again, F, H, contact geometry, and surface roughness are, sort of, “knowns” for a given connector. For connectors that are not provided in a lubricated condition, the state of lubrication is a variable dependent on the environment in which the connector is used. I suggest that, practically speaking, the contact force is the most significant parameter in k for connectors for the following reasons.
Consider Figure 1, schematically illustrating two surfaces in contact. It is important to note that on the microscale of the contact interface, all surfaces are rough. For simplicity, two contact points, or asperities, are shown. In the following discussion, it is assumed that the first contact creates interface (a in Figure 1) and as the load continues to increase, bringing the surfaces closer together, interface (b) is created.
Under these conditions, interface (a) experiences more deformation than interface (b). Given that asperity contacts are very small, the deformation will be plastic and radial flow of the asperity will occur as the top of the asperities flatten against one another. This radial flow disrupts surface films and contaminants and helps to create the desired metallic contact interface. The metallic interface created will experience some degree of cold welding. Let me simply state that “cold welding” means that the metal surfaces in contact bond to one another across the asperity interface in the same manner that metallic bonds are formed in the interior of the metal. The metal of the asperity is also work-hardened due to the deformation of the asperities. The same processes occur as interface (b) is created, but to a lesser extent. This means that interface (a) will be stronger than interface (b) because the larger amount of deformation has created a larger contact area for cold welding, and it has also experienced a greater degree of work hardening.
Given these interface characteristics, consider what happens when a shear stress is applied to the system. Because (a) is the stronger interface, the applied stress must be sufficient to break the interface at (a) and the weaker interface at (b) will follow. From a wear viewpoint, where the interfaces break is important. Consider the state of interface (a). It is cold-welded and work-hardened. In fact, due to the work-hardening interface, (a) may have a higher cohesive strength than the base metal itself, and separation of the asperities may take place within the base metal, as shown in Figure 1, rather than directly at the interface. The resulting wear particle is the wear volume, V, cited in the equation. The weaker interface at (b) may break at or near the original interface with little wear occurring. The wear process at (a) is commonly referred to as adhesive wear and that at (b) as burnishing wear. If wear tracks produced during connector mating are viewed under magnification, at 30 to 50 magnifications, adhesive wear tracks will show evidence of wear particles and appear somewhat rough while burnishing wear tracks will appear smooth and shiny. For completeness, there is an additional wear mechanism that can come into play if the wear particles produced during adhesive wear are sufficiently hardened by deformation to act as an abrasive at the contact interface, which is referred to as three-body abrasive wear.
Now back to k. Clearly, a change in wear mechanism from burnishing to adhesive wear will be reflected in a significant increase in the value of the wear coefficient k. During burnishing wear, k may increase as the contact force increases. When the contact force has increased to the point where adhesive wear becomes active, however, k will increase discontinuously and, potentially, significantly. The magnitude of contact force resulting in the discontinuous change in k will be dependent primarily on the state of lubrication of the connector. For clean surfaces, the transition range will be of the order of a few to tens of grams while for well-lubricated surfaces the transition may not occur until forces of hundreds of grams.
Consider these comments in the context of connector performance. In applications where low contact forces are acceptable/necessary, such as in low-current, low-mating-force/high pin-count, and high-durability applications, the expected wear mechanism will be burnishing wear and the rates will be low. Power applications will, in general, require higher contact forces to meet more demanding contact interface resistance requirements in both resistance magnitude and stability. In such cases, adhesive and abrasive wear mechanisms may be active. This is one reason that many power contact designs take advantage of multiple contact beams. These systems reduce the connector resistance because the multiple beams are electrically in parallel and they can also be designed with lower contact forces to reduce the potential for adhesive and abrasive wear.
Wear issues are, of course, best addressed in the design and manufacture of the connector system. Because wear rates vary inversely with hardness, tin-finished connectors will show higher wear rates than gold connectors. This relationship also indicates why so-called hard golds are typically used. Another design parameter that impacts wear in gold-finished connectors is the nickel underplate, as discussed in previous articles in Connector Supplier. The thickness of the finish influences the number of wear cycles a connector can support without wear-through. This factor should be accounted for if thin or flash gold finishes are being considered. If a connector system is found to be inadequate with respect to wear performance, a contact lubricant may be able to enhance performance sufficiently to meet application requirements.
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