Because the mechanisms causing connector wear can be very complex, this paper focuses on some simple observations of the connector wear process. Wear can be described by a simple equation, although the meaning of the parameters in the equation can be quite complex.
Connector wear has a significant impact on connector performance because the wear process removes the surface layer that provides corrosion protection for the contact materials. The most commonly used contact materials are copper alloys, all of which are susceptible to corrosion in typical connector operating environments.
Connector wear has two main causes. The most obvious is wear that occurs with each engagement as the plug and socket contact surfaces slide against each other. The second is wear caused by fretting during connector use. Fretting is small-scale movement, ranging from a few micrometers to tens of micrometers, caused by mechanical disturbances or thermal expansion mismatch forces.
First, the wear mechanism can be very complex, so this paper will limit its discussion to some simple observations of the connector wear process. Wear can be described by a simple equation, although the meaning of the parameters in the equation can be quite complex. The equation is as follows:
V=kFL/H
Where V is the wear volume (the volume of metal removed from the interface in a single wear body), k is the wear coefficient, F is the applied load (the contact normal force of the connector), L is the displacement length of the wear body, and H is the hardness of the contact metal.
The explanations for F, L, and H are relatively simple. The contact force F is a connector design parameter. The length of the wear body is the engagement length during contact, or the length of the fretting displacement. The contact engagement length is a connector design parameter, but the length of the fretting displacement is related to many parameters. In the equation, H is the hardness of the contact metal.
For this type of material, the hardness is fixed. For two different metals, it is a composite hardness. In connectors, the two contact surfaces generally have the same finish, such as gold, but the thickness of the nickel base plate and the hardness of the contact material may differ, thus a composite hardness is appropriate. However, for a given connector, F, L, and H can be considered "known".
The wear amount V requires some explanation. The parameter for connector wear is the loss of surface thickness in each wear element. V = at, where A is the area of the contact interface and t is the thickness of the material removed. A depends on the geometry. Wear occurs because the pin contacts a surface, rather than a ball bearing with a diameter of a centimeter; this is important for the geometry. Similarly, for a given connector, this association can also be considered "known".
However, k is a variable factor with many parameters. The most important parameters are contact force F, hardness H, contact geometry, surface roughness, and the lubrication state of the contact surfaces. Again, F, H, contact geometry design, and surface roughness are "known" for a given connector. The lubrication state depends on the environment in which the connector is used. I believe that, in reality, contact force is the most important parameter in k for the connector, for the following reasons.
Figure 1 illustrates two contact surfaces. It is important to note that at the microscale of the contact interface, all surfaces are rough. For simplicity, two contact points, or bumps, are shown. In the following discussion, it is assumed that the first contact point creates the interface (a) in Figure 1, and that interface (b) begins to function as the load increases, making the surfaces more compact.
Under these conditions, interface (a) undergoes more deformation than interface (b). Because the uneven contact is small, the deformation will be plastic, and radial flow from the uneven surfaces will occur as the tops of the uneven surfaces gradually flatten each other. This radial flow disrupts the surface film and surface contaminants, and helps create the desired metal-to-metal interface. The resulting metal-to-metal interface will undergo a degree of “cold welding.”
Simply put, "cold welding" means that metal surfaces are joined together through an uneven interface, just as metallic bonds are formed inside the metal. Due to the deformation of the uneven surface, the metal is also hardened. The same process occurs when creating interface (b), but to a lesser extent. This means that interface (a) will be stronger than interface (b) because the greater deformation creates a larger contact area for the "cold weld," and it also undergoes a greater degree of hardening.
Considering these interface characteristics, what happens when shear stress is applied to the system? Since (a) is the stronger interface, the applied stress must be sufficient to break the interface at (a), while the weaker interface at (b) will then appear.
From a wear perspective, interfacial fracture is key. Consider the state of interface (a). After "cold welding" and hardening. In fact, due to interfacial hardening, (a) may have higher cohesion than the original metal itself, and uneven separation may occur inside the original metal, as shown in Figure 1, rather than directly at the interface. The resulting wear particles are the wear volume V in the equation.
The weaker interface at (b) may break at or near the original interface, with little wear. The wear process at (a) is usually called adhesive wear, and the wear process at (b) is called polishing wear. If the wear trajectory generated during connector engagement is observed under a magnifying glass at 30-50x magnification, the adhesive wear trajectory will show signs of wear particles and appear somewhat rough, while the polishing wear trajectory will appear smooth and shiny.
If the wear particles generated during adhesive wear are deformed and hardened sufficiently, they will act as abrasives at the contact interface, a process known as three-body abrasive wear, where an additional type of wear occurs.
Returning to k, it is clear that the change in wear mechanism from polishing to adhesive wear will be reflected in a significant increase in the wear coefficient k. During polishing wear, k increases with increasing contact force. However, when the contact force increases to the point where adhesive wear becomes active, k increases discontinuously and potentially significantly.
The magnitude of the contact force, causing discontinuous variations in k, will depend primarily on the lubrication condition of the connector. For clean surfaces, the transition range will be from a few grams to tens of grams, while for well-lubricated surfaces, the transition may not occur until the contact force increases to hundreds of grams.
Taking into account the connector's performance, in applications where low contact force is acceptable or required, such as low current, high pin count, low engagement force, and high durability applications, the expected wear mechanism is polishing wear, and the wear rate will be very low.
Generally, power applications require higher contact forces to meet the increased demands on contact interfaces in terms of resistance and stability. In such cases, adhesive and abrasive wear mechanisms may be more active. This is one of the reasons why many power connection designs utilize multiple contact beams.
These systems reduce connector resistance because multiple beams are electrically parallel, and they can also be designed to have lower contact forces to reduce the likelihood of adhesive and abrasive wear.
Of course, wear issues require optimal solutions in the design and manufacture of connector systems. Since wear rate is inversely proportional to hardness, tin-plated connectors will have a higher wear rate than gold-plated connectors. This relationship also explains why so-called hard gold is commonly used.
Another design parameter affecting wear resistance of finished gold connectors is the nickel base plate. The smoothness of the surface affects the number of wear cycles the connector can withstand without actual wear. Thin gold or shimmering gold finishes should be considered in this regard. If the connector system is found to have insufficient wear resistance, contact lubricants can significantly improve performance to meet application requirements. Connector design must fully consider wear resistance; otherwise, the impact on connector performance will be fatal.
