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A brief analysis of connector wear!

Date:2018-10-05 Hits:79

Connector wear has a significant impact on the performance of the connector because the wear process eliminates the surface layer of the connection that provides corrosion protection for the contact material. The most commonly used contact material is copper alloys, all of which are vulnerable to corrosion in typical connector working environments.

There are two main reasons for connector wear. The most obvious one is that when the contact surfaces of the plug and socket slide against each other, wear and tear occur during each occlusal process. The second is the wear caused by fretting during the use of connectors. Fretting is a small-scale motion caused by mechanical disturbance or thermal expansion mismatch, ranging from several microns to tens of microns.


First, the wear mechanism can be very complex, so this article is limited to a few simple observations of the wear process of the connector. Wear can be described by a simple equation, although the meaning of the parameters in the equation may be quite complex. The formula is as follows:


V = k F L/H


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 normal contact force of the connector), L is the displacement length of the wear body, and H is the hardness of the contact metal.


The explanation for F, L and H is relatively simple. The contact force F is the connector design parameter. The length of the worn body is the length of the engagement or the length of the fretting displacement. Contact length is the design parameter of connector, but the length of fretting displacement is related to many parameters. In the equation, H is the hardness that contacts the metal. For such materials, the hardness is determined. For two different metals, it is composite hardness. In a connector, the two contact surfaces generally have the same finish, such as gold, but the thickness of the nickel substrate and the hardness of the contact material may be different, so the composite hardness is appropriate. However, for a given connector, F, L and H can be considered "known".


Wear volume V needs some explanation. The parameter of the connector wear is the loss of the surface thickness of each worn body. V=at, where A is the area of the contact interface, and t is the thickness of the material being removed. A depends on geometric design. Wear is due to needle contact with a surface, not a centimeter diameter ball bearing; it is important for geometric design. Similarly, for a given connector, this association can also be considered "known".


But K is a variable factor of many parameters. The most important parameters are contact force F, hardness H, contact geometry, surface roughness and lubrication state of the contact surface. Again, F, H, contact geometry design, and surface roughness are "known" for a given connector. The lubrication state depends on the environment of the connector. In my opinion, in fact, contact force is the most important parameter in the K of connectors, for the following reasons.
Electronic connector

As shown in Fig. 1, two contact surfaces are shown. It should be noted that all surfaces are rough at the micro scale of the contact interface. For simplicity, two contact points or bump points are given. In the following discussion, assume that the first contact point creates an interface (a in Figure 1) and that the interface (b) starts to work as the load increases and the surface becomes tighter.

 connector

Under these conditions, the interface (a) experiences more deformation than the interface (b). Because the uneven contact is very small, the deformation will be plastic, and the uneven radial flow will occur with the uneven top smoothing each other. This radial flow destroys the surface film and surface contaminants, and helps to create the required metal contact interface. The metal interface created will experience a certain degree of "cold welding". Simply put, "cold welding" means that metal surfaces are interconnected through rough interfaces, just as metal bonds are formed inside the metal. The uneven metal is also hardened due to uneven deformation. The same process occurs when creating an interface (b), but to a lesser degree. This means that the interface (a) will be stronger than the interface (b), because larger deformation creates greater contact area for "cold welding" and it also undergoes greater hardening.


Taking into account these interface features, what happens when shear stress is applied to the system. Since (a) is a strong interface, the applied stress must be sufficient to destroy the interface at (a), and the weaker interface at (b) then appears. From the point of view of wear, interfacial fracture is the key. Consider the state of the interface (a). After "cold welding" and hardening. In fact, due to interfacial hardening, (a) may have higher cohesion than the original metal itself, uneven separation may occur within 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. At B, the weaker interface may break near or near the original interface, and wear rarely occurs. The wear process at (a) is usually referred to as adhesive wear, and the wear process at (b) is called polishing wear. If the wear trajectory produced during the occlusion of the connector is observed under a magnifying glass, the adhesive wear trajectory will show signs of wear particles at 30-50 magnification, and appear a little rough, while the polishing wear trajectory will appear smooth and bright. If the abrasive particles produced in the adhesive wear process are deformed enough to harden, they act as abrasive on the contact interface, i.e. the so-called three-body abrasive wear, and an additional wear occurs.


Back to k, it is obvious that the change of wear mechanism from polishing to adhesive wear will be reflected in the significant increase of wear coefficient K. In the process of polishing wear, K increases with the increase of contact force. However, when the contact force increases to the extent that the adhesive wear becomes active, K increases significantly discontinuously and potentially. The discontinuous change of K will mainly depend on the lubrication state of the connector. For clean surfaces, the transition range will be several to dozens of grams, while for well-lubricated surfaces, the transition may not occur until the contact force increases to hundreds of grams.
Electronic connector

Consider connector performance comprehensively. In applications where low contact forces are acceptable/required, such as low current, high pin number, low bite force and high durability applications, the expected wear mechanism is polishing wear, and the wear rate will be very low. In general, power applications require higher contact forces to meet higher requirements for contact interfaces in terms of resistance and stability. In this case, the adhesive and abrasive wear mechanisms may be more active. This is one of the reasons why many power connections are designed to take advantage of 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, in the design and manufacture of connector system, the wear problem needs the best solution. Because the wear rate is inversely proportional to the hardness, the wear rate of the tin connector will be higher than that of the gold connector. This relationship also explains why the so-called hard gold is usually used. Another design parameter that affects the wear of gold finished connector is nickel plate. The degree of finish affects the number of wear that the connector can withstand without wear. In this regard, thin gold or Flash gold should be considered. If the wear performance of the connector system is found to be inadequate, the contact lubricant can fully improve the performance to meet the application requirements. The wear and tear problem should be fully considered in the design of the connector, otherwise the performance of the connector will be fatal.