Tribology and the Effects of Motion in Connectors
Tribology is the study of the science and engineering of interacting surfaces that are in relative motion, and includes the associated principles of friction, lubrication, and wear. So how does tribology relate to connector interfaces?
Although connector contact surfaces are usually not intended to be in continual relative motion, often the surfaces do experience “micro motion,” which can cause wear that ultimately results in connector failures, due to elevated resistance from fretting corrosion.
What is “fretting corrosion”? It is a degradation mechanism that is caused by relative sliding motion that can be as low as 3 to 4 nanometers. Fretting refers to wear (and sometimes corrosion damage) at the asperities of contact surfaces. The contact movement causes mechanical wear and material transfer at the surface, often followed by oxidation of the metallic debris and the freshly exposed metallic surfaces. This damage is induced under load and in the presence of repeated or cyclic relative surface motion. Function is impaired when corrosion results in high contact resistance at the interfaces. Electrical failures are another possible problem. These motions can be caused by vibration or differential thermal expansion.
Fretting corrosion may be minimized by designing the mating contact system to exhibit high normal forces, thereby mechanically preventing micro motion from occurring. This high force is evident in some connection systems; quick disconnect tabs and receptacles are common examples. However, it is not always practical to have connectors with high normal force because of the desire for lower insertion and removal forces of the connector. Another method of preventing fretting corrosion is to design the connector with floating as opposed to rigidly-fixed contacts. This prevents the introduction of relative motion between the mating contacts. Lubrication may also be used to reduce fretting. Tin finishes are most susceptible to fretting corrosion and degradation; however, fretting wear is also a concern on gold flash-plated contacts. The loss of a thin layer of gold because of fretting wear can expose the underlying nickel and lead to a higher resistance contact than anticipated.
Lubrication can also be used in connectors to reduce friction, thereby reducing mating forces and increasing durability. In addition to providing protection against fretting wear, lubrication has also been used successfully as a pore blocker and may allow for thinner precious metal platings to be utilized.
In summary, tribology is the study of interacting surfaces that are in relative motion. Although connectors are designed to be static (once mated together), relative motion can occur by vibration or by differential thermal expansion. This micro motion can lead to fretting corrosion and ultimately high contact resistance at the mating interface. If this motion cannot be prevented from occurring by careful connector design, lubrication of the interfaces may be an option. Lubrication may also be used to reduce friction and wear, increase durability, and act as a pore blocker to potentially allow thinner plating on contact surfaces.
Bill Garver of APEX Electrical Interconnection Consultants has 47 years experience in the connector industry, primarily in the management and direction of new product development and operational division management. He held the titles of division manager and director of development engineering at AMP. Garver developed new products throughout the full product life cycle – concept through introduction – for numerous industries including consumer, commercial, computer, industrial, communications, and medical.