The working principle of contact resistance
Observe the surface of the connector contacts under a microscope. Although the gold-plated layer is very smooth, 5-10 micron raised parts can still be observed. You will see that the contact of the mated pair of contacts is not the contact of the entire contact surface, but the contact scattered at some points on the contact surface. The actual contact surface must be smaller than the theoretical contact surface. Depending on the smoothness of the surface and the contact pressure, the gap between the two can reach thousands of times. The actual contact surface can be divided into two parts; the first is the real metal-to-metal direct contact part. That is, the contact micro-points without transition resistance between metals, also known as contact spots, are formed after the interface film is damaged by contact pressure or heat. The part accounts for about 5-10% of the actual contact area. The second is the parts that contact each other after contaminating the film through the contact interface. Because any metal has a tendency to return to the original oxide state. In fact, there is no truly clean metal surface in the atmosphere. Even if a very clean metal surface is exposed to the atmosphere, an initial oxide film layer of several microns will be formed very quickly. For example, copper takes only 2-3 minutes, nickel takes about 30 minutes, and aluminum takes only 2-3 seconds. An oxide film layer with a thickness of about 2 microns can be formed on the surface. Even the particularly stable precious metal gold, due to its high surface energy, will form an organic gas adsorption film on its surface. In addition, atmospheric dust and the like can also form a deposited film on the surface of the contact. Therefore, from the microscopic analysis, any contact surface is a contaminated surface.