The tin coating is susceptible to other corrosion mechanisms, namely "fretting" corrosion. Fretting refers to a small range of movement as small as a few microns to a few microns due to mechanical (vibration/shock) or thermal (thermal expansion mismatch) driving force. Corrosion is caused by the tin oxide wear debris accumulated on the contact interface due to fretting motion. The accumulation of wear debris will cause the contact resistance to increase over time, the speed of which depends on the number of fretting movements that occur. Our company provides SMB coaxial connector.
Insignificant corrosion degradation can lead to high resistance in as few as thousands of events. The main purpose of contact lubricant in tin surface treatment is to reduce the possibility of fretting corrosion. Contact lubricants can perform this function in two ways: by providing a protective layer to reduce the possibility of corrosion, and by reducing the wear that occurs in each fretting cycle (thus reducing the amount of wear debris that is easily oxidized).
For the sake of completeness, I must note that when nickel gold wears on, the gold flash on the nickel finish (gold plating thickness in the range of 0.1 microns) becomes prone to fretting corrosion degradation, thereby exposing nickel that is prone to fretting corrosion Floor.
In fact, the contact lubricant can be freely applied to the conductor and therefore must be non-conductive. How to create the required metal-to-metal contact interface in this situation? During the mating process, the plug contact slides on the socket contact, and the contact lubricant provides a layer that reduces the friction coefficient and wears process, thereby reducing the required mating force and wear. When the plug stops in the mating area of the socket, the contact lubricant will shift because the two surfaces deform to form the required metal-to-metal interface.
The figure below shows the structure of the contact interface in microscale. Within this range, all surfaces are rough, and surface deformation occurs only at high points on the surface close to each other. The actual metal contact area produced is usually about a few percent. The remainder of the surface consists of recesses that can accommodate displacement and contact lubricant. The valleys are connected to each other, which means that each metal-to-metal contact area is effectively isolated from the external environment by contracting lubricant. This provides the required corrosion protection in gold and tin surface connectors.
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