Customers often ask how to distinguish between good and bad quality of terminal blocks, and what are the technical requirements for selection and composition. The selection of terminal blocks mainly includes wire diameter, current, wiring direction, application scenarios, wiring methods, and other wire diameters and currents. This is a very common question. Below is a summary of the internal composition and related technologies of terminal blocks by Sanmenwan Electrical Engineer Chen Zeqiao, hoping to be helpful for everyone's future selection.  
Most electronic connectors and wiring terminals require surface treatment, which generally refers to electroplating. There are two main reasons for protecting the terminal spring substrate from corrosion; The second is to optimize the performance of the terminal surface, establish and maintain the contact interface between terminals, especially the film layer control. In other words, making it easier to achieve metal to metal contact.
Preventing corrosion:
Most connector springs are made of copper alloy and are usually corroded in the usage environment, such as oxidation, vulcanization, etc. Terminal electroplating is to isolate the spring from the environment and prevent corrosion from occurring. Electroplated materials, of course, if they are non corrosive, at least in the application environment.    
Surface optimization:
There are two ways to optimize the surface performance of wiring terminals. One is in the design of the connector, establishing and maintaining a stable terminal contact interface. The second is to establish a metallic contact. It is required that any surface facial mask layer does not exist or will break when inserted. The difference between precious metal electroplating and non precious metal electroplating lies in the absence of film layer and film layer rupture. Precious metal electroplating, such as gold, palladium, and their alloys, is inert and does not have a film layer itself. Therefore, for these surface treatments, metallic contact is "automatic". What we need to consider is how to maintain the "nobility" of the terminal surface, free from external factors such as pollution, substrate diffusion, terminal corrosion, etc.  
Non metallic electroplating, especially tin and lead and their alloys, covers a layer of oxide film, but during insertion, the oxide film is easily broken, establishing a metallic contact area.  
(1). Electroplating of precious metal terminals
Precious metal terminal electroplating refers to the coating of precious metal on the bottom surface, usually nickel. Typical connector coating thickness: 15-50u gold, 50-100u nickel. The most commonly used precious metal electroplating methods are gold, palladium, and their alloys. Gold is the most ideal electroplating material with excellent conductivity and thermal conductivity. In fact, it is corrosion-resistant in any environment. Due to these advantages, in connectors that require high reliability, the main plating is gold, but the cost of gold is high. Palladium is also a precious metal, but compared to gold, it has high resistance, low heat transfer, and poor corrosion resistance, but it has advantages in friction resistance. Generally, palladium nickel alloy (80-20) is used in the design of precious metal electroplating for connector terminals. The following considerations need to be taken into account:
Porosity: In electroplating processes, gold nucleates on numerous exposed surface stains. These nuclei continue to grow and expand on the surface, and eventually these isolated objects collide with each other to completely cover the surface, forming a porous electroplated surface. The porosity of gold plating is related to the thickness of the plating. Below 15u, the porosity increases rapidly, while above 50u, the porosity is very low, and the actual rate of decrease can be ignored. That's why the thickness of electroplated precious metals is usually in the range of 15~50u. The porosity and defects of the substrate, such as inclusions, stacking, stamping marks, incorrect cleaning of stamping, incorrect lubrication, etc., are also related to the wear of the electroplated surface of the worn terminal, which can also cause exposure of the substrate.  
The wear or lifespan of electroplated surfaces depends on two characteristics of surface treatment: friction coefficient and hardness. As the hardness increases, the friction coefficient decreases, and the lifespan of surface treatment increases. Electroplated gold is usually hard gold, containing hardening activators, among which Co (cobalt) is the most common hardener, which can improve the wear resistance of gold.  
The selection of palladium nickel electroplating can greatly improve the wear resistance and lifespan of precious metal coatings. Generally, a 3-u gold coating is applied on top of a 20-30 u palladium nickel alloy, which has good conductivity and high wear resistance. Additionally, a nickel substrate is typically used to further extend the lifespan.  
Nickel bottom layer
The nickel substrate is the primary factor to consider in precious metal electroplating, providing several important functions to ensure the integrity of the terminal contact interface.
Through the positive oxide surface, nickel provides an effective isolation layer, blocking the substrate and pores, thereby reducing the potential for pore corrosion; And it provides a hard support layer beneath the precious metal electroplating layer, thereby improving the lifespan of the coating. What kind of thickness is suitable? The thicker the nickel substrate, the lower the wear, but considering cost and surface roughness control, a thickness of 50-100u is generally chosen.  
(2) Non precious metal electroplating
The difference between non precious metal electroplating and precious metal electroplating is that they always have a certain amount of surface facial mask layers. As the purpose of connectors is to provide and maintain a metallic contact interface, the presence of these film layers must be taken into account Generally speaking, for electroplating of non precious metals, a high forward force is required to break the film layer and maintain the integrity of the terminal contact interface. The scrubbing effect is also important for terminal surfaces containing film layers.
There are three types of non-metallic surface treatments in terminal electroplating: tin (tin lead alloy), silver, and nickel. Tin is the most commonly used, silver has superiority over high current, and nickel is limited to high-temperature applications.  
Tin surface treatment.  
Tin also refers to tin lead alloys, especially tin 93 lead 3 alloys. We proposed the use of tin surface treatment based on the fact that the oxide film layer of tin is easily damaged. The surface of the tin coating will be covered with a hard, thin, and fragile oxide film. Under the oxide film is soft tin. When a positive force is applied to the film layer, tin oxide, due to its thinness, cannot withstand this load, and because it is brittle and fragile, it cracks. Under such conditions, the load is transferred to the tin layer, which is soft and flexible, making it easy to flow under the load. Due to the flow of tin, the cracking of oxides becomes wider. Through cracks and spacing layers. Tin is extruded onto the surface to provide metal contact. The role of lead in tin lead alloys is to reduce the production of tin whiskers. Tin whiskers are a layer of single crystals (tin whiskers) formed on the surface of tin electroplating under stress. Tin whiskers can form short circuits between terminals. Increasing lead by 2% or more can reduce tin whiskers. There is also a type of tin lead alloy with a ratio of tin: lead=60:40, which is close to the composition ratio we solder (63:37), mainly used in connectors to be soldered. But recently, there have been increasing legal requirements to reduce lead content in electronic and electrical products. Many electroplated terminals require lead-free electroplating, mainly pure tin, tin/copper, and tin/silver electroplating. Tin whiskers can be slowed down by plating a layer of nickel between copper and tin layers or using a non smooth and matte tin surface.
Silver surface electroplating.  
Silver is considered a non precious metal terminal surface treatment because it reacts with sulfur and chlorine to form a sulfurized film. Sulfide film is a semiconductor that forms the characteristic of a "diode".  
Silver is also soft, similar to soft gold. Because sulfides are not easily destroyed, silver does not exhibit frictional corrosion. Silver has excellent conductivity and thermal conductivity, and will not melt under high current, making it an excellent material for surface treatment of high current terminals.  
(3) Terminal lubrication
The lubrication effect varies for different terminal surface treatments, mainly serving two functions: reducing friction coefficient and providing environmental isolation. Reducing friction coefficient has two effects:
Firstly, reduce the insertion force of the connector
Secondly, improving the lifespan of connectors by reducing wear and tear
Terminal lubrication can provide environmental isolation by forming a "closed layer" to prevent or delay contact between the environment and the contact interface. Generally speaking, for surface treatment of precious metals, terminal lubrication is used to reduce the friction coefficient and improve the lifespan of connectors. For surface treatment of tin, terminal lubrication provides environmental isolation and prevents friction corrosion.
Although lubricants can be added in the next step of electroplating, it is only a supplementary operation. For connectors that need to be soldered to PCB boards, soldering cleaning may lose lubricant. Lubricants stick to dust, and if applied in dusty environments, it can increase resistance and reduce lifespan. Finally, the temperature resistance of lubricants may also limit their application.
(4) Summary of Terminal Surface Treatment
Precious metal electroplating, assuming coverage on a 50u nickel substrate, with gold being the most commonly used material, the thickness depends on the lifespan requirements, but may be subject to porous impact.  
Palladium is not recommended for use in weldability protection applications.
Silver is sensitive to rust and migration, mainly used in power connectors. Through lubrication, the lifespan of silver can be significantly improved.
Tin has good environmental stability, but mechanical stability must be ensured.