Optimization Of Terminal Blocks
Distribution box: It is a kind of metal electrodeposition process, which refers to the process in which simple metal ions or complex ions are discharged and reduced to metal atoms on the surface of a solid (conductor or semiconductor) by electrochemical methods and attached to the electrode surface to obtain a metal layer. Purpose Electroplating changes the surface characteristics of the solid to change the appearance, improve corrosion resistance, wear resistance, enhance hardness, and provide special optical, electrical, magnetic, thermal and other surface properties. Introduction to terminal electroplating knowledge
Most electronic connectors require surface treatment of the terminals, which generally refers to electroplating. There are two main reasons to protect the terminal reed substrate from corrosion; the second is to optimize the performance of the terminal surface, establish and maintain the contact interface between the terminals, especially the film layer control. In other words, it makes it easier to achieve metal-to-metal contact. Prevent corrosion:
Most connector reeds are made of copper alloys, which usually corrode in the use environment, such as oxidation, sulfide, etc. Terminal electroplating is to isolate the reed from the environment to prevent corrosion. The electroplated material, of course, should not corrode, at least in the application environment.
Surface optimization: The optimization of the surface performance of the terminal can be achieved in two ways. The first is to establish and maintain a stable terminal contact interface in the design of the connector. The second is to establish metallic contact, which requires that any surface film does not exist or will break when inserted. The difference between the two forms of no film and film breakage is the difference between precious metal plating and non-precious metal plating. Precious metal plating, such as gold, palladium, and their alloys, is inert and has no film itself. Therefore, for these surface treatments, metallic contact is "automatic". What we need to consider is how to keep the "noble" surface of the terminal free from external factors such as pollution, substrate diffusion, terminal corrosion, etc.
Non-metallic plating, especially tin and lead and their alloys, is covered with an oxide film, but when inserted, the oxide film is easily broken, and a metallic contact area is established.
(1). Precious metal terminal plating
Precious metal terminal plating refers to the precious metal covering the bottom surface, which is usually nickel. The general connector plating thickness: 15~50u gold, 50~100u nickel. The most commonly used precious metal platings are gold, palladium and their alloys. Gold is the most ideal electroplating material, with excellent electrical and thermal conductivity. In fact, it is corrosion-resistant in any environment. Due to these advantages, the main electroplating material in connectors for applications requiring high reliability is gold, but the cost of gold is very high. Palladium is also a precious metal, but it has high resistance, low heat transfer and poor corrosion resistance compared to gold, but it has advantages in friction resistance. Generally, palladium-nickel alloy (80~20) is used in the terminal of the connector. The following items need to be considered when designing precious metal electroplating: Porosity: During the electroplating process, gold nucleates on many stains exposed on the surface. These nuclei continue to grow and spread on the surface, and finally these islands (isolated objects) collide with each other and completely cover the surface, forming a porous electroplated surface. The porosity of the gold plating layer has a certain relationship with the thickness of the plating layer. Below 15u, the porosity increases rapidly, and above 50u, the porosity is very low, and the actual reduction rate can be ignored. This is why the thickness of the electroplated precious metal is usually in the range of 15~50u. Porosity and substrate defects such as inclusions, laminations, stamping marks, improper stamping cleaning, and improper lubrication also have a certain relationship. Wear of the terminal electroplated surface can also cause the substrate to be exposed. The wear or life of the electroplated surface depends on two characteristics of the surface treatment: friction coefficient and hardness. As the hardness increases and the friction coefficient decreases, the life of the surface treatment will increase. Electroplated gold is usually hard gold and contains a hardening activator, among which Co (cobalt) is the most common hardener, which can improve the wear resistance of gold. The choice of palladium nickel electroplating can greatly improve the wear resistance and life of the precious metal coating. Generally, a 3u gold plating layer is covered on a 20~30u palladium nickel alloy, which has both good conductivity and high wear resistance. In addition, a nickel base layer is usually used to further improve the life.
Nickel base layer The nickel base layer is the first factor to be considered in precious metal electroplating. It provides 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 pinholes, thereby reducing the potential possibility of pinhole corrosion; and provides a hard support layer under the precious metal electroplating layer, thereby increasing the life of the plating layer. What thickness is appropriate? The thicker the nickel base layer, the lower the wear, but considering the cost and controlling the roughness of the surface, the thickness of 50~100u is generally selected.
(2) Non-precious metal electroplating Non-precious metal electroplating is different from precious metals in that they always have a certain number of surface film layers. Since the purpose of the connector is to provide and maintain a metallic contact interface, the existence of these film layers must be taken into account. Generally speaking, for the electroplating of non-precious metals, the positive force required is high enough to destroy the film layer, thereby maintaining the integrity of the terminal contact interface. The scrubbing effect is also very important for the terminal surface containing the film layer. There are three non-metallic surface treatments in terminal electroplating: tin (tin-lead alloy), silver and nickel. Tin is the most commonly used, silver has advantages for high currents, and nickel is limited to high temperature applications.
Tin surface treatment Tin also refers to tin-lead alloys, especially tin-93-lead-3 alloys. The use of tin surface treatments is proposed based on the fact that the oxide film of tin is easily damaged. The surface of the tin plating is covered with a hard, thin, brittle oxide film. Under the oxide film is the soft tin. When a normal force acts on the film, the tin oxide, being thin, cannot withstand the load, and because it is brittle and brittle, it cracks. Under these conditions, the load is transferred to the tin layer, which is soft and pliable and flows easily under the load. Because of the flow of tin, the cracks in the oxide are wider. Through the cracks and spacers. The tin is squeezed to the surface to provide metal contact. The role of lead in tin-lead alloys is to reduce the generation of tin whiskers. Tin whiskers are a layer of single crystals (tin whiskers) formed on the surface of tin electroplating under stress. Tin whiskers form short circuits between terminals. Adding 2% or more lead can reduce tin whiskers. There is another type of tin-lead alloy with a ratio of tin: lead = 60:40, which is close to the composition ratio of our welding (63:37), and is mainly used in connectors to be welded. However, recently more and more laws have required the reduction of lead content in electronic and electrical products. Many electroplated terminals require lead-free electroplating, mainly pure tin, tin/copper and tin/silver electroplating. The generation of tin whiskers can be slowed down by plating a layer of nickel between the copper and tin layers or using a rough, 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 sulfide film. The sulfide film is a semiconductor and will form the characteristics of a "diode".
Silver is also soft, similar to soft gold. Because sulfides are not easily destroyed, there is no friction corrosion in silver. Silver has excellent electrical and thermal conductivity, does not melt under high current, and is an excellent material for high current terminal surface treatment.
(3) Terminal lubrication For different terminal surface treatments, the role of lubrication is different. There are two main functions: reducing the friction coefficient and providing environmental isolation. Reducing the friction coefficient has two effects:
First, reducing the insertion force of the connector
Second, increasing the life of the connector by reducing wear Terminal lubrication can provide environmental isolation by forming a "sealed layer" to prevent or delay the contact of the environment with the contact interface. In general, for precious metal surface treatments, terminal lubrication is used to reduce the friction coefficient and increase the life of the connector. For tin surface treatments, terminal lubrication is to provide environmental isolation to prevent friction corrosion. Although lubricants can be added in the next process of electroplating, it is only a supplementary operation. For connectors that need to be soldered to PCB boards, solder cleaning may lose lubricants. Lubricants stick to dust. If used in a dusty environment, it will increase resistance and reduce life. Finally, the temperature resistance of lubricants may also limit its application.
(4) Summary of terminal surface treatment Precious metal plating, assuming coverage on a 50u nickel base layer, gold is the most commonly used material, the thickness depends on the life requirements, but may be affected by porosity.
