Customers often ask how to distinguish between the good and the bad quality terminal block. And what technical content there is in terms of purchase and composition. The selection of the terminal block is nothing more than wire diameter, current, wiring direction, application occasions, wiring methods, other wire diameters and currents. In this article, DORABO, an LCSC supplier, will discuss how to identify the internal quality of a terminal block, which is a commonly asked question. Below, we briefly discuss and summarize the internal structure of terminal blocks and related technologies. This information will be helpful for future purchases.
DORABO electronic engineer said: Most electronic connectors and terminal blocks require surface treatment, which generally refers to electroplating. There are two main reasons to protect the terminal reed base material from corrosion; the second is to optimize the performance of the terminal surface and establish and maintain the contact interface between terminals, especially film layer control. In other words, this makes it easier to achieve metal-to-metal contact.
Prevent corrosion
Most connector reeds are made of copper alloy, which usually corrodes in the use environment, such as oxidation, sulfide, etc. Terminal plating is to isolate the reed from the environment to prevent corrosion. The electroplated material, of course, must not corrode, at least in the application environment.
Surface optimization
Optimizing the surface properties of terminal blocks can be achieved in two ways. One is the design of the connector to establish and maintain a stable terminal contact interface. The second is to establish a metallic contact, which requires that any surface film layer does not exist or will break during insertion. The difference between the two forms of no film layer and film layer rupture is the difference between precious metal plating and non-noble 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 terminal surface free from external factors, such as contamination, substrate diffusion, terminal corrosion, etc.
Non-metallic plating, especially tin, lead and their alloys, have an oxide film. But when inserted, the oxide film is prone to rupture, establishing a metallic contact area.
(1) Precious Metal Terminal Block Plating
Precious metal terminal plating means that precious metal covers the surface of the bottom layer, which is usually nickel. The typical plating thickness for connectors is 15 to 50 micrometers of gold and 50 to 100 micrometers of nickel. Common precious metal platings include gold, palladium, and their alloys. Gold is an ideal electroplating material with excellent electrical and thermal conductivity.
Due to these advantages, the main plating used in connectors for applications requiring high reliability is gold, but the cost of gold is high. Palladium is also a precious metal, but compared with gold, it has high resistance, low heat transfer, and poor corrosion resistance, but it has advantages in friction resistance. Generally, a palladium-nickel alloy (80~20) serves in the terminal posts of connectors. When designing precious metal plating, we should consider the following matters:
Porosity
During the electroplating process, gold nucleates on numerous exposed surface stains. These nuclei continue to grow and spread on the surface. These islands (isolated objects) collide with each other and cover the surface, forming a porous electroplated surface. The porosity of gold plating has a certain relationship with the thickness of the plating. When below 15u, the porosity increases rapidly. When above 50u, the porosity is very low, and the actual rate of decrease is negligible. This is why the thickness of precious metals for electroplating is usually in the range of 15~50u. Porosity and substrate defects such as inclusions, laminations, stamping marks, improper cleaning of stampings, incorrect lubrication, etc. are also related to wear and tear of the plating surface of terminals, which can also cause exposure of the substrate.
The wear or lifespan of a plated surface depends on two characteristics of the surface treatment: the coefficient of friction and the hardness. The hardness increases, the friction coefficient decreases, and the life of the surface treatment will increase. Electroplated gold is usually hard gold and contains a hardening activator. Cobalt (Co) is a common hardening agent that can enhance the wear resistance of gold.
The use of palladium-nickel plating can significantly enhance the durability and lifespan of precious metal coatings. Typically, a 20-30u palladium-nickel alloy is coated with 3u of gold plating, providing excellent conductivity and high wear resistance. Additionally, a nickel underlayer is often employed to extend the lifespan.
Nickel bottom layer
The primary consideration for precious metal plating is the nickel underlayer, which provides several vital functions to ensure the integrity of the terminal contact interface.
Nickel provides an effective isolation layer between the substrate and the pinholes through its positive oxide surface. It reduces the potential for pinhole corrosion. And it provides a hard layer beneath the precious metal plating layer. Thus, it improves the coating life. What thickness is appropriate? The thicker the nickel bottom layer, the lower the wear. However, considering the cost and controlling the roughness of the surface, a thickness of 50~100u is generally selected.
(2) Non-precious Metal Terminal Block Plating
Non-precious metal plating is different from precious metals. It always has a certain number of surface film layers. The connector is to provide and maintain a metallic contact interface. We must take into account the presence of these coatings. Generally speaking, the forward force requirement for non-noble metal plating is high enough to destroy the coating. Thus, it maintains the integrity of the terminal contact interface. The scrubbing effect is also vital for terminal surfaces containing film layers.
There are three non-metallic surface treatments in terminal plating: tin (tin-lead alloy), silver, and nickel. Commonly used tin, silver has advantages for high currents, and nickel is limited to high-temperature applications.
Tin surface treatment
Tin also refers to tin-lead alloy, especially the alloy of tin 93-lead 3. We propose the use of tin surface treatment from the fact that the tin oxide film layer is easily damaged. We will cover a hard, thin, brittle oxide film on the surface of the tin plating. Underneath the oxide film is soft tin. When a certain positive force acts on the film layer, the tin oxide, because it is thin, cannot withstand this load, and because it is very brittle and brittle, it cracks. Under such conditions, the load transfers to the tin layer, which is soft and pliable and flows easily under load. Because of the flow of tin, the cracks in the oxide are wider. Through cracks and spacers, tin is extruded to the surface to provide metal contact.
The role of lead in tin-lead alloy is to reduce the generation of tin whiskers. Tin whiskers are a layer of single crystals (tin whiskers) formed on the surface of tin plating under the action of stress. Tin whiskers can create short circuits between terminals. It can reduce tin whiskers by adding 2% or more lead. There is also a 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). We mainly use it to solder connectors. However, there is a growing legal requirement to reduce lead in electronic and electrical products. Many electroplated terminals require lead-free plating. There are mainly pure tin, tin/copper, and tin/silver plating. Plate a layer of nickel or use a matte matte tin surface to slow down the generation of tin whiskers.
Silver surface plating
Silver is considered a non-noble 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 “diode” characteristics.
Silver is also soft, similar to soft gold. Because sulfides are not easily destroyed, there is no frictional corrosion of silver. Silver has excellent electrical and thermal conductivity and will not melt under high currents. It is a material used for surface treatment of high-current terminals.
(3) Terminal Block Lubrication
For different terminal surface treatments, the role of lubrication is different. It has two main functions: reducing the friction coefficient and providing environmental isolation. Reducing the friction coefficient has two effects:
- Reduce the insertion force of the connector.
- Improve the life of the connector by reducing wear and tear.
Terminal lubrication can provide environmental isolation by forming a “sealing layer” to prevent or delay the environment’s contact with the contact interface. Generally speaking, for precious metal surface treatments, terminal lubrication is used to reduce the friction coefficient and increase the lifespan of the connector. For tin surface treatments, the role of terminal lubrication is to provide environmental isolation and prevent friction and corrosion.
Although we can add lubricant in the subsequent process after plating, it is only a supplementary operation. For those connectors that soldered to a PCB board, soldering cleaning may have lost the lubricant. Lubricant sticks to dust. If used in a dusty environment, it will increase the resistance and reduce the service life. A lubricant’s ability to withstand temperature may also limit its application.
