What plating options are available for high voltage connectors? 

There are many options for the coating of terminals, which are usually divided into four categories: precious metals, inert metals, alloys and dispersions. The first three categories are already established and used, while decentralization is just beginning to emerge in response to the need for high-power connectivity in EV charging and similar applications. We begin with a brief review of basic electroplating system concepts, then introduce the properties of precious metals, inert metals, and alloys, and finally briefly discuss the potential advantages of dispersions.

The coating system is usually a multi-layer structure, in which the top layer can be a relatively thin coating. Inert metal surfaces like tin are often found in consumer products. Industrial, military, medical and other high-performance applications often turn to precious metals. Precious metals are often used as alloys. For example, pure gold is too soft for many applications, while gold alloyed with cobalt, nickel, or iron is called "hard gold" and is used to provide higher durability (Figure 1). Other precious metals such as silver can also be used as alloys to increase hardness.

FIG. 1 The electroplating system consists of multiple layers of different materials

Precious metal

1. Gold is very conductive even at low voltages. It has excellent oxidation resistance and corrosion resistance compared to other options. It is ideal for use in environments with high humidity or frequent thermal cycles.

2. Silver performs well in high power applications. It costs less than gold and is a good choice for larger, high-current contacts.

3. Palladium has high electrical and thermal conductivity and is resistant to corrosion. It is harder than gold or silver and is a good choice for applications that require more durability.

4. Rhodium is harder than palladium and resistant to corrosion and acids. It is ideal for applications that require good wear resistance and high hardness.

Inert metal

1. Tin is non-toxic and therefore suitable for some medical applications. This is a low cost and suitable for applications that require a small number of mating cycles.

2. Nickel is very hard, durable and resistant to corrosion. The combination of its durability and high electrical conductivity makes it suitable for high-use applications.

3. Copper is one of the metals with the highest electrical conductivity, but it needs a protective layer to prevent corrosion. It is often used as the basis for subsequent metal plating.

Metal Alloy

1. White bronze is a non-magnetic alloy of copper, tin and zinc that is more conductive than nickel. It is well suited to many medical and RF connectors.

2. Palladium-nickel alloys are resistant to corrosion and have low contact resistance. It has good weldability and can handle a large number of mating cycles. Palladium-nickel coatings often flash gold.

3. Electroless nickel plating can be applied to both conductive and non-conductive surfaces. Electroless nickel plating contains up to 14% phosphorus. The higher the phosphorus content, the stronger the corrosion resistance, but the hardness is reduced.

4. Nickel-phosphorus alloys have high wear and corrosion resistance. They have good weldability, but have lower electrical conductivity than other alloys.

Silver-graphite dispersion

Silver-graphite dispersions have been developed for use as contacts in high-power applications such as electric vehicle chargers. It combines the properties of fine silver and hard silver. The fine silver matrix provides high electrical conductivity, and the graphite increases wear resistance and supports a high cycle life. Uniform and random dispersion of graphite particles is one of the keys to high durability (Figure 2).

Figure 2 X-ray spectral scan of the contact surface showing a uniform distribution of graphite on the surface (Image: Umicore)

Summary

A wide range of plating options are available to support different levels of electrical conductivity, environmental robustness, durability and other application needs. With the development of graphite dispersion plating materials, the traditional choice between precious metals, inert metals and various alloys is expanding.