In the article " RF coaxial connectors common materials and its characteristics ", we introduced the common metal materials and insulating materials in the basic materials of connectors in detail. In the actual production and application of connectors, in order to achieve the electrical performance and environmental requirements of connectors, a layer of compatible metal, commonly known as electroplating, will be coated on the surface of the substrate.

Electroplating is a process that uses electrolysis to make metal or other materials attach a layer of metal film, which can prevent corrosion, improve wear resistance, and improve aesthetics. In this article, we will introduce in detail the requirements of connector coating, characteristics of coating materials and applications.

Requirements for connector coating：

•  Good electrical characteristics and thermal conductivity

•  Reduce or eliminate surface oxidation and corrosion, and protect the conductor covered by the coating

•  Provide good contact conductivity between conductors

•  Easy to weld or solder is easy to attach to the surface

•  Increased wear resistance and hardness

Four important characteristics must be considered when selecting coating materials:

•  High current conductivity

•  Good wear resistance

•  Low contact resistance

•  Good adhesion to base materials or sedimentary layers

Common plating materials

Au

Gold (Au) is a good conductor of heat and electricity, with good ductility and strength. Generally, it does not react with air or most other substances, making it an excellent transport material. In order to prevent diffusion corrosion, a layer of nickel is often plated under gold, which is called diffusion protection layer, and can slow down the diffusion speed.

The conductor in the connector is often gold-plated to obtain better transmission, corrosion resistance and repeated plugging times.

Gold plated parts: inner conductor (center pin), outer conductor spring, test level main body, outer conductor of PCB connector (welding).

Ag

Silver (Ag) has high hardness and is cheaper than gold, and has excellent electrical and heat transfer properties. Silver is commonly used for welding contacts in radio frequency applications. It has the best transmission efficiency of all metals, the lowest contact resistance, and the lowest loss under high current load, which makes its intermodulation characteristics better. Silver will be oxidized when it comes into contact with ozone, hydrogen, sulfide and sulfur.

Silver is the substance with the highest conductivity and thermal conductivity among all metals. The order of conductivity is: silver ->copper ->gold ->aluminum.

Silver plated parts: low intermodulation and high power inner conductor (center pin), outer conductor, welding cup, contact surface parts

Ni

Nickel (Ni) is an economical coating material. Its hardness is higher than gold, and it has good malleability and ductility. It is a suitable carrier for electricity and heat.

Nickel is usually used as the deposition material of precious plating metals (such as gold) in electroplating, and is also widely used in stainless steel. The reason why it is suitable to be used as deposition layer material is that its molecular structure is crystalline. The existence of the deposit layer makes the coating material and the base material more closely combined, and also reduces the degree of oxidation.

Nickel plated parts: shell, outer conductor, crimping ring.

Ternary alloy (Cu-Zn-Sn alloy)

The commonly used ternary alloy of RF connector is composed of copper, tin and zinc (basic ratio: copper accounts for 55%~60%, tin accounts for 20%~25%, and zinc accounts for 15%~20%). It can be used as plating instead of nickel. Cu Sn Zn alloy has good electrical properties and corrosion resistance. In transmission systems (such as base stations), the non-magnetic contact area has a great impact on intermodulation. The properties of ternary alloy are close to that of silver, and the intermodulation level is less than - 155 dBc.

The contact resistance of ternary alloy can reach 1-3m Ω. The outer conductor of ternary alloy plating has high strength and corrosion resistance, and can be inserted and pulled thousands of times. Generally, the outer conductor is silver plated first and then ternary alloy plated. Silver has good transmission performance. When the frequency increases, most signals on the outer surface are transmitted from the silver layer. In the field, the corrosion resistance of ternary alloy can prolong its service life.

Parts plated with ternary alloy: shell, main body, crimping ring.

Comparison Table of Common Plating Characteristics

Note: Relative values range from ++ (very good/very low price), + (better/low price), 0 (fair), - (poor) to -- (very poor/very high price).

#The contact resistance must be as small as possible（++ means perfect）