Electrical contacts are interface regions that transmit signals or power, and are widely used in everything from smartphones and industrial machinery to transportation systems and defense equipment.
The performance of these contacts is governed largely by three key factors:

• Contact resistance

• Mechanical durability and wear resistance

• Environmental resistance (corrosion, oxidation, thermal and vibration stress, etc.)

Among these, surface treatment technology is a core method for reducing contact resistance and improving durability and environmental resistance. Plating, in particular, is the most widely used post-process after contact manufacturing.

Pre-treatment and Plating Methods

After electrical contacts (for example, solid or clad types) are manufactured, a pre-treatment process is required to remove surface oxides and contaminants. Ultrasonic cleaning and chemical etching are commonly used.
Plating is then applied, typically using one of two main methods:

• Electroplating

• Electroless plating

These methods are designed to form a uniform coating of the plating material on the substrate.

Main Plating Materials and Characteristics

Gold Plating

• Gold (Au) has excellent electrical conductivity and corrosion resistance, and is widely used for high-reliability contacts.

• It is typically classified into soft gold and hard gold. Hard gold is produced by adding elements such as cobalt (Co) or nickel (Ni) to increase hardness and improve wear resistance.

• Typical thickness: generally 0.5 to 5 µm, with 1 to 3 µm being most common.
  If the layer is too thin (for example, 0.1 µm or less), the substrate may become exposed as the coating wears, increasing the risk of corrosion. If it is too thick (for example, 10 µm or more), cost rises and problems such as brittleness can occur.

• Example from research: studies under sliding conditions have analyzed the wear and reliability of gold-plated contacts and confirmed how plating thickness and contact load influence contact lifetime.

Silver Plating

• Silver (Ag) is less expensive than gold and has higher electrical conductivity (approximately 63×10⁶ S/m) than gold (approximately 45×10⁶ S/m), making it especially suitable for high-current contacts.

• Its drawbacks include surface discoloration and oxidation due to sulfidation. To prevent this, a nickel under-plate is commonly used beneath the silver layer.

• Typical thickness: studies report silver plating thicknesses in the range of 2 to 10 µm, with around 4 µm often cited as optimal for resistance to fretting (micro-vibration wear).

Other Plating Materials

• Palladium (Pd) plating: Has properties between gold and silver, with excellent corrosion and wear resistance. It is often used in automotive electronic components.

• Nickel (Ni) plating: Commonly used as an under-plate. Applied at thicknesses of around 5 to 10 µm to protect the substrate and improve hardness.

• Tin-zinc plating: Emerging as a cadmium replacement that complies with environmental regulations such as RoHS, with strong corrosion resistance as a key feature.

Improved Corrosion and Environmental Resistance

The plating layer protects the contact surface from oxidation and corrosion.

• For example, gold-plated contacts can remain free from corrosion for more than 1 000 hours in salt-spray environments, which is reported to be more than ten times the life of unplated contacts.

• Silver plating can also provide sufficient resistance to sulfidation and oxidation when an appropriate under-plate is used.

As a result, there are industrial reports that the service life of contacts can be extended by a factor of 2 to 5.

Maintaining Low Contact Resistance

Noble-metal plating layers make it possible to maintain very low initial contact resistance.

• For example, studies have reported that gold plating with a thickness of 1 µm shows less than 20% increase in contact resistance even after aging.

• Low contact resistance is crucial to minimizing signal loss in high-speed data transmission (such as USB 3.0) and in precision measurement circuits.

Durability and Wear Resistance

Plating layers are designed to withstand wear caused by friction, insertion and removal, and vibration.

• Hard gold plating and silver plating can reduce the coefficient of friction, enabling designs that endure more than 1 000 insertion/removal cycles.

• Nickel under-plates can control surface roughness and have been reported to reduce wear rates by up to 50%.

• Designs that remain free of cracks under thermal shock conditions (for example, −55 °C to 125 °C) are becoming increasingly important.

Other Additional Benefits

• EMI (electromagnetic interference) shielding enhancement: Plating improves surface conductivity, reducing the impact of electromagnetic noise on the contact region and surrounding circuits.

• Cost efficiency: High performance can be achieved with relatively thin layers, lowering material and processing costs.

• Regulatory compliance: Lead-free plating and materials that comply with environmental regulations such as RoHS can be selected.

• Reliability improvement: Taken together, these advantages have led to more frequent designs in which the mean time between failures (MTBF) of contacts exceeds 10⁶ hours.

Need for Plating in Railway Applications

Railway systems such as high-speed trains, subways, and electric train control circuits often operate 24 hours a day, and contacts play a critical role throughout signal, power, and control circuits. The key characteristics of such environments are:

• Environmental risks: Compared to non-railway environments, corrosion progresses more rapidly due to high humidity (often above 90%), salt (coastal lines), dust, and vibration.

  • For unplated contacts, contact resistance may increase above 10 mΩ, potentially causing signal delays or short circuits.

• Mechanical and vibration stress: During operation, the system is subjected to vibration, shock, frequent mating/unmating, and fretting wear.

  • There are reports that silver plating at about 4 µm can reduce the coefficient of friction to around 0.2 and endure more than 10⁵ cycles.

• Electrical stability requirements: For high-current contacts (for example, 1 000 A or more), silver plating is advantageous in preventing overheating by improving thermal conductivity.

• Economic and safety requirements: Applying plating to contacts significantly extends component life and reduces maintenance costs.

  • For example, some reports indicate that extending life from 15 years to 25 years can reduce maintenance costs by about 40%.

In the railway sector, standards such as EN 50155 may require plated contacts to pass salt-spray tests of 500 hours or longer.

Need for Plating in Defense Applications

Defense systems operate in extremely harsh environments in applications such as missile guidance, aircraft radar, tank communication, and submarine power/signal systems. Failure of a single contact can lead directly to mission failure for the entire system. Key considerations include:

• Resistance to extreme environments: In desert environments (high temperatures of 70 °C or higher), marine environments (salt), and explosive shock conditions, there have been cases where unplated contacts corroded within 100 hours and contact resistance rose sharply.

  • Gold plating in the range of 3 to 5 µm has been reported to achieve survival rates of over 99% in 1 000-hour salt-spray tests.

  • As a cadmium replacement, zinc-nickel plating (around 5 µm) has been shown to reduce corrosion rates by roughly 80%.

• EMI/RFI shielding and electronic warfare response: In environments where high-frequency signals above 1 GHz are used, the surface conductivity of contacts contributes to shielding performance and suppression of leakage.

  • Gold and silver plating have been reported to improve shielding performance by more than 40 dB.

• Mechanical and thermal durability: Under severe conditions such as 50 g mechanical shock and thermal cycling between −55 and 125 °C, hard gold plating has been shown to survive more than 500 cycles without cracking.

  • This has been linked to data showing that the MTBF of weapon systems (for example, F-35 fighter connectors) can be improved to the order of 10⁷ hours.

• Strategic and economic aspects: There are reports that plating can extend component replacement intervals from 5 years to 10 years and cut costs by about 50%.
  In addition, plated contacts are recognized as key elements for system survivability.

Nano and Hybrid Plating Research

Recently, hybrid coatings such as gold-silver composite plating (Au–Ag) have been developed to balance hardness and conductivity.

There are also studies on designing plating layers at the nanoscale to finely control contact behavior, including wear and electrical performance.

Design and Quality Control Considerations

• The thickness of the plating layer does not simply follow the rule “thicker is better”; instead, there is an optimum value depending on contact load, number of cycles, and environmental characteristics.

• Micro-contact spots (a-spots), surface roughness, and geometry affect contact resistance, so designers must consider how the plating layer changes these structures during the design phase.

• Testing and validation under various environmental conditions (temperature changes, vibration, humidity, salt, etc.) are essential.
  Contact designers and quality engineers should evaluate, based on data, how the plating layer behaves in actual operating environments.

Conclusion

The surface plating of electrical contacts is not just a “protective film”; it is a core post-process that integrally improves electrical, mechanical, and environmental performance.

By properly designing the thickness of materials such as gold and silver and applying them in appropriate structures, the service life, reliability, and performance of contacts can be significantly enhanced.

In fields such as railway and defense, where extreme environments and very high reliability are required, plating has become an essential post-process rather than an optional one.