PCB Gold Plating: Process, Hard Gold, and Soft Gold

PCB gold electroplating uses an external electrical current to deposit gold onto selected conductive surfaces. A nickel underlayer is commonly applied between the copper and gold.

The finish is used where the connection requires specific contact, wear, or bonding characteristics. Hard gold is commonly selected for connector fingers, while high-purity soft gold may be specified for qualified wire-bonding applications.

Gold plating does not automatically improve every aspect of PCB performance. Its value depends on the deposit properties and the function of the finished surface.

What Is PCB Gold Electroplating?

During electroplating, the features to be plated are connected to the negative terminal of a power supply and act as the cathode. The plating bath contains dissolved gold species, which are reduced at the cathode to form a metallic deposit.

Unlike immersion gold, this process requires an external electrical connection to the plated features. PCB layouts may therefore need temporary plating connections or another fabrication-approved method of supplying current.

A common layer structure is:

Copper → Nickel underplate → Electroplated gold

Nickel and gold serve different purposes. Nickel is an underlayer, while gold forms the outer contact or bonding surface. Tin is a separate finishing material, not a standard constituent of PCB gold plating.

Hard Gold vs. Soft Gold

Hard Gold for Electrical Contacts

Hard gold is engineered for greater hardness and wear resistance than high-purity soft gold. Depending on the chemistry, small amounts of alloying elements such as cobalt or nickel may be incorporated into the deposit.

Typical PCB applications include edge-connector fingers and selected contact areas subject to repeated mating.

The objective is to maintain suitable contact performance over the required service life. Performance depends on coating thickness, porosity, the mating contact, contact force, and environmental exposure—not simply the presence of gold.

Umicore’s gold-nickel electrolyte documentation, for example, describes a hard, abrasion-resistant deposit with low contact resistance. These are properties of the specified coating system rather than a guarantee for every gold-plated surface. Umicore: AURUNA 8400 Gold-Nickel Electrolyte

Soft Gold for Qualified Bonding Applications

Soft gold generally refers to a high-purity, relatively low-hardness gold deposit. It can be selected where surface purity and deformability are important, including certain wire-bonding processes.

The deposit must be compatible with the wire material, bonding method, and underlying metallization. A surface that looks gold is not automatically suitable for wire bonding.

Umicore’s fine-gold technical documentation identifies high-purity, low-hardness coatings for PCB bonding and soldering applications. Actual performance still depends on the specified process and assembly conditions. Umicore: Fine-Gold Electrolyte Technical Information

Main Steps in the PCB Gold Plating Process

1. Define the Plating Areas and Electrical Connections

The fabrication data should identify which areas require gold plating and the intended finish.

For selective plating, masking protects areas that should not receive the deposit. The fabricator also plans electrical access to the selected features and how temporary connections will be removed or isolated afterward.

These requirements should be reviewed before the PCB layout is finalized.

2. Clean and Prepare the Surface

Cleaning removes oils, residues, and other contamination that can interfere with adhesion. Additional surface preparation, such as controlled microetching and activation, depends on the substrate and plating chemistry.

The aim is to create a clean, suitable surface—not to apply a generic protective film before plating. An incompatible film could prevent proper deposition.

3. Deposit the Nickel Underlayer

Nickel is commonly plated onto the prepared copper before gold deposition. It helps limit copper diffusion toward the gold surface and contributes to the properties of the coating system.

Underlayer quality matters. Gold deposited over a rough, contaminated, or poorly adhering surface cannot compensate for those defects.

4. Electroplate the Gold

The board enters the gold bath, and controlled current drives deposition onto the connected surfaces.

Important process variables include current density, plating time, bath composition, temperature, agitation, and electrical contact quality.

Thickness uniformity is not automatic. Feature geometry and current distribution can produce local variation, so tooling, masking, and process settings must be developed together.

5. Rinse, Finish, and Inspect

After plating, the board is rinsed and dried. Mask removal, removal of temporary plating connections, and edge finishing are completed as required by the fabrication sequence.

Inspection may include coating thickness, coverage, adhesion, surface condition, and dimensional checks. Contact or bonding tests should be included when required by the product specification.

A separate anti-corrosion coating is not a universal post-treatment requirement. Any additional treatment must be compatible with the intended electrical or bonding function.

How Is Electroplated Gold Different from ENIG?

ENIG stands for electroless nickel immersion gold. Its nickel and thin gold layers are deposited chemically without an external plating current.

ENIG and electroplated gold should not be treated as interchangeable simply because both produce gold-colored surfaces.

Can Gold-Plated PCB Pads Be Soldered?

Some gold-plated surfaces can be soldered, but compatibility depends on the deposit composition, thickness, solder alloy, and solder volume.

Gold dissolves into molten solder during assembly. Excessive gold in certain solder joints can contribute to brittle intermetallic formation and reduced joint reliability. Texas Instruments discusses this risk in its guidance for gold-finished hermetic package terminations. The document’s package-specific limits should not be applied indiscriminately to every PCB joint. Texas Instruments: Termination Finishes and Gold Embrittlement

Hard gold optimized for contact wear should not automatically be specified for solder pads. Where soldering is required, qualify the complete finish-and-solder combination.

Benefits and Limitations of PCB Gold Plating

Gold’s resistance to oxidation helps maintain useful electrical contact surfaces. Appropriate hard-gold systems can provide wear resistance, while suitable soft-gold systems can support bonding.

These benefits have practical limits. Porosity, damaged plating, or wear can expose underlying materials. Gold plating also adds material cost, process control requirements, and electrical-access constraints.

Nor does coating a contact with gold automatically improve high-frequency signal performance or increase the current capacity of the complete circuit. Those properties depend on the broader electrical and mechanical design.

What Should You Specify Before Ordering?

Define the function of each plated area before selecting the finish. Connector fingers, solder pads, and wire-bonding pads may require different solutions.

The fabrication drawing should specify the required gold type, underlayer, thickness requirements, plated areas, and applicable acceptance criteria. Include relevant contact-life, bonding, or soldering requirements rather than relying on the instruction “gold plate.”

For repeated-mating contacts, review the connector manufacturer’s requirements. For bonding and soldering, qualify the finish with the actual assembly process.

A clearly specified plating system gives the manufacturer a practical target and helps prevent a visually acceptable gold surface from being used for the wrong function.