Electroplating Process: How Gold Is Deposited

Electroplating looks simple from the outside — a part goes into a tank and comes out gold — but the underlying process is a controlled electrochemical reaction with a dozen variables that determine whether the deposit performs or fails. This guide explains the science of the electroplating process from first principles. If you want to see how our own six-step production workflow applies it, see our plating process.

What is the electroplating process?

Electroplating is the use of direct electrical current to reduce dissolved metal ions onto a conductive surface, building a solid metal layer atom by atom. The part being plated is the cathode, a counter-electrode is the anode, and the electrolyte solution supplies the metal ions.

When current flows, gold ions at the part's surface gain electrons and convert to metallic gold, locking onto the crystal lattice of the surface beneath them. Because deposition happens ion by ion, the layer is dense, continuous, and metallurgically bonded — fundamentally different from spraying or dipping.

What happens inside the plating bath?

Inside the bath, three things happen simultaneously: gold ions migrate to the cathode, electrons reduce them to metal, and the electrolyte replenishes ions at the surface. The balance between these rates controls deposit quality.

Commercial gold baths dissolve gold as a stable complex — most commonly potassium gold cyanide, or a sulfite complex in cyanide-free systems. The complex keeps gold in solution at a controlled concentration, typically measured in grams per liter and monitored by lab analysis. Key operating variables include:

• Current density, expressed in amps per square foot, which sets deposition rate and grain size. Too high and the deposit burns; too low and plating takes impractically long.
• Temperature, usually held between 90 and 140 degrees Fahrenheit depending on chemistry.
• pH, which affects deposit purity and bath stability.
• Agitation, which refreshes ion supply at the part surface and prevents streaking.

Faraday's law governs the math: the mass of gold deposited is proportional to the charge passed. A technician who knows the part's surface area can calculate the amp-minutes needed to reach a target thickness within a few microinches.

Why does surface preparation come before any plating?

No electroplated deposit can bond to a contaminated or oxidized surface, so every plating sequence begins with cleaning and activation. Oils, oxides, buffing compound, and fingerprints all prevent atomic-level bonding and cause blistering or peeling later.

A typical preparation train includes soak cleaning, electrocleaning (where the part itself is made an electrode to scrub the surface with gas evolution), rinsing, and an acid activation dip that removes the final oxide film seconds before plating. Preparation is so decisive to outcome that we cover it separately in surface preparation for plating — most plating failures trace back to this stage, not the gold bath.

What is a strike layer and why is it used?

A strike is a very thin, high-adhesion deposit applied at high current density and low metal concentration to establish bonding on difficult substrates. Strikes are the bridge between activation and the main deposit.

A gold strike, for example, is often applied before the main gold layer to guarantee adhesion and to protect the expensive main bath from drag-in contamination. Nickel strikes perform the same role on stainless steel, whose passive chromium-oxide layer resists direct plating. On copper alloys, a nickel underplate also acts as a diffusion barrier that keeps substrate atoms from migrating into the gold over time.

How is thickness controlled during plating?

Thickness is controlled by managing current, time, and surface area, then verified by X-ray fluorescence measurement. Because Faraday's law ties deposited mass to charge passed, a rectifier's amp-minute counter is effectively a thickness dial.

In practice, geometry complicates the picture. Current concentrates on edges and points (high current density areas) and starves recesses (low current density areas), so complex parts plate unevenly. Platers manage this with rack design, conforming anodes, shields, and robbers — sacrificial features that draw current away from edges. Specifications such as ASTM B488 define where on a part the minimum thickness must be met, called the significant surface. For a full treatment of thickness classes and measurement, see gold plating thickness.

What does the full process sequence look like?

A production gold plating sequence typically runs eight to twelve stations from raw part to finished deposit. A representative flow for a copper-alloy part looks like this:

1. Inspection and masking of areas that must stay bare
2. Soak clean to remove oils
3. Electroclean to strip remaining films
4. Rinse, then acid activate
5. Nickel underplate, typically 50 to 200 microinches
6. Rinse and activate again
7. Gold strike for adhesion
8. Main gold deposit to specified thickness — for example, 24K gold plating at 30 to 100 microinches
9. Recovery rinse (reclaiming gold drag-out), final rinse, and dry
10. XRF thickness verification and final inspection

Each rinse matters: cross-contamination between tanks is a leading cause of bath failure and rejected parts.

How is electroplating different from electroless plating?

Electroplating requires external current; electroless plating deposits metal through a chemical reducing agent in the solution itself, with no rectifier. Electroless deposits grow at the same rate on every wetted surface, which gives them perfectly uniform thickness even inside bores and blind holes.

Electroless nickel is common as an underplate for exactly that reason. Immersion gold — the thin gold in the ENIG finish used on circuit boards — is a displacement reaction limited to roughly 2 to 8 microinches. For deposits thicker than that, electrolytic gold remains the workhorse because it builds thickness quickly and economically.

Ready to put the process to work?

Understanding the process helps you write a better plating requirement — but you do not need to master bath chemistry to get parts plated. Send us photos and dimensions of your components and our Vista, CA lab will spec the preparation, underplate, and gold thickness for you, with fast turnaround and capacity for high-volume runs. Request your free quote online or call (760) 458-3299 to talk through your job with a plater. Minimum order $500; pricing from $100 per square inch.