PCB Electroplating: Process, Chemistries and Thickness Control

Metal Deposited by Current

Electroplating deposits a metal layer onto a circuit board by passing current through a solution containing that metal. It is the process that connects the layers of a multilayer board, protects copper from corrosion, provides a wear resistant surface on edge connectors and, in the case of gold over nickel over copper, defines how many times a connector can be mated before it fails.

It is also a process whose results are invisible from the outside, which is why understanding what it does and how it is controlled matters more than understanding how it looks.

Electroplating Against Electroless Deposition

The distinction is fundamental. Electroplating requires an external current, which means the board must be connected to a rectifier and the deposit grows wherever current flows. Electroless deposition relies on a chemical reaction and needs no current, which allows it to coat features that current cannot easily reach. In practice both are used together: a thin electroless copper layer makes the hole walls conductive, and electrolytic plating then builds the copper to the required thickness.

Electroplating is also used for more than copper. Nickel provides a diffusion barrier and mechanical hardness, gold provides corrosion resistance and low contact resistance, and tin provides solderability.

The Process

• Surface cleaning. Acid or alkaline cleaning to remove oils, oxides and contamination. Every subsequent step depends on this one.
• Activation. Chemical treatment to make the surface receptive so the metal adheres rather than sitting on top of a film.
• Immersion in the plating bath. The board is placed in a solution containing the metal ions, typically copper sulphate for copper plating.
• Current driven deposition. Direct current causes metal ions to deposit onto the board surface and into the hole walls.
• Rinsing and drying. Removing residual chemistry so that it does not contaminate later steps.
• Inspection. Checking thickness, adhesion and uniformity against the applicable IPC requirements.

Typical thicknesses are 20 to 50 micrometres of copper for through holes and surface plating, 3 to 6 micrometres of nickel as a diffusion barrier, and 0.03 to 0.1 micrometres of gold for corrosion protection and solderability. Those ratios are not arbitrary: the nickel layer exists to stop copper diffusing into the gold, and a nickel layer that is too thin fails at elevated temperature.

Chemistry and Equipment

Four chemical groups dominate. Copper sulphate solution for copper plating. Nickel sulphate and nickel chloride for the barrier layer. Gold solutions, historically cyanide based, for gold deposition. And organic additives that control brightness, hardness and deposition rate.

The equipment is equally standard: a plating tank holding the electrolyte and the boards, a rectifier delivering controlled direct current, a filtration system removing contamination from the bath, and agitation to keep the ion concentration uniform around the board. The two variables that decide the result are current density distribution across the panel and bath chemistry control, and modern lines address both with automation and real time monitoring rather than operator judgement.

What Plating Delivers

• Higher conductivity. Plated copper lowers resistance on the traces and the vias, which matters both for power delivery and for high speed signal integrity.
• Corrosion resistance. Nickel and gold layers prevent oxidation of the underlying copper, which is what gives a gold finger connector its long service life.
• Better solderability. A uniform plated surface wets more reliably than a bare or uneven one.
• Wear resistance and durability. Critical for edge connectors and any interface that is mated repeatedly.
• Thermal performance. Plated copper spreads heat as well as carrying current, which is why power boards rely on it. The techniques involved are described under heavy copper PCB manufacture.

Where It Matters Most

Automotive electronics including control systems, infotainment and driver assistance. Aerospace and defence, where the board has to survive temperature, vibration and humidity simultaneously. Consumer electronics, where gold plated boards appear in phones, tablets and notebooks. Medical devices, including implantable and diagnostic equipment where reliability is the whole specification. And industrial equipment including high current power supplies and robot control systems, which depend on plated copper for durability. The quality requirements common to all of them are described under quality management.

Challenges and Limitations

• Cost. Precious metal plating is expensive, and the cost scales with the plated area rather than with the board count.
• Environmental burden. Plating produces hazardous effluent that requires treatment, which is a real cost and a regulatory obligation rather than a formality.
• Process control difficulty. Uneven thickness causes signal loss on high frequency paths and poor solderability on assembly pads, and it is not visible without measurement.
• Economics of small batches. Plating lines are most efficient at medium to large volume, because setup and bath maintenance are shared across the batch.

Those four factors are why plating is a capability rather than a commodity. Two suppliers can quote the same board and produce measurably different results, with the difference traceable to bath control and current distribution.

Plating Compared With Other Finishes

• Plating against electroless deposition. Electroplating needs current but deposits faster and thicker; electroless coating reaches complex geometry without current. Most boards use both.
• Plating against ENIG. ENIG gives a very flat surface suited to fine pitch surface mount assembly. Electroplated hard gold is more durable and is the choice for edge connectors and repeated mating.
• Plating against immersion tin and immersion silver. Immersion finishes are cheaper but offer less corrosion resistance and much less wear resistance. Electroplated finishes provide better long term reliability where the interface is mechanically stressed.

The selection follows the requirement: assembly surface, connector durability or corrosion resistance. The fabrication context sits under PCB manufacturing.

Cost

Plating cost depends on three variables: the metal and thickness, the plated area, and the order quantity. Copper plating is inexpensive per unit area; gold is not, which is why a board with a long edge connector costs materially more than the same outline with a short one. Where a design requires thick copper for current carrying, the cost rises again because plating time and etch difficulty both increase. Reviewing the plated area and the specified thickness against the actual requirement is the most effective way to control this line item, and the overall pricing structure is described in our notes on custom PCB pricing.

Where the Technology Is Going

Four directions are visible. Cyanide free gold chemistries, driven by environmental and safety regulation. Automated monitoring with real time thickness feedback, which improves consistency and reduces the process window that operators must manage manually. Multi-metal composite plating, where copper, nickel and gold layers are engineered together for high end boards rather than deposited as independent steps. And increasingly precise deposition for miniaturised products, where the feature sizes in wearable and connected devices demand plating control that was previously reserved for high density interconnect work, described under HDI PCB fabrication.

FAQ

What copper thickness is typical? Between 20 and 50 micrometres for through holes and surface plating, depending on the current and thermal requirement.

Is electroplating suitable for small batch production? It is more efficient at medium to large volume because of setup and bath maintenance, but it is still applied to high performance prototypes where the specification demands it.

Does plating extend board life? Yes. Plating resists oxidation, wear and mechanical stress, which is why it is used on every interface that has to survive repeated use.

Why is gold used over nickel rather than directly over copper? Because nickel acts as a diffusion barrier. Without it, copper migrates into the gold layer and the contact degrades, particularly at elevated temperature.

What limits plating quality? Current density distribution across the panel and bath chemistry control. Both are invisible on the finished board, which is why process control matters more than final inspection.

Summary

PCB electroplating uses current to deposit copper, nickel, gold or tin onto a board, connecting layers, protecting copper from corrosion, improving solderability and providing a wear resistant surface where connectors mate repeatedly. Copper is plated to 20 to 50 micrometres, nickel to 3 to 6 as a diffusion barrier, and gold to 0.03 to 0.1 for corrosion resistance and stable contact resistance. The process requires clean surfaces, correct activation, controlled bath chemistry and even current distribution, all of which are invisible in the finished product. Cost scales with the metal, the plated area and the quantity, and the environmental obligations around effluent treatment are a real part of the production budget rather than an administrative detail.