Edge Plating and Gold Fingers: Process and Design

Edge plating places metal on the side wall of a printed circuit board, either as a continuous ground connection or as a gold contact surface for an edge connector. Both are used to remove a connector, to improve shielding or to make a board plug into a slot, and both require the plating to wrap around the cut edge rather than stopping at the top surface.

What Edge Plating Is Used For

The common application is the gold finger: a row of contacts on the board edge that plugs into a socket. The same construction appears as a castellated tab, as a shielding connection between two ground planes, and as a low impedance return for a module that must be grounded to a chassis along its side.

A second application is the wrapped edge, where copper is continued around the board profile to join the top and bottom layers. This is used where an antenna feed or a shielding wall has to be continuous, and it requires the copper to be present on the side wall after profiling.

Gold Fingers and Contact Requirements

Contacts that will be inserted repeatedly need a hard, wear resistant surface, so a nickel gold plating sequence is standard. The nickel provides the mechanical support and the barrier against diffusion, while the gold provides a low contact resistance that does not oxidise.

Thickness and hardness are specified together with a nickel thickness range and a gold thickness range, and the fingers are usually separated by a bevel or a chamfer so that the connector does not damage them during insertion.

Why Wrapping the Edge Is Difficult

Plating follows the surface it can reach. A flat copper area is easy to plate, but the vertical side wall of a board is only exposed if the copper there is uncovered and if the plating solution can physically contact it.

The problem is that the side wall is created by profiling, which happens late in the process, after the surface has already been patterned and covered. Where the edge must be plated through a cut rather than on the original surface, the process has to arrange for the wall to be exposed to the chemistry before the cut is made, or accept that the plating will not reach it.

Trenching the Copper Before Plating

One approach is an etching step that cuts a narrow trench in the copper along the intended edge, so that the side wall that will eventually be plated is exposed while the board is still flat and the plating bath can reach it. The trench follows the contour of the area that has to be wrapped.

Plating then covers the top surface of the copper area and the exposed side wall in the same operation, so the gold wraps around the edge rather than stopping short of it. The trench is removed later when the board is profiled, leaving the plated wall on the finished edge.

Dry Film Openings and Chemistry Access

The plating resist has to be opened over the area to be plated and slightly beyond it, so that the solution can flow into the trench. An opening that is too small restricts the flow and produces a thin or missing deposit on the wall.

An opening a few thousandths of an inch larger than the plated area on each side is typical, since it allows the chemistry to reach the wall without exposing features that should remain covered. The pad design and the resist artwork therefore have to be planned together with the edge requirement.

Isolated Pads and Plating Bars

Electroplating requires a continuous electrical path, so every feature to be plated has to be connected to the plating supply. Pads that are isolated after the board is complete need a temporary connection, known as a plating bar, which is removed once plating is finished.

Where the entire edge is one continuous copper area, that connection already exists and the process is simpler. The design of the plating bar, and the point at which it is cut, are decisions that belong on the fabrication drawing rather than being left to the fabricator.

Design Rules for Edge Features

Keep the copper set back from the outline where plating is not required, and keep the plating area clear of the board edge where it is. The width of the plated band, the chamfer angle on gold fingers and the clearance to any adjacent copper all need to be stated.

The outline itself is part of the specification, as described in general outline and mounting practice. A profile that passes through the plated area has to be cut cleanly, and the tolerance on that cut determines whether the plated band is the width the design intended.

Inspection and Acceptance

Inspection checks the presence and continuity of the plating on the wall, the thickness of the nickel and gold layers, and the appearance of the contact surface. Cross sectioning a coupon is the only way to confirm the wall thickness reliably.

Continuity testing verifies that the edge plating connects the layers it is supposed to join. Where the plating is a shielding connection rather than a contact, the requirement is electrical rather than cosmetic, but the acceptance criteria should still be written down.

Applications and Alternatives

Edge plating is chosen where a connector would add height or cost, where a shield must be closed along the board side, or where the product must slide into a chassis rail. It is not a substitute for a connector where many insertions are expected.

Alternatives include castellated holes, which give a solderable joint rather than a contact surface, and a separate metal frame bonded to the board. Each has a different assembly consequence, so the decision should be made with the mechanical design rather than after it.

Surface Finish and Cleaning

The finish applied after plating protects the contact surface and prevents oxidation before assembly. Gold on a finger is normally left bare, while the rest of the board takes a finish suited to soldering, so the two requirements are separated by masking rather than by choosing one finish for everything.

Cleaning matters for the same reason. Residue trapped in a trench or against the side wall is difficult to remove once the board is profiled, so the cleaning step has to be effective while the board is still in its panel and the surfaces are accessible.

Working With the Fabricator

Edge wrapping is not a standard capability in every shop, and the sequence varies between suppliers. Sending the requirement early, with a sketch of the edge and the extent of the plating, allows the fabricator to confirm the method and to price the work realistically.

Ask specifically how the wall is protected before plating and how the profile is cut afterwards. The answers describe the real sequence, and they usually explain any difference in price between suppliers quoting the same specification.

Common Defects and Their Causes

Incomplete plating on the wall comes from restricted chemistry access, which is a resist opening problem. Rough or torn plating comes from profile cutting after the metal has been deposited. Poor adhesion is usually a preparation issue at the plating stage.

Each of those has a process remedy rather than a design remedy, which is why the questions to ask are about sequence rather than about machinery. A fabricator that can describe the plating defects it avoids and how, is describing a controlled process.

FAQ

What is the difference between a gold finger and ordinary gold plating? A gold finger is designed to be inserted into a connector and must resist wear, so it uses a thicker hard gold over nickel, while ordinary gold plating is intended for soldering or bonding.

Can the edge be plated after the board is profiled? Only with difficulty, because the plating has to reach a vertical surface that was created by cutting. The practical approach is to arrange for the wall to be exposed to the chemistry before profiling.

Is edge plating compatible with castellated holes? They can appear on the same board, but they are produced by different steps and each has its own tolerance, so the drawing should describe both explicitly.