Edge Plating: Castellations and Sidewall Connections

Edge plating is a conductive layer deposited on the sidewall of a board, and it serves three purposes: connecting a ground plane to the chassis, forming the half holes of a castellated module, and providing a solderable edge for a connector. Each purpose sets a different requirement on the same process. The three requirements pull in different directions, and the specification has to say which one governs.

The difficulty is that plating a sidewall is not like plating a hole. There is no enclosed geometry to concentrate the current, no uniform field, and no easy way to measure the result without cutting the board. The result is a coating whose quality varies along the edge and around the panel.

What Edge Plating Is For

A ground connection through the edge gives a short return path between the plane and the enclosure, and it is usually specified as a band along part of the perimeter. The plating has to be thick enough to carry the current and continuous enough to make contact. A band interrupted by a routed slot breaks the connection, so continuity is a design requirement.

A castellated module uses half holes on the edge so the module can be soldered down like a component. The plating there has to be solderable, and it has to survive the reflow that attaches the module to its carrier. The half hole also has to accept solder without wicking it away from the joint.

Castellations and Half Holes

A castellation is a plated half hole, produced by drilling a full hole at the panel edge and routing through its centre after plating. The remaining half carries the plating deposited in the hole, and its quality depends on what happened to the full hole first. The plating in the hole is measured before the cut, and the cut is expected to reduce its quality rather than improve it.

The hole is therefore plated to a barrel specification and then cut in half, which leaves the plating with an exposed edge. How cleanly that edge is cut decides whether the plating stays attached to the laminate and whether it can be soldered. A cut that leaves a ragged edge produces a joint that looks sound and fails in thermal cycling.

Preparing the Edge Before Plating

Plating adheres to a surface that has been cleaned and, in some processes, roughened in a controlled way. The edge of a routed board carries a smear of resin that has to be removed before the plating step. A resin smear is the residue of the routing operation, and it insulates the copper from the plating.

The preparation is mechanical or chemical and has to reach the full depth of the edge. A preparation that only affects the surface leaves a layer that fails later, and the failure appears as a peel rather than as a missing coating. The adhesion side is described in the notes on hole wall roughness and plating adhesion. The preparation is verified on a sample by looking at a cross section rather than at the surface.

Plating Thickness on a Sidewall

Thickness on a flat sidewall is set by the same current density that controls the rest of the plating, and it is measured on a coupon because the production board cannot be cut. Current density is described in the notes on current density control. A band at the edge of a panel also shares its current with the rest of the panel, which changes the local rate.

The distribution along the edge is less uniform than in a hole, because the edge is an open geometry and the field is not confined. Thickness at the ends of a plated band is usually different from the thickness in the middle. A minimum thickness is therefore specified at the weakest point rather than as an average.

Routing After Plating

Routing after plating is what forms the castellation, and the tool and the feed rate decide the quality of the cut. A worn tool tears the plating instead of cutting it, and the tear appears as a lifted edge after soldering. The cut is made after plating so that the coating exists on the surfaces the cut passes through.

The direction of the cut matters too. A cut that pushes plating away from the laminate leaves a burr, while a cut that pulls it can lift the plating over a longer length. Tool life is tracked as for any routing operation, and the discipline is described in the notes on drill regrind and bit life.

Inspection and Measurement

Inspection of a plated edge is visual and dimensional. A plated band is checked for continuity and coverage, and a castellation is checked for the plating remaining on the half hole and for the condition of the cut.

Thickness is measured on a coupon processed with the panel and on a section cut from a sample. A coupon gives the thickness and not the distribution, while a section gives both on one board. The section is authoritative and it is destructive, so it is taken on a sample from each lot.

Common Defects

The recurring defects are a missing or thin coating at one end of a band, a lifted edge after routing, a smear of resin beneath the plating, and voids in the coating over a rough edge. All four are visible on a section and only some are visible on the surface.

The causes are distributed across the process. A thin end points to current distribution, a lifted edge to the routing tool, a smear to the preparation step, and a void to the surface condition of the laminate.

Process Control

Control starts with the drawing. The area to be plated, the minimum thickness and the tolerance on the castellation are stated together, because they interact and a change to one affects the others. A drawing that specifies a plated area without a castellation tolerance leaves the fabricator to guess.

The panel layout also matters, because a plated band at the edge of a panel is affected by the current distribution across the whole panel. Panels with edge plating are usually run at a current chosen for the edge rather than for the average.

Records and Qualification

Qualification includes a section, a thickness measurement and a solderability check on the plated edge. The solderability check shows whether the edge will accept solder during assembly, and it is what catches a contaminated surface before production. The check is run on the edge because the coating there is thinner and more likely to be contaminated.

The records hold the preparation method, the plating parameters, the routing tool and the section result. With those fields a lifted edge can be attributed to the tool or to the plating rather than being argued about. The record is what allows a field failure to be compared with a section taken at the time of manufacture.

FAQ

Can a plated edge be repaired? A local repair is possible on a band and not on a castellation, because the geometry of a half hole cannot be recreated by hand.

Is the plating thickness the same all along an edge? No. The ends of a plated band are usually thinner than the middle, because the current distribution is not uniform.

Why does a castellation lift after soldering? The cut edge has a mechanical weakness that the reflow cycle exposes. A cleanly routed edge and a sound preparation reduce the risk.