Castellated Holes: Edge Plating for Modules and SMT

A castellated hole is the feature that lets a small circuit board be soldered onto a larger one as if it were a component. It appears on wireless modules, sensor boards, power converters and LED drivers, and it is the reason those parts can be dropped into a design without a connector. This article covers what the feature is, how it is produced, the design rules that decide whether it will solder reliably, and the yield limits that drive its cost.

What a Castellated Hole Is

A castellated hole is a plated through hole that is cut in half by the board edge, leaving a plated semicircular channel on the edge of the board. The result is a solderable feature on the edge rather than on the face of the board, which allows a small module to be soldered directly onto a carrier board as though it were a component.

The technique is standard on wireless modules, sensor modules, power converters and any subassembly that needs to be mounted as a device rather than connected by a cable or a connector. The castellations carry the signals, the ground and the power between the module and the carrier, and because they are soldered rather than inserted, the connection is compact and mechanically robust.

It is a manufacturing feature rather than a design abstraction: the hole is drilled and plated as a normal through hole, and then the board outline is routed through the centre of the hole. The quality of the result depends on controlling that cut precisely relative to the plated hole.

Why It Is Used

• Space: the connection occupies the edge of the module rather than any of its face area, which matters when the module is small.
• Cost: it removes the connector from both the module and the carrier board, which removes two parts, two assembly operations and two sources of failure.
• Low profile: a soldered castellation adds almost nothing to the height of the assembly, unlike a header or a mating connector pair.
• Mechanical robustness: the solder joint wraps the plated channel and the adjacent pad on the carrier, which distributes the load better than a single surface mount pad.
• Inspection: the joint is visible from the side, unlike a ball grid array connection.

The trade is that the module becomes permanently attached in most cases, which is acceptable for a device that is not intended to be serviced separately and is a real limitation for anything that is.

How It Is Made

Manufacture follows the standard fabrication sequence with three extra controls. The castellation holes are drilled to a finished size larger than an ordinary via, then plated like any other through hole so the barrel and the land on both faces are copper. The board is then routed along the edge that carries the holes, and the cut runs through the centre of the row so each hole is halved and each half keeps its plated wall, its land on the top face, its land on the bottom face and the copper that joins them.

Two tolerances rule the process: how accurately the router follows the row, and how well the plating covers the barrel before it is cut. Because the holes sit on the panel edge, plating current distribution is worse there than in the middle of the board, so a thicker wall costs more.

Design Rules

Rule one is the pitch. Castellations are normally placed on a 1.0 mm or 1.27 mm pitch. The supplier minimum depends on the finished hole size, the board thickness and the routing tolerance, and thinner boards allow a tighter pitch because a thin stack routes more cleanly.

Rule two is the annular ring. The land around the hole has to survive the cut, so the pad should extend at least 0.15 mm beyond the hole radius on every side. Rule three is the keepout: no traces, planes or vias within roughly 0.3 mm of the cut line, and no copper pour touching the castellation row, or the router will clip it and the plating will tear.

Rule four is the board edge. Castellations are normally placed only on straight edges, because routing a curved edge through a hole row is far less controllable, and they should be kept away from corners and from panel tooling features. All four numbers belong on the fabrication drawing, together with the finished hole size, the plating thickness and the routing tolerance.

Assembly onto the Carrier Board

Castellated modules are placed and reflowed like any other surface mount part, which is the main reason they are popular: the carrier board sees a row of small plated pads along the module outline.

Three details decide whether the joint is reliable. The module needs support beyond the castellations, usually a large central ground pad. The paste volume has to be enough to wet the vertical wall of the castellation, which often means a slightly thicker stencil or an upstep. And the pad on the carrier should extend about 0.2 mm to 0.3 mm past the castellation wall so a visible fillet can form.

Inspection is the awkward part. The joints are vertical and mostly hidden, so lines rely on side view, X-ray at the interface or electrical test through the module, and qualify the process with a first article cross section rather than inspecting every unit. A gap at the base of the castellation is the classic failure, which is why paste volume and pad extension matter so much.

Where the Technology Is Used

The most familiar example is the wireless module. Wi-Fi, Bluetooth, LoRa and cellular modules are often supplied as small boards with castellations on two or four edges and a shield can on top, so a product designer can add connectivity without designing an RF section.

Sensor modules follow the same logic, because the sensing element needs a specific stackup, cavity or material that the customer would rather buy than design. Power conversion is the third large category: a buck, boost or isolated converter sold as a castellated module lets a system designer add a rail without owning the thermal design, and the castellations carry several amps without the contact resistance of a socket. LED modules are a smaller but growing case, where a driver board and an emitter board have to sit flush against a heatsink.

Castellation, Connector or Edge Plating

The three options solve the same interface problem in different ways, and the choice comes down to size, current and how many times the joint will be made.

• Castellated module: soldered once, lowest profile, no connector body, best in volume when the joint never has to be separated.
• Board to board connector: separable and easier to rework, but adds height, cost and a second component to place.
• Edge plating with a card edge: excellent for many mating cycles, but needs a controlled edge profile and a mating receptacle.

Edge plating and castellation are often confused because both put metal on the board edge. A card edge is a continuous plated strip used for insertion; a castellation is a row of discrete half holes used for soldering. Some boards use both on different edges, but the process controls differ and should be specified separately. Where current is high a large solder joint usually beats a small connector contact; where the assembly has to be serviced in the field, the connector wins regardless of the electrical argument.

Quality Limits and Yield

The difficult part of castellation production is not the plating, it is the routing that cuts the holes in half. When the router wanders, the remaining wall is uneven from hole to hole, and if it wanders far enough the cut enters the barrel and tears the plating, leaving a ragged surface that can wick solder during assembly.

Burrs and smear are the second problem. A router or laser cut leaves a burr on the copper unless the process includes a deburring step, and a burr on a vertical wall can bridge to a neighbouring castellation or lift during reflow.

Plating coverage on the cut wall is the third factor. A castellation is plated before it is cut, so the cut wall exposes the cross section of the barrel and the annular ring rather than fresh plated copper. That is by design, but it limits the finish on the wall, which is why a solderability check on the first article matters more here than on a conventional board. Electrical test is also weakened because some nets are only reachable through the carrier, so test coupons on the production panel are often the only way to confirm continuity before routing.

What Drives the Cost

Castellation adds cost in two places: drilling and plating, and routing. The holes need a finished size and a wall thickness specified separately from the rest of the board, and they sit on the panel edge where plating current distribution is worst, so the panel has to be plated longer or with more copper than the middle of the board needs.

Then the cut that halves them has to be held to a tolerance measured in tens of microns while leaving a consistent land, and a castellated panel gives up the edge strips that would normally carry test coupons and tooling, so panel utilisation drops. On a small order the tooling and first article dominate and quotes come back per panel rather than per square metre. In volume the premium over a plain board of the same size settles to a modest amount, and the advantage over a connector plus cable assembly is usually large.

FAQ

• Can castellations be used on a flexible circuit? Yes, but the edge is polyimide rather than FR-4 and the mechanical behaviour of the cut is different, so it is usually handled by a dedicated flex assembly process rather than the standard rigid flow.
• How many layers can a castellated module have? Castellation is common up to eight layers and is possible on thicker stacks, but the cut becomes harder to control as the board gets thicker and the achievable pitch loosens.
• Is a castellation the same as a half hole? In practice yes. Half hole is the shop floor term for the same feature, and either name on a drawing will be understood.
• Can the joint be reworked? It can, with hot air and a small nozzle, but the vertical geometry makes it slower than reworking a leaded part, so it is treated as a non reworkable joint in most production planning.

For a module that has to be small, low profile and permanently attached, castellation remains the most economical interface available, and it is supported by the same SMT assembly flow as a conventional board. Designers who plan the footprint, the paste volume and the inspection method before the first build usually find that the assembly partner can qualify the joint with a single first article, and that the electrical test strategy carried over from the capabilities review is enough to catch the outliers.

Summary

Castellated holes turn the board edge into a solderable interface. The feature is simple to specify and awkward to produce, because it combines two demanding processes in one place: a plated through hole with a controlled wall, and a routing cut that halves it without tearing the plating. Fix the pitch, the finished hole size and the routing tolerance early, give the carrier a pad wide enough for a fillet, and qualify the joint with a first article cross section. The result is a module interface smaller, cheaper and more reliable than any connector; leaving the geometry to the supplier risks finding the problem only after the module footprint is committed.