Castellated PCB: Half-Hole Modules for Board-to-Board Assembly

Small radio modules have a recognisable shape: a rectangular board with a row of half-round plated holes along each edge. Those half-holes are the connector, and they are what allow a module to be soldered directly onto a larger board without a socket, a header, or a cable. The construction is called a castellated PCB, after the battlement shape it resembles.

What castellated means

A castellated PCB has semicircular plated holes machined along its edges. Each half-hole is a connection point: when the module is placed on a motherboard, the half-hole rests against a matching pad, and a reflow solder joint forms between them.

The technique is used for daughter boards and modules, and it replaces a connector pair with a direct solder joint. That single substitution is the reason the construction appears in so many compact products, because it removes both the cost of the connector and the space it would occupy.

How the connection works

A castellated module is designed to be soldered onto a larger motherboard. Its edge half-holes are plated with copper and align with corresponding pads on the motherboard. Paste is printed on the motherboard, the module is placed, and the assembly passes through reflow soldering, which forms all the joints in one operation.

The result is mechanically strong and electrically short. There is no connector interface to add inductance, no cable to route, and no separate assembly step, which makes the approach well suited to modular products built in volume. The same alignment discipline that governs any fine-pitch placement applies here, and the relevant rules are covered in our guide to PCB pad design standards.

Alignment is the whole joint. A half-hole that sits half a pad off centre produces a joint that passes visual inspection and fails under vibration.

Why the construction is used

Integration is the first advantage, because the module is placed with the same surface mount process used for every other component. Testability is the second: the half-holes remain exposed at the edge, so a probe can reach them without access features on the motherboard.

Prototyping benefits from replaceability, since a module can be swapped to test a variant without redesigning the carrier. Space is the fourth advantage, because a row of half-holes occupies less area than any connector that carries the same number of signals. And cost follows from all of them: fewer components, less labour, and a simpler assembly sequence.

Typical applications

Wireless modules are the largest category, covering Wi-Fi, Bluetooth, and low-power mesh radios. Satellite navigation receivers use the same construction, and IoT sensor and controller boards follow, because a module that can be replaced without touching the carrier board shortens both development and service.

Microcontroller daughter boards use castellations to expose the full pin set on a compact outline, radio-frequency power amplifier modules use them to carry high-frequency signals with a short connection, and small video and audio transmission modules use them to keep the assembly flat. Any design that values modularity, replaceability, and a low profile is a candidate.

Design rules

Hole diameter and pitch are the first specification. A diameter around 0.6 mm with a pitch of at least 1.2 mm is a common starting point, and it leaves enough pad area for a reliable joint while keeping the hole within the capability of the plating process.

Alignment between the module half-holes and the motherboard pads is the second requirement, because the joint is formed by the two surfaces meeting rather than by a pin entering a barrel. Plating inside each hole is the third, since conduction travels through the plated wall, and an unplated half-hole is a mechanical feature with no electrical function.

Solder mask is the fourth and the one most often overlooked. Mask around the edge holes must be kept back, because a mask dam next to a castellation blocks solder flow and produces a partial joint. Mechanical support is the fifth: large or heavy modules should include screw holes in addition to the solder joints, since solder alone is not a structural fastener. The outline and mounting arrangement is discussed further under board outline and mounting design.

A mask dam beside a half-hole is invisible on a schematic and decisive in assembly. It is the most common reason a castellation solders poorly.

Manufacturing process

Production begins by drilling the through-holes at the positions along the board edge where the castellations will be. The barrels are then plated with copper, which is what makes each half-hole conductive. Routing follows along the hole centrelines, cutting each drilled hole in half and leaving the semicircular profile that gives the construction its name. A surface finish such as electroless nickel immersion gold or hot air levelling is then applied to improve solderability and corrosion resistance.

The process is not difficult to describe and it is demanding to execute. Edge-to-copper spacing has to be controlled to a fraction of a millimetre, and the routing step determines whether every half-hole is symmetrical. Not every fabricator offers a stable castellation process, which is why the supplier question is usually answered by asking how many such designs they build.

Cost structure

Size, layer count, and quantity set the price, with precision adding to it. A prototype order is expensive per board because tooling and setup are carried by a small number, and a production order of the same module reduces that unit price substantially.

Multilayer castellations cost more than single or double-sided ones because the plating and registration requirements rise with the layer count. Non-standard materials, impedance control, and special surface finishes each add their own share, and the same logic applies here as anywhere: specify the finish and the material the product actually requires, and no more.

Selecting a supplier

Five questions separate a supplier who can build castellations reliably from one who cannot. Does the shop have specific experience with the process, particularly the routing step? Can it hold edge spacing control within a fraction of a millimetre? Does it accept small-batch prototyping without a large minimum order quantity? Does it provide design review and short lead times? And does it work to a recognised quality standard?

The alignment and placement problems that appear on small module assemblies are described in our guide to SMT component shift causes, and most of them apply to a castellated module with one addition: the joint depends on the edge profile as well as on the pad.

A note on the carrier board

The module is only half of the design. The motherboard has to provide a matching pad pattern, a solder mask opening wide enough for the joint, and a keep-out that stops tall components from obstructing the module. Reviewing the two boards together, as one assembly rather than as two, is what prevents the alignment problems that appear at reflow.

FAQ

Can a castellated module be multilayer? Yes. Two, four, and six layer versions are all produced, with the choice driven by the routing density the function requires.

Why do the joints sometimes fail? Usually a pad misalignment, insufficient solder paste, or an uneven board edge. Mask in the wrong place and a warped module also produce partial joints.

Can a soldered module be removed? Yes, with a proper rework process. The heating has to be controlled, because the pads are small and a careless removal lifts them from the carrier board.