Castellated Hole: Preparation, Placement and Process Control

A castellated hole is a plated hole that has been cut in half so that the plating is exposed on the board edge. It lets a small module be soldered onto a carrier board like a component, with connections on the side rather than underneath. The feature is simple to describe and demanding to make, because it exposes the inside of the barrel to the routing operation and to the assembly process. This article covers the design rules, the fabrication sequence and what has to be controlled at each step.

What Castellations Are Used For

Castellations are used to convert a module into a surface mountable part. A radio module, a power module or a sensor assembly can be built on its own small board, tested, and then soldered onto the main board by the same process that places other components. The connection is visible from the side, which allows inspection, and the joint tolerates vertical misalignment.

Edge plating is the related feature where the board edge itself is plated rather than a set of holes. It is used for shielding, for a ground connection to a chassis and for a robust edge contact. The two are often specified together on a module that needs both signal connections and a grounded edge.

Design Rules for Castellated Holes

The hole diameter sets the barrel width that will be exposed, and it has to be large enough to give a solderable surface after routing. Small holes are harder to plate uniformly and leave a narrow contact. The pitch between holes has to leave a strip of laminate between the barrels so that the module edge retains its mechanical strength.

The annular ring should be adequate on both sides. Because the hole is cut, the ring becomes a half ring, and a thin ring leaves little surface for the solder fillet. The layer count also matters, since a castellated hole in a multilayer board exposes all the layer connections on the edge, which is sometimes desirable and sometimes a short circuit waiting to happen.

Fabrication Sequence and Its Consequences

The sequence determines the quality of the result. The hole is drilled and plated as a normal plated hole, and then the board edge is routed through it. Routing after plating means the cutter passes through the plated barrel and can drag copper, leave a burr or smear the plating across the laminate.

The alternative is to route first and plate afterwards, which avoids the cutting damage but requires the edge to be plated in a way that reaches the exposed half barrels. Most processes use the first sequence with a carefully controlled routing step, because plating an edge selectively is more difficult. The control points are the cutter condition, the feed rate and the support of the board during cutting.

Plating Thickness and Barrel Quality

The plating thickness in a castellated hole has to be sufficient to survive the routing and to carry the current of the joint. A thin deposit is cut away or cracked by the cutter, which leaves a connection that looks complete and fails later. The thickness should be specified as a minimum rather than as a target, and it should be verified on a sample after routing rather than before.

Barrel quality matters for the same reason. A barrel with voids, a rough deposit or an incomplete wrap cannot be relied on once it is cut open. The general requirements for the plated wall, described in copper in the plated hole, apply here with the additional constraint that half the barrel will be exposed.

Routing, Tooling and Edge Quality

Routing is where most of the damage occurs. A worn cutter tears the copper rather than cutting it, and a feed rate that is too high leaves a ragged edge with a burr that can short two adjacent castellations. The support condition of the panel also matters, because vibration during cutting produces a rougher edge.

The finished edge should be inspected for burrs, for smeared plating and for laminate damage. Burrs are particularly important on a module with a fine castellation pitch, since a burr can bridge two adjacent barrels and create a short that only appears after the module is soldered down. The dimensional tolerance of the edge, covered in board outline tolerance, affects how well the module aligns with its pads on the carrier.

Assembly and Solder Behaviour

The solder joint on a castellated hole is formed on a half cylinder. It wicks up the exposed barrel, which gives a visible fillet from the side, and it requires enough paste to fill the joint. The stencil aperture has to deliver that volume, and the pad on the carrier board has to be wide enough to allow the fillet to form without overflowing onto the neighbouring feature.

The self alignment of the module during reflow is weaker than for a conventional package because the joints are on the side rather than underneath. Placement accuracy therefore has to be better, and the design should avoid a castellation layout that produces an unbalanced surface tension which pulls the module to one side. Stencil design considerations for those apertures are in solder paste volume and stencil design.

Panel Design for Cheap Castellations

Castellated modules are usually small, so several can be built on one panel and separated at the end. The panel design affects both cost and quality. Rails on the edges that carry the castellations should be avoided, because the castellation edge must be routed cleanly without the extra material of a rail interfering. Where the modules are separated by a tab, the tab should be placed on an edge that carries no castellations so that breaking it does not damage a plated half barrel.

Spacing between modules should allow the router to cut each edge in a single clean pass and to leave enough laminate for handling. Dense packing reduces material cost but makes the routing step harder and increases the chance of damaging an adjacent module, so the panel should be reviewed with the router path in mind rather than only with the material yield in mind.

Inspection and Acceptance

Inspection of a castellated joint combines visual and X-ray checks. The side fillet is visible, which allows a direct judgement of wetting and of joint height. The region under the module, where the castellation meets the pad, needs X-ray to confirm that the joint is complete and not voided.

The acceptance criteria should state the required fillet height, the maximum allowable void and the limits for a partially plated barrel. The gopcb fabrication and assembly teams agree these criteria at the design stage for module products, because the same feature is created by two different processes and the responsibility for a defect is not always obvious from the symptom. A record of the agreed criteria in the data package, following the practice described in PCB fabrication notes, avoids that ambiguity later.

FAQ

Can any hole be castellated? Any plated hole can be cut, but the result is only reliable if the plating is thick enough and the hole is large enough to leave a solderable half barrel.

Is edge plating the same as a castellation? No. Edge plating covers the board edge itself, usually for shielding or grounding, while a castellation exposes a plated hole for a solder connection.

How is a damaged castellation repaired? Usually it is not. A torn or partially plated barrel cannot be rebuilt reliably, so the module should be scrapped rather than patched.