Castellated Holes for Module Edges

What Castellated Holes Are

A castellated hole is a plated through hole that has been cut in half at the edge of the board, leaving a plated semicircular channel along the edge. The result is a connection that can be soldered flat onto a pad on a carrier board, in the same way a surface mount component is, while still providing a through-board connection inside the module itself.

The appearance is distinctive: a row of half-cylinders along the edge, each with plated copper on the wall and a pad on the top or bottom surface. The technique is common on small modules such as radio, sensor, and power modules, where the module is designed to be soldered onto a larger board as a component.

The advantage over a connector is size and cost. Castellated edges take no additional height on the module, they eliminate the connector, and they allow the module to be handled by the same surface mount equipment that places the other components. The trade-off is that the module is soldered to the carrier and is not removable.

Why Modules Use Them

The first reason is form factor. A module that must fit within a tight height budget cannot afford a connector, and the castellations are effectively free in height. The second is assembly simplicity: the module can be placed, reflowed, and inspected alongside the other components, with no separate process step.

The third is electrical performance. A castellated connection is short and has low inductance compared with a pluggable connector, which matters for high-frequency modules and for power modules where the current path is part of the design. The plated half-hole also provides a robust mechanical bond when the solder fillet forms on both the edge and the pad.

The trade-off is serviceability. Once soldered, the module cannot be removed without rework, and the castellations are not designed for repeated mating. Where a product needs a replaceable module, a connector is still the right choice.

Design and Fabrication Rules

The castellation is formed by drilling and plating a full hole and then routing the board so that the hole is cut along its centreline. The plating on the wall has to survive the routing, which is why the process is more demanding than a standard plated hole. The fabrication drawing should state the castellation requirement explicitly, because a board shop that is not told will simply route the edge and leave an open half-hole with no plating.

The hole diameter and the remaining plating determine the strength of the connection. A larger hole gives more plated area but less room for the pad, while a smaller hole limits the solder fillet. The pad around the castellation should provide enough land for the solder to form on the top and bottom surfaces, and the spacing between castellations should allow the solder to wet each one without bridging.

The edge rout must be accurate enough that the cut is centred on the hole. An offset cut leaves an asymmetric castellation with less plating on one side and a weaker joint. The routing tolerance is therefore part of the design requirement, and the drawing should specify it.

Assembly Considerations

The module is placed like any other surface mount component, but the paste requirement is different. The castellation needs enough paste on the pad to form a fillet up the plated wall, and the stencil aperture should be sized for the volume the joint needs. Too little paste leaves a partial fillet; too much produces a ball at the edge or a bridge to the next castellation.

The module body also influences the process. A module with a large thermal mass draws heat from the castellations during reflow, and the joints at the centre of the array may not reach liquidus while the outer ones do. The profile should be developed on the assembled module rather than on a bare board.

Alignment matters because the castellations have to sit on their pads. A misalignment that is small in absolute terms can be significant relative to the castellation width, and the resulting joint is weak. The placement machine’s vision system should be used with a fiducial on the module or with the castellations themselves as the reference.

Inspection and Quality

The visible joint is the fillet on the pad and the plating inside the castellation. A good joint shows solder wetting the plated wall and forming a concave fillet on the pad, with the module edge seated flat against the pad. A joint that shows solder on the pad but not up the wall has not wet the castellation, and one that shows a ball at the edge has too much solder or insufficient heat.

X-ray is useful for the joints that cannot be seen, and it can show whether the solder has flowed into the castellation. Where the module has a shielded or opaque body, the edge joints are often the only ones visible, and those must be inspected carefully because they carry the mechanical load as well as the current.

Mechanical testing is part of qualification. A shear or peel test on a sample module shows whether the castellation joints can carry the load they will see in service, and it also reveals process problems such as incomplete wetting that a visual inspection would pass.

Common Problems

Unplated castellations are the most serious defect, and they occur when the fabrication drawing does not specify the process or when the routing cuts away the plating. The result is a joint that relies on the pad alone, with much lower strength and a poor electrical connection. This defect should be caught at incoming inspection with a simple visual or microsection check.

Offset routing is the second problem. A castellation that is not cut on centre has less plating on one side, and the joint is weaker and more variable. The routing tolerance should be specified and the incoming inspection should check the cut position on a sample.

Solder bridging between adjacent castellations is the third. The spacing and the paste volume have to be balanced so that each joint forms independently, and the stencil aperture should be sized accordingly. Once bridging occurs, the repair is difficult because the castellation cannot be easily reworked.

FAQ

What is a castellated hole? A plated through hole that has been cut in half at the board edge, leaving a plated half-hole that can be soldered flat to a carrier board.

Are castellated edges reworkable? They are soldered like surface mount joints and can be reworked with hot air or a similar tool, but the process is difficult and the module is not designed for repeated removal.

How is a castellation made? The hole is drilled and plated, and the board is routed along the hole centreline so that half the plated barrel remains on the edge.

What happens if the castellation is not plated? The joint relies only on the pad, with much lower mechanical strength and a degraded electrical connection. The requirement must be stated explicitly on the fabrication drawing.

How do I control the solder volume? Size the stencil aperture for the fillet the castellation needs, and develop the profile on the assembled module to account for its thermal mass.

Conclusion

Castellated holes let a module be soldered onto a carrier like a component, with no connector, no extra height, and a short electrical path. The process depends on the plating surviving the edge rout and on the paste volume and profile being matched to the castellation, and the requirement has to be stated clearly for the board shop. For related topics, see our notes on PCB manufacturing, SMT assembly, PCB assembly, and quality management for how module assembly is controlled in 2026.