Castellation PCB Modules: Design and Assembly Guide

A castellated module is a small board with plated half holes along its edge, so that it can be soldered onto a carrier board like a surface mount component. It is the standard way to package a radio, a power converter or a sensor subsystem so that the customer can treat a tested design as a single part instead of re-engineering it.

What a Castellation Is

A castellation is a plated half hole formed at the edge of the module. The hole is drilled and plated while the module is still part of the panel, and the outline is then routed through the holes so that half of each barrel remains at the edge of the finished board.

The exposed half cylinder is solderable, so the module can be placed on a carrier board and reflowed, with the solder forming a fillet onto the castellation and the corresponding pad. The joint is visible from the side, which makes it inspectable in a way that a fine pitch area array joint is not.

Pad and Hole Geometry

The castellation needs a pad on the module, and the plated barrel has to be wide enough to form a reliable joint. The typical approach is a pad whose diameter is larger than the finished half hole, so that the copper surrounds the barrel and gives it mechanical support.

The spacing between castellations has to allow for the routing tolerance and for the solder fillet. Adjacent castellations that are too close will bridge during reflow, and the module edge must leave enough material between holes to resist cracking during singulation. The pad design rules that apply are the same as those described in PCB pad design standards.

<img src="https://www.gopcba.com/wp-content/uploads/2024/10/pcb-coloro.jpg" alt="Castellated module PCB with plated half holes along the edge” />

Panel Design and Singulation

Castellated modules are produced in a panel with the modules connected by tabs, and the outline is routed through the centre of the drilled holes. The routing position is therefore critical: if it is offset, the castellations on one side of the panel become larger than those on the other.

The pick and place location of the module within the hole pattern has to be controlled by the panel design, and the fabricator needs the drill and the outline to be registered to each other rather than to separate references. A common mistake is to specify the outline to the nominal hole centre without leaving tolerance for both, which results in an edge that is visually and electrically inconsistent.

Design Rules for the Module Itself

The circuit inside the module follows normal design rules, with two additions. First, the castellations must be treated as the only electrical and mechanical interface, so the ground and power connections have to be distributed properly rather than concentrated in one corner. Second, the module has to be testable before it leaves the factory, which usually means test points on the bottom face or on the castellations themselves.

The mechanical form also matters. A module that is too long relative to its width will be difficult to keep flat during reflow, and one with a heavy component near the castellations will put stress on those joints. Where a large component must be fitted, it should be placed towards the centre of the module where the board is stiffest.

The Solder Joint

The joint between the castellation and the carrier pad is a fillet rather than a flat surface mount connection, and it forms during the same reflow as the other components. Paste is printed on the carrier pad, the module is placed, and the solder wets both the pad and the half cylinder.

Joint quality depends on the paste volume and on the gap between the module and the carrier. A module that stands off the carrier because of its own solder mask or a component on the underside will produce an unreliable joint. Lead-free alloys wet slightly differently from leaded ones, and the implications are the same as on any other board, as described in lead-free versus leaded solder.

Assembly Considerations

The carrier board layout has to provide a keep out around the module footprint, both for the component placement and for the soldering. Since the joint is on the module edge, the carrier needs a pad that extends slightly beyond the module outline, and the surrounding components must not obstruct the fillet or the inspection.

The module placement uses the same pick and place equipment as any other component, with a nozzle sized for the module. Its weight and size mean the placement speed is lower and the vision alignment matters more, so the carrier should provide fiducials close to the module position rather than only at the panel corners.

Inspection and Test

Inspection is visual with magnification for the fillet, and the acceptance criteria relate to the fillet shape and to the visible wetting of the castellation. Where the carrier has a high component density, an automated optical inspection system can be programmed for the module edges.

Rework is practical, because the joints are accessible. Hot air with a shield to protect the module, or a localised soldering iron for a single connection, allows a mispositioned module to be removed and replaced. That is a significant advantage over a fine pitch package and one of the reasons castellated modules remain popular.

Mechanical and Environmental Performance

The joints between a module and its carrier are subjected to shear when the assembly bends, and to fatigue when the product undergoes thermal cycling. Because the fillet is on the edge, the stress concentrates at the interface between the castellation and the module substrate.

Where the application involves vibration or wide temperature swings, the module outline and the distribution of the castellations both affect the life of the joint. A module with castellations distributed along all four edges shares the load better than one with them only on two. The mechanical considerations for the outline are the same as those in board outline and mounting design.

When to Use a Castellated Module

The approach makes sense when the function is self contained, the design is difficult to reproduce in-house, and the customer wants a tested and certified subsystem. Radio modules, power supplies and sensor packages are the classic examples.

The trade off is cost and footprint. A module costs more per unit than the components on it, and it occupies more board area than the equivalent circuit would. Where the volume is high and the design is fully understood, integrating the function onto the main board is usually cheaper, and the decision should be reviewed as the product matures.

FAQ

How many castellations can a module have? As many as the edge length allows, subject to a minimum spacing and to the mechanical strength of the remaining material. The practical limit is usually set by the panel and the routing tolerance.

Can a castellated module be soldered by hand? It can, for prototype work, but the joints are difficult to make uniform by hand and the result is not representative of production. Reflow with a stencil is the reliable route.

Is a castellation the same as a plated edge? No. A castellation is a plated half hole used as a connection point, while edge plating is a continuous conductor along the board edge, usually for shielding or grounding.