Castellated PCBs: Design the Module Edge and Host Footprint Together

Castellations are plated features at a PCB edge, often made by routing through plated holes. They allow a module board to connect to pads on a host board through soldered edge interfaces. They are one board-to-board connection method, not the only way that two circuit boards can be joined.

Treat the module as a component

Define a controlled outline, pad numbering, orientation mark and host footprint for the module. Include underside features and any keepout areas that the host design must respect. A module drawing should identify the physical and electrical interface as clearly as a packaged component drawing.

For RF modules, preserve the antenna and ground-related integration requirements from the module designer. A convenient castellated connection does not remove those requirements or automatically transfer every certification claim to the host product.

Coordinate the fabricated edge

• Identify castellated holes explicitly on the fabrication drawing.
• Define the intended relationship between holes and routed outline.
• Review pad support and inner-layer treatment.
• Select a supported surface finish.
• Reserve suitable panel attachment areas.
• Agree edge-quality and dimensional inspection.

Eurocircuits’ castellation guidance explains that hole plating, edge routing and mechanical support must work together. Its recommendations are process-specific examples; obtain the chosen fabricator’s requirements before finalizing dimensions.

Review solder access on the host board

Coordinate host lands and solder-paste definition with the assembly process. Consider how the module is placed, supported and inspected after soldering. Keep nearby components from obstructing the required access.

If a castellated pad also connects to a complete hole for an optional header, review how solder might move into that hole. The dual-use feature changes the soldering geometry and should not be added without considering its effect on the module-to-host joint.

Verify the complete module assembly

Check fit and orientation before production. Use representative first assemblies to inspect the edge joints and verify the module’s required functions on the actual host. A module that works in a separate test fixture may behave differently with the host’s power, grounding or interface conditions.

Record the module and host revisions together with the assembly settings and results. If either footprint changes, review the mating relationship again. A seemingly minor outline adjustment can alter alignment or joint inspection access.

For servicing, define whether the module is intended to be replaced and how rework will be controlled. Do not assume a soldered module has the same replacement convenience as a socketed component.

Primary reference: Eurocircuits’ castellation manufacturing guidance. See the panelization guide and assembly-file guide for the supporting release data.