PCB Edge Plating: Castellated Holes for Module Design

Edge plating and castellated holes are specialised PCB manufacturing techniques that allow modules and breakout boards to be surface-mounted directly onto a carrier PCB. If you have ever used an ESP32 module, a GPS module, or a sensor breakout board, you have seen castellated holes in action — those half-moon shaped plated cutouts along the board edge. This guide explains how castellated holes work, design rules for creating your own modules, and manufacturing considerations for Indian fabricators.

Table of Contents

• What Are Castellated Holes
• Applications and Use Cases
• Design Rules for Castellations
• Full Edge Plating
• Assembling Castellated Modules
• Testing and Quality
• Manufacturer Capability
• Frequently Asked Questions

What Are Castellated Holes

Castellated holes (also called castellation or half-holes) are plated through-holes placed at the board edge, then cut in half during the board outline routing process. The result is a half-moon shaped plated contact on the board edge that can be soldered to pads on another PCB.

The manufacturing process:

1. Drill standard plated through-holes at the board edge locations
2. Plate the holes with copper (standard PTH process)
3. Route the board outline through the centre of the holes, cutting each hole in half
4. The remaining half-hole has a smooth, plated concave surface

Applications and Use Cases

• Wireless modules: ESP32, ESP8266, nRF52, LoRa modules — castellated edges allow reflow soldering onto a carrier board
• Sensor modules: IMU, barometric pressure, gas sensor breakouts designed for PCB-to-PCB mounting
• Power modules: DC-DC converter modules with castellated inputs and outputs
• Daughterboard connections: Plug-in sub-boards for modular product designs
• Production testing: Test jigs with castellated contacts for bed-of-nails probing

The key advantage over pin headers is that castellated modules sit flat on the carrier board, reducing overall height and providing a more robust mechanical connection. They also support automated reflow assembly, whereas pin headers require manual or wave soldering.

Design Rules for Castellations

Critical design considerations:

• Place the drill hole centre exactly on the board outline. The router will cut through the centre, leaving a half-hole
• Add a copper pad extending inward from each castellated hole for soldering and trace routing
• Include a ground plane connection to at least 2-3 castellated pads for EMI shielding on wireless modules
• The inner pad should be solder mask defined (SMD) to prevent mask from encroaching on the castellation

Full Edge Plating

Full edge plating covers the entire board edge with copper, not just at specific hole locations. This is used for:

• EMI shielding: A continuous copper edge connected to the ground plane creates a Faraday cage when the board is enclosed
• Edge connectors: Custom edge connectors with specific contact patterns
• Grounding: Direct ground connection through the board edge to a metal enclosure

Full edge plating is more expensive than castellated holes because it requires a separate plating step after the board outline is routed. Not all fabricators offer this service — confirm capability before designing it in.

Specify edge plating in your fab drawing by indicating which edges require plating. Include a cross-section drawing showing the plating extending from top copper to bottom copper along the board edge.

Assembling Castellated Modules

Reflow soldering (production):

1. Apply solder paste to the carrier board pads using a stencil
2. Place the castellated module onto the pads using a pick-and-place machine or by hand with tweezers
3. Reflow in the oven — solder paste melts and wicks up into the castellation concave surface
4. Inspect joints visually or with AOI — good joints show a smooth fillet from pad to castellation

Hand soldering (prototype):

1. Apply flux to both the carrier board pads and the module castellations
2. Position the module on the pads and tack one corner with the soldering iron
3. Solder each castellation by touching the iron to the pad and feeding solder — the concave castellation draws solder by capillary action
4. Use a fine tip (1.2mm chisel or smaller) for castellations on 1.27mm pitch

Testing and Quality

• Visual inspection: Look for complete solder fillets on all castellations. Incomplete wetting indicates insufficient flux or temperature
• Electrical continuity: Test each castellated connection with a multimeter in continuity mode
• Pull test: For reliability verification, a properly soldered castellation should withstand at least 1kg of pull force before the solder joint fails
• X-ray inspection: For production quality, X-ray can verify solder fill inside the castellation cavity

Manufacturer Capability

When ordering, explicitly mention “castellated holes” or “half-holes” in your order notes. Some manufacturers require you to add the castellated holes as a special instruction rather than relying on automatic Gerber interpretation.

Frequently Asked Questions

Can I use castellated holes on a 2-layer board?

Yes, castellated holes work on any layer count. For 2-layer boards, connect the castellation pad to traces on both top and bottom layers for maximum joint strength. Most wireless and sensor modules use 2 or 4 layer boards with castellations.

How do I create castellated holes in KiCad?

Create a custom footprint with plated through-holes placed on the board edge. Set the hole centre exactly on the Edge.Cuts line. Add pads extending inward. When the board is routed at the fabricator, the holes will be cut in half. In KiCad 7+, there is a “castellated” pad type specifically for this purpose.

What is the maximum current through a castellated connection?

A typical 0.8mm castellated hole with 25µm plating can handle approximately 2-3A. For higher currents, use multiple castellations in parallel or larger hole diameters (1.0mm+). Always verify with thermal analysis for power applications.

Are castellated holes reliable for vibration environments?

Yes, castellated solder joints are more reliable than pin headers in vibration environments because the entire castellation surface is soldered. The joint area is larger and distributes stress more evenly. Many automotive and industrial modules use castellated mounting for this reason.

Find prototyping boards, soldering supplies, and module components at Zbotic PCB & Prototyping — shipping across India.