Castellated holes and gold fingers both live on the board edge, and both are usually specified in one line on a drawing: "castellations, gold fingers". They are different processes with different failure modes, and both are easy to specify in a way that cannot be manufactured.
This article covers how each is made, the dimensions that matter, and where the cost comes from.
Castellated holes
A castellated hole — a half hole — is a plated through hole that is later cut in half by the board outline. The remaining half barrel forms a solderable castellation on the edge of a module.
The process is:
- Drill and plate the hole as a normal PTH.
- Route the board outline through the center of the hole.
- Clean and finish the edge.
The result is a half-cylinder of plated copper. It is used on modules that solder onto a carrier board, such as RF modules and power modules.
DFM rules for castellations
- Hole diameter and pad diameter must leave enough annular ring after the cut. A common starting point is a 0.6–0.8 mm hole with a 1.0–1.2 mm pad.
- The cut must pass through the hole center; offset cuts leave a thin wall on one side.
- Castellated holes need a minimum spacing to the next feature, because the router leaves a tolerance.
- The hole must be plated before routing; plating after the cut is not possible.
- Solder mask should not enter the castellation; it prevents wetting.
- Specify the finished half-hole tolerance, not only the drill.
A castellation that looks correct from the top can have a thin or missing barrel after the cut. Cross-sections at first article are the only reliable check.
Gold fingers
Gold fingers are edge contacts plated with hard gold over nickel. They plug into a connector or a mating slot, so wear resistance and contact resistance matter more than solderability.
The process is:
- Pattern the fingers on the outer layer.
- Plate nickel, then hard gold.
- Bevel the board edge.
- Protect the fingers during solder mask and finishing.
DFM rules for gold fingers
- Specify hard gold thickness, typically 0.3–0.5 µm over 3–5 µm nickel. ENIG is not a substitute for a wear surface.
- Specify the bevel angle, commonly 20–45 degrees, and which edge is beveled.
- Keep solder mask off the fingers with a defined clearance.
- Avoid plated through holes inside the finger area; they create bumps and voids.
- Define the finger length and the insertion depth.
- State whether the fingers are gold over nickel or gold over copper; the former is standard for connectors.
Gold thickness is the most common dispute. A "gold finger" with 0.05 µm of immersion gold will wear through quickly. Hard gold is a different process and a different price.
Where the cost comes from
| Feature | Cost driver | Note |
|---|---|---|
| Castellated holes | Extra routing pass, tighter tolerance | Per hole or per edge |
| Half-hole plating | Plating before routing, inspection | Cannot re-plate after cut |
| Gold fingers | Hard gold plating, bevel, masking | Gold thickness drives price |
| Bevel | Extra routing step | Angle and edge count |
| Edge tolerance | Tighter tolerance, slower routing | Affects yield |
Both features add handling and inspection. A board with castellations and gold fingers is usually quoted with a separate edge-processing line item.
Common mistakes
- Specifying "castellations" without a hole and pad size.
- Putting a castellation too close to a gold finger; the router damages the finger.
- Using ENIG for a connector edge and expecting wear resistance.
- Forgetting the bevel, so the board cannot seat in the slot.
- Letting solder mask wick into the castellation.
- Specifying a half-hole tolerance tighter than the router can hold.
FAQ
Can castellated holes be plated after routing?
No. The barrel is plated before the cut. That is why the plating thickness and wall quality must be right before routing.
Is ENIG acceptable for gold fingers?
No, for a wear surface. ENIG is thin immersion gold. Gold fingers need hard gold over nickel.
How deep should the castellation cut be?
Through the hole center, so the remaining half barrel is uniform. An offset cut leaves a weak side.
Do castellations work on flexible boards?
Yes, but the flex material and adhesive need a controlled routing process. Review the stackup with the fabricator.
What to send for an edge-feature review
- Castellated hole drawing with hole, pad and pitch
- Gold finger length, thickness and bevel angle
- Edge tolerance
- Connector or mating slot specification
- Solder mask clearance
- Acceptance class
Send your board edge drawing for review, or review our rigid-flex capability and flex PCB capability.
