Besides the board outline, many designs carry internal features: a window for a connector, a hole for a transformer core, an isolation slot for creepage. Most of these are cut on a CNC router with a rotating end mill. Because the cutter is round, the resulting geometry follows fixed rules. A drawing that ignores them usually comes back with an engineering query.
This article covers internal routing geometry only. For outline tolerances and V-scoring see PCB profiling and outline machining tolerance.
Every internal corner has a radius
As the cutter follows the contour, its edge can only trace an arc at an inside corner. The smallest internal corner radius equals the radius of the tool. A 90° corner drawn on the print still comes out round.
Outside corners are not limited this way. The cutter passes around them, so they can be sharp.
What this means for the design:
- A square part or square terminal dropped into a square cutout will sit on the rounded corners.
- A smaller cutter gives a smaller radius, but small tools break more easily and run slower, so they do not suit every contour.
- Thicker boards usually narrow the choice of small tools. The actual tool size is confirmed during engineering review.
Fitting a square part: dog-bone relief
If the corners of a part must drop fully into the cutout, the usual answer is a dog-bone or T-bone relief: an extra small circle routed beyond each inside corner so the sharp corner of the part lands inside it. The cost is four small notches on the cutout edge, which need to be checked against nearby copper and pads.
The other option is to relax the part side. If the part already has chamfered corners, simply allowing the inner radius may be enough.
Slot width is limited by the tool
A slot is cut by running one end mill along its centerline, or around its contour. The slot cannot be narrower than the tool. Narrower slots are sometimes made as a row of drilled holes plus trimming, which leaves a scalloped edge. Slot ends are semicircular, not square.
Call out slots as "width × length" and state whether length is overall (including both semicircular ends) or center-to-center. The two readings differ by one tool diameter.
Plated versus non-plated slots
- Non-plated slots (NPTH) are routed after plating. The wall is bare laminate and the size comes straight from the tool.
- Plated slots (PTH) must be routed before plating, then metallized and plated. The finished width is smaller than the routed width by twice the plating thickness, so the tolerance is usually wider than for non-plated slots.
The manufacturing tolerances page lists Slots and Internal Cutouts with separate plated and non-plated values. Design to the finished size.
How to draw it in the print and Gerber
- Draw the finished contour on the mechanical layer, not the tool centerline. Contours must be closed, and cutouts must be clearly separated from the outline by layer or line type.
- State the maximum acceptable inner corner radius. This is clearer than drawing a sharp corner and adding "radius allowed".
- Mark plating on the drawing: which slots are PTH and which are NPTH. Do not leave it to be inferred from the drill file.
- Use the same datums as the outline. Dimension cutouts from tooling holes or board edges; see tolerance callouts in PCB drawings.
- Name one source of general tolerances: IPC-2615 or ISO 2768, not a mix of both.
- Keep copper back from routed edges. Copper near a routed edge can lift or become exposed; inner layers need more clearance than outer layers.
Common reasons for an engineering query
- A square cutout drawn with sharp corners for a part without chamfers, and no relief.
- Slot width smaller than the available tool, or slot length undefined.
- A PTH slot not marked as plated, or a drill file supplied without a mechanical layer.
- A cutout overlapping the outline, or a mechanical contour that is not closed.
How SKPCB handles it
During engineering query (EQ) review SKPCB checks mechanical contours, inner corner radii, slot widths and plating attributes, and raises a question before production when a feature does not fit routing geometry. Standard tolerances for outline, slots and internal cutouts are on Manufacturing Tolerances, the overall process range is on Capabilities, and special cutouts can be submitted with a mechanical drawing via Get a Quote.
Summary
The shape of an internal routed feature is set by the tool: inner corner radius equals tool radius, slot width is at least the tool diameter, and plated slots finish narrower. Draw the finished contour and state the maximum corner radius and plating, and one round of engineering questions goes away.
FAQ
Q: Can a router make a true 90° inside corner?
A: Not with a rotating cutter. The inner radius is at least the tool radius. Use a dog-bone relief when a part must drop fully in.
Q: What is the minimum inner corner radius?
A: It depends on the smallest usable tool, which in turn depends on board thickness and material. Put the largest acceptable radius on the drawing and let engineering confirm it.
Q: Why do plated slots have a wider tolerance?
A: They are routed before plating, so variation in wall plating thickness adds to the finished width.
Q: How should slot length be dimensioned?
A: State whether it is overall length including the semicircular ends or center-to-center. Otherwise the readings differ by one tool diameter.
Q: Can the Gerber show the tool centerline?
A: Not recommended. The mechanical layer should show the finished contour; tool compensation is done in the fabricator's CAM.
Q: How far should copper stay from a routed edge?
A: Follow the design tips on the manufacturing tolerances page. Inner-layer copper needs more clearance than outer-layer copper.