Cavities in a PCB: depth-controlled milling, limits and how to design them
A cavity is a pocket milled into the board without going through it. What we can make, which boards it suits and what to write in the request.
What a cavity is
A cavity (also called a pocket or recess) is an area milled into the board from one side to a set depth, without cutting through it. The board stays whole and keeps a thin floor under the cavity. This is depth-controlled milling: the depth is the important number, not the outline alone.
What cavities are used for
- A recess for a part: a component, a connector body or a sensor sits lower in the board, which reduces the height of the assembly.
- Heat: a pocket for a thicker metal part or a heat spreader.
- Mechanics: a recess for a screw head, a nut or a part of the enclosure.
Our limits
| Feature | Limit |
|---|---|
| Depth tolerance | ±0.2 mm |
| Minimum thickness left under the cavity | 0.3 mm |
| Suitable boards | Thicker boards, from 1 mm |
Because at least 0.3 mm must remain under the cavity, a cavity needs a board of enough thickness: from 1 mm up. On a thin board there is not enough material for a pocket and a floor. Board thicknesses are in standard PCB thickness.
Design checklist
- Draw the outline of the cavity in the mechanical layer and say on which side it is milled.
- Give the thickness that remains under the cavity (at least 0.3 mm), not only the depth of the cut. The board thickness and the milling depth both vary a little, and the remaining thickness is what protects the board.
- Allow for the depth tolerance of ±0.2 mm in your design, especially where a part sits in the cavity.
- Keep copper out of the area the milling reaches, unless you want it exposed, and tell us in that case (stack-up guide).
- Keep components, pads and traces away from the cavity walls. The inside corners of a milled cavity are rounded, because the bit is round.
- Mention the cavity in the notes of the request, as one of the special features (what to put in a request).
Cavity, slot or hole
A through slot or a hole goes all the way through the board; a cavity does not. For holes see vias and hole sizes, and for the other milled features see the design rules.
How to order
Upload the files in the price calculator, choose a board thickness suited to the cavity and write the cavity in the notes with the side, the outline and the remaining thickness. We confirm in the quote that it can be made. For a new design, contact us with your files before you order.
Frequently asked questions
What depth tolerance do you hold for a cavity?
±0.2 mm.
How much board must remain under a cavity?
At least 0.3 mm.
Can I make a cavity in a thin board?
Rarely. With 0.3 mm left under the cavity, cavities suit thicker boards, from 1 mm.
Should I give the depth or the remaining thickness?
Give the remaining thickness under the cavity, at least 0.3 mm, as it is what the board must keep after milling.
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