Electronic Design Automation

Cutout

Q: What is the difference between a cutout and a copper keep-out?

A cutout removes physical board material: the dielectric and all copper layers are routed away, leaving an actual hole or slot through the laminate. It is defined on the board-outline or mechanical layer and produced by the fabricator with a router bit or laser. A copper keep-out, by contrast, removes only copper in a defined region while the dielectric stays intact; it is a layout-tool rule that prevents traces, pours, and planes from entering the zone. Engineers use cutouts for through-board clearance, antenna isolation, and mounting, and use copper keep-outs to control fill near antennas, sensitive RF traces, or high-voltage gaps without weakening the board mechanically.

Q: What minimum internal cutout size can a PCB fabricator route?

For routed internal cutouts, most fabricators specify a minimum slot width equal to the router bit diameter, commonly 0.8 mm to 1.0 mm, with internal corner radii matching the bit (a 1.0 mm bit leaves a 0.5 mm corner radius). Position tolerance on a routed feature is typically plus or minus 0.10 mm to 0.15 mm. Smaller or sharper internal cutouts down to about 0.20 mm width with square corners require laser cutting or controlled-depth machining, which raise cost. Always pull the chosen shop's design rules: minimum cutout dimensions and corner radius are fabricator-specific and should be confirmed before final layout.

Q: Why are cutouts placed under or around RF antennas?

Removing copper and dielectric beneath or beside an antenna eliminates parasitic ground coupling and lossy substrate that detune the element and lower radiation efficiency. A clear cutout in the ground plane under a patch or chip antenna keeps the feed impedance close to its designed value and prevents the FR-4 or laminate from absorbing fields at 2.4 GHz, 5 GHz, or millimeter-wave bands. Antenna vendors publish a recommended keep-out and ground cutout footprint; following it preserves the antenna's rated gain, return loss, and bandwidth. The cutout also reduces detuning when the board is mounted near metal enclosures.

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Defined on the board-outline or mechanical layer of an EDA layout, a cutout is a routed void where the laminate and every copper layer are removed, leaving a physical hole or slot through the board. RF designers add cutouts to clear tall components, create antenna isolation slots, relieve heat, and form mounting features. Unlike a copper keep-out, which strips only copper while the dielectric remains, a cutout removes material entirely. Common internal cutouts route at a 0.8 to 1.0 mm slot width with a position tolerance near ±0.10 mm, and they interact closely with the ground plane and any nearby copper pour.

Category: Electronic Design Automation

Min Slot Width: 0.8 to 1.0 mm

Position Tol.: ±0.10 mm

How Cutouts Shape an RF Board Layout

In EDA tools such as Altium, Cadence Allegro, or KiCad, a cutout is drawn as a closed contour on the board-outline layer (sometimes a dedicated milling or mechanical layer) rather than on a signal layer. During fabrication the shop interprets that contour as material to remove, routing it with a carbide bit or, for fine features, cutting it with a laser. Because the dielectric itself is gone, a cutout changes both the mechanical and the electromagnetic behavior of the board: it severs return-current paths, breaks ground continuity, and removes the lossy substrate that would otherwise sit beneath a radiating element.

The most demanding RF application is the antenna keep-out. Chip and patch antenna vendors specify a ground cutout and a copper-free zone so the element sees free space instead of a reflective, lossy plane. Removing the laminate under a 2.4 GHz or 5 GHz antenna prevents detuning, holds the feed impedance near 50 ohms, and preserves rated efficiency. At millimeter-wave frequencies the same principle applies to launch transitions and connector clearances, where even a small unwanted dielectric tab shifts return loss by several dB.

Cutouts also serve purely mechanical roles: clearance for a connector body or shield can, a window for a battery or display flex, a slot that lets the board flex, or a thermal break that isolates a hot power stage from sensitive analog circuitry. Every cutout must respect the fabricator's minimum internal corner radius, which equals the router-bit radius, so square internal corners are not achievable by routing alone.

Cutout Geometry and Milling Limits

Internal corner radius (routed):
r_min ≈ d_bit / 2

Effective slot width:
w_slot ≥ d_bit (typical d_bit = 0.8 to 1.0 mm)

Antenna ground cutout (vendor-driven):
L_cut × W_cut ≥ the antenna datasheet keep-out footprint

Where d_bit = router bit diameter. The ground-cutout size for a chip or patch antenna is set by the vendor's published keep-out (a near-field dimension), not a fixed wavelength fraction, so always use the datasheet footprint rather than a generic rule of thumb. Example: a 1.0 mm bit leaves r_min ≈ 0.5 mm internal corners, so a square pocket must be laser-cut or corner-relieved.

Cutout Type Versus Production Method

Cutout Type	Typical Method	Min Feature	Corner	RF Purpose
Routed internal slot	Carbide router	0.8 to 1.0 mm wide	r ≈ 0.5 mm	Connector / shield clearance
Antenna ground cutout	Router or laser	Per vendor footprint	Radiused OK	Detuning and loss control
Laser-cut pocket	UV / CO₂ laser	~0.20 mm wide	Near-square	Tight mmWave keep-outs
Thermal break slot	Carbide router	1.0 mm wide	r ≈ 0.5 mm	Isolate hot power stage
Copper keep-out (no cut)	EDA layout rule	Design-defined	Any	Limit fill, keep dielectric

Common Questions

Frequently Asked Questions

What is the difference between a cutout and a copper keep-out?

A cutout routes away the dielectric and all copper, leaving a real hole or slot through the laminate, and is defined on the board-outline layer. A copper keep-out removes only copper in a region while the dielectric stays intact and is enforced as a layout rule. Use cutouts for through-board clearance, antenna isolation, and mounting; use copper keep-outs to control fill near antennas or sensitive RF traces without weakening the board.

What minimum internal cutout size can a PCB fabricator route?

Routed internal cutouts typically need a minimum slot width equal to the router bit, commonly 0.8 to 1.0 mm, with internal corner radius equal to the bit radius (a 1.0 mm bit leaves r ≈ 0.5 mm). Position tolerance is around ±0.10 to 0.15 mm. Sharper or smaller pockets down to about 0.20 mm with near-square corners require laser cutting, which raises cost. Always confirm the chosen shop's design rules first.

Why are cutouts placed under or around RF antennas?

Removing copper and dielectric near an antenna eliminates parasitic ground coupling and lossy substrate that detune the element and cut radiation efficiency. A ground cutout under a patch or chip antenna keeps the feed impedance near 50 ohms and stops the laminate from absorbing fields at 2.4 GHz, 5 GHz, or mmWave bands. Following the vendor's recommended cutout footprint preserves rated gain, return loss, and bandwidth, and reduces detuning near metal enclosures.

Related Terms

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