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What is a PCB slot or cutout?

A PCB slot is an elongated opening in the board. A cutout is the broader category of an internal opening and may have a more complex contour. Mechanical slots can provide clearance, locating access, adjustment travel or separation between regions. Their useful definition is the finished open shape in relation to the parts that must pass through or around it. A line drawn on a mechanical layer is not a complete specification.

This guide focuses on unplated mechanical openings and routed contours. When the opening must have a conductive wall for a terminal or another electrical function, use the PCB plated-slot guide. When only part of the board thickness is removed, define a cavity PCB feature with a floor and depth. These distinctions determine what the manufacturer must build and inspect.

Start with the mating component or enclosure drawing. Identify the maximum envelope, assembly direction, permitted movement and the datum that locates the opening. Then define the board feature with tolerances that support that task. 鈥淢ake a slot here鈥� leaves too much unresolved for a repeatable fabrication and assembly process.

Overall length, end radius and usable opening

For a conventional obround slot with semicircular ends, width W sets the end radius at W/2. Overall length L includes both rounded ends. The distance between the centers of the end arcs is L 鈭� W. For example, a 4.00 脳 12.00 mm opening has a 2.00 mm nominal end radius and an 8.00 mm end-center distance. Stating 12.00 mm as a centerline distance would define a different opening.

The nominal area of this ideal contour is A = W 脳 (L 鈭� W) + 蟺W虏/4. This is a geometric area, not a machining-cost estimate. The actual routing time depends on the agreed process, cutter, access, number of passes and handling. A larger nominal area is not necessarily a proportional increase in manufacturing cost.

Not every slot is an obround. A rectangular opening with rounded corners needs its own contour definition. A keyhole, dog-bone relief or irregular access opening cannot be completely described by one width, one length and an assumed semicircle. Use a closed dimensioned profile for those features and identify any corner that is functionally critical. The tool on this page deliberately rejects a radius inconsistent with its simple obround model.

A width that fits nominally may still interfere

Consider a 4.00 mm slot with a 0.10 mm negative width allowance and a mating feature that can be 3.40 mm wide. The minimum total width clearance is 0.50 mm. If the feature is centered, that gives 0.25 mm per side. If the worst-case relative lateral offset is 0.15 mm, the smaller-side clearance becomes 0.10 mm. This is a useful width-section check, but it is not a complete fit proof.

The offset must represent the relevant relative tolerance chain. If both the board opening and the mating feature can move, account for their combined adverse displacement according to the product's tolerance method. Do not enter only one contributor and assume the other is exact. Rotation, assembly tilt and deformation can require additional analysis beyond this straight-wall model.

Length and corners remain independent checks. A rectangular part can contact the rounded ends even when its centerline and width look acceptable. An adjustable fastener may need useful travel after subtracting the head, washer and enclosure constraints. Verify the full envelope throughout the intended motion, including the assembly path. A simple minimum gap at one cross-section cannot capture all these conditions.

Calculate an obround opening and local fit margin

The worksheet makes the overall-length convention visible and checks a supplied semicircular radius. It calculates the nominal area and the smaller-side width clearance after an entered relative offset. Use it while preparing a drawing or reviewing a mating dimension. It does not generate a compensated routing program or select a cutter.

Keep the source drawing and revision with the entered dimensions. Negative results remain visible as interference in the modeled width section. A positive result means only that this section has an arithmetic margin. You still need to review overall length, end shape, position in the other direction, rotation and the remaining board structure. Download the note if it helps the review, or use Contact directly for an unresolved geometry.

Communicate a closed finished contour

For an internal cutout, the output should define the opening unambiguously using the format and representation agreed with the fabricator. Depending on the workflow, that may be a board-outline contour or a route representation with an associated drawing. Do not supply two conflicting versions and expect CAM to infer which one is authoritative. Label plating intent separately from the shape.

The explains board-outline and fabrication-output functions. The general lesson for any editor is to inspect the exported package, not only the source design. Reopen the contour alongside copper and drill data, check the origin and units, and confirm the opening appears once in the intended place. Software support varies by feature and output type.

A dimensioned PDF is valuable for intent and inspection, while machine-readable data carries the geometry used by CAM. Keep their revisions aligned. A screenshot with a measurement cursor is a weak substitute because it can omit scale, datums, tolerances or hidden contours. Use the PCB file-format guide when assembling the complete manufacturing package.

Preserve copper clearance and mechanical support

Inspect the slot boundary on every layer, including planes and features that are not visible in the top view. The manufacturing envelope includes contour tolerance and location variation. Nearby copper must remain consistent with the electrical and mechanical requirements throughout that envelope. A line-to-line clearance at nominal geometry can disappear when both features move toward one another.

The remaining laminate webs need their own review. Long narrow webs can be vulnerable during panel processing, depanelization, assembly and use. Board material, thickness, slot length, support location and product loading affect the result. There is no universal web width in this guide. Ask for a construction-specific review where the opening creates a fragile bridge or lies near a mounting load.

If the purpose is electrical separation, do not convert slot width into an automatic creepage or clearance rating. The applicable product standard, working voltage, environment, material and actual three-dimensional path all matter. Copper geometry around the ends may control the relevant path. Use the slot drawing as an input to the electrical safety assessment, not as a substitute for it.

Routing details that affect the result

Rotary routing naturally produces a finite internal radius. The selected process and tool determine what contour can be made, while the customer drawing defines the finished function. Very narrow or deep openings, multiple orientations, small retained pieces and difficult support conditions can complicate the operation. Agree these features before assuming they are equivalent to a simple round drilled hole.

An internal opening also affects panel handling. Consider when the material is removed, how the board remains supported and whether assembly fixtures need to bridge the opening. A large cutout near an edge can alter stiffness even if the final outline is unchanged. These questions belong in the manufacturing and assembly discussion when they affect the design's practical behavior.

Inspect the opening that the product actually needs

Define which measurements establish acceptance: width and length at specified locations, position to datums, contour profile, or a functional gauge where appropriate. A gauge can test a particular fit but may not separately report every dimension. Agree its role and limits rather than assuming it replaces the drawing. For critical mating interfaces, include representative hardware or a controlled model in the first-article discussion.

Send the outline, drill or route data, drawing, board construction and mating requirements through Contact. The PCB DFM review can address nearby copper and weak structural regions. Keep unresolved requirements explicit so the quotation can identify a realistic process and inspection scope. Record any approved contour change in the CAD source as well as the purchase documentation.

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