PCB Panelisation: V-Score, Tab Routing, and Fiducials Explained

Detailed Explanation

A modern SMT pick-and-place machine handles boards on a conveyor belt: the board enters on rails, moves to the placement head, gets components placed, and exits. If a board is too small, it falls between the rails, tilts, or jams. The reflow oven has the same constraint — a small board can topple or shift in the oven's air turbulence.

The solution is panelisation: arranging multiple copies of the board (or multiple different boards) in a larger array that the assembly line can handle reliably. After soldering, the boards are separated by snapping apart the panel along pre-scored lines or perforated tabs.

Why Panelisation Matters Beyond Board Size

Even for boards large enough to travel the assembly line individually, panelisation is often used in production for throughput reasons: placing 6 copies of a board in a single panel means one paste print, one pick-and-place run, and one oven pass per 6 boards — the setup time (stencil alignment, program load) is amortised across all 6.

Additionally, the assembly house's minimum order fee often applies per-panel-run rather than per-board, so panelising more boards per panel directly reduces unit cost at volume.

V-Score (V-Groove Separating)

V-scoring uses a rotary blade to cut a V-shaped groove partway through the board from both the top and bottom, leaving a thin web of material connecting the boards in the panel. After assembly, the boards are snapped apart by hand, roller scoring, or a pneumatic depanelling tool.

Rules and constraints:

• V-score cuts must be perfectly straight — they cannot turn corners or follow curves.
• The cut extends to the edge of the panel in a straight line, meaning all boards in a V-scored panel must have the same outline shape and be aligned in rows.
• Typical V-score geometry leaves 1/3 of the board thickness as the web (e.g. 0.53 mm web on a 1.6 mm board). Thinner webs break during handling; thicker webs require more force and can stress components near the board edge.
• Component clearance for V-scoring: Keep all components at least 0.5 mm (preferably 1 mm) from the board edge where a V-score cut will be made. Components right at the edge can be damaged by the mechanical stress of the snap — especially tall or rigid components (connectors, electrolytic capacitors).

Use V-scoring when: Boards are rectangular, all copies in the panel have the same outline, and a clean, flat edge is needed after separation.

Tab Routing (Mouse Bites)

Tab routing uses a CNC router to cut slots around the board outline, leaving small uncut tabs connecting the board to the panel frame. The tabs are typically pre-perforated with a series of drilled holes called "mouse bites" — small holes close together along the tab that create a perforation line. After assembly, the boards are broken free by hand or depanelled with a router.

Rules and constraints:

• Can follow any board outline shape (including curved or irregular outlines) — unlike V-score.
• Each tab is typically 3–5 mm wide with 0.8–1.0 mm mouse-bite holes spaced 0.8–1.0 mm apart.
• The tab breakout leaves a slightly rough edge (the remaining hole material). For enclosures or connectors requiring a smooth board edge, factor in post-assembly edge cleanup or tolerance on the edge dimension.
• Component clearance for tab routing: Keep components at least 1.5–2 mm from the tab locations. The mechanical stress of snapping a tab is locally higher than V-scoring and is applied to a specific point rather than along the full edge.

Use tab routing when: The board has an irregular outline, the board design has components extending to (or over) the edge, or a mix of board shapes are being panelised in the same array.

Panel Frame and Rails

The panel typically includes a surrounding frame of panel material that is discarded after assembly. The frame provides:

• Edge rails: The conveyor grips the frame edges, not the board edges. Most assembly machines need at least 5 mm of clearance on two opposing sides (the conveyor rail sides). A common convention is 5 mm rails on the top and bottom of the panel (in the direction of travel).
• Fiducial markers and tooling holes (see below)
• Barcodes and identifiers: Many assembly houses add a panel barcode for traceability; leave space on the frame for this.

Fiducial Markers

Fiducial markers are copper circles (typically 1 mm diameter, sometimes 2–3 mm) with a clear copper-free courtyard area around them (typically 3× the fiducial diameter). They appear in the copper layer but have no solder mask opening. The pick-and-place machine's vision system uses fiducials as reference points to precisely locate the panel and correct for any rotation or stretch.

Types:

• Global fiducials: Located on the panel frame, used to align the entire panel. A minimum of two global fiducials are required (three is preferred for non-square panels). They should be placed diagonally opposite each other (or at three corners) for maximum angular correction accuracy.
• Local fiducials: Located on each individual board within the panel, used for fine-correction of placement accuracy on that board. Required when tight-placement components (BGAs, QFPs with sub-0.5 mm pitch) are present. Place at least two per board.

Fiducial location rules:

• On the panel frame (or the board itself, if no separate frame)
• Clear of all components and traces by at least 3 mm
• Free of solder mask (the fiducial must be visible bare copper with no mask opening — not solderable)
• Not on the V-score cut line

Tooling Holes

Tooling holes (also called registration holes or mounting holes) are drilled unplated through-holes in the panel frame used to register the panel accurately in wave soldering fixtures, test fixtures, and depanelling equipment.

Standard tooling hole diameter is 3 mm (sometimes 2.8 mm for tight tolerances). Typically two tooling holes are placed at diagonally opposite corners of the panel frame, within the border area. They must be unplated (no copper barrel) — plated holes tolerate less positional variation.

Edge Clearance Rules

Components placed too close to the board edge cause two problems: they may overlap the V-score or tab route area (causing assembly damage during depanelling), and they may overhang the conveyor rail area (preventing the board from lying flat on the assembly machine's rail surface).

General rules:

• V-score edge: 0.5–1.0 mm minimum from any component body to the V-score line
• Tab route edge: 1.5–2.0 mm minimum from any component body to the nearest tab
• Conveyor rail edge: 5 mm minimum from any component body to the panel rail edge (the 5 mm rail must be free of components)

Designing the Panel vs Letting the Fab Panelise for You

Letting the fab house panelise: Most fab houses will accept an individual board Gerber set and create a panel for you. They choose the arrangement, breakout method, and frame dimensions. This is fast and low-effort but gives no control over component-to-edge clearance, tab locations, or array dimensions.

Designing the panel yourself: Add a panel outline layer in your EDA tool, replicate the board in an array, add the frame, V-score or tab route markers, fiducials, and tooling holes as separate layers or copper features, and include these in the Gerber package. KiCad's GerberZIP or KiKit panel tool and Altium's Panelisation view both support this workflow.

For a production run repeated over many orders, designing the panel once is the more cost-effective path and ensures consistent component clearances across every production run.

Design Considerations

• Panel size constraints: Most assembly machines accept panels up to 460 × 460 mm or larger; most fabs produce panels in 250 × 330 mm or similar standard sizes. Confirm the maximum panel size with your assembly house.
• Odd-shaped boards require tab routing: If your board outline is not rectangular, V-scoring is not possible — design the tab routes and mouse-bite perforation at the initial layout stage, not as an afterthought.
• Test point clearance on panels: If bed-of-nails ICT testing is used after assembly, test points must land within the PCB outline and not be blocked by tab material or panel frame — factor in the panel design when placing test points.

Common Mistakes

• Placing components within 0.5 mm of a V-score line, leading to component damage when the panel is depanelled.
• Forgetting global fiducials on the panel frame, requiring the assembly house to add temporary markers or proceed without fine-correction.
• Using V-scoring on a board with a non-rectangular outline — the router cut must follow straight lines only.
• Placing tooling holes in plated-through positions on the PCB interior, then wondering why the assembly fixture doesn't register correctly (tooling holes should be unplated, on the panel frame).