PCB panelization: rails, V-score and tab routing, boards per panel, utilization, and a worked example
Boards are not made or assembled one at a time: the fab builds them on a working panel and the assembler runs a panel of them through the line, held by rails along its edges, and breaks them apart afterwards. How many boards fit, in which orientation, with what wasted, and therefore what each costs, is a small sum worth doing before the outline is frozen. This calculator takes the board, the panel, the rails and the separation method, and gives the boards per panel in the better orientation, the utilization of the panel and of its usable area, the leftover strips, the V-score lines or routing tabs, and the cost per board, with the panel drawn to scale.
How to use the panelization calculator
- Type the board's width and height and the spacing between boards: 0 for V-scoring, about 2 mm for tab routing (the switch sets it).
- Pick a panel from the presets or type its size, choose the rail width, whether rails run on two or four sides, and whether the board may be rotated. The count, the grid, the utilization and the leftover appear.
- Add the panel price for the cost per board. The Panel table tab tries the same board on every common panel, each with a Use button.
Rails, fiducials and tooling
The assembly line's conveyor and the pick-and-place clamps hold the panel by two strips along its long sides, the rails, 5–10 mm wide and free of parts; thin or flexible panels get rails on all four sides. The rails carry three fiducials (copper dots the placement camera finds the panel by, in an L so that the orientation is unambiguous), tooling holes for the test fixture and the router, and a label. They are scrap once the boards are broken out, and the calculator subtracts them before laying the boards.
V-score or tab-route
Two ways to make the boards separable. A V-score is a pair of V-grooves cut from the top and bottom along a straight line across the whole panel, leaving about a third of the thickness to snap: the boards touch, nothing is wasted between them, and the edges come out clean; it needs straight edges that line up across the panel, boards over about 1 mm thick, and parts at least 1 mm clear of the line. Tab routing cuts a slot around each board with a 2 mm router bit and leaves small tabs, perforated with mouse bites, to hold the board in: any outline will do and parts can sit at the edge, at the cost of the routed gap, the stubs where the tabs break, and the routing time. Many panels use both, scoring the straight edges and routing the rest.
Counting the boards
across = ⌊(usable width + s) / (board width + s)⌋ down = ⌊(usable height + s) / (board height + s)⌋
With a spacing s between boards, n of them in a row take n × board + (n − 1) × s, so adding s to both the usable length and the board gives the count directly. The calculator tries the board upright and rotated and takes the better, unless the orientation is fixed because a connector edge has to reach a rail or the parts dictate the conveyor direction. A mixed layout, with the leftover strip filled by a row turned the other way, can beat both grids but only with tab routing, since V-score lines must run straight across the panel; the calculator flags when such a strip exists.
Utilization and cost
The fab prices the panel, so the cost per board is the panel price over the count, and utilization, the boards' area over the panel's, is money: 70–85 % is typical, over 90 % means the board's size was chosen with the panel in mind. The leftover strips tell you where the next row is: when one is nearly a board wide, a few millimetres off the outline, a narrower rail or a different panel size gains a whole row at once, and the count moves in steps, never smoothly.
Your panel, step by step
- Usable: 250 × 184 mm inside 8 mm rails on two sides.
- Grid: upright 4 × 4 = 16, rotated 6 × 3 = 18 → 18 boards, rotated.
- Utilization: 18 × 2400 mm² / 500 cm² = 86%; leftover 10 mm across and 4 mm down.
- Separation: 11 V-score lines; $48.0 per panel is $2.67 per board.
Worked example: 60 × 40 mm boards on a 250 × 200 mm panel
A 60 × 40 mm board, V-scored, on a 250 × 200 mm assembly panel with 8 mm rails along the long sides, at $48 a panel. The usable area is 250 × (200 − 16) = 250 × 184 mm. Upright, ⌊250 / 60⌋ × ⌊184 / 40⌋ = 4 × 4 = 16 boards; rotated, ⌊250 / 40⌋ × ⌊184 / 60⌋ = 6 × 3 = 18, so the boards go on rotated: 18 per panel, 18 × 2400 mm² = 432 cm² of 500 cm², 86% of the panel, with 10 mm left across and 4 mm down; 11 score lines separate them. At $48 a panel each board is $2.67. Tab-routed with 2 mm gaps instead, the same panel holds 18 boards (86%) with 4 tabs each; on a 350 × 250 mm panel the V-scored grid grows to 25 boards at 69%.
Questions
What panel size should I use?
Ask the assembler: the line has a maximum, commonly 350 × 250 or 450 × 350 mm, and a minimum of about 50 × 50. Within that, the size that divides your board best; the Panel table shows the candidates.
V-score or tab-route?
V-score for rectangular boards over 1 mm thick with nothing within 1 mm of the edge: it wastes nothing and leaves clean edges. Tab-route for odd outlines, thin boards, edge-mounted parts or connectors that overhang. Many panels do both.
Do I need rails on all four sides?
Two along the long sides are enough for the conveyor and most clamps. Four when the panel is thin or long enough to sag, or when parts overhang the short edges and would foul the rails' neighbours.
How does this relate to the fab's price?
The fab builds on its own working panel, typically 18 × 24 inches, and your assembly panels are cut from it; its quote is for the working panel's area including what it cannot use. A board or panel that divides into the working panel cleanly is cheaper per board, which the table's fab-panel rows let you check.