Vision alignment system for PCBs: cameras that find fiducial marks and correct board offset, rotation and scale
PCB fiducial alignment · Mark size and keep-out, distance from the clamped edge, two or three global marks, local marks at fine-pitch parts
Before a printer, placement head or test fixture touches a circuit board, a downward-looking camera finds two or more small copper dots on it and the machine works out where the board really is. Those dots are fiducial marks, and the alignment can only be as good as the marks the board designer put down: a mark hidden under a conveyor clamp, crowded by a nearby track or placed symmetrically can make the vision step fail or, worse, succeed on the wrong answer.
Camera-based fiducial alignment of printed circuit boards and panels in SMT printing, placement and test equipment, and the fiducial mark design it depends on: mark size, keep-out, edge distance, global and local marks, and the corrections they allow. Solder paste printers as machines are covered on the Solder paste printer page and placement gantries on the XY motion system of an SMT placement machine page; camera alignment for stacking transformer laminations on the Alignment vision system for stacking core laminations page; lens loading vision on the Vision/sensor system for lens loading page.
CNFX editorial research · Sources checked October 17, 2026 · 3 sources
A 1 mm copper dot in a clear ring, kept away from the clamped board edge
LCSC recommends a solid circle with a diameter of “1.0 mm for most production scenarios” and describes a layout of a 1 mm pad with a 3 mm solder-mask opening. [1] Bittele's 7PCB guide gives 1 mm as the minimum diameter with a 3 mm diameter clearance area, a solid copper mark with the same finish as the rest of that layer, and no pads, holes or traces inside the clearance. [2]
The keep-out figures differ between sources. LCSC asks for a clearance radius at least twice the mark radius and says recognition is better at three times; the OpenPnP project calls a 1 mm pad with a 2 mm keep-out ideal and allows the keep-out to be relaxed on crowded boards if nearby features cannot be mistaken for a round mark. [1][3] Editorial reading: for a 1 mm mark these give keep-out diameters of 2 mm (OpenPnP, and LCSC's minimum) and 3 mm (7PCB, and LCSC's preferred). Ask the assembler which one its machines are set up for.
LCSC wants the mark centre more than 3.85 mm from the board edge so that the 3 mm clamp rails of some placement machines do not cover it, and suggests putting marks on a 5 mm tooling strip where the board is tight. [1]
Put the mark size, keep-out and coordinates in the fabrication and assembly data, and check them against the conveyor clamp width of the line that will build the board.
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Two marks fix shift and rotation; a third adds separate X and Y scale and skew
OpenPnP documents what its locator can estimate from each number of marks: with two, the board's position, rotation and one average scaling difference; with three, position, rotation, separate X and Y scaling and non-squareness; with more than three, the same estimates averaged to reduce measurement noise. It recommends at least four where possible and asks that the camera's units-per-pixel calibration be checked first. [3]
The 7PCB guide asks for at least two global marks at opposite ends of the board or panel, as far apart as possible, and three in a triangle when non-linear distortion such as scaling, stretching or twisting has to be compensated. Local marks next to a part's land pattern correct that part alone: one in the centre of the pattern for translation, or two placed diagonally inside the pattern perimeter for rotation. [2]
LCSC warns that two marks placed symmetrically, for example at the top-left and bottom-right corners, may let the machine accept a board loaded 180° rotated and place parts wrongly. [1]
Editorial calculation: if two global marks 200 mm apart are found 0.10 mm out of line, the board is rotated by atan(0.10 / 200) ≈ 0.029°. Left uncorrected, a pad 100 mm from the rotation centre moves about 100 × 0.10 / 200 = 0.05 mm sideways. Distances are assumed for illustration.
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Board data the alignment needs
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
Fiducials: [diameter; keep-out; finish; global count and coordinates]
Local fiducials: [which parts]
Board or panel: [size; edge clearance; tooling strip]
Process: [printing / placement / test]
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Compiled and source-reviewed by the CNFX editorial team.
Each manufacturer above links to the page on that company's own website where the product is listed, with the date we checked it. How we source and review
· Page last updated 17 October 2026
Buyer questions
What size should a PCB fiducial be?
LCSC and Bittele's 7PCB guide give 1 mm; OpenPnP also uses a 1 mm round pad. [1][2][3]
How much clearance does a fiducial need?
Sources differ: OpenPnP calls a 2 mm keep-out ideal, LCSC asks for at least twice the mark radius and prefers three times, and 7PCB gives a 3 mm clearance diameter. [3][1][2]
Why use three fiducials instead of two?
OpenPnP says three let it estimate separate X and Y scaling and non-squareness, which two marks cannot. [3]
Can symmetric fiducials cause errors?
LCSC notes that two symmetric marks may let a 180°-rotated board pass alignment. [1]
Sources and what still needs a direct check
Sources checked 2026-10-17. Mark design figures come from two board services, LCSC and Bittele Electronics (7PCB). What each number of marks lets the software estimate is taken from the documentation of OpenPnP, an open-source pick-and-place control project.
