PCB fiducial marks: size, placement and clearance for assembly
Fiducials are small reference marks that let a pick-and-place machine find the exact position of a board. Here is how to design them so assembly runs smoothly.
What a fiducial is
A fiducial is a small, round copper mark with no component and no function in the circuit. The camera of a pick-and-place machine reads it to find where the board really is and how it is rotated, so that every part lands on its pad. The same idea is used on a whole panel and on single fine-pitch parts.
Global and local fiducials
- Global fiducials mark the position of the board or the panel. Put at least two on the board, in opposite corners, or three for the best accuracy.
- Local fiducials sit next to one fine-pitch part (a QFP or BGA, for example), usually two in opposite corners of its outline. Use them when the parts are small or the board is large.
- Panels: the rails of a panel carry their own fiducials, so the machine can align the whole panel before it reads the boards (see panelization).
Size and clearance (common practice)
| Item | Common value |
|---|---|
| Copper dot diameter | 1 mm |
| Solder mask opening | About 2 mm, concentric with the dot |
| Clear zone around the dot | No copper, silkscreen or other marks within the opening |
| Distance from the board edge | At least 3 mm |
| Surface | The same finish as the pads, kept flat |
These are general assembly guidelines, not limits of PCB-ELECTRA. Your assembler may ask for other values: follow the assembler’s data sheet when you have one.
Design rules
- Keep them round and copper. A bare, flat copper dot gives the camera the best contrast.
- Use a mask opening, not a mask-covered dot. Mask over the dot changes its apparent size and colour.
- Do not put anything near them: no silkscreen, traces or vias inside the clear zone.
- Place them asymmetrically (not in a perfect rectangle) so the machine can tell if the board is turned the wrong way.
- Repeat them on both sides if components are mounted on both sides.
Fiducials at PCB-ELECTRA
When you define a client panel in the calculator, the technological margins receive fiducial marks automatically: three on the top side and three on the bottom side. For fiducials on the boards themselves, add them in your CAD tool, as copper and as a mask opening, and send them with the Gerber files (Gerber guide). Check the layout against the DFM checklist before you upload it to the price calculator.
Frequently asked questions
What is a fiducial on a PCB?
A small round copper mark, with no electrical function, that a pick-and-place machine uses to find the exact position and rotation of a board or a part.
How big should a fiducial be?
A common choice is a 1 mm copper dot with a solder mask opening of about 2 mm. Your assembler may specify another size.
How many fiducials does a board need?
At least two, in opposite corners, or three for better accuracy. Add local fiducials next to fine-pitch parts when needed.
Does PCB-ELECTRA add fiducials to a panel?
Yes, on a client-defined panel the technological margins receive three fiducials on each side, automatically.
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