PCB Fiducial Placement: Mark Point Design for Assembly
A fiducial is a small copper mark that the placement machine uses to find the board. Without it, the machine relies on the panel edges and the tooling holes, and any variation in routing, lamination shrink or conveyor position is transferred directly into the placement position. Good fiducial placement costs almost nothing and removes an entire class of assembly defects.
What the Vision System Needs
A placement machine locates a circuit by viewing two or more fiducials and computing the offset and rotation of the pattern. Both the position accuracy and the rotational accuracy of every subsequent placement depend on that calculation, so a fiducial that is hard to see produces a systematic error rather than a random one.
The camera needs contrast. A mark point is normally a copper pad exposed through an opening in the solder mask, often with the same surface finish as the pads so that it does not oxidise. A fiducial covered by mask, or one placed on a dark, uneven copper pour, may not be recognised at all, and the machine will either reject the panel or place from a poor reference.
Size and Shape
A round fiducial between 0.5 mm and 3 mm in diameter works on most machines, with 1 mm being the usual choice for global marks and 0.5 to 1 mm for local marks beside a fine pitch device. Square marks are sometimes used because the vision algorithm can find the centre of the flat edges, but round marks are more tolerant of lighting variation.
The copper must be clear of any other feature. Keep at least 2 mm of free area around the mark so that the camera sees an unambiguous shape, and keep traces, vias and text out of that zone. A fiducial that touches a neighbouring trace loses its defined outline and may be interpreted as part of a larger copper region.
Solder Mask Clearance and Contrast
The mask opening around a fiducial is typically twice the diameter of the copper mark, which gives a clean ring of laminate between the copper and the mask. That solder mask clearance is what produces the contrast the camera needs, and shrinking it to save space usually causes recognition failures.
Surface finish affects contrast as well. A bare copper mark that has oxidised becomes dark and blends with the background, so the mark should be finished with the same process as the pads, whether that is an organic coating, immersion silver or a nickel gold layer. If the board is coated after assembly, the fiducials must be protected or a different alignment method used, because the machine cannot see through an opaque layer.
Global, Local and Panel Fiducials
Global fiducials are placed on the circuit itself and align the pattern to the machine axes. Local fiducials sit close to a fine pitch package and compensate for the small dimensional changes that occur between the fiducial and the part of the board being populated.
Panel fiducials belong to the panel rather than to a single circuit. They are used for the first alignment and for processes such as solder paste printing and depaneling, and they should be positioned so that they survive every handling step before assembly. The relationship between panel geometry and circuit geometry is part of the general panel design work, and the layout rules that keep a panel manufacturable are collected in the manufacturable design guidelines work, and the mechanical rules that keep a panel flat and clampable are covered in the guidance on board outline and mounting design.
Placement Rules and Keep-Outs
Two fiducials are enough to define translation, rotation and, with a third, scale. Place at least three per circuit and arrange them asymmetrically so that the machine cannot mistake a rotated panel for a correct one. A common pattern is three marks near the corners, offset so that no two are on the same diagonal.
Keep marks at least 5 mm from the board edge where the conveyor clamps the panel, and avoid placing them under a connector body or a heat sink that will be fitted later. They should also stay clear of the area used by the stencil and by any test fixture, because a probe that lands on a fiducial damages the very feature the machine depends on. Standard pad geometry practice applies to fiducials as well, as set out in the notes on PCB pad design standards.
Common Design Mistakes
The most frequent mistake is covering the mark with solder mask, usually because the mask layer was generated automatically without an opening. The second is placing the fiducial on a copper pour without the isolation ring, which removes the contrast. The third is forgetting that the panel will be routed into separate circuits and placing all the marks on the tabs.
Another common problem is a fiducial placed inside a dense component area where the shadow of a tall part or the nozzle path crosses the mark. Vision systems need a clear line of sight, and a fiducial that is visible in the CAD file may be unreachable in the machine.
Verification and Process Control
After the first panel is assembled, record the reported offset and rotation from the machine. A consistent offset of more than about 0.05 mm across a batch suggests that the fiducial data in the placement program does not match the fabricated board, which usually points to a scaling or origin difference between the CAD file and the Gerber set.
If the machine reports frequent recognition failures, examine the marks under the same lighting the machine uses. Problems that look impossible on a bench with bright light often become obvious when the board is viewed with the machine’s own camera, and the fix is usually a larger mask opening rather than a new algorithm.
Fiducials in the Assembly Program
A fiducial only helps when the placement program knows where it is. The coordinates in the program come from the same CAD data that produced the Gerber files, so any scaling applied during data preparation shows up as a constant offset at the machine. Comparing the reported offset against the expected value is the quickest check that vision alignment is working as intended.
Machines also need to be told how many marks to use and in what order. A program that searches for a fiducial in the wrong place will either fail to find it or lock onto a similar feature, such as a large test pad, and place the entire board from a false reference. Naming the fiducials in the CAD file, rather than adding them as anonymous copper, avoids that error.
When a product is transferred to a different line, the fiducial scheme should transfer with it. Marks that work on one machine may be marginal on another with a different camera or lighting arrangement, so the first article on the new line should be checked for recognition reliability before volume production begins.
FAQ
How many fiducials does a board need? Three per circuit, arranged asymmetrically, plus panel fiducials if the circuits are assembled in a panel. Fewer marks work for coarse placement but leave no way to detect a rotated or misidentified panel.
Can a test point be used as a fiducial? Not reliably. A test point is usually covered by mask in a different pattern and has no isolation ring, so the contrast and outline differ from what the vision system expects.
Do fiducials need a specific surface finish? They need the same finish as the pads so that the copper does not tarnish. Bare copper that has oxidised loses contrast, and a mark that cannot be seen is worse than no mark at all.
