PCB Fiducials: Design and Placement Rules

What a Fiducial Is For

A fiducial is a small, sharply defined mark that a machine uses to find its position on a board. The stencil printer, the pick and place machine and the automated optical inspection system each need to know where the board actually is before they can do anything accurate, and they determine that by imaging fiducials and comparing the measured positions with the programmed ones. Without them, the machine indexes off its own mechanics and the board’s tolerance, and the placement error becomes the sum of every tolerance in the chain. With them, the machine corrects for board position, rotation and, where the machine supports it, a small amount of scale error.

Size, Shape and the Clear Surround

The standard fiducial is a solid round dot of bare copper, typically one millimetre in diameter, with a clear area of at least twice the diameter around it in every direction, free of copper, mask, traces and legend. The clear area is what gives the vision system the contrast it needs to find the edge, and a fiducial surrounded by other copper is unreliable no matter how well the dot itself is made. Square and cross shaped marks exist and some machines prefer them because a cross gives a better rotational reference, but the round dot is the most widely supported. The dot should be defined on the copper layer and excluded from the solder mask, and it should be free of the surface finish that covers the pads only if the machine works better with a matte surface; in most cases a standard finish over bare copper is acceptable, and what matters is that the mark contrasts with the mask and is not covered by it.

Global Fiducials and Local Fiducials

Global fiducials are placed on the board, or on the panel, so that the machine can establish a single coordinate system for the whole array. Three of them are the normal requirement, placed asymmetrically or in a triangle so that rotation can be resolved unambiguously, near the corners of the board but inside the area the machine can reach and away from the edges where handling damage occurs. Local fiducials are additional marks placed close to a specific component, usually a fine pitch device, so that the machine can correct for local distortion of the board that the global alignment cannot see. Distortion comes from lamination, from reflow and from the way the panel is supported, and on a large board with a fine pitch BGA the local error can easily exceed the placement tolerance of the package. The usual rule is that any device with a pitch below about 0.5 millimetres, or any package where the placement tolerance is a large fraction of the pad size, should have local fiducials.

Panel Fiducials and the Stencil

The stencil printer aligns the stencil to the board using the same fiducials, so they must not be covered by the stencil aperture and must be visible with the stencil in place. On a panel, the printer usually aligns to panel level fiducials rather than to individual boards, which means the fiducials must be positioned accurately enough that the paste aligns across every board in the array. Where the panel contains many small boards, local fiducials on each board are impractical and the panel alignment has to carry the accuracy, which is one of the reasons panel fiducial placement and panel flatness matter so much. The fiducials used for the stencil are normally the same ones used for placement, so the same clear area and contrast requirements apply twice.

Placement Rules in Practice

Place the global fiducials near the corners of the panel or the board, but far enough from the edge that the handling system does not damage them and the conveyor does not obscure them. Keep them away from the panel border if the border is used by the machine’s clamp rather than its camera, and keep them out of any area that will be covered by a component, a shield can or a conformal coating. Keep the region around them free of vias, because a via in the clear area changes the contrast. Place local fiducials in diagonally opposite corners of the component footprint, close enough to be in the same field of view as the package if the machine requires it, and keep them clear of the paste apertures. On a board that will be tested, keep the fiducials away from the test fixture’s probe field, since a probe mark on a fiducial destroys the contrast that the vision system needs.

The Mistakes That Cause Alignment Failures

The most common defect is a fiducial that is covered by solder mask, which usually happens when the mask layer was copied from the copper without an exclusion. The second is insufficient clear area, often because the copper pour was routed too close. The third is poor contrast, which occurs when the fiducial is placed on a dark mask over a dark surface or when the finish is too reflective. The fourth is a missing local fiducial on a fine pitch device, which shows up as a placement offset that varies across the board and is blamed on the machine. The fifth is a fiducial placed under a component that is placed early in the sequence, which makes it invisible for the rest of the run. All five are visible in a design review if the fiducials are checked as a group rather than as individual marks.

FAQ

How big should a fiducial be? One millimetre of bare copper is the standard, with a clear area of at least twice that diameter around it, free of copper, mask, traces and legend.

How many fiducials are needed? Three global fiducials, placed asymmetrically so that rotation is unambiguous, plus local fiducials near any fine pitch component.

Should fiducials be covered by solder mask? No. The mask must be excluded so the copper is visible and contrasts with the surrounding board surface.

When do I need local fiducials? For devices with a pitch below about 0.5 mm or where the placement tolerance is a large fraction of the pad, because board distortion varies across the panel.

Can fiducials be placed on the panel border? Only if the machine can see them there. If the border carries the clamp or the conveyor, the fiducials must be inside the board area instead.

Conclusion

Fiducials are cheap marks that decide whether an expensive placement machine can do its job, and they are governed by three rules: enough bare copper, enough clear area, and enough contrast. Place three global fiducials asymmetrically, add local fiducials wherever the pitch is fine, keep the marks clear of probes, coatings and components, and check them as a group before releasing the design. The material and mask constraints that affect fiducial contrast are part of PCB capabilities, the placement conventions belong in PCB design and layout, and the assembly process that uses them is covered by SMT PCB assembly. A prototype PCB assembly run is where an alignment problem becomes visible before it becomes a yield loss in 2026.