Panel Fiducial Design

Fiducials are the marks a placement machine looks for to decide where the board actually is. They are small, cheap and completely unforgiving: if the vision system cannot find them reliably, the machine will place every part with an offset that no amount of program tuning can remove.

What Fiducials Have to Achieve

A fiducial has to give the vision system a repeatable centroid under the lighting the machine uses. That means high contrast against the surrounding surface, a defined shape and a size that fits the field of view at the magnification the machine selects.

The system also has to be able to distinguish it from every other feature on the board. A fiducial that resembles a via, a test pad or a silkscreen dot will be confused eventually, usually on the one panel where the confusion is expensive.

Local and Global Fiducials

Global fiducials sit on the panel or on the board outline and establish the coordinate frame for the whole assembly. Local fiducials sit close to a fine pitch part and correct for the local distortion of the panel after reflow or handling.

A board with a large BGA or a fine pitch connector benefits from local marks, because panel stretch between the global fiducials can be a fraction of a millimetre and that is enough to shift a fine pitch part off its pads. Our article on placement accuracy capability explains how much of that budget is available.

Geometry and Contrast

The classic shape is a solid circle of copper with the solder mask cleared around it, so the mark is a bright disc against a dark field. A square or a cross is also used where the machine’s algorithm expects it, but the shape should match the library rather than be invented.

Contrast comes from the surface finish as much as from the mask. An oxidised copper pad is a poor target, and a mark that is covered by a coating or a conformal film may disappear entirely depending on the finish.

Placing Fiducials on the Panel

Fiducials should be placed so that they are visible after every operation that needs them, which means away from rails, clamps and the areas a pallet masks. On a panelised board, the fiducials usually belong on the panel rails rather than on the individual boards, so that they survive routing.

Spacing matters as well. Marks that are too close together give a poor angular reference, and marks that are too far apart may fall outside the machine’s search window. Our article on odd shape panelisation covers the layout of irregular panels.

Common Design Mistakes

The most frequent mistake is to place a fiducial under a component or under the shadow of a tall part, where the vision system cannot see it at the height it operates. The second is to place it too close to a board edge, where the mask and the copper are less controlled.

A third is to leave the fiducial out of the solder mask artwork entirely, so the mark is covered and the finish is the only contrast available. Each of these problems is invisible at design review and obvious at the machine.

Verification at First Article

The fiducials should be checked on the first article with the actual lighting and the actual camera that will be used in production, and the machine’s reported centroid should be recorded. A mark that is found with a large search offset is a mark that will be lost when the panel is marginally different.

It is also worth checking the marks after the panel has been through a thermal cycle, because the appearance can change. Our article on PCB design release checklist places this check in the wider release sequence.

Documentation

The fabrication drawing should state the fiducial size, the mask clearance and the finish, and the assembly drawing should show where they are. A note that says fiducials are required without a size is a note that will be interpreted differently by every shop.

Keeping the definition in one place and referring to it from both drawings prevents the situation where the bare board is built to one size and the assembly program is taught to another.

Checks Before Release

Handling between operations is part of the process, and the damage it causes is often attributed to the operation that preceded it. A change that is not recorded is a change that cannot be explained when the result moves, which is why the record is part of the process.

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where an operation cannot be verified afterwards, it has to be controlled during the operation, and that control has to be visible in the record.

Consumables have a life measured in cycles, and the replacement point should come from the measurement rather than from a failure. The sequence of operations is part of the specification, because a different order produces a different result from the same steps.

Points to Confirm at First Article

Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one.

FAQ

How many fiducials does a panel need? At least three for a full two dimensional correction, and often two diagonally opposite marks plus local marks near fine pitch devices. The machine’s requirements should be checked before the layout is frozen.

Can a via or a test pad be used as a fiducial? Sometimes, but it is a poor practice. A feature that has another function will change appearance for reasons unrelated to vision, and the machine may confuse it with its neighbours.

Does the fiducial need a specific surface finish? It needs the contrast the machine expects, which usually means bare copper or a defined finish with the mask cleared. A mark under a coating may not be visible at all.