Fiducial Design and Placement for Pick and Place Alignment

A pick and place machine has to know where the board actually is before it can put a component in the right place. Fiducials are the marks that give it that reference, and their design and placement decide how accurately the machine can align itself to the panel. A poorly placed fiducial, a low contrast one or one surrounded by copper does not fail loudly; it simply shifts every placement by a small amount that later becomes an assembly defect.

What Fiducials Are For

The machine locates a fiducial with a camera and calculates the position and rotation of the board relative to the machine coordinate system. From that calculation it derives the offset and the small angular error of the panel, and it applies both to every placement. Without a fiducial, placement accuracy depends on the mechanical repeatability of the conveyor and the tooling pins alone.

The correction is what makes a fiducial valuable. A panel that is loaded at a small angle would otherwise produce a rotational error that grows with distance from the origin, which is why the effect is worst at the far corner of a large panel. The correction is applied to every placement, so a small alignment improvement at the fiducial is multiplied across the whole assembly.

Global, Local and Panel Fiducials

A global fiducial is placed on the board outline or on the panel rail and is used to align the whole assembly. A local fiducial sits next to a fine pitch component and is used to refine the alignment of that specific part. Panel fiducials on the rails are used for machine alignment, while board level fiducials inside each board image are used for the same purpose at a finer scale.

The three levels serve different corrections. Global marks correct the panel position, local marks correct the position of a component relative to the copper underneath it, and the two are not interchangeable because the laminate itself can move between the panel level and the local level.

Shape, Size and Contrast Requirements

A fiducial is normally a solid copper dot with a mask free area around it, so that the camera sees a bright copper circle against a dark background. The size has to suit the camera field of view: too small and the centroid calculation is noisy, too large and the field of view cannot contain the whole mark at the required magnification.

Contrast is the property that actually determines performance. A mark that is partially covered by mask, or one on a surface finish that reflects like the mask, gives a soft edge and a poorly defined centroid. The finish specified for the board has a direct effect on how well the fiducial reads, which is why the fiducial requirement should be stated alongside it. The fabrication note should state the mark size, the mask opening and the copper coverage so that the result is repeatable from panel to panel.

Clearance and Keep-Out Rules

The area around a fiducial has to be free of anything that could be mistaken for the mark or that could obscure it. A common rule is a clear zone around the dot that is several times its diameter, kept free of copper, traces, vias, mask features and legend. Any copper inside that zone changes the contrast and can pull the centroid off centre.

The keep-out also applies to the stencil and to the paste print, because a fiducial that is covered with paste is invisible to the camera. If the fiducial must sit inside the printed area, the stencil has to have an aperture that leaves it clear.

Placement of Global Fiducials

Global fiducials work best when they are far apart and not collinear. Two marks at opposite corners of the panel give the machine a long baseline, which reduces the error in the calculated rotation. Two marks close together give a short baseline, and a small error in either centroid then produces a large angular error across the panel.

They should also be positioned so that they are visible with the panel loaded in the normal orientation. A fiducial that sits under a clamp or outside the conveyor width is not available when it is needed. The position should be checked against the actual machine and tooling rather than against the drawing alone.

Local Fiducials for Fine Pitch

Local fiducials are used where the placement tolerance is tighter than the panel level alignment can deliver. They are placed diagonally opposite each other near the component so that the machine can compute the local offset and rotation, and they need the same contrast and clearance as the global marks.

They cost board area, so they are used selectively. A board with one or two fine pitch areas benefits from local marks at those locations, while a board full of them needs a different conversation about panel level tolerance. Our land pattern notes cover the pad geometry that the local alignment has to match.

Panel and Tooling Fiducials

The panel needs its own alignment features for the fabrication and assembly tooling. Tooling holes provide the mechanical datum for the fabrication processes, and the fiducials provide the optical datum for assembly. Both should be placed on the rails so that they are removed with the frame, and both should be consistent from panel to panel.

Our tooling hole notes describe how the mechanical datum is established, and the fiducial layout should be derived from the same coordinate system so that the optical and mechanical references agree.

Common Mistakes That Degrade Accuracy

The most common mistake is a fiducial placed too close to copper, so that the machine sees a shape with an irregular edge. The second is a fiducial with insufficient contrast, usually because the mask opening is the same size as the dot and the mask edge overlaps it. The third is a fiducial hidden under a component or covered by the stencil.

A fourth mistake is a fiducial design that changes between the prototype and production panels. The mark should be treated as part of the fabrication data and released with the same control as the copper layers. If the production panel uses a different mark or a different position, the assembly program has to be changed, and a change that is not recorded produces a shift that is difficult to diagnose.

Verification and Maintenance in Production

The assembly program should be verified by placing a board and checking the actual position of the components against the copper, not just against the program coordinates. A small residual offset that is constant across the panel points at a program origin rather than at the fiducials.

<img src="https://www.gopcba.com/wp-content/uploads/2026/09/237-scaled.png" alt="Pick and place vision system aligning to a fiducial mark” />

In production, the fiducials should be checked on a sample of panels for condition and contrast, because a mark that has been scratched or partially covered will degrade the alignment without generating an error message. Our AOI notes describe how the resulting placement errors are detected after reflow, and our quality guide describes how a fiducial defect is classified at gopcb.

FAQ

How many global fiducials are needed? Two is the minimum, placed as far apart as possible, and three allows the machine to detect a scaling error as well as an offset and a rotation. More than three is rarely useful for a rigid panel.

Can a via or a pad be used as a fiducial? Not reliably, because the shape and the contrast vary. A dedicated mark with a defined size and a clear zone is what the vision system is designed to find.

Do fiducials need to be on every board in the panel? Only if the panel level alignment is not accurate enough for the placement tolerance. For most products, panel level marks plus local marks at fine pitch areas are sufficient.