Fiducial Design and Placement for Vision Alignment

A fiducial is the only feature on the board that the placement machine is allowed to interpret, and everything the machine does afterwards is referenced to it. When a mark is undersized, partly covered by mask or placed too close to copper, the error enters the machine datum and appears later as a placement defect that no amount of machine calibration will remove. This article covers the geometry, the contrast and the count of marks, and how to tell a fiducial problem from a machine problem.

What the Vision System Needs from a Fiducial

A fiducial is a shape the machine can find and centre on, and the algorithm works by contrast rather than by interpretation. A 1.0 mm copper dot with a 2.0 mm clear area is the usual default, and both dimensions matter: the dot sets the centroid, and the clear area stops a nearby trace from dragging that centroid sideways.

Placement machines typically resolve the centroid to between 10 and 25 micrometres on a good mark, and that figure is the floor of the machine’s placement accuracy. A mark that is smaller than the specified size, or partly covered by mask, moves the centroid by more than the machine error, and the placement specification is lost before the first production board is built.

Size, Shape and Clearance Rules

Global fiducials are normally 1.0 to 3.0 mm across with a clearance of at least twice the mark diameter. Local fiducials, used where a fine pitch device needs its own alignment, are smaller, commonly 0.5 to 1.0 mm, and they sit within about 10 mm of the component they serve.

Square marks suit some algorithms and round marks suit others, and the choice should follow the machine rather than preference. Whichever shape is selected, it has to be identical on every board in the panel, because the machine learns the shape from the first one and applies the same threshold to the rest. Placement capability is discussed further in our capability notes.

Clearance is measured on the finished panel, not on the artwork. Etch and mask registration both close the window that was drawn, and the finished clearance is what the camera actually sees.

Contrast and Surface Finish

The mark has to differ from its surroundings under the machine’s own illumination. Bare copper carrying a thin oxide reads differently from copper covered by ENIG, and on a board with a dark solder mask the mask itself can be as dark as the copper under red light.

The reliable combination is a copper mark with the mask cleared from the mark and its clearance band, so the camera sees copper against laminate. Where the finish is bright, the illumination and exposure belong in the machine recipe, and they should be verified on a sample of the production finish rather than on a bare coupon. The fabrication notes set out how that finish and the mask opening are called out on the drawing.

Panel Fiducials and Board Fiducials

Panel fiducials locate the panel in the machine and board fiducials locate the individual board inside the panel. Where boards are separated before assembly, the board marks carry the alignment; where the panel is assembled and separated afterwards, the panel marks establish the datum and the board marks refine it.

Both sets have to survive every process that precedes assembly. A mark that is routed away at depaneling, or covered by a pallet, is not available when it is needed, so the fiducial pattern belongs on the panel drawing beside the rails and the tooling holes.

Number and Position Around the Board

Three fiducials are the practical minimum, one at each of three corners, because three points define scale, rotation and offset together. Two marks cannot separate a stretch from an offset, and a board aligned from two marks will show errors that vary with direction across the panel.

Spacing should be as wide as the board allows, since angular accuracy improves with the lever arm between marks. On a long narrow board the marks belong at opposite ends rather than clustered at one end, even though that means a wider search window in the machine programme.

Interaction with Solder Mask and Copper Balance

Clearing the mask around a fiducial leaves an exposed copper dot, and the mask opening has to be large enough that no sliver of mask sits at the edge of the mark. A sliver shifts the apparent centroid by a few micrometres, and a few micrometres is significant on a 0.4 mm pitch device.

The copper around the mark also matters at fabrication. An isolated dot inside a large clearance etches differently from one connected to a plane, so the mark should be detailed with the same clearance on every layer to keep the etch consistent across the panel. Finish effects on the exposed dot are described in our plating guide.

<img src="https://www.gopcba.com/wp-content/uploads/2026/05/G9.jpg" alt="Placement machine camera viewing a panel fiducial” />

A fiducial that is connected to a plane by a thin spoke etches to a different diameter from one that is isolated, and the difference is visible in the finished mark.

Separating Fiducial Error from Machine Error

Placement accuracy is measured with a glass plate or an X-ray system, comparing the placed position with the programmed position over a sample of boards, using the measurement approach described in our X-ray and AOI guide. The figure that matters is not the mean error but the spread, expressed as a capability index against the placement tolerance.

When the spread is wider than the specification, the cause is normally divided among the fiducial centroid, the machine’s own repeatability, and the dimensional stability of the board through reflow. A fiducial study separates the three, and it costs less to run than a placement study on real product.

Fiducials on Flexible and Thin Boards

A thin or flexible board moves during placement, so the mark seen at the start of the cycle is not where it is when the nozzle arrives. Local fiducials beside the fine pitch device reduce that error, because the distance between the mark and the placement position is short and the accumulated movement is smaller.

Where the board is held in a pallet, the marks have to be visible through the pallet openings, and those openings have to be large enough that the clearance band is not in shadow. A pallet that covers a mark changes the machine datum without raising any alarm.

Documentation and Verification

The fabrication drawing should state the fiducial diameter, the clearance, the finish under the opening, the position tolerance and the number of marks. The assembly drawing then states which set is the datum and how the marks are illuminated on the machine.

First article inspection verifies the marks by measuring the diameter and the clearance band on the finished panel and by running the vision routine on the production machine. A mark that a bench system finds easily can still fail in production if the illumination recipe has not been updated for the new finish or the new mask colour.

FAQ

Can a via be used as a fiducial? No. The barrel and the mask ring change the apparent shape and the contrast, and a centroid taken from an annular ring is not repeatable enough for fine pitch placement.

How many fiducials does a panel need? Three per board as a minimum so that scale and rotation are both defined, plus three on the panel where the panel is the machine datum, placed as far apart as the dimensions allow.

Why does placement drift across a panel? Usually a fiducial problem rather than a machine problem: a mark that is partly covered, a clearance band that is narrower than drawn, or a datum established from two marks instead of three.