PCB Fiducial Design Guide

A pcb fiducial is a small mark on the board that exists purely so that a camera can find it. The placement machine measures the mark, compares its position with the programmed position, and shifts the whole placement program to match. Without the mark, the machine places parts against the mechanical edge of the panel, and the tolerance of that edge is several times worse than the tolerance of the copper.

What a Fiducial Is

A fiducial is a solid area of copper with a defined shape, surrounded by an area where the copper and the solder mask are both absent. The mask opening is what gives the contrast that the camera needs, and the copper gives the mark a defined edge.

The camera does not measure the board, it measures the mark. Everything that the machine does afterwards is referenced to that mark, so a mark that is badly placed or badly made shifts every component on the board.

The mark is also the reference for the solder paste printer, and on some lines for the automated optical inspection system. One set of marks can serve the whole line if it is placed where every machine can see it.

Because the mark is not part of the circuit it is often omitted by a designer who is short of space. The saving is a fraction of a millimetre and the cost is a loss of placement accuracy that no amount of machine tuning can recover.

Global and Local Fiducials

A global fiducial is placed on the panel or on the board outline and is used to align the whole program. Two marks are the minimum for a translation and a rotation, and three are needed if the machine also corrects for scale.

A local fiducial is placed close to a fine pitch component and is used to align that component alone. The local mark corrects for the distortion of the board between the global marks, which matters when the part has a pitch below about half a millimetre.

A board with a fine pitch ball grid array should have local marks on at least two opposite corners of the device. The marks are read at each placement, which costs a little machine time and recovers the accuracy that the laminate has lost.

The rule of thumb is that the local mark is worth adding when the placement tolerance is a significant fraction of the pad pitch. Below that threshold the global marks are enough.

Size, Shape and Contrast

The usual mark is a round dot of about one millimetre in diameter, which suits the field of view of most cameras. The exact size is set by the machine vendor, and the specification should come from the assembly house rather than from a general rule.

A square or a cross shaped mark is used where the camera has to detect an orientation, and a three mark pattern with one offset mark is a common way to detect a rotated panel. The shape has to be recognisable after the mask and the finish have been applied.

The contrast is provided by the mask opening, which has to be larger than the copper by a margin of about the mask registration tolerance. A mask that overlaps the copper edge blurs the mark and the camera either fails to find it or finds it in the wrong place.

The finish matters as well. A mark under a matt finish has a different contrast from one under a bright finish, and a mark that is covered by a coating or a label is invisible. The outline rules and the finish choice both interact with the fiducial.

Clearance and Keep Out

The area around a fiducial has to be clear of anything that the camera might mistake for the mark or that might confuse the recognition. Vias, traces and text are all noise in the camera image.

The usual clearance is between two and three times the mark diameter, measured from the edge of the copper, and it applies on every layer that is visible through the mask opening. A trace that passes under the opening changes the contrast and defeats the purpose.

The keep out also has to consider the height of the components. A tall part beside the mark can shadow the illumination, and a part that is placed close to the mark can overhang it. The fiducial has to remain visible after the first side is populated.

Where a mark has to sit near a dense area, the recognition can be improved by choosing a larger mark rather than by moving it. A larger mark is easier to find and it is less affected by a neighbouring feature.

Placement on the Board and Panel

Global marks belong on the panel rails as well as on the individual boards where the panel is assembled as a whole. A panel with the marks only on the rails will place parts correctly on the panel and incorrectly on a board that is distorted within the panel.

The board level marks should be placed on the diagonal, as far apart as possible, and in a position that survives the assembly. A mark in the corner next to a breakaway tab is a mark that will be damaged by the separation.

The marks should be identical on every board of the panel, at the same relative position, so that the machine can use the same offsets for each. A design that varies the mark position between boards forces a separate program for each.

The tooling holes and the fiducials work together, since both are references for the same machines. Their positions should be defined in the same drawing and checked against the tab design before the panel is released.

Design Review

The review starts with the machine specification. The mark size, the clearance and the shape should come from the assembly house, and the layout should be checked against that specification rather than against a template that came from another product.

The second item is the count and the placement: at least two global marks per board, three for a large or a distorted panel, and local marks for every fine pitch device. The count should be listed in the documentation.

The third item is the keep out, checked on every layer. A fiducial with a trace underneath is a defect that only shows up when the machine refuses to recognise it.

Finally, the first article should be checked with the real camera, and the recognition should be recorded as part of the release package. A misread fiducial is a systematic placement error, and the shift causes list begins with exactly this problem.

FAQ

How many fiducials does a board need? Two at minimum for translation and rotation, and three where the panel can distort. Fine pitch devices need local marks in addition.

Can a via be used as a fiducial? No. A via has a hole and a smaller contrast area, so the camera cannot find its centre reliably. A dedicated solid mark is needed.

Does a fiducial have to be on the panel rails? It should be on both the rails and the boards. The rail marks align the panel, while the board marks correct for distortion within the panel.