Panel Fiducial and Tooling Strategy for PCB Assembly

A placement machine finds the board by looking for marks printed on it. Those marks are fiducials, and the holes that locate the panel in the machine are tooling holes. Both belong to the design, and a board with poorly specified references will be placed accurately in one position and inaccurately in another, no matter how good the machine is. Getting them right is a small design task with a large effect on assembly yield.

Why the Machine Needs References

Every machine has its own coordinate system, and the board has another. Unless the two are related by a measurement, the machine can only assume that the board is where it expects it to be, and a panel that is offset by a tenth of a millimetre will place every component with that error.

Fiducials provide the measurement. The vision system finds them, calculates the offset and the rotation of the board, and applies the correction to every placement that follows, which is what makes an accurate machine accurate in practice. Without a fiducial the machine relies on a mechanical stop and on the assumption that every panel is identical, which holds only until the first supplier or process change.

Global and Local Fiducials

Global fiducials are placed at the corners of the panel or of a large board and are used to locate the whole assembly. Two are the minimum for a correction in X, Y and rotation, and three are used where the board may also be scaled or stretched by the process.

Local fiducials are placed close to a fine pitch device and are used to refine the position of that one component. They are needed on boards where the accumulation of tolerance across a large panel is significant, or where the component pitch leaves no margin for a panel level error. Where placement accuracy is critical, a local fiducial is often the difference between a process that works and one that needs constant adjustment.

Fiducial Shape, Size and Contrast

A fiducial is normally a round copper pad, sometimes with a clear area around it, and the vision system recognises it by its contrast against the surrounding surface. A square or triangular shape can be used as long as both the design and the program agree on it.

The size should be large enough to be recognised reliably and small enough to fit the space available, and the contrast around it has to come from an absence of copper and of solder mask. A fiducial surrounded by traces or covered by a thin mask is a fiducial the machine cannot find. A clear area of about three times the fiducial diameter is a common requirement, and it should be stated on the drawing so the fabricator does not fill the space with a copper pour.

Tooling Holes and Their Tolerances

Tooling holes are used for mechanical location in the printer, in the test fixture and in the depanelling equipment. They are drilled or punched to a tolerance that is tighter than the rest of the outline, because their position defines where the panel sits.

The holes and the fiducials have to agree with each other, since a machine that locates the panel from its tooling holes and then finds the fiducials will report a discrepancy if the two sets of features were not positioned in the same operation.

Panel Design and Breakaway Tabs

The panel is what the assembly line actually handles, so its size, its rails and its tabs are part of the design. A breakaway tab that is too thin will break during handling, and one that is too thick will be difficult to remove without stressing the board.

Support and clamping also depend on the panel, because the rails are what the conveyor and the support pins act on. A panel with a wide rail is easier to support than one that has been trimmed to save material. The rails also carry the tooling holes, so their width and their position affect every machine that handles the panel.

Registration Between Fabrication and Assembly

The features a machine uses are created during fabrication, and their accuracy depends on the same tooling that positions the inner layers and the solder mask. A fiducial that is out of position relative to the pads is worse than no fiducial, because the machine will place the parts consistently in the wrong place.

The registration tolerance between the fiducial and the pad layer should be stated on the drawing, and it is usually a fraction of the placement tolerance. The relationship between tooling and layer registration is described in the guide to tooling holes and registration.

Common Problems: Missing, Covered and Crowded Fiducials

The most common problem is a fiducial that has been covered by solder mask or removed from the artwork to make room for a trace. The second is a fiducial that is placed too close to a component, so the nozzle or the board support touches it.

A third is a set of fiducials that are placed asymmetrically, which makes it difficult for the program to determine the rotation unambiguously. The layout should follow a simple pattern that the machine can interpret without special handling. Asymmetric marks also make it impossible for the program to verify that the panel has been loaded in the correct orientation.

Specifying Fiducials on the Drawing

The fabrication drawing should state the fiducial diameter, the clearance around it, the surface finish and the mask treatment, and the position tolerance relative to the pad layer. It should also state the tooling hole diameter and tolerance, and which of the two sets of features is used for what.

Leaving a fiducial to be defined by the assembly house is a common cause of delay, because the decision affects the artwork and cannot be made after the board is fabricated. Defining it in the release package costs nothing and prevents the argument later. The same applies to the tooling holes, which should be dimensioned and toleranced in the release package rather than left to the fabrication house to interpret.

Verification and Records

Verification is a first article check: the machine reports the offsets it measured, and those figures should be inside the tolerance the design allows. A board where the vision system struggles to find the marks will show a large offset or an intermittent failure to detect.

The recorded results, together with the fiducial specification and the tooling drawing, form the evidence that the process is under control. The checks that belong in a design release are listed in the guide to the design release checklist.

FAQ

How many fiducials are needed? Two are enough to correct offset and rotation, and three are used where scaling or distortion has to be measured. Local fiducials are added for fine pitch devices that need a tighter reference.

Can a via be used as a fiducial? It can if it is open, has a clear area around it and is consistent across the panel. A tented via or one that is partly covered by mask will be unreliable.

Do fiducials have to be copper? No, they can be formed in the solder mask or in another layer as long as the contrast is sufficient and the position is defined. Copper with a clear mask opening is the most common choice because it gives the best contrast.