PCB Fiducials: Placement Rules for Machine Alignment

A fiducial is a small copper mark on the board that exists only for a machine to look at. It carries no signal, it is not part of the circuit, and if it is missing, covered or badly placed, the entire assembly line loses its ability to position components accurately. It is one of the cheapest features to add and one of the most expensive to omit.

What a Fiducial Is For

A pick and place machine, a solder paste stencil printer and an automated optical inspection system all need to know where the board actually is. The board is presented on a conveyor with some positional error, and the artwork on the panel may be offset relative to the panel edge. Vision alignment solves both problems at once by finding known marks and calculating the transformation needed to place features correctly.

That transformation corrects translation, rotation and scale. Without it, the machine relies on mechanical stops and the panel edge, which is accurate enough for coarse work but not for a 0.4 mm pitch device where the placement tolerance is a fraction of the pad width.

Fiducials are also used away from the placement machine. Stencil printers use them to align the stencil aperture to the pads, optical inspection systems use them to align the camera to the board, and some test fixtures and depaneling machines use them as well. Each of those users has the same requirement: a mark that is unambiguous, high contrast and in a known position.

Types of Fiducial

Three kinds are in normal use, and they serve different scales.

Panel or global fiducials are placed on the panel frame, outside the individual boards, and they align the machine to the panel. On a large panel, three or more are used so that scale error and rotation can both be corrected.

Board fiducials are placed on each individual board, normally at least three, and they align the machine to the board rather than to the panel. Where a panel is depaneled before assembly, the board fiducials become the primary reference.

Local fiducials are placed close to a fine pitch component and are used for its placement specifically, which corrects for the local distortion of the board after reflow. They matter on large boards, on thin boards and on assemblies that have already been through one reflow cycle, because the board shrinks and moves during the thermal excursion.

Size and Clearance Rules

The numbers that make a fiducial work are stable across the industry, and the standard is generous because a fiducial costs almost nothing.

• Mark diameter: 1.0 mm is the standard, with 0.5 mm as a practical minimum for dense boards and up to 3 mm on panels.
• Clear area: the mark needs a mask free and copper free zone around it, typically giving a total clear diameter of about three times the mark. A 1 mm dot therefore sits in a 3 mm clear area.
• Contrast: the vision system needs the mark to differ from its background. The usual arrangement is bare copper surrounded by bare laminate, or a copper mark under mask surrounded by mask free laminate. What must not happen is a mark that is the same colour as everything around it.
• Distance from the board edge: keep fiducials at least 3 mm, and preferably 5 mm, from any edge, and away from tooling holes and panel rails.
• Number: three is the usual minimum for a board, placed so that they form a wide triangle.
• Symmetry: place them asymmetrically relative to the board outline, so the vision system can distinguish a 180 degree rotation error from a correct orientation.

The clear area rule is the one most often broken. A fiducial with a trace running beside it, or one placed next to a component pad, produces a variable background that the camera cannot threshold reliably. Likewise a fiducial covered by solder mask but with no opening will simply not be found, because the machine looks for the copper feature through the opening.

Local Fiducials and Fine Pitch

On a board with a large fine pitch device, global and board fiducials may not be enough. The board changes shape during the first reflow, and the placement error at the far side of a large package can exceed the tolerance the component allows.

Local fiducials solve this by giving the machine a reference close to the component. Two or three are placed near the corners of the package footprint, typically in the same clear area arrangement as a normal fiducial but smaller, and the machine uses them for that component only.

The rules are the same as for board level fiducials with one addition: the local fiducials should be close enough to the pads that the correction is valid at the pads, which in practice means within a few millimetres, and they must not be covered by the component body or by a stencil aperture. A local fiducial under a connector or hidden by a shield can will not be visible when the machine needs it.

Mistakes That Break Alignment

Most fiducial problems come from a small set of errors, and all of them are visible in the CAD data before the board is built.

The mark is left covered by solder mask with no opening, so the copper is invisible to the camera. The clear area around the mark is filled with a copper pour or crossed by a trace, which destroys the contrast. The fiducials are placed too close to the edge of the board, where the panel rail or the conveyor clamp obscures them. They are placed on a part of the board that will be covered by a component, a shield or a label. Only two are provided, so rotation cannot be distinguished from scale error, or the third is placed in line with the other two so the triangle has no area. And the marks are placed symmetrically, so a 180 degree rotation looks identical to the correct orientation and a whole panel can be populated backwards.

The last error is worth emphasising, because it is catastrophic and silent. A symmetric fiducial pattern gives the machine no way to detect an inverted board, and the mistake may only be found after a full panel has been assembled.

Who Else Uses Them

Three other processes rely on the same marks, and it is worth checking each one when the fiducials are placed.

The stencil printer aligns the stencil to the board using fiducials, and it needs marks with high contrast on the paste side of the board. Where the board is printed on both sides, both sides need fiducials.

Automated optical inspection uses fiducials to align the camera and the inspection program, and it benefits from the same clear area and contrast. Because AOI images the whole board, the placement of the marks relative to the inspection field also matters, and a common practical rule is to keep them inside the area the machine will scan.

Depaneling and test fixtures sometimes use fiducials for the same reason as the placement machine. Where a board will be depaneled before assembly, a fiducial on the individual board is more useful than one on the panel.

Do You Always Need Them

For hand assembly, no. For any machine placement, yes, and the cost is zero: a fiducial is copper on a layer that already exists, and the mask opening is part of a stencil that is already being made.

The only case where they are omitted is a board so small that a 1 mm mark with a 3 mm clear area would consume a significant part of the surface. Even then, a smaller mark with a proportionally smaller clear area can usually be found, and the alternative is a placement process that relies on the board outline, which is a much less accurate reference.

FAQ

• How many fiducials does a board need? Three is the standard minimum, placed to form a wide triangle and positioned asymmetrically.
• Should the fiducial be bare copper or covered by mask? It has to be visible to the camera, so either bare copper with a mask opening around it, or a copper mark with a defined opening. A mark with no opening at all will not be found.
• Can a fiducial be placed under a component? No. It must be free of components, connectors, shields and labels, and visible when the machine looks for it.
• Do fiducials affect the electrical performance? No. They are isolated copper with no connection to any net.
• Are panel fiducials and board fiducials interchangeable? They serve different references. Panel fiducials align the machine to the panel; board fiducials align it to each board. A panel that is assembled before depaneling normally uses both.

Summary

A fiducial is a vision target, and the rules that make it work are about visibility rather than about geometry: a mark of about 1 mm, a clear area of roughly three times that, high contrast against the surrounding surface, a position well inside the board outline, and a pattern of at least three marks arranged asymmetrically so rotation errors are detected.

They cost nothing to add and they are difficult to add later, because the mask opening and the panel layout are both fixed by the time the design is released. Getting them right is part of designing for the assembly process at the same time as the circuit, and the checks belong in the same review as the layout clearance rules and the panelisation that the fabricator will build to. On a prototype build it is easy to hand place around a missing fiducial; on a production line there is no such option, and a missing or obscured mark stops the line.