Fiducial Design and Machine Vision Alignment Guide

A fiducial is a small copper feature that exists only so that a machine vision system can find the board. Without it the placement machine, the paste printer and the automated optical inspection system all have to guess the position from the board edge or from the tooling holes, and neither of those is accurate enough for the fine pitch components in use today. A fiducial costs nothing to add at the design stage and is expensive to add afterwards, because it changes the artwork and every fabrication tool that derives from it.

What a Fiducial Does

The vision system photographs the fiducial, calculates its centre to a fraction of a pixel, and uses two or more of them to build a coordinate system for the whole panel. Every subsequent placement and every inspection measurement is then made in that coordinate system.

The accuracy of that system depends on the fiducials being clearly visible, precisely located and consistent across the panel. A fiducial that is partly covered by mask or that sits on a background with poor contrast reduces the accuracy of everything downstream of it. The system relies on the fiducial as its origin, so an error at that point is applied to every feature placed afterwards.

Global, Local and Panel Fiducials

Global fiducials are placed on the panel or on the board and define the overall coordinate system. Local fiducials are placed next to a fine pitch component and define the position of that component within the global system, which compensates for local distortion of the laminate. That compensation matters most on a large panel, where the laminate can move by more than the placement tolerance across the diagonal.

Panel fiducials are used when several boards are arrayed on a working panel, so that the machine can align to the panel first and then to each board. The hierarchy matters, because a machine that aligns only to the panel will not correct for the movement of an individual board within the array. The hierarchy should be described in the design notes so that the assembly house configures the program the same way.

Shape, Size and Clearance

The common shapes are a solid circle and a square, with the circle generally preferred because its centre is unambiguous regardless of rotation. A typical size is around one millimetre across, with a clear area around it at least as wide as the fiducial itself.

The clear area is the part that is most often neglected. A fiducial surrounded by traces or by a plane does not present a clean edge to the camera, and the vision algorithm may lock onto the surrounding feature instead of the fiducial itself. Keeping the clear area free of copper, mask and legend is the simplest and most effective fiducial rule there is.

Contrast and Surface Finish

The vision system works on contrast, so the fiducial has to be bright against a dark background or dark against a bright one, consistently across the panel. Bare copper with a good finish reflects well, while a heavily tarnished surface or a solder mask over the copper reduces the contrast.

The finish chosen for the board affects the fiducial as well. A dark finish such as a black or a matte mask reduces the contrast against bare copper, while a very reflective finish can blow out the image at the wrong exposure. The interaction should be checked with the assembly house. Where a dark finish is required for cosmetic reasons, a small bright fiducial can be left without the finish to preserve contrast.

Number and Placement

Two fiducials are the minimum to define a plane, and three or more are normally used so that the system can compensate for scaling and for a slight rotation. Placing them as far apart as the panel allows gives the best angular resolution. A long baseline also reduces the effect of a small error in the centre calculation of any single fiducial.

Placement should avoid the edges and the areas that are handled most, because a fiducial that is scratched or contaminated will fail intermittently. Keeping them away from the vacuum holes and the conveyor rails also prevents a systematic offset. Consistency of position across a family of products makes the assembly program easier to create and to verify.

Fiducials and the Solder Mask

The mask opening around a fiducial has to be larger than the fiducial itself, and the relationship between the two is defined by the mask registration tolerance. If the mask encroaches, the visible area of the fiducial becomes asymmetric and the calculated centre shifts. The mask opening should be sized from the fiducial plus twice the mask registration tolerance, measured at the worst case.

This is a common cause of a small, consistent placement offset that appears on one side of a panel. Our land pattern guide covers the related pad and mask geometry rules. Checking the fabricated panel under a microscope before production is a quick way to confirm the openings are correct.

Common Vision Failures

The failure modes are a fiducial that is not found at all, one that is found at the wrong position, and a system that passes its check while drifting slowly. The first two usually come from a contrast or a clearance problem, while the third indicates wear in the machine rather than a defect on the board. Separating a board problem from a machine problem is the first step of any vision alignment investigation.

A fiducial that is missing from one board in an array is a frequent finding, because the feature is often removed by accident when the array is edited. An automated check of the fabrication data catches that before the panels are made. A design rule check that counts the fiducials on every board in the array is a small script that prevents a large problem.

Design Rules Checklist

The useful checklist is short: at least three global fiducials, at least two local fiducials per fine pitch component, a clear area around each one, a mask opening larger than the fiducial, and a finish that gives good contrast. Our tooling guide explains how the fiducials relate to the tooling holes and the panel datum. Where the panel datum is used for the drill and the fiducials for the placement machine, the relationship between them has to be defined on the drawing.

Where the board will be inspected by an optical system, the fiducials for that system should be included as well, because the inspection and the placement systems may use different reference points. A shared set of fiducials is preferable where the machine can use it, because it removes a conversion between two coordinate systems.

Process Control Points

The controls are the artwork check against the fiducial rules, the mask registration, the finish and its contrast, and the machine vision setup itself. The exposure and the lighting of the camera should be part of the machine’s setup record rather than adjusted by the operator. A lighting change that makes one fiducial easy to see can make a different one invisible on the next product.

A periodic check of the placement offset on a production panel confirms that the fiducials are still doing their job. Our quality documentation describes how these results are recorded at gopcb, and a fiducial that has been degraded by handling shows up there first. Fiducial design is a small part of the layout and it decides whether the assembly process can start at all.

FAQ

How many fiducials does a board need? At least three for the panel or the board itself, and at least two local ones beside each fine pitch component. Three allow the system to correct for scale and rotation as well as position.

Can a fiducial be covered by solder mask? It should not be. A mask opening larger than the fiducial keeps the visible shape symmetrical, and mask encroachment shifts the calculated centre and produces a placement offset.

Why does one fiducial fail to be recognised? Usually because the clear area around it is not wide enough, or because surrounding copper confuses the algorithm. Reducing the surrounding features normally solves it.