Fiducial: Key Checks Before Release

Every automated machine on an assembly line finds the board before it does anything else. The fiducial is the mark it finds, and the accuracy of the whole operation depends on how well that mark is made. A printer, a placer, an optical inspection system and an X-ray machine all use the same marks, and a board whose fiducials are poorly designed produces alignment errors that are blamed on the machines.

This article explains what a fiducial must provide, how its size and contrast are chosen, where the marks are placed, and how they are protected through fabrication.

What A Fiducial Must Provide

A fiducial is a feature that a vision system can locate to sub pixel accuracy and that has an unambiguous centre. It has to be flat, of high contrast against its surroundings, and surrounded by a clear area so that the edge of the mark can be found without interference from other features. Anything that makes the edge uncertain, such as mask over part of the mark, a neighbouring trace or a variation in the copper beneath, reduces the accuracy of the alignment.

The mark also has to be in the same coordinate system as the features that will be placed. That means it is created from the same artwork as the pads, etched in the same step and plated in the same pass, so that any error in one appears in the other and cancels. A fiducial that is added as a separate operation on the finished board cannot do that.

Shape, Size And Contrast

The standard shape is a solid circle, with a diameter of about 1 millimetre for a global fiducial and 0.5 to 1 millimetre for a local one. A circle is preferred because its centroid is independent of the orientation of the camera, and because edge detection around a curve is less sensitive to the direction of illumination than detection along a straight line. Squares and crosses are used where a machine requires them, but they are the exception.

Contrast is produced by exposing bare copper, by covering the copper with a mask of a contrasting colour, or by leaving the mark as a hole in a copper plane. Exposed copper oxidises and its contrast falls, so the exposed mark is usually finished with the same surface finish as the pads. A mark that is covered with mask on a copper plane gives the most stable contrast, because the geometry does not change with the finish.

Global And Local Fiducials

A global fiducial is placed at the corners of the board or the panel and is used to establish where the board is in the machine. Three marks are the minimum and four are usual, because three define a plane and the fourth confirms that the board is not distorted. Local fiducials are placed near a fine pitch component and are used to refine the alignment for that component alone, compensating for the local distortion of the laminate after pressing and reflow.

The two are used at different stages. The printer uses the global marks, the placer uses the global marks to find the board and the local marks before placing a fine pitch device, and the inspection equipment uses whichever set gives it the accuracy it needs. A design that provides only global marks will place fine pitch parts to the accuracy of the whole board, which on a large panel is not enough.

Where To Place Them

Global fiducials belong in the corners of the board or of the panel, at least 5 millimetres from the board edge and from any tooling hole, and clear of the rails where the conveyor will grip. Each one needs a clear area around it, typically twice its diameter, which is often called the quiet zone, and no part of that area may carry mask, legend or another feature.

Local fiducials are placed diagonally opposite each other near the component, within about 10 to 15 millimetres of the outermost pads. Two are the minimum and three are safer where the laminate is thin. Where a panel carries several boards, the fiducials are placed on the panel frame as well as on the individual boards, so that the machine can align to the panel first and to the board second. The layout rules that keep those areas free are described under board outline and mounting design.

Fabrication And Mask Requirements

The fiducial is drawn on the copper layer and has to survive etching, plating and mask. Etching changes its diameter by the same amount as the traces, so its size is compensated in the artwork, and the compensation is the same as for the surrounding features to keep the contrast uniform. Plating adds thickness and can make the edge less sharp if the current density at that point is different from its surroundings.

The mask opening over a fiducial is a common failure. An opening that is too small leaves a ring of mask around the mark, which reduces its apparent size and shifts its apparent centre if the ring is not concentric. An opening that is too large exposes the laminate, which gives good contrast but can lift during reflow. Both are avoided by stating the opening explicitly on the mask layer rather than relying on a rule. The general rules that make the design manufacturable are collected under manufacturable design guidelines.

Verification

Verification is done on the machine rather than on the board. The placement machine reports the position it found for each fiducial and the residual error after alignment, and a board whose marks are marginal shows a larger residual, more variation between runs and a placement accuracy that is worse than the machine is capable of. Recording those figures over a period is the way to tell whether the marks are working.

A visual check on the first article confirms that the marks are the intended size, that the quiet zone is clear and that the mask opening is correct. The check is quick, and it prevents a whole production run from being placed with an alignment error that is invisible on the finished board and only shows up as a field failure. The placement process as a whole is described under placement order and pad positioning.

Common Mistakes And Their Cost

The first mistake is a fiducial under a component. A mark that is covered by a part after assembly cannot be used for the second side, cannot be found by an inspection system and is a waste of the space it occupies. The second is a mark placed too close to the board edge, where the conveyor clamps it and the machine cannot see it during part of the cycle. The third is a mark that is a different size from the one the machine was taught, which happens when the artwork is changed and the camera program is not.

The cost of each is paid at the line rather than at the design. A missing mark means the machine falls back to a pin or to a manual alignment, which reduces the placement accuracy for the whole panel and slows the line. A mark that is inconsistent means the line runs at a reduced speed to allow more vision time. Both are avoided by treating the marks as features that have a function rather than as artwork that is added at the end.

FAQ

How many fiducials should a panel have? At least three per panel and two per board that carries a fine pitch component, with the panel set placed on the frame rather than inside the boards.

Can a via be used as a fiducial? It can be used for a coarse alignment, but the mask opening and the annular ring make the centre less certain than a solid circle, so it is not a substitute for a proper mark on a fine pitch product.

Should the fiducial be bare copper or covered with mask? Both are used. A mask covered mark on a copper plane gives the most stable contrast, while a bare copper mark with the same finish as the pads is easier to inspect and is the more common choice.