PCB Fiducial Guidelines Improve Assembly Outcomes

Key Takeaways

• Fiducials are targets for optical systems to assess distance, plane skew, and board warp/twist.

• Fiducials can occur at multiple levels of the design: the panel, the board, and individual fine-pitch components.

• Fiducial placement should be towards the corner of designs but with enough clearance from the edge to prevent clamp coverage in automated equipment mounts.

PCB fiducial guidelines ensure that assembly goals remain on target.

Watching PCB manufacturing is a dizzying, high-speed affair; perhaps the most impressive aspect is the precision maintained with machinery that cannot technically see its performance. While technological advancements may make deep neural network learning a reality sooner than later, today’s machines rely on fiducials to navigate space accurately. While fiducial placement occurs typically at the tail end of design, PCB fiducial guidelines will help maximize computer vision performance for DFM builds.

Fiducial Function by Count

Understanding Fiducial Functionality

Fiducials are targets that assist in aligning the optical equipment used in production, like pick-and-place, stencil printers, and automated optical inspection (AOI). Broadly, two categories of fiducials perform the same function at different levels of the design:

• Global fiducials are those found in the circuit artwork that indicate position and direction. Designers will want to place at least two pads – ideally three for nonlinear distortion compensations, such as board stretch and twist that can disrupt planarity during automated assembly – in any corners of the board for any layer with components to aid computer vision; multiple fiducials account for translational and rotational offsets. To maximize the offset resolution capabilities of the computer vision system, it’s important to space fiducials as far apart as possible. Global fiducials can also extend to the panel, supplementing board-level fiducials.

• Local fiducials belong to individual fine-pitch (less than 20 mils) components found on any layer that use automated component placement. The latter can be mistaken for a polarity/pin one indicator (in effect, they operate similarly), but it’s important to distinguish between them. For one, the indicator prevents backward (or otherwise incorrect) installation that could damage the component, board, or user. Second, local fiducials are found in pairs, as a single marking would provide little assistance to a computer vision system.

The fiducials' placement is essential regarding the board dimensions and the stackup. Although it’s not a requirement, designers may prefer to have the lower-left-hand corner fiducial and origin coincide, thus providing an apparent visual reference. For layer assignment, a fiducial should be on copper layers only, as mask or silkscreen layers do not possess the same level of accuracy during manufacturing.

Much like panel tooling holes, fiducials represent manufacturing requirements that have little (if any) impact on the design. Designers could leave placement up to the manufacturer, but best practices reduce the work required before the board can enter production. For HDI designs where fiducial placement may have a more moderate effect on performance, designers can avoid unnecessary revision pre-production and confirm fiducial location before sending documentation to the manufacturer.

PCB Fiducial Guidelines: Design Rules

A fiducial requires considerable keep-out area: computer vision needs the fiducial to be distinct from its background, and any nearby traces or copper pours can undermine it. The radius of this clearance should be at least twice that of the fiducial (according to SMEMA 3.1, the fiducial diameter should be between 1.00 mm / 40 mils and 3.00 mm / 118 mils) with the fiducial mark and keep-out areas concentric. For comparison purposes, fiducial sizes must match within 25 microns / 1 mil, but this is a trivial requirement, assuming designers simply repeat the placement of the padstack/part combo from the library; performing a part search during the final pre-documentation checklist can determine identicality. The board/panel edge clearance must be greater than the summed 4.75 mm / 187 mils and the fiducial mark clearance. Placement closer to the edge risks the clamps of the pick-and-place machine covering the fiducials once engaged.

Although the fiducial is an unconnected pad and thus has no complications about oxidization affecting performance (i.e., solderability, continuity, etc.), DFM should strongly consider a protective layer over the fiducial. Computer vision requires a high degree of contrast for the fiducial; accordingly, a cover layer prevents oxidation or other tarnishing that can confuse optical systems. Nickel/tin plating or HASL between 5 and 10 microns (not to exceed 25 microns) can ensure fiducial distinguishability throughout processing and handling. In contrast, solder mask coverage over the fiducial is liable to wholly or partially obscure the fiducial, and the area should retain a solder mask opening along the keep-out dimensions. Additionally, the solder mask finish is relatively glossy, and reflections from the surface can disrupt the computer vision; the bare board material is less likely to inhibit operations.

Because fiducial placement has minimal effect on the design, it usually occurs during the wrap-up stage of the board layout. Placing a part padstack with suppressed inner layer pads (or any layers without automated component placement) should be a relatively straightforward task, given that a fiducial’s general location overlaps with board edge clearance constraints that prevent routing placement in the immediate area. Note that plane pours underneath a fiducial are entirely acceptable, as the “floating” pad has no electrical net connection and cannot appreciably influence the pour’s performance.

Cadence Design Solutions Ensure Your Project Hits Its Targets

Following PCB fiducial guidelines can improve assembly outcomes and other visual-based automated equipment.  While the fiducial may be a simple feature occupying a small board area, designers must reserve appropriate space and configure design rules accordingly to optimize performance. Cadence’s PCB Design and Analysis Software suite gives design teams a comprehensive ECAD environment with powerful tools and customization options that support DFM. With OrCAD PCB Designer, staying on the same page with manufacturers has never been easier with a constraint-driven design.

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