A PCB fiducial mark is a deliberately simple optical reference that lets solder-paste printers, pick-and-place machines and inspection systems relate their coordinate system to the physical board. The usual implementation is a round, exposed copper target surrounded by a contrasting clearance area. A camera locates the target, compares it with the programmed coordinates and compensates for translation, rotation and—in equipment that supports it—small scale or distortion errors.
Fiducials look minor, but they sit at the boundary between CAD data and real manufacturing. Poor contrast, inconsistent geometry or an ambiguous arrangement can turn a capable assembly line into a recurring alignment problem. This guide explains the design decisions that should be agreed with the assembler before release.
What Fiducial Marks Do#
An automated line cannot assume that every board enters in exactly the same position. Conveyor tolerances, panel tooling, print setup and board fabrication introduce small offsets. Vision software uses known fiducial coordinates to transform machine coordinates into board coordinates before printing paste, placing components or running automated optical inspection.
One recognized mark can establish an origin or local offset. Two separated marks can also resolve rotation. Three non-collinear marks provide a stronger reference for checking scale, stretch, skew or board-to-program mismatch when the equipment and process support those corrections. The exact recognition strategy is machine-dependent, which is why the assembly supplier's documented capability is part of the design input.
Global, Panel and Local Fiducials#
| Type | Reference area | Typical use |
|---|---|---|
| Panel fiducial | Entire manufacturing panel | Align the panel for printing, placement and inspection before individual boards are separated. |
| Global board fiducial | One PCB image | Locate the individual board and correct its overall position and rotation. |
| Local fiducial | One fine-pitch or area-array component site | Improve registration close to a placement whose pitch or geometry leaves little alignment margin. |
A panel can contain both panel-level and board-level marks. Panel marks help the line locate the full array; board marks let equipment correct each image when required. Local marks are not automatically necessary for every BGA or fine-pitch package—modern machines may achieve the needed placement accuracy from robust global references—but they remain useful when the assembler, component application guidance or process capability calls for them.
Recommended Fiducial Geometry#
A practical starting design is a solid circular copper pad approximately 1 mm in diameter, exposed through the solder mask, with a clear area around it. A 2 mm mask opening around a 1 mm target is widely used by assembly suppliers, but this is not a universal substitute for the assembler's specification. IPC-7351 includes figures for global, panel and local fiducials as well as size and clearance requirements; it should be consulted together with the factory's equipment rules.
The strongest target is intentionally boring:
- a true circle, not a logo, cross, via or text character;
- solid exposed copper with no drill hole;
- a uniform solder-mask exclusion that produces strong optical contrast;
- no silkscreen, copper traces, pads, test points or board text in the keepout;
- the same size, finish and construction for all marks used by the same alignment step.
Do not cover the target with solder mask. A fiducial is also not a tooling hole: the former is an optical feature, while the latter mechanically locates a panel or fixture. The two serve different error-control functions.
Surface Finish and Optical Contrast#
Fiducials normally receive the same exposed-copper surface finish as other pads on the board. ENIG, immersion silver, immersion tin, OSP and HASL can present different brightness, flatness and reflectivity to a camera. Consistency matters more than making the mark unusually shiny. An uneven deposit, contamination or solder on the target can shift the detected center or make recognition intermittent.
Keep the clearance ring free of visually competing features. With dark solder-mask colors, exposed metal often creates clear contrast; with lighter or reflective surfaces, the vision recipe may need adjustment. If a nonstandard finish or mask is planned, send representative samples or confirm the target design with the assembler rather than assuming the default library will work.
How Many Fiducials Should a PCB Have?#
For a typical rectangular PCB, three global fiducials arranged in a non-collinear pattern are a robust starting point. Two widely separated marks can resolve position and rotation, while a third provides redundancy and supports additional geometric checks. A single mark may be enough for a limited manual or machine process, but it cannot independently resolve every alignment error.
Place the marks as far apart as practical because a long baseline makes angular measurement less sensitive to a small center-location error. Avoid putting all three on one straight line. An asymmetric arrangement also helps prevent a 180-degree board orientation from looking identical to the vision system.
Placement Guidelines#
- Use the component side being processed. For a double-sided assembly, provide suitable marks on each side that will be printed or placed.
- Spread global marks across the board. Put them near separate corners while respecting edge handling, rails, clamps and component keepouts.
- Avoid collinearity and symmetry. Three marks should form a clear triangle rather than a straight line or an easily reversed pattern.
- Keep them on the same rigid reference. Do not place a critical mark across a flexing tab, breakaway connection or region likely to warp relative to the placement area.
- Protect the camera view. Tall components, labels, adhesive, pallets and fixtures must not shadow or cover the target at the process step that needs it.
- Maintain an uncluttered keepout. Nearby pads, copper shapes and silkscreen can be interpreted as part of the target or reduce edge contrast.
For arrays, consider the complete PCB panelization strategy: tooling rails, breakaway tabs, V-score lines, conveyor support and fiducials should be designed together. A perfect mark placed on a rail that the assembler removes before a later operation cannot serve that later operation.
Local Fiducials for Fine-Pitch Components#
Local fiducials are placed close to a demanding component so the placement system can correct for local board stretch or registration error. They are most relevant when component pitch, package size or the demonstrated machine-to-board accuracy leaves limited tolerance. Area-array, fine-pitch QFP and high-density connector sites are common candidates.
Use at least two separated local references when the process must resolve both translation and rotation around that site, unless the assembler specifies another method. Do not squeeze local marks so close to the footprint that their clearances conflict with paste apertures, escape routing or inspection access. If there is insufficient room, discuss whether global references and the equipment's component-centering system provide adequate capability before forcing a poor local design.
Fiducials in the PCB CAD Library#
Treat a fiducial as a controlled footprint, not an improvised copper circle. The library definition should include:
- copper target diameter;
- solder-mask opening and explicit clearance;
- assembly-layer identification;
- component or mechanical classification appropriate to the EDA tool;
- a courtyard or placement keepout that prevents accidental encroachment;
- a descriptive reference such as FID1, FID2 and FID3.
Exclude fiducials from purchasing and ordinary placement quantities in the bill of materials, but keep them visible in the centroid/position data if the assembler's import workflow expects fiducial references. The released fabrication layers—not just the schematic symbol—must contain the intended copper and mask geometry.
Panel Fiducials and Step-and-Repeat Data#
When boards are supplied in an array, panel fiducials belong on stable rails or panel areas that remain available during the automated steps. The assembly drawing should distinguish panel, board and local marks. Coordinate data should use a documented origin, units and rotation convention so the factory does not have to infer which targets belong to which image.
If the fabricator creates the production panel, provide board-level intent and let the fabricator and assembler coordinate the rail geometry. Do not assume a fabrication panel is automatically suitable for assembly; assembly rails may need fiducials, tooling holes, coupons, support clearance and sufficient width for conveyor handling.
Common Fiducial Design Mistakes#
| Mistake | Why it causes trouble | Better approach |
|---|---|---|
| Using a plated hole as the target | The drill, plating and annular ring add edges and reduce center consistency. | Use a solid, undrilled copper disc. |
| Mask covering part of the mark | The detected edge can shift or contrast can disappear. | Create an explicit, concentric mask opening. |
| Silkscreen or copper inside the keepout | The camera may see competing shapes. | Enforce a clear optical zone in DRC and documentation. |
| Three marks in a straight line | Geometric correction and orientation checking are weakened. | Use a widely spaced triangular arrangement. |
| Only one side has marks | The second-side print or placement step lacks equivalent references. | Add side-specific fiducials for each automated process. |
| Marks hidden by fixtures or labels | The target exists in CAD but is unavailable to the camera. | Review the full process sequence and fixture envelope. |
| Coordinates omitted or ambiguous | Setup becomes manual and error-prone. | Release consistent centroid, drawing and panel data. |
Do All PCBs Need Fiducials?#
Not every board requires the same fiducial strategy. A hand-assembled prototype with large through-hole parts may not need optical alignment marks. A modern pick-and-place machine can sometimes use board features, pads or panel references when its software and process have been qualified to do so. Conversely, a dense production assembly with fine-pitch packages normally benefits from explicit, repeatable targets.
The decision should be based on placement accuracy, board size, panel format, component pitch, expected volume and the actual equipment. Removing fiducials simply to save a few square millimeters can shift cost into setup time, reduced process margin or inspection failures.
Design Review Checklist#
- Confirm the assembler's preferred target diameter, mask opening and clearance.
- Provide global or panel references for every automated side and operation that needs them.
- Use a non-collinear, asymmetric arrangement with the widest practical spacing.
- Check that rails, clamps, pallets, labels and components will not obstruct camera access.
- Add local marks only where package/process capability justifies them.
- Verify copper and mask output in the final Gerber or IPC-2581 data—not only in the CAD view.
- Keep fiducial identities and coordinates consistent across assembly drawings and placement files.
- Inspect the marks on first articles for size, finish, contamination and recognition stability.
The final release should be checked in an online Gerber viewer and then confirmed through the supplier's DFM review. Fiducials are part of the assembly process plan, just like stencil apertures, component orientation and the reflow profile.
Authoritative References#
- IPC-7351B table of contents, which identifies global/panel, local, location, size and clearance fiducial guidance.
- IPC document revision table for current design and assembly standard status.
- Texas Instruments Array QFN Design Guide, which notes that placement accuracy depends in part on fiducial marks, datum location, board size, flatness and mounting equipment.
- Texas Instruments QFN and SON PCB Attachment for package-to-board assembly context.
Frequently Asked Questions#
What is a PCB fiducial mark?
It is an optical reference—usually a round exposed-copper target with a clear surrounding area—that automated equipment uses to align physical boards with programmed coordinates.
What size should a PCB fiducial be?
A 1 mm copper target with an approximately 2 mm mask opening is a common starting point, but the correct dimensions are the assembler's qualified machine specification and applicable design standard.
How many fiducials are needed?
Three widely spaced, non-collinear global marks are a robust general design. Some processes use two or one, while demanding local placement may use additional marks. Confirm the required recognition method with the assembler.
Should fiducials be covered with solder mask?
No. The copper target is normally exposed through a larger, concentric solder-mask opening to create a clear optical boundary.
Are fiducials required on both sides?
If both sides undergo automated printing, placement or inspection, each processed side should have accessible references appropriate to those operations.
Are tooling holes the same as fiducials?
No. Tooling holes provide mechanical location for panels or fixtures. Fiducials provide an optical coordinate reference for vision systems. A production panel may need both.