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Direct answer: A fiducial mark is a small exposed-copper circle on the PCB that gives the placement machine's camera a fixed reference point. Before placing a single part, the machine photographs two or three of these marks, works out where the board really sits — its X–Y offset, its rotation, and any stretch — and shifts every placement coordinate to match. Get the fiducials right, and the machine's stated accuracy is what you actually get. Get them wrong, and every part on the board inherits the same error, usually surfacing later as tombstoned passives, bridged fine-pitch leads, or a job the machine refuses to start.
Placement accuracy is usually discussed as a machine specification: ±0.035 mm, Cpk ≥ 1.0, and so on. But that number is a system property, not a machine-only property. The machine can only place as accurately as it knows where the board is — and the only thing that tells it is a set of copper circles a few millimetres wide, drawn on the board long before it reaches the line. This article explains what fiducial marks do, the three kinds you will meet, where they belong on a board, how a small error in them becomes a large placement error, and how to check them on first article.
A fiducial mark is a solid circle of exposed copper, deliberately left free of solder mask, so that a machine-vision camera sees a bright disc against the darker solder mask around it. The camera does not read the mark the way a person reads a label. It finds the centre of the disc — the centroid — and that centre becomes a coordinate the machine can trust. The same marks are used at three points on the line: the solder paste printer aligns the stencil to them, the pick-and-place machine aligns the board to them, and the AOI system compares the finished joints against positions derived from them.
Three things a fiducial is not. It is not a component pad: nothing is ever soldered to it. It is not a test point: no probe touches it. And it is not a tooling hole: a drilled hole gives the machine a mechanical edge, not an optical centre, and mechanical referencing is far less repeatable than optical referencing. A board that has tooling holes but no fiducials can still be assembled, but for fine-pitch work the fiducials are effectively mandatory — the machine needs an optical reference it can measure to a fraction of a pad width.
The mark itself costs nothing. It is copper the fabricator was etching anyway, in space the board already had. That is the point worth remembering throughout this article: fiducials are the cheapest accuracy on the whole SMT line, and also the easiest accuracy to lose by accident.
Not all fiducials do the same job. A board can carry three kinds at once, and confusing them — in the design file or in the machine program — is one of the commonest causes of a whole batch being placed off-target.
| Kind | Where it sits | How many | What it corrects |
|---|---|---|---|
| Global fiducial | On the board itself, near the corners | Minimum three per assembled side | Board-level X–Y offset, rotation and scale for the whole board |
| Local fiducial | Beside a critical component, at diagonally opposite corners of its footprint | Two per component | Local distortion near that component, which global fiducials cannot see |
| Panel fiducial | On the panel rails, outside the individual boards | Minimum three per panel | Panel-level registration, so each board in the panel is found in turn |
Global fiducials establish the board's master coordinate system. Every placement coordinate in the program is defined relative to them. Local fiducials exist because a circuit board is not a perfectly rigid, perfectly stable object: FR-4 expands and contracts with temperature and handling, and over the distance from a global fiducial to a component on the far side of a large board, that small movement is enough to shift a fine-pitch BGA or QFN off its pads. Local fiducials let the machine re-check position right next to the part that needs it most. They are needed near components at roughly 0.5 mm pitch or below — BGAs, QFNs and fine-pitch QFPs. Panel fiducials serve the same purpose at panel level: without them, the machine corrects the first board and then assumes the rest of the panel is identical, so placement error accumulates board by board down the panel.
The sequence is the same on every modern placer, and it happens before a single component is picked:
Two fiducials are enough to determine X–Y offset and rotation. Three are needed to detect scale as well — the board being slightly longer or shorter than the design, which happens after etching, lamination and thermal processing. That is why the minimum is three, and why they must not sit on a straight line: three points that form a triangle define a plane; three points on a line do not.
The same reference chain runs at the printer. The JHIMS M8 solder paste printer reads the board's marks with a 1.3-megapixel CCD camera using geometric pattern matching, recognising eight mark types from 0.1 to 6.0 mm in a 150 ms cycle, and aligns the stencil before every print. If the printer and the placer read the same marks, paste and parts agree with each other; if they read different marks, or if one of them cannot read the marks at all, paste and parts disagree by exactly the amount the board moved between the two stations.
The idea extends inside the machine, too. The Yamaha YSM40R carries a vision system described as fiducial, multi-camera, side-view and smart recognition, and its MACS accuracy compensation reads in-machine correction marks during production to compensate for thermal drift and mechanical variation. The machine, in other words, uses reference marks to correct itself in the same way it uses your board's fiducials to correct the board.
Fiducial placement is not a matter of taste. The rules below are the common industry practice described in PCB design guides and in the IPC-7351 and IPC-2221 standards. Treat the dimensions as typical practice to confirm against your machine's manual and your fabricator's capability — there is no single universal fiducial size, and a number copied from another factory's board is not a specification.
For size, a global fiducial copper diameter of about 1.0 mm is the common choice, with a usual range of roughly 0.8–2.0 mm; larger boards and panels often use 1.5–2.0 mm so the camera can find the mark from further away, while dense designs sometimes drop to 0.8 mm to fit the available space. Local fiducials are smaller, typically 0.5–1.0 mm, to fit beside the component. Again, these are typical practice ranges, not a standard: the deciding numbers are the ones in your placement machine's manual and your printer's accepted mark range — the M8, for example, accepts marks from 0.1 to 6.0 mm.
Most fiducial failures do not announce themselves as fiducial failures. They show up as placement defects, and the fastest way to diagnose them is to match the pattern of the defect to the mark that caused it.
| Symptom on the line | Likely fiducial cause | What to check first |
|---|---|---|
| Machine alarms “fiducial not found” and refuses to start | Mark missing, covered by a clamp, label or stiffener, or outside the camera search window | Look at the board: is the mark there, uncovered, inside the clamp zone? Compare the program's fiducial coordinates with the current board revision. |
| Whole board shifted uniformly in one direction | One global fiducial misread — its centroid pulled by nearby copper or silkscreen inside the clear zone — or the program read panel fiducials where board fiducials were expected | Inspect the clear zone around each global mark; confirm which set of marks the program is using. |
| Parts placed rotated or mirrored, whole board backwards | Symmetric fiducial layout let the machine accept a 180°-rotated board | Check the fiducial pattern: is it asymmetric? Change the layout to an L-pattern on the next revision. |
| Fine-pitch BGA or QFN bridges or opens while passives are fine | No local fiducials: global-only correction cannot see local board distortion near that part | Add two local fiducials at diagonally opposite corners of the problem footprint. |
| Good boards and bad boards alternate in the same batch | Fiducial surface varies board to board: HASL dome, oxidation or contamination changing contrast | Compare the marks under magnification across good and bad boards; consider ENIG or immersion tin for the marks. |
| Placement drifts across a long panel, worst at the far end | Panel fiducials missing: the machine corrected the first board and assumed the rest | Add panel fiducials on the rails and enable panel-level correction in the program. |
| Errors grow with distance from one corner of the board | Rotation error: the board is slightly rotated and the machine's correction is based on fiducials too close together | See Section 6. Move the global fiducials further apart on the next revision. |
One failure mode deserves emphasis because it is a process choice, not a design error: when the machine alarms on a fiducial, the tempting fix is to teach the machine to skip that mark, or to widen the acceptance window until the job runs. That trades a visible alarm for an invisible error. The board that follows is placed against a correction the machine no longer trusts, and the defect shows up at AOI or — worse — at the customer. Fix the mark or fix the program; do not silence the check.
A rotation error is the most expensive kind of fiducial error, because it multiplies with distance. A board that is shifted 0.05 mm in X is 0.05 mm wrong everywhere. A board that is rotated by a small angle is slightly wrong near the rotation centre and badly wrong at the far corner. The relationship is simple geometry: the shift at a distance L from the centre is approximately L × sin(θ).
Illustrative calculation — geometry, not a measurement from any machine:
This is why the placement rules in Section 4 insist on fiducials as far apart as possible: the machine's angular uncertainty shrinks as the baseline between its two measuring points grows. It is also why a rotation problem looks the way it does in the table above — parts near one corner land correctly and parts at the opposite corner are off by a growing amount. If your defects grow with distance from a corner, suspect rotation before you suspect the feeder, the nozzle or the program.
Fiducials are verified once, on the first board of a new job or a new board revision. Eight steps, ten minutes, and they catch nearly every failure in Section 5 before it becomes a batch of rework.
Most fiducial problems are decided at the design desk, not on the line. If you send boards to an assembler, put these six items in the assembly notes or the fabrication drawing so the question never has to be asked mid-job:
A placement machine's accuracy figure only means something if the fiducials are readable. The machines below state their accuracy on the assumption that the board gives the vision system a clean mark to find. That is the honest limit of any machine specification: the board designer owns the marks, and the machine reads them.
Yamaha YSM10 — a compact high-speed mounter with a mounting accuracy of ±0.035 mm (±0.025 mm) at Cpk ≥ 1.0 (3σ). Its scan camera handles components up to 12×12 mm on the fly, with an optional multi camera for larger parts, and its e-Vision system generates component recognition data automatically. Its side-view detection verifies pickup status during the transport cycle, catching missing or misaligned components before placement.
Yamaha YSM40R — an ultra-high-speed modular mounter with the same ±0.035 mm (±0.025 mm high-accuracy) placement accuracy at Cpk ≥ 1.0 (3σ). Its vision system is listed as fiducial, multi-camera, side-view and smart recognition, and the side-view camera detects pickup errors, tombstoning and flipped parts. Its MACS accuracy compensation reads in-machine correction marks during production to compensate for thermal drift — the machine correcting itself against reference marks, the same principle as the board-level correction described above.
JHIMS M8 solder paste printer — the first machine on the line to read the fiducials. Its 1.3-megapixel CCD camera with geometric pattern matching recognises eight mark types from 0.1 to 6.0 mm in a 150 ms cycle, with a print accuracy of ±0.025 mm. When the printer and the placer read the same marks, paste and parts agree; the printer is where that agreement is established.
If you are choosing between inspection steps after placement, the downstream check is covered in SPI or AOI First? A Buy-Order Decision Guide for SMT Lines, which explains where each inspection step pays back. But no inspection step can put a part back where it belonged: that decision is made by the fiducials, before placement starts.
Fiducial marks are the cheapest accuracy on an SMT line: a few copper circles, no components, no cost, drawn once and used by every machine the board passes through. Design them once — asymmetric, clear of copper and silkscreen, the right size for the machines that will read them, on a flat finish — verify them on the first article of every new revision, and record the offsets. When placement goes wrong, check the marks before you blame the feeder, the nozzle or the program: the machine can only place as well as it knows where the board is, and the fiducials are the only thing that tells it.
A minimum of three global fiducials per assembled side, arranged asymmetrically, plus two local fiducials at diagonally opposite corners of each fine-pitch component footprint (roughly 0.5 mm pitch or below), plus panel fiducials on the rails if the board is panelised. Fewer than three global marks cannot correct rotation and scale at the same time.
Some machines can reference board edges or tooling holes, but mechanical referencing is far less repeatable than optical referencing, and the error is the board-loading variation itself. For fine-pitch work, fiducials are effectively mandatory.
Almost always a symmetric fiducial layout: with four symmetric marks, the vision system cannot distinguish a board loaded rotated 180° from a correctly loaded one. Change the layout to an asymmetric L-pattern on the next revision, and add a board-orientation check to the first-article procedure.
Yes. The printer reads the same marks to align the stencil before every print. If the printer cannot read them, the paste lands off the pads and no amount of placement accuracy afterwards can fix it.
Ask for the accepted mark range of their printer and placer cameras — the M8 printer, for example, accepts marks from 0.1 to 6.0 mm — then enlarge the copper diameter or the solder-mask opening on the next revision, keeping the clear zone around the mark. Do not shrink a mark to save board space: a mark the camera cannot find is worse than no mark at all, because the machine may read something else instead.
Questions about fiducial reading on the YSM10, YSM40R or M8, or about setting up a first-article check for your boards? Contact JHIMS at [email protected] for technical information.
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