Fiducial Markers on PCB: Complete Guide to Types, Placement & Design Rules

If you’ve ever wondered why your pick-and-place machine occasionally misaligns components—or worse, refuses to run at all—the culprit might be sitting right under your nose: fiducial PCB markers. After 12 years of designing boards and troubleshooting SMT assembly issues, I’ve learned that these tiny copper dots are one of the most overlooked yet critical elements in PCB design.

In this guide, I’ll walk you through everything you need to know about fiducial marks—from basic concepts to advanced placement strategies that will save you headaches (and money) during production.

What Is a Fiducial PCB Mark?

A fiducial mark is a small, precisely positioned copper reference point on a printed circuit board that helps automated assembly equipment determine the exact location and orientation of your PCB. Think of it as a GPS beacon for pick-and-place machines.

When your board enters the SMT line, vision cameras on the stencil printer and placement machines scan for these fiducial marks. By comparing the actual position of the fiducials against the expected CAD coordinates, the equipment can calculate:

• X and Y axis displacement
• Rotation angle (theta)
• Board stretch or shrinkage (with 3+ fiducials)

This compensation happens in milliseconds, but without it? You’re looking at misaligned components, solder bridges, and a pile of rejected boards.

Why Can’t We Use Other Features as Reference Points?

I’ve seen engineers ask: “Why not just use a via or drill hole?” Here’s the thing—fiducials are etched in the same production step as your circuit traces. This means their positional accuracy relative to SMT pads is guaranteed. Drill holes and silkscreen are added in separate operations, introducing registration errors that can throw off fine-pitch placement.

A typical 0.4mm pitch BGA has pads measuring just 0.254mm with 0.15mm spacing between them. At those geometries, even a 50-micron registration error from using drill holes as references can cause assembly failures.

Types of Fiducial Marks on PCB

There are three main categories of fiducial marks, each serving a distinct purpose in the assembly process.

Global Fiducials (Board-Level)

Global fiducials are placed near the corners of an individual PCB and serve as the primary reference for the entire board. They allow the assembly machine to calculate the overall position, rotation, and scale of your board.

Key characteristics:

• Located at diagonal corners of the PCB
• Typically 3 fiducials arranged in an “L” pattern
• Used by both stencil printers and pick-and-place machines
• Required for every SMT board

Local Fiducials (Component-Level)

Local fiducials are smaller markers placed near specific components that require extra placement precision. They’re particularly important for fine-pitch components where global alignment alone isn’t sufficient.

When to use local fiducials:

• BGA packages with pitch ≤0.5mm
• Large QFP/QFN components (40+ pins)
• Components with pitch ≤0.5mm
• Any critical component where placement accuracy is paramount

Local fiducials are typically placed at two diagonal corners of the component, allowing the machine to verify rotation angle right before placement.

Panel Fiducials (Array-Level)

When multiple PCBs are manufactured on a single panel (multi-up array), panel fiducials are placed on the tooling rails/frame. These align the entire panel with the SMT equipment before individual board assembly begins.

Fiducial PCB Design Specifications

Getting the specifications right is crucial. Here’s what IPC and SMEMA standards recommend—and what I’ve found works best in real production.

Size Requirements

The 1:3 ratio (1mm copper pad with 3mm solder mask opening) is what I use as my default. It provides excellent contrast for camera recognition while leaving adequate clearance.

Shape Requirements

Circles work best. A circular shape maintains a consistent geometric center regardless of rotation angle, making centroid detection fast and reliable. Some designers use squares or diamonds, but these require specific machine settings and can cause recognition issues.

Avoid:

• Complex or irregular shapes
• Shapes with sharp corners
• Hollow rings (unless specifically required by your assembler)

Surface Finish Considerations

The fiducial surface must provide high contrast for camera recognition. Here’s how different finishes perform:

ENIG provides the best long-term reliability, especially for boards that may sit in storage before assembly. If your boards are assembled immediately after fabrication, immersion tin works well at lower cost.

Flatness Requirement

This one often gets overlooked: fiducial surfaces must be flat within 0.015mm (0.0006″). Uneven surfaces caused by solder buildup or contamination can throw off the camera’s ability to locate the center accurately.

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Fiducial Placement Guidelines

Proper placement is just as important as proper design. Here are the rules I follow for every project.

Global Fiducial Placement

The Three-Point Rule

Always use three global fiducials arranged asymmetrically. Why three instead of two?

• Two fiducials can only correct for X/Y displacement and rotation
• Three fiducials add the ability to compensate for board stretch/shrink
• Asymmetric arrangement prevents 180° orientation confusion

Place your fiducials in an “L” pattern—for example, lower-left, upper-left, and upper-right corners. Never place fiducials in all four corners; this creates symmetry that can confuse the vision system.

Edge Clearance Requirements

The 5mm minimum accounts for the SMEMA standard transport clearance used by conveyor systems. If your fiducials are too close to the edge, they’ll be obscured by the clamping mechanism.

Local Fiducial Placement

For components requiring local fiducials:

1. Place two fiducials at diagonal corners of the component
2. Position them outside the component footprint but as close as practical
3. Ensure fiducials remain visible after component placement (not hidden under the package)
4. Include local fiducial coordinates in your pick-and-place file

A common mistake is placing local fiducials too close to the component, where they get shadowed during optical inspection. I typically maintain at least 1mm clearance from the component body.

Reference Designators

Include fiducials in your pick-and-place file with proper designators:

• Top side: FD1, FD2, FD3 (or FID1, FID2, FID3)
• Bottom side: FD4, FD5, FD6

This helps your assembler’s equipment automatically identify and locate your fiducials.

When Do You Need Local Fiducials?

This is where experience really matters. Not every fine-pitch component needs local fiducials, and adding unnecessary ones wastes board space. Here’s my decision framework:

The key factor is angular error accumulation. On a large board, even a small rotation error at the global fiducials translates to significant positional error at components far from center. Local fiducials reset the reference point right where it matters.

Common Fiducial PCB Design Mistakes

After reviewing hundreds of designs, these are the errors I see most frequently:

1. Solder Mask Covering Fiducials

If your solder mask accidentally covers the fiducial—even partially—the camera may fail to detect it. Always verify your solder mask gerber shows proper openings around each fiducial.

2. Symmetric Placement

Placing fiducials in all four corners or in a perfectly symmetric pattern prevents the machine from detecting incorrect board orientation. Use three fiducials in an “L” pattern.

3. Insufficient Clearance

Putting traces, vias, silkscreen, or other copper features within the clearance zone reduces contrast and can cause recognition failures. Keep the area completely clear.

4. Inconsistent Sizes

All fiducials on the same board should be identical in size (within 25 microns). Mixing sizes confuses the vision system’s pattern matching algorithm.

5. Missing Local Fiducials for BGAs

That 0.4mm pitch BGA might assemble fine on small prototype boards, but when you scale to production with normal process variations, you’ll wish you had added local fiducials. The cost of two small copper dots is trivial compared to BGA rework.

6. Placing Fiducials Too Close to Edge

Fiducials within 5mm of the board edge will be obscured by conveyor rails and clamping fixtures. This is a surprisingly common issue on compact designs.

How to Add Fiducials in PCB Design Software

Most EDA tools have straightforward methods for adding fiducials. Here’s the general approach:

1. Create a pad: Add a non-plated SMD pad with your chosen diameter (typically 1mm)
2. Set solder mask opening: Configure the solder mask expansion to achieve your target clearance (typically 1mm expansion for a 3mm total opening)
3. Place on copper layer: Ensure the fiducial is on the layer that contains SMT components
4. Define clearance: Add a keepout zone equal to the solder mask opening diameter
5. Add to pick-and-place data: Include fiducial coordinates with proper reference designators (FID1, FID2, etc.)

Double-check that your fiducials appear correctly in the gerber output—I’ve caught errors where the solder mask opening didn’t export properly.

Useful Resources and Downloads

Here are some references I keep bookmarked for fiducial design:

IPC Standards:

• IPC-7351: Generic Requirements for Surface Mount Design and Land Pattern Standard
• IPC-2221: Generic Standard on Printed Board Design
• IPC-A-610: Acceptability of Electronic Assemblies

Manufacturer Guidelines:

• Check with your specific PCB assembler for their fiducial requirements
• Many assembly houses publish DFM guidelines on their websites

Design Software Libraries:

• Most EDA tools (Altium, KiCad, Eagle, OrCAD) include fiducial footprints in their default libraries
• Verify the footprint matches your assembler’s requirements before use

FAQs About Fiducial PCB Marks

1. Can I use vias or drill holes instead of fiducials?

Technically yes, but it’s not recommended. Vias and drill holes are created in separate manufacturing steps from the copper pattern, which introduces registration errors. For fine-pitch components, this can cause placement failures. Fiducials are etched with the copper traces, ensuring consistent positional accuracy relative to SMT pads.

2. How many fiducials do I need on my PCB?

For most boards: three global fiducials arranged asymmetrically (not in all four corners). For double-sided assembly, add three on each side. For fine-pitch components (≤0.5mm pitch BGAs, QFPs), add two local fiducials per component at diagonal corners.

3. What happens if my PCB doesn’t have fiducials?

Some assemblers can use alternative features (pads, vias) as references, but this reduces accuracy and increases setup time. Many automated lines will reject boards without proper fiducials, especially for fine-pitch assemblies. You may also face higher assembly costs due to manual intervention required.

4. Do I need fiducials for through-hole only boards?

Not typically. Fiducials are primarily for automated SMT assembly. If your board only has through-hole components that are placed manually or with insertion equipment, fiducials aren’t necessary. However, if you might add SMT components in the future or use automated optical inspection, it’s good practice to include them anyway.

5. Why does my assembler require different fiducial sizes than IPC recommends?

Vision system capabilities vary between machines. Older equipment may need larger fiducials for reliable detection, while newer high-speed lines can work with smaller marks. Always confirm requirements with your specific assembler before finalizing your design. When in doubt, 1mm copper with 3mm solder mask opening is widely accepted.

Wrapping Up

Fiducial PCB marks might seem like a minor detail, but they’re fundamental to successful automated assembly. Taking the time to design them correctly—proper size, shape, placement, and clearance—will pay dividends in production yield and reduced rework costs.

Remember the key points:

• Use three global fiducials in an “L” pattern for board-level alignment
• Add local fiducials for components with ≤0.5mm pitch
• Maintain 5mm minimum distance from board edges
• Keep the clearance area completely free of other features
• Include fiducials in your pick-and-place data file

If you’re unsure about specific requirements, reach out to your PCB assembler before finalizing your design. A five-minute email can save you a five-thousand-dollar respinning.