Fiducial in Flex PCB: Purpose, Types, and Design Guidelines
- Flex Plus Tech team
- Jul 13
- 3 min read
Fiducials in flex PCB are reference marks used by automated assembly equipment to accurately locate a flexible printed circuit board during manufacturing. They serve as optical alignment points for SMT pick-and-place machines, solder paste printers, AOI systems, and other inspection equipment.
Although fiducials are small copper circles, they play a major role in ensuring accurate component placement, especially on flexible PCBs where the material can expand, shrink, or deform more easily than rigid PCBs.
What Is Fiducial in Flex PCB?
A fiducial is a simple, exposed copper pad with no electrical function. Vision systems recognize these marks to establish the PCB's exact position, rotation, and scale before assembly begins.
On a flex PCB, fiducials are even more important because:
Polyimide material can slightly expand or contract during processing.
Flexible circuits may not remain perfectly flat during SMT assembly.
High-density components require precise alignment.
Panelized flex PCBs often undergo multiple manufacturing processes before assembly.
Without reliable fiducials, placement accuracy decreases, increasing the risk of solder defects or component misalignment.
Types of Fiducials Used in Flex PCB
Global Fiducials
Global fiducials identify the overall position of the PCB or manufacturing panel.
Typical recommendations include:
Three fiducials per PCB whenever possible
Positioned near the board corners
Maximum spacing between fiducials for improved accuracy
Not placed too close to board edges or tooling holes
Global fiducials establish the coordinate system used by the assembly machine.
Local Fiducials
Local fiducials provide additional alignment for high-density components such as BGAs, CSPs, Fine-pitch QFPs, Large ICs with tight pin spacing. They compensate for small local dimensional changes that global fiducials cannot fully correct.
Panel Fiducials
When multiple flex PCBs are manufactured in a panel, additional fiducials are placed on the panel frame. Panel fiducials help equipment align the entire production panel before locating individual circuit fiducials.
Recommended Fiducial Design
The following dimensions are commonly used throughout the PCB industry.
Feature | Typical Recommendation |
Copper diameter | 1.0 mm |
Solder mask opening | 2.0 mm |
Copper shape | Solid round circle |
Surface finish | ENIG or HASL |
Clearance to copper | At least 2 mm |
Number of global fiducials | 2–3 per board |
A round, exposed copper pad provides the highest recognition rate for machine vision systems.
Where Should Fiducials Be Placed?
Good fiducial placement is just as important as the fiducial itself.
General guidelines include:
Place global fiducials as far apart as practical.
Avoid placing them near board edges that may be routed away.
Keep fiducials away from silkscreen, copper traces, and solder mask irregularities.
Ensure the surrounding area remains clear so cameras can easily recognize the mark.
On flex PCBs with stiffeners, place fiducials in stable regions whenever possible.
For flexible circuits with multiple SMT areas, each assembly section may benefit from its own local fiducials.
Why Fiducials Are Especially Important for Flex PCB
Compared with rigid PCBs, flexible circuits introduce additional assembly challenges.
These include:
Material movement during reflow
Dimensional variation after lamination
Slight stretching during handling
Vacuum fixture positioning differences
Panel deformation
Fiducials allow the assembly equipment to compensate for these variations automatically, improving placement accuracy and production yield.
Common Fiducial Design Mistakes
Several design errors can reduce recognition accuracy.
Common mistakes include:
Covering fiducials with solder mask
Using square instead of circular marks
Placing fiducials too close to copper pours
Adding silkscreen over or near fiducial
Using only two poorly positioned fiducial
Omitting local fiducial around fine-pitch components
Locating fiducials on unsupported flexible sections that may bend during assembly
Avoiding these issues helps improve machine vision performance and reduces assembly defects.
Do All Flex PCBs Need Fiducials?
Not always. Simple flex circuits assembled manually or containing only a few large components may not require fiducials.
However, fiducials are strongly recommended for:
Automated SMT assembly
Fine-pitch ICs
BGAs and CSPs
Double-sided assembly
High-volume production
Tight component placement tolerances
For most modern flex PCB designs intended for automated manufacturing, fiducials are considered a standard design feature.
Conclusion
Fiducial in flex PCB may occupy only a small amount of board space, but they have a significant impact on assembly accuracy and manufacturing yield. Properly designed global, local, and panel fiducials enable automated equipment to compensate for the unique dimensional changes of flexible circuits. By following standard fiducial design and placement guidelines, manufacturers can reduce placement errors, improve solder joint quality, and achieve more consistent production results.
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