Where Quality Meets Innovation in PCB Assembly

PCB Design For Manufacturing (DFM) Tips


Tip 01:

Importance of Panel and Board Fiducials

What are Fiducial Marks?

Fiducial Marks are easily identifiable markings usually printed on to your circuit board along with the circuit pattern. They provide common measurable points for all steps in the assembly process. This is especially important for automated equipment as it will use the fiducial marks to calculate where the PCB and individual sections of the PCB are located.

Ensure both your PCB and PCB Panel have 3 fiducial marks each; most equipment in the PCB assembly process will use them for PCB alignment.

When adding fiducials you should ensure that the solder mask opening is large enough to provide a good contrast and stand to keep the fiducial clear of the solder mask. This will ensure accurate identification by all optical recognition based alignment systems.

The recommended shape for a fiducial mark is a solid circle bet ween 1mm and 3mm. Fiducial marks for a given board should be the same size otherwise optical recognition may have a problem identifying them. We recommend that they do not vary in size by more than 25 microns.

Again in order to increase optical recognition compatibility we also recommend a clearance (of at least the size of the chosen diameter) between any fiducial marks and any other board markings. Any over laps may also cause an issue.

As a guideline; most optical recognition systems rely on a consistent high contrast being present between fiducial mark and the base material used for the printed circuit board.

Why three you ask?

Adding a 4th fiducial can mean that, if a board is inserted back to front, it will be assembled incorrectly; 3 fiducials give enough data for the machine to calculate the PCB and Panel XY position and board skew.

Ensure that the fiducials on your Printed circuit board assembly have a solder mask window around them to prevent solder mask misregistration affecting fiducial alignment and that panel fiducials are kept 4mm away from panel edges so they are not obscured by conveyor clamps.

Clear reliable fiducials can mean better accuracy for your SMT / SMD component placement.

Types of Fiducial Marks?

There are two type of Fiducial marks that are used; Global Fiducials and Local Fiducials.

Global Fiducials are used to locate the position of an entire Printed Circuit Board. If an assembly is being Panelised then this will be the position of the entire Panel.

Local Fiducial marks are used to locate the position of an individual component, this is not usually required but is required for components that require a more precise placement such as a 0.020” pitched IC in a quad flat package.

All fine pitch components should have two local fiducials designed into their land patterns.

Each fiducial can be; bare copper, bare copper protected by anti oxidation coating, nickel or tin plating or solder coated (HASL). Again we do recommend that whichever is chosen is applied to all fiducials in order to make them the same just to help with optical recognition.


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