Fiducial Marker "Fiducials" (PCB) What are fiducials and how are they placed?
Fiducials in electronic manufacturing
In highly automated electronics production, machines with image recognition systems (vision systems) are utilized. These measure the actual positioning of the circuit structure on the circuit board before solder paste printing, SMD assembly or AOI inspection.
During the measurement, the fiducials on the printed circuit board are approached and their position is determined. Subsequently, these positions are compared with the mathematical image and any possible expansion, compression or rotation of the circuit board can be compensated.
What happens if there are no fiducials available?
Missing fiducials lead to an increased set-up effort during the placement as alternative markings such as vias or boreholes have to be used. Fiducials are more precise compared to the alternative marks since they are etched in the same production step as the circuit structures.
It is not uncommon that manual reworking is necessary in order to be able to assemble the circuit boards automatically. In inopportune cases, fine pitch elements cannot be assembled if there is insufficient accuracy for position detection.
The optimal key data for fiducials
We recommend the following key data for an optimal production of your product:
The optimal size and shape of fiducials
- Fiducials have a diameter of 1 mm,
- Fiducials lie flat and are free of solder mask,
- Fiducials are exposed in the solder mask 3 mm in diameter or square 3x3 mm.
If space is limited, it is also possible to place smaller fiducials. However, under no circumstances, they should be smaller than 0,5 mm. Moreover, they should have enough contrast through being surrounded by open area.
Examples
| OPTIMAL | OPTIONAL | MINIMAL | |
|---|---|---|---|
| Diameter Fiducial Mark | 1 mm | 1 mm | 0,5 mm |
| Circumference of open area | 1 mm | 0,5 mm | 0,25 mm |
| Diameter (Fiducial including open area) | 3 mm | 2 mm | 1 mm |
Positioning of fiducials
Global and Panel Fiducials
- Three fiducials per single PCB are arranged in “L”.
- - For a multiple circuit board in a panel, three additional fiducials are arranged in “L” on the panel edge.
- All fiducials are arranged as far on the outside as possible in order to create maximum distances. However, at least 5 mm to the edge (transport edge!) are needed.
- Recommended reference designation: FD1 to FD3 for TOP side, FD4 to FD6 for BOTTOM side (these should be indicated as components in the Pick & Place file).
- Fiducials should also be indicated in the solder paste data (these are lasered from below during the production of the stencil).
Local fiducials
We recommend using local fiducials for components with a pin distance (pitch) ≤ 0,5 mm.
- Three fiducials for each corresponding component should be arranged (minimum two diagonally),
- Coordinates should be indicated in the same way as components (reference FDx-IC#) in the Pick & Place file,
- Enough distance to the next component should be ensured (fiducials should still be visible after the assembly).
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