|The minimum recommended size for fiducials is 1.0 mm. |
It is good practice to keep all fiducial marks the same size as some assembly equipment is less flexible in its ability to recognize different size marks on the same printed circuit board.
There are several common used shapes. Some say that the optimal fiducial mark is a solid filled circle.
The minimum diameter of the fiducial mark should be 1 mm (0.040 in).
The maximum diameter of the fiducial should be 3 mm (0.120 in).
Fiducial marks on the same PCB should not vary in size by more than 25 microns (0.001 in).
A clear area devoid of any other circuit features or markings should be maintained around the fiducial mark.
The size of the clear area should be equal to the radius of the mark. A preferred clearance around the mark is equal to the mark diameter.
The distance from a fiducial mark to the edge of a printed circuit board or fabrication panel shall not be less than the sum of 4.75 mm [0.187 inch] (the SMEMA Standard Transport Clearance) and the fiducial mark clearance.
The prefered base material of the fiducial mark should be bare or bare copper protected by a clear anti-oxidation coating.
It may also be nickel or tin plated, or solder coated (HASL).
For optimum performance, there should be a high degree of contrast between the surface of the fiducial mark and the adjacent printed circuit board base material.
Plating and coating Thickness:
When used, the thickness for the bare-copper fiducial mark plating or coating should be from 5 to 10 micron [0.0002 to 0.0004 inch].
The thickness of a solder coating should never exceed 25 microns [0.001 inch].
The flatness of the surface of the fiducial mark should be within 15 microns (0.0006 in).
The fiducial marks should be used in conjunction with land patterns that have been designed in accordance with the requirements of IPC-SM-782.