Global and/or panel fiducials should ideally be located on a three point grid based system, with the lower left fiducial located at the 0,0 datum point and the other two fiducials located in the positive X and Y directions.

Global fiducials should be located on all PCB layers that contain components to be mounted with automated equipment.
This is true even if the circuit design contains no fine pitch (<= .020” pitch) components, as most modern assembly equipment uses vision recognition for PCB alignment.
While other circuit objects (via-holes, etc.) can be used, this can compromise accurate component placement.

Offsets:

A minimum of two global fiducial marks shall be provided when it is necessary to correct for translational (X- and Y-position) and rotational offsets (theta-position). These global fiducial marks should be located diagonally opposite one another and as far apart as possible on the printed circuit board or fabrication panel.

Nonlinear Distortions:

A minimum of three global fiducial marks shall be provided when it is necessary to correct for nonlinear distortions, such as scaling, stretch and twist.
These global fiducials should be located in a triangular pattern and should be located as far apart as possible on the printed circuit board or fabrication panel.

Figure 1.	Global and Local Fiducials.

A fiducial mark (or marks) used to locate the position of an individual land pattern or component that may require more precise location, such as a .020” pitch QFP.

Translational Offsets:

One or more local fiducial marks shall be provided when it is necessary to correct for translational (X- and Y-position).
These local fiducial marks shall be located inside the perimeter of the land pattern, preferably at the center of the land pattern.

If space is limited, a minimum of one local fiducial mark may be used to correct translational offsets (x and y position).
The single fiducial should be located in the center of the land pattern.

Three panel fiducial marks should be provided when it is necessary to provide the most accurate correction for both translational and rotational offsets.
For these applications the three panel fiducial marks should be in a triangular pattern and should be located as far apart as possible outside the perimeters of the end-product printed circuit boards.

Figure 2.	Panel Fiducials.

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.

Shape:

There are several common used shapes. Some say that the optimal fiducial mark is a solid filled circle.

Figure 3.	Common used Fiducials.

Size:

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).

Clearance:

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.

Figure 4.	Fiducial Clearance.

Edge Clearance:

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.

Base Material:

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].

Flatness:

The flatness of the surface of the fiducial mark should be within 15 microns (0.0006 in).

Land Patterns:

The fiducial marks should be used in conjunction with land patterns that have been designed in accordance with the requirements of IPC-SM-782.

Source IPC SMEMA and TKB-4u.com