Hi Satish,

Per the IPC Designers Council standard cited above, fiducials can be either local or global. Local fiducials relate to a single land pattern (component), while global fiducials relate to the whole board. Sometimes fiducials relate to all of the boards on a single panel, in which case they're called panel fiducials, but they are still global fiducials (i.e. the global fiducial rules apply).

The required minimum number of fiducials depends on what you want to correct for. If you want to correct for translational errors (x and y position) and rotational errors (theta) then a minimum of two fiducials are required. They should be placed diagonally as far apart as feasible (opposite corners of the board or panel, or opposite corners of the land pattern).

If you may also need to correct for such errors as stretching, scaling or twist you should add a third fiducial to an additional corner at maximum separation. In practice this will only apply to global fiducials (boards or panels), not local fiducials (individual components).

Finally, if space is limited you can use a single fiducial as a local fiducial, but it should be placed in the center of the relevant land pattern. In such a case the fiducial will only allow for translational correction (x and y position), not rotational (theta) positioning, but this isn't really a problem. Most modern SMT assembly shops use pick and place machines with vision systems that will use the features of the land pattern itself as fiducials and will thus correct for rotational positioning, making a second fiducial superfluous. In fact, unless you are requiring the position of a component to a critical precision less than the limits of the land pattern you don't really need any local fiducials. The machine is going to automatically center the part in the land pattern at placement, and the surface tension forces are going to recenter the part during reflow. So if you really do need the part to be critically positioned within the land pattern, you also need to glue it down before reflowing.

I hope this helps. Good luck,

Joe