Adjacent Layers of Fiducial Marks

[design_sentinel]

Can someone please help me with my question.

If I place a fiducial mark on L1 of a board is it necessary that its adjacent L2 covered with plane and must not contain any slits?
If this is so what is the explanation behind it?

Thanks in Advance.

 

[senilicus]

Fiducial Marks -
Fiducial marks are used to provide visual alignment points for the automatic assembly of a PCB. They serve as reference points for the optically aligned pick-and-place CNC machine that picks up
and places all of the surface mount components on the PCB board. Each Panel needs at least 3 fiducial marks to provide alignment for all of the parts on the panel. These marks are known as the
global fiducial marks.

Local fiducial marks are also required around all large, fine pitched surface mount chip to provide at least two local points of reference to ensure good alignment between the part and the pads.

There is no relations with any feature on inner layers.

 

[marce]

Possibly someone is worried about light and patterns causing the camera to not register the fid on the pick n place machine, shouldn't be a problem today.

 

[design_sentinel]

Thanks for the replies seniculus and marce. . .

Thanks seniculus for the description. . . I already know that. . . I'm asking this question because a colleague which is currently based in Japan keep insisting there is a need for the adjacent layer of the fiducial mark to be covered with a plane and must not have slits. . .and honestly I really think there is no need for those. . .

@marce can you please elaborate your answer. . .I think you're up with something there. . .I just want to understand it more. . .thanks in advance

 

[marce]

some cameras use a back light (light shining through the PCB) a fid is a dark circle with a lighter annulus aroud it. The camera detects the fid by contrast, if you have tracks or other shapes under neath the fid crossing the annulus they can create shadows with a back lit pcb confusing the camera, as it is looking for a round shape. Alot of todays equipement use camera mounted lights so shine onto the PCB so this should not be the prolem it was.