I'm a bit confused about the correct fiducial design. I've been reading about pick and place machines, and some suggest that fiducials should be round while others say they should be a cross. Also, sometimes I read that it should have paste on top and sometimes not.
So what is a standard design for such a fiducial? This was my approach until I read about it:
Check this link for guidelines: http://www.accutroninc.com/pdf/download/fiducial.pdf
1mm diameter, round, with 2mm diameter of solder mask removed is the IPC guideline.
There's a bit more about it at http://denatechnologies.com/capabilities/pcb-fiducial-guidelines My impression is that various manufacturers may have slightly different preferences for the ideal one, so it's probably a good idea to find out if/what your manufacturer has a its own preference/guideline... assuming you're making the design for only one mfg. Dena tech recommends the R-3R (1mm/3mm) variant.
Accutronic in the document linked in the other answer also recommends the R-3R version rather than the R-2R [IPC] one, which they still support though.
The SMEMA std can be found here for example. Note that they say R-2R is the minimal design:
The size of the clearance area shall have a radius that is at least twice that of the fiducial mark and shall be concentric with the center of the fiducial mark
(emphasis mine).
The other answers are excellent. In addition, it is common practice to put a feature in the solder paste layer that matches the fiducial's copper layer. This is to assist aligning the solder stencil over the PCB.
