Where to place fiducial symbols in PCB design and how to do it?

hi!

A guy from the SMT-assembly lab said me, that I should place fiducial simbols to my PCB design.

Questions:
-In which layer should I place them? Somewhere someone wrote something about placing in copper... but I think, it is much visibler when I place it in the top silkscreen, because of the contrast of the silkscreen and the solderstop colors.

-What simbols should I use? I have found some, but I am not shure in that all will be good for all machines.

-the size of the simbol: is it fix?

-should I place two? As I know, the first, I should place to the left bottom corner. And what about the second mark? Should I use the same simbol, or should I use different?

Added after 5 minutes:

If I use pick-and-place machine: Is it necessary to use silkscreen layer?
If not, how can I make the fiducial simbols from the copper and the solder-resist?

fiducial simbols

The fiducials are used to calibrate the position of the components relative to the board. There are two kind of fiducials: global and local.
Global fiducials are placed usually on oposite corners of the panel (a panel can contain a matrix of several identical boards). These fiducials are used to determine the relative position of the components on the entire board, and it works well for regular SMT components.
However, for fine and very fine pitch components, global fiducials are not enough, and local fiducials must be used. These are usually placed on the oposite corner of the component for which you require very precise positioning, and they must be placed on all the boards in a panel (if it is the case).
Fiducials are always placed on the copper layer, and above them, the solder mask must be cut. Silk layer is not good at all, because the silk layer is deposited with high offset errors relative to the copper layer. The pick and place machine recognises them, and the quality of the recognition is proportional with the contrast of the image. A solder mask over the fiducial will decrease the quality of the recognition.
Usually, in the user manual of the pick and place machine there are tutorials and examples with the recommended fiducials, which the machine recognises better. I think you can use circular fiducials (regular round pad), without any trouble. In the PCB design sw, you can create the fiducial as a component (a smd round pad, with enough solder mask clearance ). Just make sure to cover the paste mask layer over it, you don't need to deposit solder paste over the fiducial, this will compromise the recognition.
See: **broken link removed**

/pisoiu

Re: fiducial simbols

THe PCB assembly Company said me that fiducial marks must be round (2mm of cooper without plating) a clearence to the solder mask of 1 mm (4 mm diameter from fiducial center)
However, I think the shapes is different for many Pick&Place machines, so You shoud ask your assembler.

fiducial simbols

Here is the link to the SMEMA standard for Fiducials: https://www.smema.org/smema3.1.pdf

fiducial simbols

thanks guys.

And, how can I place local fiducials at components? inside the QFP square? or outside? -how far? how big?

should be the same simbol in the opposite corners?
should be the same simbol for local and global?

And what about BGAs? I can not place anything inside the square.

Is it necessary to use two for local?