I've read that fiducial should be placed on the corners of the PCB.
And that no more 3 should be placed. Below are some questions I can't seem to find online.
1) Would it be a problem if there are some component farther away from fiducials, and very close to the edge of the PCB? For example in the picture below what happens to the components that are placed on the area "A"?.
2) If you have 2 fiducial on the board and you add 2 more close to a fine-pitch component like a BGA or a big QFN, why doesn't that violate the "3 fiducial maximum" rule?
3) What happens in a non square PCB? Like in a T-shapped or an L-shapped board?
This is very similar to question #1
For example in this picture. Would it be any problem if there were components on the area that the arrows points?
Best Answer
The most qualified to answer your question is the EMS that has to assemble your PCBAs.
Usually it is at least 2-3 fiducial marks. I don't think there is an upper limit to how many marks you can place. The EMS will just ignore them if they don´t need them.
The fiducial marks are used for alignment/registration of the pick'n'place machine. I usually place a few (4 or 5) on each side of the PCB (depending on the size).
There is no problem for the machine to place components in the "A" region of your figure.
In your last figure I would place on near the point of the arrow.