Fiducial marks (or fids for short) are a physical identifier for an imaging system that allows a point of reference. The fid essentially gives the image system a known zero reference point. For PCB assembly, these are generally small marks on the top and bottom layers of copper.
The pick and place locates the fids and generates an offset from the machine zero to the PCBs fids. Without the fids, the pick and place will not accurately place parts on the board as the board will only be centered by the mechanical stops on the conveyor. Figure 1 shows a 50 mil fid being located by the My200 pick and place.
Fiducials are generally 1-2mm or ~50-100 mil in diameter on the copper layer. The copper layer is used instead of the silkscreen because sometimes the silkscreen will be offset from the copper layer. Using the silkscreen as a fid can result in the parts being placed offset from their centers. A pull back of the solder mask is recommended by 2-3mm or ~100-150mil. Most solder mask finishes are glossy which might obscure the boundary between the fid and the solder mask. Pulling back the solder mask increases the contrast of the copper to the surrounding area which increases the level of confidence the machine has in finding the fids.
Figure 3 shows a 50 mil diameter fid with a 100 mil diameter pull back on the solder mask. This is the same fid as shown in Figure 2. In most EDA tools you can make a fid by placing a round SMD pad. Be sure to remove the paste layer (cream in Eagle) and increase the solder mask resist (stop in Eagle).
The minimal number of fids per PCB is two but three is preferred. One fid corrects the X-Y location of the board. Two fids corrects any skew of the board in the conveyor. Three fids corrects any shrinking or stretching over the entire PCB. The third fid is generally not important for smaller boards. For larger boards or boards with small parts and features it is highly recommended to keep placement errors low across the entire board.
Sometimes its useful to place a local fid in the center of a large IC, as shown in Figure 5. This increases the accuracy of the placement of the part and makes it easy to check if the parts X-Y location data is correct. This is not always necessary on small boards.
A helpful tip is to make the fids a part in your EDA tool, so when you export your XYRS placement data the fids are included. In the value field for the fid you should include the diameter of the fid. In our MF_Aesthetics Eagle Library there are some examples of fids we have used.