a fiducial is a coordinate mark used by the pick and place robot to align the stage to the board.
You need 3 of them and they need to be placed in an L shape. ( that means : at three corners of the board )
The one in the corner of the L has a different shape than the others.
you typically put the corner one on coordinates o.o
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the square one is centered at 0:0
the others sit at 0,x and y,0
You need these on top and bottom lyer ( if you have parts on both sides )
You need to make 2 pcb footprints. one with a square SMD pad and one with a round SMD pad. They don;t neeed to be large. i use 20 mil diameter pads. The soldermask peelback needs to be 15 mils so the robot gets a good contrasted view. You cannot have ANYTHING in a circle 20 mils around the fiducial.. no copper, no parts, no text .
You place one square and two rounds on top layer , and one square and two rounds on bottom layer. DO NOT USE a MULTILAYER pad with diameter set to zero. Let me explain why :
You want the fiducials to be exported as a SMT pad. that way the pick and place file will contain the center coordinates for these pads. so 3 parts on top , 3 parts on bottom. this gives you 6 lines in the PAP file containing the center coordinates. this makes it easy for the assembly people to calibrate the robot. The PAP line item contains layer index and x/y . if you call your fiducials FIDU-SQUARE and FID-ROUND this is a que for them to pick up the right one during programming of the robot.
During board registration in the robot it will automatically go to 0:0 , look down with the camera and calculate the offset between the center of the square and where it is positioned. This is called nulling of the stage. The reason to peel back the soldermask is so the imager can see a sharp contrast between bare FR4 and the copper of the pad in the fiducial. The imager traces the outline of the square pad and calculates the geometrical center. any X-Y offset in the stage is compensated.
In the next step the stage ( the stage is the X-Y arm that moves the turret around ) will go find the two round ones , the camera will register these x-y coordinates and can now detect if there is an annular offset in the board registration. At this point the numbers are used to compensate the entire coordinate system and the stage movements are now perfectly aligned to how the board sits in the machine. the machine can now translate the coordinates from your cad data to its internal coordinate system.
If you use panelized boards you can put the fiducials in the panel ring ( the individual boards don't need them in this case. you simply align the whole panel. )
Fiducials are important if you have lots of fine pitch components as it increases placement precision. some robots go and check after placing x amount of parts to see if they have not accumulated any offset ( maybe a miscount in the optical encoder on the stage ) and re-null the stage. As parts count o a board increases the fiducials become vey important.
Oh, and another thing : DO NOT PUT SOLDER PASTE on the fiducial marks.. it thows off the nulling. the camera may focus on the 'blob' of solder and be thrown ff center.
so in short
make 2 PCB footprints
one called FIDU-ROUND one called FIDU-SQUARE
each contains a single TOP layer SMD pad. Pad number 1 Hole size 0 , Paste mask expansion -20 (no paste mask) solder mask expansions : 20 mils
Set origin of the symbol to 'center'
Make two schematic symbols (FIDU-round and fidu-square) and link them to the pcb symbls.
in your top sheet you simply place 4 fidu-round and two fidu-squares. once the push to pcb is done :
set one fidu square at your board origin (0:0) on top layer , the second one you change layer (hit the L key on your keyboard) and also place at origin
place two round one at outlying corners on top layer. place two round ones on bottom layer outlying corner.
Since they are comonents, and their origin is set to the center of the pad they will be exported in the PAP file ( you suppres them in the BOM )
so you will (for example) find a line
FIDU-SQUARe , TOP LAYER, 0:0
FIDU-SQUARE, BOTTOM LAYER, 0:0
FIDU ROUND, TOP LAYER 0,1000
FIDU ROUND,top layer 100,0
fidu round, bottom layer 0,-100
fidu round bottom layer -1000,0
in the PAP. So now it's really easy for the people doing the setup of the robot. all info is right there in the setup file.