EEVblog Electronics Community Forum

Electronics => General PCB/EDA/CAD Discussions => Altium Designer => Topic started by: rockybyt on April 04, 2012, 09:41:18 AM

Title: Fiducials
Post by: rockybyt on April 04, 2012, 09:41:18 AM
My question for the experimented guys goes here: What is a fiducial mark? I mean... i know what it is and why it is so useful but, how do you place fiducials in Altium Designer?

Do i have to create a custom component (just a simple round pad on the top layer) or is something more complex than that? Do i have other options? Has Altium a default fiducial footprint?

I would appreciate your help guys. Thanks!
Title: Re: Fiducials
Post by: hans on April 07, 2012, 07:32:44 PM
I don't know how to place them or whether there is a built in function. I would place a circle pad with a lot of solder mask expansion. Unfortunately, I don't know what typical sizes are good..
Title: Re: Fiducials
Post by: Psi on April 07, 2012, 08:14:22 PM
i'm not sure either but i suspect it's part of the Altium "panelization" process.

I think Dave might have mentioned some info about their size in this video.
But i haven't rewatched it to confirm. (!
Title: Re: Fiducials
Post by: Short Circuit on April 07, 2012, 08:18:13 PM
Usually, you make a component for this.
In Altium this can be labelled as Standard (No BOM), so it does not pollute the BOM.

The shape is just a single round SMD pad, somewhere between 0.8 and 1.5mm diameter.
Set the paste mask expansion fixed to 0mm, and the soldermask expansion approximately
equal to the pad radius, so it becomes twice the diameter. Set the clearance such that no
traces are allowed in the SM area.
Exact values would depend on your board density and parts used. Working on a next gen
iPad, you probably want as small as possible. But for general boards you'd go for the larger
sizes, as you do not want the fiducials to be the limiting factor for the production line...
Title: Re: Fiducials
Post by: rockybyt on April 12, 2012, 09:20:07 AM
Thank you very much guys :)
Title: Re: Fiducials
Post by: free_electron on April 12, 2012, 11:20:20 AM
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

/                          \
| O                         |
|                           |
|                           |
|                           |
|                           |
|                           |
|                           |
| _                         |
||_|                      O |

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 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.
Title: Re: Fiducials
Post by: JuKu on April 12, 2012, 03:34:25 PM
Good post by free-electron. the only ting I can add is that if you have a big board, it is a good idea to put a couple of extras close by to the finest pitch parts. The machine can use a "local" reference when extra precision is needed.
Title: Re: Fiducials
Post by: free_electron on April 12, 2012, 10:55:09 PM
Correct. I have fiducial marks in my device footprints for larger 'monsters' like a TQFP208 or 240.
Anything with .6 mm pich pins or below i add fiducals on the corners.
Title: Re: Fiducials
Post by: SeanB on April 13, 2012, 05:42:45 AM
That explains nicely why I see them on HDD boards then. Sadly the most reliable part is the mechanicals, with the electronics being the part most often failing.
Title: Re: Fiducials
Post by: free_electron on April 13, 2012, 07:05:26 AM
That explains nicely why I see them on HDD boards then. Sadly the most reliable part is the mechanicals, with the electronics being the part most often failing.
HEY ! no badmouthing HDD electronics ! It's commonly the heads that fail due to electrostatic strike or the mechanical controller fries due to a failing power supply ( and you can't fight a SMPS from a coumputer that can pump 50 ampere through the system. Nothing can stop that ... )
If we'd have clean , well behaved well regulated power supplies in computers , like ANTEC or DELTA power supplies, you will have very long life from your drive.
Title: Re: Fiducials
Post by: SeanB on April 13, 2012, 02:41:35 PM
I am not, just that the mechanical parts are now so well made and reliable. I have done a few data recoveries just by doing a controller swap with another board, and have gotten the data off.
Title: Re: Fiducials
Post by: free_electron on April 13, 2012, 03:25:17 PM
Just joking. I design the drive controllers for a living :)
Title: Re: Fiducials
Post by: SeanB on April 14, 2012, 01:03:13 AM
I have seen the designs you put out. They look very nice, and from the back almost bare, until you look at the hidden side, and see all the detail in it.
Title: Re: Fiducials
Post by: free_electron on April 14, 2012, 02:20:23 AM
ehh.. i'm not doing the PCb (i do help out with those and i make the reference layouts)... i'm actually inside the slabs of silicon on the drive :)
Title: Re: Fiducials
Post by: guru_florida on September 21, 2012, 04:13:55 PM
I used to use keep-out to ensure no copper gets into the border surrounding the fiducial. Problem was it also left a solder mask hole on the bottom of the board! Also the keep out layer conflicts with components/pads underneath.

Now I setup another "Clearance" rule that specifies the 26mil clearance needed and is limited to just these fiducials. Copy your standard Clearance rule and specify this as an advanced rule in the rule editor:
Obviously, replace 'Fiducial-1.5mmS' with the name of your fiducial component.

Now the copper pours or traces will only keep back on the top layer (...the fiducials layer).

btw: Don't forget to adjust your rule priorities after you create the rule, else you'll see no difference if the default Clearance rule has a higher priority.