Questions On Fiducial Marks

[chromium]

1. I've been taught that fiducial marks at the board level should not be exactly diagonally symmetrical, for by doing that one of them is rendered useless since if you know the coordinate of one, you'd know that of the other. Is that true?
2. I've also been taught that the fiducial mark should have a paste mask layer for alignment purposes. But some say that by adding solder paste the contrast is lowered. Which one is correct?

 

[kevin54]

The first one might make sense. but I think not for the reason you state, I think it might be a protection against a board being off by 90 degrees.

Here's what our assembler asks for. They do ask for fiducials to be tinned which agrees with your second question.

Fiducials
Fiducials shall meet the following requirements.
1. Need to be tinned and have a 1 - 1.5 mm diameter circle.
2. Solder mask opening should be at least 1mm greater diameter than fiducial.
3. Must have a minimum distance of 1mm from any fiducial to any land, trace or hole.
4. Fiducials should have a minimum distance of 1.5mm to any silk screen marking.
5. There should be a minimum of 4 mm distance from fiducials to any board edge
6. Ideally 3 or 4 fiducials are to be used for each board.
7. 3 fiducials should lie on two lines that intersect at a right angle.
8. A minimum of 2 fiducials is required for each PWB.
9. Space fiducials as far apart as possible within all restrictions stated above.
10. Place fiducials on each side of PWB that contains SMT components.
11. Place fiducials on each individual board.
12. Also place a set of fiducials on each panel array in the margins using above restrictions.

 

[chromium]

Thanks a lot. I guess different assemblers have somewhat different requirements.

 

[marce]

No they dont.
There is an organisation allied with IPC call SMEMA (Surface Mount Equtpment Manufacturers Association) they have a variety of standards, including one on fiducials:
**broken link removed**
Also in the IPC-2221A spec there are notes on fids.
Again, you dont have to tin fids, as long as the solder mask is cleared the vision systems on all of todays SMT equipement will pick them up no problem, it does help to have the fids imaged onto the solder paste screens though. These can be laser imaged onto the paste screen without creating a hole all the way through.

 

[chromium]

No they dont.
There is an organisation allied with IPC call SMEMA (Surface Mount Equtpment Manufacturers Association) they have a variety of standards, including one on fiducials:
**broken link removed**
Also in the IPC-2221A spec there are notes on fids.
Again, you dont have to tin fids, as long as the solder mask is cleared the vision systems on all of todays SMT equipement will pick them up no problem, it does help to have the fids imaged onto the solder paste screens though. These can be laser imaged onto the paste screen without creating a hole all the way through.

I've read the documents that you recommended. However it doesn't seem to mention anything at all in regard to my first question. What can you comment on that?

 

[marce]

I always use 3, as stated by Kevin above, forming a right angle triangle, with one leg longer than the other, to avoid diagonal symetry.
WHY! because I once did a board where 2 of the fids were symetrical, and the production dept. only used 2 fids. The boards were panelised, with arrows showing the direction of travel through the SMD line. They still tried to build the boards the wrong way round and the pick and place was only set up to use the fids for location! So about 100 boards later, they noticed that they were the wrong way round and things were not quite right8-O
So now I do everything I can to avoid any possibility of a human error creeping in downstream, so I insist production use 3 fids to locate the boards for solder paste printing and component placement. And as said above I use non diagonaly symetrical fiducials. Work on the principle of minimising any ambiguities with your designs, it helps avoid human error.:-D