8
\$\begingroup\$

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"?.

enter image description here

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? enter image description here

\$\endgroup\$
  • 9
    \$\begingroup\$ 99 questions raised and only 12 have attracted formal answer acceptance. Something seems amiss. \$\endgroup\$ – Andy aka Oct 16 at 11:22
  • \$\begingroup\$ @Andyaka I have found plenty of information about their size, shape, the reasons they're used but not much about the questions above \$\endgroup\$ – MrBit Oct 16 at 12:22
  • 3
    \$\begingroup\$ Laziness then because I can see some pretty good answers that you have neither accepted not left a comment for clarification. \$\endgroup\$ – Andy aka Oct 16 at 12:43
  • 12
    \$\begingroup\$ I think what Any's getting at is that you've asked a lot of questions on the site, but apparently don't like many of the answers you've gotten since you've not accepted many of them. This pattern of behavior ends to dis-incentivize some people here from providing you with more answers. \$\endgroup\$ – brhans Oct 16 at 12:46
  • 2
    \$\begingroup\$ It's not too late for you to go and accept answers to all of your old questions. We don't show the accept percentage anymore like we used to, so it's usually less of an issue now. \$\endgroup\$ – Reinstate Monica Oct 16 at 19:21
8
\$\begingroup\$

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.

\$\endgroup\$
  • \$\begingroup\$ According to this guide worthingtonassembly.com/blog/2014/12/29/… should be placed no more than 3 fiducial. How do place 4? Do you mean 4 or 5 in total? \$\endgroup\$ – MrBit Oct 16 at 11:40
  • \$\begingroup\$ You EMS prefers 2-3 marks. I have worked with several that doesn't care as long as there is enough marks. It's all a matter of making your PCB documentation compatible with those using it in the next production step. \$\endgroup\$ – Peter Karlsen Oct 16 at 11:57
  • 6
    \$\begingroup\$ I think the rule about no more than 3 fiducials means no more than3 in the corners of a rectangular board. This helps them detect a board loaded in the wrong orientation. 3 corner fiducials plus local fiducials near fine pitch components should not be a problem. \$\endgroup\$ – The Photon Oct 16 at 14:59
12
\$\begingroup\$

You've got answers to most of your question, but I'll add more detail on this one part,

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?

What you call the "3-fiducial rule" is really "Don't put fiducials in all 4 corners". The reason is that if there are fiducials in only 3 corners it's much easier for the vision system to detect if the board has been loaded in the wrong orientation, rotated 90 or 180 degrees from the way it's expected to be.

(Why your vendor is okay with 2 fiducials in opposite corners, which has the same problem, I'm not sure. The vendors I work with prefer fiducials in 3 corners)

Additional fiducials in the middle of the board (near fine pitch parts) don't create unwanted symmetry in the fiducial arrangement so they don't cause problems.

\$\endgroup\$
5
\$\begingroup\$

You need to check with the company that places your components what the requirements are but typically you will have three to allow for the artwork being slightly over or undersize and for the artwork being slightly rotated in relation to the PCB. These should have a clearance in the solder resist around them to ensure the machine locates on the copper and nothing else. In addition you may require local fiducials around fine pitch components but these may not be required if the board is small enough that the main ones suffice.

For panels of small boards I have often placed the fiducials of the break-off region instead of each individual artwork to save space.

Fiducials are not usually required for boards consisting of Pin Through Hole (PTH) components only or hand built boards and if you have surface mount on both sides of the PCB they are required on both sides.

Note: too many fiducials is not a problem as the board-stuffer will just ignore any they do not need.

\$\endgroup\$
2
\$\begingroup\$

They are used to check what the actual position of your board in the machine is and adjust the settings. The shape of the board does not mater. More than two are used to see if the size of the board matches the programmed one.

You do not have to place it on the board. If the PCB is panellized you can place them on the panel borders instead (I do not recommend it personally, but sometimes on the very dense boards there is no space for them).

\$\endgroup\$
  • 1
    \$\begingroup\$ On the very dense boards, it's probably even more important to try to find space on the boards, especially if there are fine pitch parts. I suppose it makes no difference if you can tolerate poor yield. \$\endgroup\$ – Scott Seidman Oct 16 at 16:22
  • \$\begingroup\$ If it is the panel and you have them just outside the board it does not make any difference \$\endgroup\$ – P__J__ Oct 16 at 16:32
  • \$\begingroup\$ That would depend on how many panels are on the board, and if there are runners on all 4 sides. \$\endgroup\$ – Scott Seidman Oct 16 at 16:35
  • \$\begingroup\$ So you will have many as needed and on the both sides \$\endgroup\$ – P__J__ Oct 16 at 16:56

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.