Tag Archives: fiducials

What Makes a Good Fiducial?

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Accountants may have a fiduciary responsibility, but that really has nothing to do with PCB assembly. Change the “ry” to a “ls,” however, and you get fiducials, which does have something to do with PCB assembly.

A fiducial is essentially an alignment mark for surface-mount assembly machines. High-volume assembly requires them to ensure accurate registration and parts placement. Low-volume assembly, like we do at Screaming Circuits, doesn’t necessarily require them. (Some low-volume shops do, so ask before assuming.) Even if they aren’t required, they still help and are always a pretty decent idea.

The basic idea, explained in this blog post here, is to create a non-reversal pattern with two or three fiducial marks on the board or panel. As you can see in the image above, the designer placed three fiducials around the board in a non-reversible pattern. (To protect the confidentiality of the board design, I obscured the circuit detail with this convenient robot head.)

In terms of the specific construction of a fiducial, two things are most important: contrast, and accuracy of position.

Contrast comes from it being bare copper – make it 1 to 2mm in diameter. Don’t cover it with solder mask. Make the mask opening 2 to 5mm larger than the copper.

The image on the left shows closeup detail. This particular fiducial mark uses a square cutout in the silk screen. Most use a round cutout, but the shape isn’t all that important. The copper pad should be round, though.

Making it out of copper gives the positioning accuracy. I’ve been asked why silk screen markings aren’t acceptable. Silk screen isn’t always registered consistently, and is therefore won’t ensure accurate alignment. Don’t use silk screen as a fudicial or positioning mark of any kind.

Again, they’re generally required for high-volume manufacturing. We (Screaming Circuits) don’t require them for low-volume, but some assembly houses do. Even when not required, they’re still a good idea.

Duane Benson
Fiducial on the roof is a long movie
But at least it stays in place

http://blog.screamingcircuits.com

Fiducials and Odd Boards

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One of the handy aspects of getting boards assembled at Screaming Circuits is that we don’t require fiducial marks for standard process boards. I would say that we build far more boards without fiducials than with. That’s cool, but there are sometimes when fiducials really are a good idea. In fact, if you’ve got room on the PCB, they’re always a good idea (just because something isn’t required doesn’t mean that it’s not a good idea).

Some boards are more in need of the marks than others. For example, not long ago, we got a rigid flex board in. It had three separate rigid boards connected by flex, designed to be folded into a stack. It looked pretty similar to the mockup in this image:

The boards didn’t have any fiducial marks. Normally, what we do, is find a via hole, through-hole pin hole, or some similar feature to use as a fiducial. That usually works, but not always. In this case, the length of the flex varied slightly from board to board. The PCB color was also very low contrast, which made it difficult for the machine to consistently recognize any mark we picked.

That meant our machines had a hard time finding the “home” spot, and we had to reset for each of the connected boards. Finding a spot on one board did not guarantee that we’d know where to place parts on the other two boards in the set.

In this case, it would have been far better if the boards were a consistent distance apart, and if each of the three boards had a set of fiducial marks.

What makes a good fidicual?

Most CAD packages have fiducial marks in their components library. Basically, it needs to be a metal dot surrounded by an area without any copper or solder mask. More than one is best. It should be an asymmetrical pattern that can only be oriented one way.

I’ve got some more details in this article here.

Duane Benson
Routed up like a fiducial
Another rigid flex in the night

http://blog.screamingcircuits.com

Fiddling with Fiducials Again

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I recently posted a note about fiducials but I didn’t have any images. Here’s a couple of examples:

IPC acceptable fiducialsThis first example shows what IPC would like to see. If this is an individual board, this would be it. If it were part of a panel, you would follow the same pattern on the panel rails and also put it on each individual board in the panel.

As I wrote in the earlier post, we don’t require these, but it’s always a good idea. You’ll need them once you go into volume manufacturing anyway.

The next example won’t make IPC happy, but it will make Screaming Circuits happy:

Also acceptable fiducialsIt only uses two fiducial dots, but it isn’t reversible. Reversibility is okay for jackets, but not for circuit boards. Since one of the dots is offset, it can’t be placed on the machine and recognized as correct in any way except in the proper orientation.

The important aspect of both of these examples is that they remove ambiguity. There can’t be any uncertainty, which is good because uncertainty is your enemy. It’s a subtle enemy. It might not bother you 99.9 times out of a hundred, but then, when you’re not looking, it can strike. So, give a hoot and stomp out ambiguity.

Duane Benson
False data can act only as a distraction. Therefore, I shall refuse to perceive.

http://blog.screamingcircuits.com/