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Fiducial Shapes---

#22690

Fiducial Shapes--- | 12 December, 2002

I would like to know the reason behind using soild disks and donut shapes as the most commonly used fiducial shapes in the Industry.Why not use square shape or diamond shapes fiducial and many more instead of circular shape or solid disks. Thank you

Mani

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#22691

Fiducial Shapes--- | 13 December, 2002

recognition and computation methods plus commonality.

Thanks

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#22692

Fiducial Shapes--- | 13 December, 2002

I'm no math wizard, but this is my understanding

Did you ever have a spirograph as a kid? Took you about 5 minutes and endless number of lines to create one circle ( and some pretty cool patterns too ). To create a square ( or diamond ) you only need 4 straight lines, so the vision system only has 4 lines ( so to speak ) as a reference to ensure your fiducial is correct. A circle or donut, using straight lines to create it, has hundreds of lines, which gives the system more to work with.

Maybe a confusing analogy, but then again it took me days to figure out my spirograph as a kid.

Any one know of a consulant who offers spirograph lessons???

Phil

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RDR

#22693

Fiducial Shapes--- | 13 December, 2002

One thing I have found when using square or diamond etc... Fiducials is, if it is a HASL finish board, the finish may be thick and the fid camera will see them having rounded edges different sizes etc.. and will cause quality failures.

Russ (My spirograph won't make squares either)

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MA/NY DDave

#22696

Fiducial Shapes--- | 13 December, 2002

Hi

I hope more post on this one. Since it interests me and I haven't read enough on this topic in recent years.

Donoughts with a dot in the middle I have seen.

The dot in my opinion could be over or under etched or plated with the center of the dot always staying in the same place. Also the dot could be a single flash with the artwork generation machine. The dot should also give a nice reflection or transmission to optics and then simple algorithims can easily find the center compared to other shapes.

The above could also hold true for some other shapes and there might be more reasons that are more historical so I am interested in reading more inputs.

Also interested in internal shapes.

YiE, MA/NY DDave

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#22699

Fiducial Shapes--- | 13 December, 2002

Since you asked for more here is my $ .02. There are many different vision systems, camera and software combinations that will make each make and model machine work differently. For instance a 1999 Siplace machine might work best with a 1.5mm round FID but a 2000 circa machine might be more accurate with a double diamond. Most of the better equipment manufactures will tell you what is best for your machine. Phil�s analogy is correct, the better systems will work better with more lines or surface edges to look at I also believe they will also do better with more right angles such as the double diamond shape. Consistency is the key to success no matter how good your vision system is. Make sure that you have as few different fiducial shapes as possible.

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#22709

Fiducial Shapes--- | 13 December, 2002

Thank you friends for your discussion Regarding the reason behind using Solid disk or donut fiducials.Still I am understanding clearly the reson behind using Solid disk or donut fiducial shape. In all the above discussions we had we did not discuss about the manufacturing of fiducial shapes on the board.I doubt whether any manufacturing intricacy makes solid or donut fiducial to be used more commonly...Please throw some insight about the problems in manufacturing of Fiducials on the circuit board.Is there any advantage from manufacturing point of view to use the soild disk or donut fiducial rather than square or diamond or double square. Thank you Mani

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#22710

Fiducial Shapes--- | 13 December, 2002

In theory you are right, but the camera CCD chip has only small squares as pixels. The edges of the circle lit the pixels only by a certain percentage. In order to find the edges, you will have to use sub-pixels or greyscales. A circle is also prone to create a dome shape after pre-tinning and reflect the direct light that the circle diameter image appears to be smaller. I doubt that the circle is a good fiducial for any machine vision system.

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MA/NY DDave

#22712

Fiducial Shapes--- | 14 December, 2002

Hi

Thanks for this info.

Hope someone new also enters.

Funny I just picked up an article, too small, that writes that dots on some known pattern are used to measure distortion and then it shows distortion for square shapes (barrel and pincushion distortion) that can result from the algorithms and the vision system.

If someone wants to write to the author she is at jwagner@edmundoptics.com. Her article was in Quality Dec2002 Page 8.

YiE, MA/NY DDave

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