That PCBway video around 02:10 is about bad examples of fiducials.
The fiducials used in KiCad are a circle on one copper layer inside a solder mask cutout. Whether other types of fiducials exist is irrelevant in this case.
The PCBway tutorial at 2:10 showed that a fidicual is visible on both sides of a PCB.
I repeat:
And repeat again to get over the silly 20 char *&^%$#@!~.
Something like that, and to be honest I don’t understand at all why. Note that in their explanations and most of the images they assume and show a surface pad (although they seem to have 4 through hole pads, too). None of the sources I have seen mention a through hole fiducial, and if you happen to know how fiducials work, you understand a hole isn’t good for fiducial due to optical recognition. It needs flat, clear, semi-reflective surface. Here’s another video which explains this: https://www.youtube.com/watch?v=XNdey1B4mNA.
The barrier to log in to PCBWay is too high for me to write a comment there, but maybe someone is willing to do that.
This may be a historical event, as markers were once needed to put together the top and bottom photographic plates so that the PCB could be placed between them for exposure. Making a holes in all corners through all layers was probably one of good solutions.
I think 3 is minimum to allow for calibrate X and Y scale, but you can have more of them. I have read somewhere (don’t remember where) that for special precision elements additional fiducials near them are used.
I read this thread again quickly to see where the discussion is heading to. It looks like the KiCad related questions have been answered, the rest is about fiducials in general. There have been enough pointers to further information (see one of my previous posts linking to an older thread) which helps understand why the KiCad library fiducials are what they are, and how to position them in a board. Distilled:
- Use (optimally) 3 fiducials, in each side of the board which has SMD components.
- Position them in a coarse grid if possible, preferably in integer 1 mm or more coarse grid. Position them along the same X and Y lines if possible, but
- They should be positioned so that if the board is rotated or flipped their locations always indicate the direction of the board, i.e. 4 symmetrically positioned fiducials is bad. It’s also bad if they are identical when the board is flipped (this can happen even with 3 fiducials).
- It’s a round SMD pad, diameter about 1…3 mm if possible, with large mask and copper clearance, with no paste.
- It must be exported to the SMD position file to be useful for assembly.
If anyone has something important and informative to add, do it soon.
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From JLCPCB (similar info as @eelik posted)
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