Sizes difference in Footprint and Printed Footprint

Hello all.

I am new to PCB CAD, so forgive me for asking. I would like to design a PCB, but I am not sure about the footprint distances, because they differ in the printed state from the measurement results used in the tool, even though they are printed 1:1. I am not sure about the deviations. I do not want to make the board with wrong distances.

What would I like to create as a footprint? Arduino Mega 2560
KiCad Version: 7.0
Operating system: Windows 11

What did I do to create the footprint?

  1. eagle files downloaded from: Arduino Mega 2560 Rev3 — Arduino Official Store
  2. started KiCad exe ‘C:\Program Files\KiCad\7.0\bin\pcbnew’ and opened the ‘EAGLE board’ with it.
  3. removed everything except the outline, holes for screws and the pins.
  4. copied the board and saved it in the normal KiCad as a new footprint. Otherwise I changed NOTHING about the distances.

How was the printing done?
Clicked print, set all layers included and scale 1:1.
In print settings under Windows 11 printer features says ‘Scale: None’.

Which dimensions do not fit?
According to KiCad, the board is about 101.6 mm, which also fits the part. After I printed the board, it only has a length of 98.5mm. Therefore the distances of the pins are not correct anymore. (picture attached)

Does anyone of you have an idea what the problem is and how I can solve it? Is it more correct in KiCad than the printer despite 1:1 scale? I would like to print it out and test it before, because I also have other components where I need to create own blueprint without a board file.

Thank you:)

The machinery used to produce PCBs from the fabrication outputs are far more accurate than your printing system. Trust KiCad.

As for what to do about it. Draw a dimension on your layout using the dimension tool, then tweak your printing scale until this is life size on paper.

Actually make that two dimensions one along each axis because printers can vary along both. And it’s affected by the state of the paper, printer mechanism, etc.

Is that a PDF? Have you tried printing from plot?

Yes, I think that may be a clue. You would think that 2.54mm pitch is a standard. However connectors are available in 2.54mm (1/10th inch) pitch as well as 2.50mm. Have a look at RS, Mouser, DigiKey, etc.
I guess a wrong footprint was used.
Or (as someone said earlier) OP’s printer scaled is down by a fraction, thus the difference.

EDIT: Sorry I didn’t read the part about the board dimensions being scaled down too. I guess this is a printed fitting the page in the printed area and it’s not 1:1 what KiCAD exported.

I don’t remember ever printing pcb to check anything. But I always use gerber viewer to see if PCB is ok. In much more precision way than you can get printed it. Generated gerbers are different than you see at KiCad PCB Editor.

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OP wrote:

If board outline is wrong size than you should not expect connector pitch is accurate.

Your printout apparently has an error of 101.6/98.5 = 1.03147 so about 3% too small.

That is the best reference you have.
Consumer printers are not made for accurate scale. It’s quite possible (although a bit unlikely) that your printer has a 3% error itself. It is also possible that there is some silly convertion along the path between KiCad and your printout. Maybe some hidden “print to paper size” in your printer driver.

Long ago in the DOS age, I worked with autocad, which had a nice printer calibration procedure. I think it printed out a big square (maybe 200mm), then you had to measure the square and put those numbers into autocad, and then it applied scaling for accurate printing. Maybe your printer driver has such a feature too?

But when in doubt, just the on-sceen measurements you make in KiCad. Those are the numbers you send to your PCB manufacturer, and those will be the sizes you get back from them

So you square some values just to make the data match?

Some speculation can be useful, to get an starting point to figure out where the difference may have originated, but beyond that it has little value.

Suffice it to say that a consumer grade printer is not a paragon of dimensional accuracy. I mean who notices a 3% variation in a paper report?

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It looks more like a dogs’ post marking contest anyway…