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Re: gEDA-user: Fiducial
Jeff VR wrote:
I'm working on laying out my first PCB board. I understand that the
pick and place machine uses reference points on the board called
fiducials when placing my components. I'm planning on having my board
assembled by a MyData 12 machine. So how do I incorporate this symbol
and element on my PCB. I found a similar question on this mailing
list dated a couple of years ago but it had no response.
I searched the schematic symbol library and footprint libraries and I
couldn't find anything obviouse. A little guidance from a seasoned
PCB deasigner would be greatly appreciated.
Thanks,
Jeff
I suggest creating a schematic element and a footprint for a fiducial
and instantiate it in your schematic.
I can't comment on that particular machine. In general though, you
should use a minimum of 2 "global fiducials" and preferably 3. With 2
you can correct for x,y offsets and rotational offsets. With 3 you can
correct for some nonlinear distortions like scaling, stretch, and twist.
This is paraphrasing part of the IPC-7351 document. By global, I mean
they're not for some particular part on the board. You should place
these at 3 of the 4 board corners. The document recomments that you
locate 2 diagonally across the board and the 3rd one has one x
cooridinate and 1 y coordinate in common with the others. For example
(X1, Y1), (X2, Y2), (X1, Y2)
You may or may not need local fiducials near some high pin count fine
pitch parts. If you do, place 2 of them diagonally across from each
other just outside the package corners.
The prefered shape and size is a filled circle of copper with solder
mask completely removed. The diameter should be 1.0 mm. The diameter
area free of soldermask should be 2x the diameter of the fiducial.
I'd probably make an element with 1 pin to put in schematic and layout.
-Dan
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