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Footprints: how to set pick-and-place origin of a subcircuit

fp_pnp_orig by Tibor 'Igor2' Palinkas on 2020-xx-xx	Tags: howto, pnp, pick, place, pick-and-place, origin, center, diamond, grab
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Abstract: Some footprints are asymmetric or have features on the top so the best point for grabbing them in a pick-and-place machine is not the geometrical center or the point that is practical for grabbing the subcircuit for GUI editing. This howto describes manually setting a differnt pick-and-place origin for those cases.

  For this example, we are going to use a stripped down copy of the standard library SOT325 footprint. The library version does have a pick-and-place origin, the stripped down version does not.

This example SOT325 placed on an example board at 200;200mil (grabbing at it's subc origin, the "diamond" mark) will yield a pcb XY file with the following coordinates: 200.00,4800.00 (in mil). The 200 mil X is the subc origin diamond's coordinate; the 4800 Y too, but from the bottom left corner of the 5000mil high board. Which means XY file tries to grab the part by pin 1, which is clearly wrong.

We could configure the XY exporter to grab the part at the geometric center or at the center-of-mass calculated by terminals - but that would ignore the body of the part, which also changes the center-of-mass.

So the best method is to manually specify the pick-and-place origin point, which is the point within the subcircuit where the pick-and-place machine shall grab it.

To modify the footprint, make a copy, e.g.

cp plain.fp pnp.fp

and open that file with pcb-rnd. That will get you the "footprint editor", which is really jsut pcb-rnd with loose-subc enabled, saving footprints instead of whole boards. Then add the pick-and-place origin object, following these steps.

1. Turn off the visibility of the top silk layer so it won't interfere

2. Increase grid to 0.1mil or finer

3. Right click on an empty part of the subcircuit, context menu, Edit layer bindings

4. The objects interesting for us are on the subc-aux layer, but that layer is normally invisible; find it in the layer binding dialog and change it from "virtual" to "copper" (now that the subc-aux layer is the only top copper layer bound, whatever we draw on the top copper layer will end up on the subc-aux layer of the subcircuit)

5. Enable orthogonal moves (with pressing {m c o} or from the mode, cursor/snap menu

6. Draw a line starting from the point that should be the pick&place origin, make it at least 25 mil long (the coordinate readout on the top right will help), then press esc to leave the line drawing tool

7. Set back the grid to 25 mil

8. Grab the far endpoint of the line and move it to the starting point, reducing the line length to 0, effectively making it a circle; the 25 mil grid and snapping (default on) will help with this

9. Right click on this point, select the "Edit properties" menu

10. In the property editor dialog that opens, click on the 'add' button (on the bottom); this will create a new attribute on the zero-length-line object

11. The attribute key should be subc-role and the attribute value should be pnp-origin . Warning: these are both case sensitive!

12. Close the dialogs, repeat step 4 and change back the binding of the subc-aux layer from copper to virtual; this will hide the subc-aux objects

13. Save the footprint (using {f s} or File menu, Save layout).

Now pnp.fp contains an extra subc-aux object, which is a zero-length line with attributes set up so that the XY exporter understands it marks the pick-and-place point. (In fact the actual point is the middle of the line, but as the line is zero length, it's the same as the start and end point of the line.) If you reproduce the same example board with pnp.fp placed at 200;200 mil this time, then do the same XY export the new coordinates will be 225.60,4842.00 mil, which is slightly right and slighlty up to the original point - right where we placed the subc-origin point within the footprint.