Footprint

Every part on a board sits on a patch of copper shaped for it. That patch is the footprint, one pad per leg, in the right places and at the right size, plus the outline printed on the silkscreen and the area around it that nothing else may occupy.

It is the layout’s half of a part. The schematic carries a symbol, which says what the part is and what its pins are called. The layout carries a footprint, which says where its metal goes. Neither can be derived from the other, because a symbol has no dimensions and a footprint has no idea what any of its pads do.

Two other things get called a footprint, and the confusion costs real boards.

The first is the package, the moulded body the vendor ships. That one belongs to the vendor. You draw the footprint to receive it, so a single package has several valid footprints. A hand-soldered prototype wants longer pads than a reflow line does, and a board that has to survive vibration wants more copper under the part than one sitting on a desk. If you order the right package and draw the wrong footprint, the parts will not sit on the board. Pins and packages is the longer version of that story.

The second is the reference designator, which names the part rather than describing its metal. People merge the two in conversation mostly because they are printed next to each other on the silkscreen. The designator also joins the schematic to the board. R5 on the sheet becomes R5 on the layout, and the layout’s R5 has a footprint.

Because the designator is the join, a footprint depends on the rules that protect the join. unannotated-components reports parts still carrying a placeholder like R?, and a part with no name has no line on the BOM and no footprint waiting for it. duplicate-ref-des reports two distinct parts claiming one name. The name merges their two footprints, and the layout cannot tell them apart. unconnected-component catches the part wired to nothing, which still occupies its footprint’s area and still costs a line on the BOM. symbol-unresolved is the schematic-side version. A symbol file fails to open, and the placement is left with a designator and no pins at all.

A test point is the smallest footprint there is, one pad with no part above it.

Agni models the footprint itself in the physical tier of the IR, which is provisional. The Footprint message exists and no reader fills it in yet. The board readers do produce the geometry that a footprint places. They read its pads, its silkscreen and its courtyard, keyed back to the part by its designator.

The course does not teach it yet. The word appears once, inside The decision procedure, where a resistor between two signals might be “a footprint nobody stuffed”, and the course never says what one is.