Footprint

A footprint is the land pattern for one part: solder landings, a keep-clear frame, and marks, stored as a named drawing that is stamped onto a board.

The wrong footprint is a part that does not solder. A SOIC-8 and an MSOP-8 are not interchangeable even when the editor icon looks similar. A footprint is the board cousin of a cell, a master placed by an instance. It holds pad copper, soldermask openings, silkscreen, and often a courtyard and a keepout. One master, R_0805, might carry two 1.0 by 1.2 millimeter pads on a 2.5 millimeter pitch, plus a silk box. Place fifty resistors and the board still has that one master. Footprints live in a library. They are not a schematic symbol, though the tools pair the two. Chip pad cells are cousins that go to a different shop.

Pads on two sides of a body. No part is sitting on the pads.

A pad stack is copper, mask, paste, and drill together. A through-hole pad might show a 0.8 millimeter copper land and a 0.4 millimeter hole. An NSMD BGA pad can be smaller than the mask opening. Check the vendor drawing's recommended land, not only the body outline. A library footprint that is merely close is how a reel of parts fails solder. Editing one placement's pads to fix silk can edit the master, and then fifty parts move. If only the copy moves, the library and the board no longer agree.

See footprints are almost cells, cells and instances, and a PCB stackup.