In electronics manufacturing, an Excellon file is a text-based file format which is used to control the actions of a CNC drilling machine, commonly used in the drilling of printed circuit boards (PCBs). The Excellon file format is a variant of standard RS-274C. It consists of commands to instruct a CNC drilling machine to drill holes of specific diameters at specific locations on a PCB.
The name Excellon comes from the Excellon Automation Company, founded in Torrance, California, USA in 1963 by inventor Joseph Smith. The company was later bought by Esterline, and then sold to Turning Point Capital in 2003. During the 1980s Excellon held approximately a 90% market share in the sales of PCB drilling and routing machines.
The Excellon driller was designed specifically for the purpose of drilling holes in printed circuit boards. The holes in early, single- and double-sided PCBs were used to allow the insertion of the wire leads on electronic components. Following the invention of multilayer printed circuit boards, the holes were used both for the insertion and connection of wire component leads, and for interconnecting the circuit traces between the multiple copper layers of the boards.
Holes created for the sole purpose of interconnecting traces on different layers of a multilayer circuit board are called via holes. Since about 1999, most of the holes drilled into circuit boards are via holes, and the components are built without wire leads, but solderable pads instead. These components are called surface-mount components.