PCB NC formats

The Excellon format is widely used to drive CNC drilling and routing machines in printed circuit board (PCB) manufacturing. It is a variant of the EIA RS-274-C (G-code) standard.

The name Excellon format is derived from the Excellon Automation Company, which was the market leader in PCB drilling and routing machines during the 1980s, and whose proprietary format became widely used.

The IPC organization defined the IPC-NC-349 format as standard NC format. It is similar to the Excellon format.

Usage

The Excellon format was designed for and is primarily used to drive CNC drill and route machines. It is suitable for that task as it can specify machine specific information such drill feed and speed.

An example:

 M48
 METRIC
 T01C0.6
 T02C1.0
 %
 G05
 T01
 X8.01Y4.8
 X8.01Y2.85
 X6.54Y2.85
 X6.45Y4.8
 T02
 G00X8.01Y3.825
 M15
 G01X6.54Y3.825
 M16
 M30

The Excellon format is also used to transfer design information from CAD to CAM. It is not adequate for that task: essential information such as plating and drill span is missing. Furthermore, the Excellon output in CAD systems is often poorly implemented, resulting in poor registration between drill holes and copper layers and other problems. For data exchange between CAD/CAM systems it is more practical to describe the drill data in Gerber format as the quality of the Gerber file output software is usually better and Gerber supports attributes to transfer meta-information such as which are the via holes.

The XNC subset

Graphicode, Ucamco, KiCad and Pentalogix formed a consortium to define a strict subset of the IPC-NC-349 specification targeted at data exchange between CAD and CAM. The name XNC format stands for Exchange NC format. As a strict subset, it is highly compatible with existing software. The specification is brief and very detailed, with the purpose of being easy to understand and unambiguous. Its purpose is to address the current chaos of different subsets and incomplete NC files, and define a simple common standard. The specification is freely available at the Ucamco download page.

Excellon-like files

The name Excellon format or Excellon file is also commonly used for files that only vaguely follow any specification. These files contain a few Excellon commands, but follow neither the Excellon nor the IPC-NC-349 specification. It would be more appropriate to call them Excellon-like or generic NC files. Commands are not used properly, or are used in a syntactically incorrect way, and binary data objects may be included. Sometimes the historic EIA or EBCDIC character encoding is used instead of ASCII as required by the standards. Usually the header is incomplete: essential information such as the scale or the tool diameters is missing. Sometimes there is no header at all, and the file only contains tool numbers, with an unspecified diameter, and X,Y coordinates, in an unspecified unit. An example:

 %
 T01
 X006272Y001092
 X006354Y001093
 X006653Y001092
 ...
 T02
 X008091Y001754
 X-002028
 M30

These files are meaningless without additional information, typically put in a free format human readable tool file. This information must be re-entered manually by the CAD/CAM operator. When critical information is missing (e.g. omitting leading or trailing zeroes without properly documenting if and which zeroes are omitted) the file is ambiguous. Still, such Excellon-like files are sometimes used, resulting in unnecessary manual labor and risk of delays or errors. The operator may substitute his own judgement or stop the job pending clarification.

See also

• Gerber format, which can also transfer drill information

External links

• IPC-NC-349 specification (payable)
• Former Excellon format description (archived from the original on 2007-10-30)
• PCB Fabrication Data - A Guide