PCB NC formats

Key takeaways

PCB NC formats belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect PCB NC formats to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of PCB NC formats from memory before moving on to harder problems.

Reference excerpt

PCB NC drill files convey PCB drilling and routing information. The NC formats were originally designed by CNC drill and route machine vendors as proprietary input formats for their equipment, and are known under their company name: Excellon, Hitachi, Sieb & Meyer, Posalux, etc. These formats are similar as they are based on RS-274-C and related to G-code. In 1985 IPC published a generic standard NC format, IPC-NC-349. Later XNC was designed, a simple strict subset of IPC-NC-349, designed not for driving machines but for exchanging drill information between CAD and CAM. They are collectively referred to as (PCB) NC files.

The NC files are primarily used to drive CNC machines, and they are adequate for that task. They are also used to exchange design information between CAD and CAM, for which they are not adequate: essential information such as plating and drill span is missing. Furthermore, the NC output in CAD systems is often poorly implemented, resulting in poor registration between drill holes and copper layers and other problems. To exchange data between CAD and CAM it is more preferred to use the Gerber format. The quality of the Gerber file output software is typically good, and Gerber supports attributes to transfer meta-information such as plating and span.

IPC-NC-349 format The IPC-NC-349 format is the only IPC standard governing drill and routing formats. XNC is a strict subset of IPC-NC-349, Excellon a big superset. Many indefinite NC files pick some elements of the IPC standard. A digital rights managed copy of the specification is available from the IPC website, for a fee. It is targeted at input for drill/rout machines, not CAD to CAM data exchange.

XNC format The XNC format is 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. Its purpose is to address the current chaos of different subsets and incomplete NC files, and define a simple common standard. The XNC subset was defined by a consortium formed by Graphicode, Ucamco, KiCad and Pentalogix. The specification is freely available among others at the Ucamco download page. The specification is brief, detailed, easy to understand and unambiguous. It is targeted at CAD to CAM data exchange, not at input for drill/rout machines.

An example:

Excellon format The name Excellon format is derived from the company Excellon Automation (not to be confused with Excellon Software), which was the market leader in PCB drilling and routing machines during the 1980s, and whose proprietary format became widely used. There are actually two Excellon formats, the older Excellon 1 and Excellon 2. Excellon 2 is a superset of IPC-NC-349. Commands from both are sometimes confusingly mixed in the same file. Excellon Automation stopped publishing the specification of its format, without statement on the IP and usage rights of the format. An archived copy exists. The specification is ambiguous, leading to different interpretations, especially in the area of coordinates and units. It is targeted at input for drill/rout machines, not CAD to CAM data exchange.

Indefinite NC files CAD/CAM data exchange often uses NC files that do not conform to any specification. These files contain a few IPC-NC-349 commands, but follow neither the Excellon nor the IPC-NC-349 specification. 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. Usually the header is incomplete: 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. They are often called Excellon files although they are undefinite NC files. No specification exists. An example:

The tool diameters and how to interpret the coordinates is not specified. This file is 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, with unnecessary manual labor and risk of delays or errors. Astonishingly, such enigmatic files are still used.

See also Gerber format – Standard file format used for designing printed circuit boards EDIF – File format for electronic designs STEP (file format) – Widely used CAD 3D data exchange file formatPages displaying short descriptions of redirect targets

References

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

Worked examples

Example 1 — a first encounter with PCB NC formats

Start with the simplest possible case. Write down what PCB NC formats claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to PCB NC formats before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about PCB NC formats ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of PCB NC formats

In research: PCB NC formats appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses PCB NC formats in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: PCB NC formats is common in secondary-school and first-year university syllabi. It links to neighbouring topics Printed circuit board manufacturing, so understanding it makes those chapters shorter.
In everyday life: Look for PCB NC formats outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study PCB NC formats in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what PCB NC formats means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain PCB NC formats out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is PCB NC formats in simple terms?

PCB NC drill files convey PCB drilling and routing information. The NC formats were originally designed by CNC drill and route machine vendors as proprietary input formats for their equipment, and are known under their company name: Excellon, Hitachi, Sieb & Meyer, Posalux, etc.

Why does PCB NC formats matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study PCB NC formats?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on PCB NC formats.