Ensure Your PCB Design Integrity - Don't Trust Your Gerber Files

By Simon Garrison, Numerical Innovations

WARNING: Your Gerber Files may contain hundreds of hidden mistakes!

What are Gerber files?

For the past 30 years, Gerber has been the de-facto standard file format used to manufacture all PCB’s (printed circuit boards). The Gerber format itself is simply an ASCII text file containing X,Y coordinates, and shapes (i.e apertures/dcodes). There are essentially 4 flavors of Gerber data: RS-274D, RS-274X, Fire9000, and the new X2 format.

Sample screen capture of Gerber data inside FAB 3000 software.

Gerber data is a simple, generic means of transferring printed circuit board information to a wide variety of devices that convert the electronic PCB data to artwork produced by a photo-plotter. Virtually every PCB CAD system generates Gerber data because all photo-plotters read it. It is a file structure consisting of X,Y coordinates supplemented by commands that define where the PCB image starts, what shape it will take, and where it ends. In addition to the coordinates, Gerber data contains aperture information, which defines the shapes and sizes of lines, holes, and other features.

Gerber RS-274X: Extended Gerber Format, which is also called RS-274X, provides enhancements that handle polygon fill codes, positive/negative image compositing, and custom apertures, and other features. RS-274X also encapsulates the aperture list in the header of the Gerber data file and therefore allows files to pass from one system to another without the need to re-input the aperture table. RS-274X produces a variety of Gerber data called X data.

The Problem - Gerber files can be Difficult to Handle: Gerber data is unique in the fact that each file usually represents a single layer, something different from most other EDA formats that may contain multiple layers & designs/blocks all inside a single file. Many Gerber files are not written to specification; Thus many "unwritten rules" must be made to properly load in these files to the authors intention.

Here is a list of some common difficulties found inside Gerber files: Transcoding of apertures - when multiple gerber files define the same aperture dcode, but require different shapes, Handle complex custom aperture definitions. Handle 'user-defined' custom aperture macros ($). Circular Interpolation: Arcs (G75), & Quadrant Arcs(G74) Poly-Fill routines(G36,G37) Support composite merging(LPD/LPC) Photo Expose mode (G55) Step/repeat codes(SR) Offsets(OF),Units(G70,G71),Mirror(MI) Zero Suppression with leading or trailing zeros. Format type: Incremental(G91)

Many gerber software tools have a hard time loading, & rendering gerber files attributes properly (ie. rounded paths, custom flashes, composite layers, and step/repeat). When loading gerber files, make sure to use a commercial quality tool (with thousands of hours invested refining & perfecting the gerber import process) to ensure your gerber data files load successfully.

Don't Blindly Trust Your Gerber Files!

Learn How to Protect Yourself from Common Gerber File Mistakes

It is estimated, every year PCB Designers waste over $30 Million (USD) on failures & repairs to PCB's that had common Gerber file mistakes. That averages to $500 (USD) lost each year per PCB designer! How can this happen? Most PCB design software packages are very liberal in their interpretation of the Gerber format, and usually output CAM files (i.e. Gerber, drill, ODB++, etc.) that are not to specifications. There's a very good possibility your Gerber files have not been exported correctly, and may be interpreted erroneously by your PCB Fabricator's CAM software. To protect yourself from these common Gerber file mistakes typically requires you have CAM Software or at least a Gerber Viewer. Lets compare the benefits of each:

CAM Software:

CAM Software is to Gerber files what Print Preview is to Word Processor documents. Just as you wouldn't think of printing (or publishing) your documents without previewing them first, the exact same should be held true with your Gerber files. CAM Software automates all Gerber file checks to ensure your PCB design integrity. Should any mistakes be detected, reports can be generated making it easy to go back to your original PCB layout and correct any problems. RESULT: Using CAM Software will protect you from unknown mistakes hidden in your Gerber files, improve yields, lower manufacturing costs, and increase your time to market.

Gerber Viewer:

Almost every PCB designer has used a Gerber viewer before to look for problems in their Gerber files before sending them out... but unless you have eyes like a hawk, simply staring at Gerber files inside a Gerber viewer (especially with hundreds of signals & drilled holes) is just an invitation for mistakes. It may have been understandable in the past for a designer to rely solely on a Gerber viewer but nowadays in these competitive economic times you can't afford to just stare at Gerber files and assume everything will be okay. RESULT: Using a Gerber Viewer is at best an average solution, and prone to mistakes.

Let's take a look at a customer's first-hand experience to solve the problems found in their Gerber files using FAB 3000 (CAM Software):

"I have been looking for a product like this for a long time. We have
been "bitten" three times in the past three years by gerber errors. One
time was so bad I had to write an article about it on EDN.com:
http://www.edn.com/electronics-blogs/tales-from-the-cube/4313195/Conspiracy-theory

Most board houses we have used can not do an IPC netlist compare. The
one I have been using lately charges $70 per design (over & above the
charges for electrical test). So FAB 3000 pays for itself almost
immediately! I figure it's my responsibility anyway to provide correct
gerbers, and not depend on outsiders to check my work.

I have tried other products that claim to do netlist compare, but they
can't handle split planes, or have other problems that make them unusable.

I keep a small library of 'problem' designs that typically generate
either false positives or false negatives. FAB 3000 got them all right,
with only one false positive on one board that usually gets dozens.

I didn't need 30 days to evaluate FAB 3000. I ran it through my test
cases in half a day and was amazed. The other DFM checks it does
besides netlist compare, are just gravy."

Garry Motter, Engineering Manager - SciMeasure Analytical Systems, Inc.

Here's a demonstration movie to easily check the design integrity of your Gerber files using FAB 3000 (CAM Software):

I export ODB++ from my PCB Layout. Do I need to worry about these Gerber problems?

Yes. The potential for mistakes is always present no matter what format you choose to export for manufacturing. For example, your PCB layout tool can just as easily output a thermal pad incorrectly in ODB++ as it could in Gerber. Please remember for all design and engineering work, it's always important to double-check your data.

I've never had a problem with my PCB's before. So why should I start checking my Gerber files now?

That's like saying: "I've never had a car accident before, so why start wearing my seat belt now?"

It only takes one mistake to ruin your awesome reputation as a PCB designer savant, frustrate customers, and loose significant money. Why not spend a few extra moments to perform a complete check using CAM software to ensure the design integrity of your Gerber files.

Buy Numerical Cloud for only $75/month. Includes FAB 3000 (CAM Software) on up to 3 computers. Easily perform all Design Integrity Checks on your Gerber and ODB++ files. No contracts and cancel anytime.