Best Practices in PCB Design: Exporting Gerber Files

Publish Date: Mar 04, 2009 | 4 Ratings | 4.25 out of 5 |  PDF

Overview

This document provides Ultiboard users with practical tips and recommendations on what to do when exporting a PCB design to fabrication.

This document will discuss how to best work with the Ultiboard design environment to generate Gerber files and NC drill files to send to a fabrication house such as Sunstone Circuits.

This is the second in a series of articles that:
  1. Help you finalize for fabrication in Ultiboard
  2. Discuss exporting Gerber files for prototype fabrication
  3. Provide a step-by-step check-list before prototype fabrication

Table of Contents

  1. Creating the Gerber files for PCB Manufacturer
  2. Creating Drill Files for PCB Manufacturer
  3. Export Gerbers and NC Drill File
  4. Drill Tables
  5. Next Step

1. Creating the Gerber files for PCB Manufacturer

After verifying all components and completing the final PCB checks, the Gerber files will need to be generated and  uploaded to the board manufacturer’s web page.

For more information on the Gerber Export dialog process, please view this article which provides more context to the dialog box, and what the various settings mean.

Exporting Gerber Files from NI Ultiboard

 

Gerber Properties

To begin generating the PCB files, the settings for each of the various file types will need to be established.  The first files needed are the Gerber files which allow the manufacturer to create the basic artwork for each of the layers.  From the menu:

  1. Launch the Export setup window from the menu by selecting File > Export….
  2. In the Export dialog box select the Gerber RS-274X format
  3. Click on the Properties button. 

Figure 1 - Export Dialog Box

 

The RS-274X standard is preferred as it is the most current and widely used Gerber format and automatically assigns the D-code aperture setting whereas the RS-274D requires the manual setup of aperture settings. Aperture D-codes set the basic drawing tool sizes and styles used by the manufacturer to physically create the various layers of the PCB.

In the left side of the Gerber properties window, select the following Available Layers items and press the right-arrow (red box - Figure 2) to move them to the right-side window.

  1. Board Outline
  2. All copper layers (Copper Top, Copper Inner 1, Copper Inner 2, Copper Bottom, etc…)
  3. Drill (may be optional for quick turn PCB services using standard drill sizes)
  4. Drill Symbols (may be optional for quick turn PCB services using standard drill sizes)
  5. Silkscreen Top and Silkscreen Bottom (for single sided boards only the top silkscreen layer may be required)
  6. Solder Mask Bottom and Solder Mask Top (may be optional for quick turn PCB services)

Figure 2 - Export Properties Dialog

 

It is important to complete the following steps to finalize export: 

    • In the Units section select Imperial (Mil) (blue box - Figure 2)
    • In the Digits section, select integer 2 and decimal place 4 (magenta box - Figure 2)
    • Check “Allow Non Convex Polygons” in the Options (green box - Figure 2)
    • Oversize the Soldermask based on PCB manufacturer’s recommendations (some quick-turn PCB manufactures suggest over sizing the PCB mask to allow for adequate spacing around pads for the solder mask tolerances).

Note: Some manufacturers may require the Board Outline to be merged with one of the copper layers.  In this case select the Board Outline and Copper Top layer on the right hand window (Export Layers) and press the Merge button to create a Composite layer. It is recommended that this layer is renamed back to Copper Top when saved so that it can be easily identified in the documentation sent to the PCB manufacturer.

 

 

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2. Creating Drill Files for PCB Manufacturer

Next the Gerber and NC Drill files will need to be generated.  The NC Drill files are reports indicating the hole sizes and locations for all holes within the design.

  1. Launch the Export setup window from the menu by selecting File > Export….
  2. In the Export dialog box select the NC Drill option
  3. Click on the Properties button. 

You will now be able to set the Drill Export properties. Generally the default options are appropriate for board manufacturers.

Figure 3 - NC Drill Export Properties

 

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3. Export Gerbers and NC Drill File

To begin your final exporting proces:

  1. Select the Gerber RS-274X and NC Drill items by pressing the <Ctrl> key to select both items. 
  2. Press the Export button and multiple windows will open in sequence allowing each of the selected files to be saved. 
  3. For the first set of Gerber files an additional Create aperture mapping popup window will launch allowing the default D-code aperture settings to be changed; simply press OK to continue with the default settings.   
  4. Each Gerber layer (or merged layer) will generate a file with a ‘.gbr’ extension and two NC drill files with a ‘.drl’ and a ‘.rep’ extension.

Once the save operation is completed, reorganize the files as required by the board manufacturer.  Some manufacturers require the files to be zipped into a folder with a simple file naming format with just the layer names for each file type.  For instance a file named “AmpProject - Copper Top.gbr” may need to be changed to “Copper Top.gbr” before sending.

 

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4. Drill Tables

In any PCB, there are typically many holes that need to be drilled for things such as through-hole parts, vias and mounting holes.   There are multiple files relating to the drilling of the holes in the PCB.  The one essential in the creation of the PCB is the NC Drill file (NC = Numerically Controlled).  The NC Drill export selection will create two files, a ‘.drl’ and ‘.rep’. 

The rep is a report file listing a summary of the drill sizes and quantities.  The drill sizes are associated with an individual drill bit (i.e. “tool”) so the tool numbers (“T#”) are shown as each individual sized hole.   If the number of drill sizes can be kept to a minimum, the cost of the PCB will be reduced since it will take less time to switch between sizes when the PCB is being made.

The drl file shows the exact locations of each hole.  It is sorted by the tool number tool number so the PCB manufacturer knows the size and location of all the holes for a specific drill bit size.

In addition, there are two Gerber files that are related to PCB drilling.  The Drill and Drill Symbols are created when the Gerber RS-274X is selected and subsequently these files are used in documentation such as the assembly drawing to verify all hole sizes and drill locations are correct.

The Drill Gerber file shows round images at each hole with the radius of the image the same as the hole radius.  When viewing this layer, the user can observe the hole sizes, locations and relations to other locations on the PCB.

The Drill Symbols Gerber file has symbols shown for each tool.  For example, if there are 5 different hole sizes needed for drilling into the PCB, there will be 5 different symbols on this Gerber layer.

 

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5. Next Step

Now that you have finalized your design it is time to review the files for fabrications. This final article in this series is a prototyping check-list for any engineer looking to use a quick-turn PCB fabrication house, such as Sunstone Circuits.

 

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