What Are Gerber Files?

Gerber files, also known as Gerber format or Gerber X2 format, is an open ASCII vector format for 2D binary images. It is the de facto standard used by PCB fabrication houses. A Gerber file describes the copper layers, solder mask, legend, and drill holes of a PCB.

Some key facts about Gerber files:

• Developed by Gerber Systems Corp. in the 1960s
• Based on numerical control (NC) commands
• Conveys images, apertures, and flashes to expose photoresist and copper
• File extensions are typically .GBL, .GBS, .GBO, .GBP, .GTP, etc.
• Current version is X2, which replaced the older RS-274X format

For each PCB design, you’ll need to generate a set of Gerber files, one for each layer of the board. The exact files depends on the number of layers and options of your particular PCB. A typical 2-layer PCB might have the following Gerber files:

File	Layer
.GBL	Bottom Copper
.GBS	Bottom Soldermask
.GBO	Bottom Silkscreen
.GTP	Top Paste
.GTO	Top Silkscreen
.GTS	Top Soldermask
.GTL	Top Copper
.TXT	Drill File

Step 1: DRC and ERC Checks

Before generating Gerber files from your Eagle PCB design, it’s important to run both a Design Rule Check (DRC) and Electrical Rule Check (ERC). These will verify that your PCB Layout meets manufacturing constraints and has no electrical issues.

To run a DRC:

1. Click the DRC icon in the top toolbar
2. Select the “Check All” radio button
3. Adjust the DRC constraints if needed for your PCB fab
4. Click “Check”
5. The DRC Errors window will report any issues

To run an ERC:

1. Click the ERC icon
2. Click “Check”
3. Review any warnings in the ERC Errors window

Fix any DRC or ERC errors before proceeding. Common issues include traces/pads too close together, traces crossing planes without thermals, missing or overlapping connections.

Step 2: Run CAM Processor

Once your design is error-free, you’re ready to generate the Gerber files using Eagle’s Computer Aided Manufacturing (CAM) Processor.

1. Switch to the PCB Editor window
2. Select File > CAM Processor
3. In the CAM Processor window, select “gerb274x.cam” from the “Open > Job” menu
4. This will load a default Gerber conversion setup

The CAM Processor allows you to select which layers to export and customize the output. The default gerb274x job includes the common files needed.

Step 3: Select Layers

In the CAM Processor window, you’ll see a list of layers with checkboxes. Select the layers you want to include in your Gerber output. For a standard 2-layer PCB:

• Check Top, Bottom, Pads, Vias, Dimension layers
• Check tPaste, tSilk, tMask for top stencil, legend and mask
• Check bPaste, bSilk, bMask for bottom stencil, legend and mask
• Keep Holes Vias Unplated and Holes checked

Consult with your PCB manufacturer if you’re unsure which layers to include. You can select and deselect layers by clicking their checkboxes.

Step 4: Configure Output

Next, you may need to configure the parameters for each layer in the Gerber file output:

1. Select a layer in the CAM Processor window
2. Click the “Wheel” icon to open the Device Setup window
3. Set the Device to “GERBER_RS274X”
4. Click “File” to set the output filename

Repeat this for each layer you’re exporting. By default, Eagle uses file extensions (e.g. .GBL, .GBS) to identify each Gerber layer.

You can also configure settings like:

• Format: Leading/trailing zeros, absolute/incremental coordinates
• Aperture: Embedded apertures or aperture file
• Attributes: Image polarity, rotation, mirroring

The default settings are usually fine, but check with your manufacturer for any specific requirements.

Step 5: Process Job

When you’ve configured all the layers, you’re ready to generate the Gerber files:

1. In the CAM Processor, click the “Process Job” button
2. Select an output directory to store the generated files
3. Click “Accept”

Eagle will process the job and create all the specified Gerber files in the output folder you selected. You should end up with a set of files like:

ExamplePCB.GBL
ExamplePCB.GBS  
ExamplePCB.GBO
ExamplePCB.GTP
ExamplePCB.GTO
ExamplePCB.GTS
ExamplePCB.GTL
ExamplePCB.TXT

These files are now ready to send to your PCB manufacturer for fabrication. The Gerber format is an industry standard, so any manufacturer should be able to accept them.

Step 6: Verify Gerber Files

Before submitting your Gerber files, it’s a good idea to double-check them for any issues. You can use a free Gerber Viewer program to visually inspect the files. Some popular options:

• Gerbv
• ViewMate
• ZofzPCB
• CAMtastic

Load your Gerber files into the viewer and carefully review each layer. Look out for any missing traces, incorrect pad sizes, overlapping elements, etc. Catching problems now can save you time and money.

If everything looks good, package up your Gerber files (usually in a ZIP archive) and send them off to your manufacturer! You’ll typically also need to provide a drill file, a readme with PCB specs, and pick options like solder color, surface finish, etc.

Frequently Asked Questions

What is the current Gerber file format standard?

The current Gerber format is known as X2 or Gerber X2. It extends the older RS-274X format with new commands and attributes. Always use the X2 format for best compatibility.

What are the typical Gerber file extensions?

Common Gerber file extensions include:

• .GBL – Bottom copper layer
• .GBS – Bottom soldermask
• .GBO – Bottom silkscreen
• .GKO – Board outline
• .GTP – Top paste
• .GTO – Top silkscreen
• .GTS – Top soldermask
• .GTL – Top copper layer

Do I need to generate an aperture file?

In older Gerber RS-274D, an aperture file (.APR) was used to define the shape and size of flashes. But in the modern X2 format, apertures are embedded in the Gerber file itself. An external aperture file is no longer required.

What are some common issues with Gerber files?

When generating Gerber files, watch out for:

• Missing layers or files
• Incorrect file extensions
• Overlapping traces or pads
• Clearance violations
• Acid traps and starved thermals
• Incorrect hole sizes
• Reversed polarity

Always carefully review your Gerber layers in a viewer before submitting them for manufacturing. Consult your manufacturer if you’re unsure about any settings or options in the CAM Processor.

How do I submit my Gerber files for manufacturing?

To send your Gerber files to a manufacturer:

1. Package all the Gerber files into a single ZIP archive
2. Include a drill file (usually .TXT format)
3. Provide a readme.txt with PCB specifications
4. Select options like PCB Thickness, copper weight, color, surface finish, etc.
5. Upload the files and place your order

The process varies a bit between manufacturers, but most follow a similar flow. Make sure to carefully review your design before placing the final order.

With your Eagle PCB design successfully converted to a set of Gerber files, you’re ready to get your boards manufactured. Gerber is a universal standard, supported by all PCB fabs. This guide has walked through the key steps in Eagle’s CAM Processor to configure, generate, and verify your Gerber manufacturing files. Always remember to run DRC and ERC checks first, carefully review your output files, and clearly communicate your requirements to your manufacturer.