Notes for Gerber Files Generated from Eagle 9.20

Introduction

Autodesk Eagle is one of the most widely used PCB design software tools in the electronics industry, favored by hobbyists, students, and professional engineers alike. When Eagle released version 9.20, it introduced several improvements to the CAM (Computer-Aided Manufacturing) processor, making it more convenient to generate Gerber files directly from board designs. However, these updates also brought some default settings that can cause problems if not properly configured before exporting your manufacturing files.

Gerber files are the industry-standard format used by PCB manufacturers worldwide to produce circuit boards. These files contain all the necessary information about each layer of your PCB, including copper traces, solder mask, silkscreen, drill holes, and board outline. Any errors in these files can result in manufacturing defects, delays, or even completely unusable boards. Therefore, understanding how to correctly generate Gerber files from Eagle 9.20 is essential for anyone looking to have their PCB designs manufactured.

This comprehensive guide will walk you through the critical settings and configurations you need to be aware of when generating Gerber files from Eagle 9.20, helping you avoid common pitfalls and ensure your boards are manufactured correctly the first time.

Understanding the CAM Processor in Eagle 9.20

The CAM Processor is Eagle’s built-in tool for converting your PCB design into manufacturing files. When you open your board file (.brd) and access the CAM Processor, you’ll find a streamlined interface that allows you to select which layers to export and in what format. Eagle 9.20 provides preset jobs that can generate standard Gerber outputs, but these presets contain default settings that may not be appropriate for all manufacturing scenarios.

Before diving into the specific issues, it’s worth understanding what happens during the Gerber generation process. The CAM Processor reads your board design and translates each layer into a separate Gerber file. These files use either the RS-274X format (also known as Extended Gerber) or the newer RS-274X with comments format. Most PCB manufacturers accept both formats, though RS-274X is more universally supported.

Issue 1: Soldermask Layer Polarity Settings

One of the most critical issues to address when generating Gerber files from Eagle 9.20 involves the soldermask layer settings. The soldermask is the protective coating applied over the copper traces on your PCB, typically appearing as green, blue, red, or black depending on your color selection. The soldermask layer in your Gerber files defines where the mask should NOT be applied—essentially creating openings for pads and vias where soldering will occur.

In Eagle 9.20, the CAM Processor has a default setting called “Negative Polarity” that is automatically enabled for soldermask layers. This setting fundamentally changes how the soldermask data is interpreted. When negative polarity is enabled, the meaning of the soldermask layer is inverted—areas that should be masked become exposed, and areas that should be exposed become masked.

To correct this issue, follow these steps:

1. Open your board design file (.brd) in Eagle 9.20
2. Navigate to File → CAM Processor, or click the CAM Processor button in the toolbar
3. In the CAM Processor window, locate the soldermask layer outputs (typically named “Soldermask Top” and “Soldermask Bottom” or similar)
4. Click on each soldermask output to view its settings
5. Find the “Negative Polarity” checkbox and ensure it is UNCHECKED
6. Repeat this process for both top and bottom soldermask layers if your design is double-sided

Failing to correct this setting can result in several manufacturing problems. Your PCB manufacturer may reject the files outright if they detect the polarity issue during their design rule check. In worse cases, the boards might be manufactured with inverted soldermask, leaving copper traces exposed where they should be protected and covering pads where soldering needs to occur. This would render the boards completely unusable and require a costly re-order.

Issue 2: Board Outline and Profile Layer Configuration

The board outline, also known as the profile or mechanical layer, defines the physical boundaries of your PCB. This information tells the manufacturer where to cut or route the board from the larger panel during production. An incomplete or incorrect board outline can lead to boards being cut to the wrong size or shape, or manufacturers rejecting your files due to ambiguous boundary definitions.

In Eagle, the board outline information can be distributed across multiple layers, primarily the Dimension layer and the Milling layer. The Dimension layer typically contains the main board outline, while the Milling layer may contain additional cutouts, slots, or internal routing paths. For the manufacturer to have complete information about your board’s physical boundaries, both layers need to be included in the Profile output.

To properly configure the Profile layer in Eagle 9.20:

1. Access the CAM Processor from your board design
2. Locate the Profile or Board Outline output in the job list
3. Click on the output to view and edit its layer assignments
4. Ensure that both the #Dimension layer and the #Milling layer are added to this output
5. Verify that no other unnecessary layers are included that might cause confusion

After making these adjustments, you can select your preferred Gerber format. Most users choose “Gerber RS-274X” for maximum compatibility with PCB manufacturers worldwide. The “RS-274X with comments” option adds human-readable comments to the files, which can be helpful for debugging but doesn’t affect how manufacturers process the files.

Once your settings are configured correctly, click “Process Job” to generate the Gerber files. Eagle will create separate files for each layer, typically saving them in the same directory as your board file or in a designated output folder.

Issue 3: Generating Drill Files for Multi-Layer PCBs

For simple two-layer or four-layer PCBs with standard through-hole vias, the default drill file generation in Eagle 9.20 usually works correctly. However, when working with more complex multi-layer boards that incorporate blind vias or buried vias, special attention must be paid to drill file generation.

Blind vias connect an outer layer to one or more inner layers without passing through the entire board. Buried vias connect two or more inner layers without reaching either outer surface. These advanced via types require separate drilling operations during manufacturing, and each drilling operation needs its own drill file with the appropriate layer span information.

To generate proper drill files for multi-layer PCBs with blind or buried vias in Eagle 9.20:

1. In the CAM Processor, locate the Drill output section
2. Right-click on “Drill” and select “Excellon” to access the drill generation options
3. Choose “Generate Excellon outputs based on PCB stackup”
4. Click “Process Job” to generate all necessary drill files

This option instructs Eagle to analyze your board’s layer stackup and automatically create separate drill files for each unique via span. For example, a six-layer board might have through-hole vias (layers 1-6), blind vias from the top (layers 1-2), blind vias from the bottom (layers 5-6), and buried vias (layers 2-5). Each of these would require a separate drill file.

The generated drill files will typically be named to indicate their layer spans, making it easy for manufacturers to identify which drilling operations are required. Always verify that all expected drill files are generated before submitting your design for manufacturing.

Best Practices and Additional Recommendations

Beyond the three main issues discussed above, there are several best practices to follow when generating Gerber files from Eagle 9.20:

Always perform a visual verification of your generated Gerber files before sending them to a manufacturer. You can use Eagle’s built-in Gerber viewer or free online tools like PCBWay’s Online Gerber Viewer to inspect each layer and ensure everything appears correct. Pay particular attention to the soldermask openings, pad alignments, and board outline.

Keep your Eagle software updated to the latest version when possible. Autodesk regularly releases updates that fix bugs and improve the CAM Processor functionality. However, always test your workflow after updating to ensure no settings have changed unexpectedly.

Create a custom CAM job file with your corrected settings and save it for future use. This prevents you from having to manually adjust the settings each time you generate Gerber files and reduces the risk of forgetting an important configuration.

Finally, maintain clear communication with your PCB manufacturer. If you’re unsure about any aspect of your Gerber files or manufacturing requirements, most reputable manufacturers offer engineering support and will review your files before production begins.

Conclusion

Generating correct Gerber files from Eagle 9.20 requires attention to several default settings that may not be optimal for standard PCB manufacturing. By unchecking the negative polarity option for soldermask layers, including both Dimension and Milling layers in your Profile output, and properly generating drill files based on your PCB stackup, you can avoid common manufacturing errors and ensure your boards are produced correctly. Taking the time to verify these settings before submitting your design will save you time, money, and frustration in the long run.