Navigating Gerber File Extensions in PCB Design
Understand common Gerber file format extensions (.GTL, .GBL, .GBR) and their roles in PCB fabrication. This guide explains how clear naming and the X2 format prevent manufacturing errors.
You might think specific gerber file format extensions are a strict rule, but the official standard does not require them. What truly matters for successful manufacturing is a descriptive filename and the correct internal format, like RS-274X or X2. Flawed naming protocols are a common source of error, leading to confusion and production delays. This guide deciphers common gerber file format extensions and modern naming practices, helping you deliver clear, fabrication-ready designs every time.
- Gerber files are blueprints for making circuit boards. They show each layer of the board.
- Gerber X2 is the best format to use. It helps prevent mistakes by adding important details to the files.
- Clear names for your files are very important. They tell the manufacturer what each file is for.
- Always send all needed files together in one ZIP folder. This includes Gerber files, drill files, and other important documents.
- Always check your files with a Gerber viewer before sending them. This helps you find and fix errors early.
Before you can master file naming, you need to understand what a Gerber file is and how it has evolved. These files are the bridge between your design software and the physical manufacturing process.
A Gerber file is a 2D pictorial representation of a single layer of your PCB design. Think of it as a universal blueprint for fabrication machines. You provide a collection of these files to your manufacturer, with each file detailing a specific aspect of the board. This collection typically includes:
- Copper layers for electrical traces and pads.
- Solder mask layers to define where the protective coating goes.
- Silkscreen layers for component labels and other text.
- An outline file that defines the board’s physical shape.
Each file contains precise coordinates and commands that instruct machines where to add or remove material, turning your digital design into a physical circuit board.
The Gerber format is named after Joseph Gerber, who pioneered its use for vector photoplotters in the PCB industry. Early formats were simple, containing only basic coordinates and commands. This required significant manual input from the manufacturer. Over the years, the format gained more intelligence to support the growing complexity of PCB designs. This evolution led to the standards you use today.
For many years, RS-274X was the go-to standard. Its major advantage was combining the image data and all configuration parameters into a single file for each layer. This eliminated the need for separate aperture files and reduced setup errors. However, RS-274X has limitations. It only provides a graphical picture of each layer.
Note: With RS-274X, the file itself does not state its function (e.g., “this is the top copper layer”). The manufacturer must rely entirely on your filename or other documentation to know the correct layer order.
Gerber X2 is the current and recommended standard. It solves the main ambiguity of its predecessor by adding a rich set of metadata directly into the file. While it is backward-compatible with RS-274X, X2 includes critical attributes that make your design data intelligent. These attributes specify:
- File Function: Clearly identifies the file as top copper, bottom solder mask, etc.
- Part and Layer Context: Distinguishes between single PCBs and panelized arrays.
- Pad Attributes: Provides extra data about pads for testing and assembly.
Using X2 removes guesswork from the fabrication process. It makes your design package clearer and significantly reduces the risk of manufacturing errors related to incorrect gerber file format extensions or ambiguous naming.
When you export your design files, you will encounter two main strategies for naming them. One method uses a unique file extension for each layer, while the other uses a generic extension for all layers. Both approaches work, but understanding them helps you communicate clearly with your manufacturer. The choice often depends on the default settings of your PCB design software.
Many design tools, like Altium Designer, use a system where the file extension itself tells you the layer’s function. This method creates a clear and organized file set right away. You can quickly identify the top copper layer from the bottom solder mask just by looking at the extension.
This approach is very popular because it reduces ambiguity. A manufacturer seeing a .GTL file immediately knows it is the Top Copper Layer.
Here is a table of common layer-specific gerber file format extensions you will see:
| Extension | Layer Description |
|---|---|
.GTL | Top Copper Layer |
.GBL | Bottom Copper Layer |
.GTS | Top Solder Mask |
.GBS | Bottom Solder Mask |
.GTO | Top Silkscreen (Overlay) |
.GBO | Bottom Silkscreen (Overlay) |
.GTP | Top Solder Paste |
.GBP | Bottom Solder Paste |
.GKO | Keep-Out or Board Outline |
.GML | Board Outline (Alternate) |
💡 Tip: While the extensions in the table are common, different software might use slight variations. For example, some programs use
.SMTfor Solder Mask Top and.SMBfor Solder Mask Bottom. Always check your software’s documentation.
An equally valid approach is to use a generic .gbr extension for all Gerber files. Modern CAD software like KiCad often defaults to this method. When you use this strategy, the filename becomes critically important. The manufacturer’s CAM system relies on the descriptive name you provide to identify each layer correctly.
For this method to succeed, you must name your files with absolute clarity. A filename like layer1.gbr is confusing and invites error. Instead, you should use a descriptive name that leaves no room for doubt.
Good examples include:
project-name_top-copper.gbrproject-name_bottom-soldermask.gbrproject-name_board-outline.gbr
This naming convention makes your project understandable to any person or CAM system processing the order.
As you work with different designers or older projects, you might encounter less common gerber file format extensions. These are often tied to specific software or older fabrication workflows. For instance, some systems, like Cadence Allegro, may generate files with a .art (artwork) extension. You might also see .pho (photoplotter) files from very old systems.
Do not worry if you see these extensions. They serve the exact same purpose as a .gbr or .gtl file: they define a 2D image of a PCB layer. The key is that the file’s contents follow the Gerber format standard.
Let’s look at two practical examples to see how these naming strategies work.
1. Two-Layer Board with Layer-Specific Extensions
For a simple two-layer board, your file list might look like this. Notice how the extension identifies the layer.
my_project.GTL // Top Copper
my_project.GBL // Bottom Copper
my_project.GTS // Top Solder Mask
my_project.GBS // Bottom Solder Mask
my_project.GTO // Top Silkscreen
my_project.GKO // Board Outline
my_project.TXT // Drill File
2. Four-Layer Board with Generic .gbr Extensions
For a more complex four-layer board, using descriptive filenames with a generic extension keeps everything organized.
audio-amp_top-copper.gbr
audio-amp_inner-power.gbr
audio-amp_inner-ground.gbr
audio-amp_bottom-copper.gbr
audio-amp_top-soldermask.gbr
audio-amp_bottom-soldermask.gbr
audio-amp_top-silkscreen.gbr
audio-amp_edge-cuts.gbr
audio-amp_drills.drl
In both cases, the goal is the same: to provide a complete and unambiguous set of files for manufacturing.
A complete manufacturing package needs more than just Gerber files. You must also provide several companion files. These files give your manufacturer critical data for drilling, component purchasing, and assembly.
The drill file tells the fabrication machine exactly where to drill holes in your board. It is one of the most important files in your design package. Most design software generates this data in a format called Excellon, which is a type of NC (Numerical Control) drill file. You will often see these files with extensions like .drl or .txt.
Your drill file also specifies whether a hole is plated or non-plated. This detail is crucial for manufacturing.
| Hole Type | Purpose |
|---|---|
| Plated (PTH) | Creates electrical connections between layers. |
| Non-Plated (NPTH) | Used for mechanical mounting or alignment. |
The Bill of Materials (BOM) is a complete list of every component needed to assemble your PCB. You typically export this as a spreadsheet file, like a .csv or .xlsx. A clear BOM prevents delays by ensuring the assembler orders the correct parts.
What to Include in Your BOM: For each component, you should list its:
- Reference Designator (e.g., R1, C5, U2)
- Manufacturer’s Part Number (MPN)
- Quantity
- Description and Footprint (e.g., 0603, SOIC-8)
The pick-and-place file, also called a centroid file, guides the automated assembly machines. It tells the machine the exact coordinates to place each component on the board. This file usually comes in a text-based format like .csv or .txt. It contains the X/Y location, rotation, and board side (top or bottom) for every component, identified by its reference designator.
While Gerbers guide machines, fabrication drawings provide instructions for humans. These drawings include critical details that are not in the Gerber files. You should include a layer stack-up diagram, a drill table, and notes specifying materials (like Tg values), surface finish, and required quality standards (e.g., IPC-6012). This document helps prevent misunderstandings about your design intent.
Following best practices when you submit your design files helps prevent costly errors and delays. A clean and complete package makes the fabrication process smoother for everyone involved.
You should use the Gerber X2 format whenever possible. X2 files contain intelligent data that tells the manufacturer the exact function of each layer. This removes guesswork and reduces mistakes. Using X2 offers clear advantages:
- Fewer Errors: It minimizes the risk of misinterpreting your design.
- Faster Turnaround: Manufacturers can generate quotes and start production more quickly.
- Less Back-and-Forth: Clear data means fewer questions and delays.
This modern format streamlines the entire process from design to manufacturing, ensuring your board is made correctly the first time.
Clear filenames are essential, especially if you are not using layer-specific gerber file format extensions. You should create a consistent naming system for your projects. A good name clearly identifies the project and the layer.
Example Naming Convention: Use a prefix for all files in a project. This keeps them organized.
project-name_top-copper.gbrproject-name_top-soldermask.gbrproject-name_drills.drl
This simple habit makes your files easy for any CAM system or engineer to understand.
You should always package all your manufacturing files into a single ZIP archive. This keeps everything together and simplifies the upload process. Your ZIP file should contain all Gerbers, drill files, and other necessary documents.
For extra clarity, you can include a README.txt file inside the archive. Use this file to list any special instructions, such as required materials, board thickness, or impedance control needs.
Never submit your files without a final check. You must use a Gerber viewer to inspect your design package. This step allows you to catch errors before they reach the factory. Many free and powerful viewers are available, such as ViewMate, GerberLogix, and ZofzPCB.
When you review your files, check these key items:
- Board Outline: Make sure it is a single, closed shape on its own layer.
- Drill Holes: Confirm that all plated and non-plated holes are present and correctly sized.
- Layer Alignment: Load all layers to see if they stack up correctly.
- Scale and Units: Verify that the dimensions are correct.
This final check is your last chance to ensure your digital design will become a perfect physical board.
You now know that clear filenames and the modern Gerber X2 format are more important than the file extensions you choose. A complete and well-documented package is the foundation for a successful manufacturing run. This prevents confusion and costly errors.
✅ Final Check: Always open your entire file package in a Gerber viewer before submission. This final step ensures all layers and data appear exactly as you intend, guaranteeing a smooth fabrication process.
You must always include all Gerber files for each layer and a drill file. Your Gerbers define the copper, mask, and silkscreen layers. The drill file, often an Excellon file (.drl or .txt), tells the factory exactly where to place every hole on your board.
You should use Gerber X2 whenever possible. It reduces errors by embedding layer data directly into the file. While most manufacturers still accept the older RS-274X format, using X2 provides a safer and more modern workflow for your project.
Do not worry. The file extension is less important than the filename and the file’s content. As long as the file follows the Gerber standard, your manufacturer can use it. Just make sure your filenames clearly describe the layer’s purpose (e.g., project_top_copper.gbr).
A Gerber viewer lets you see exactly what the manufacturer will see. It helps you catch critical errors before production.
✅ Common errors to check for:
- Missing drill holes
- Incorrect board outline
- Misaligned layers
This final check saves you time and money.