In PCB manufacturing, Gerber files are the universal language between designers and manufacturers, translating your circuit designs into physical products.
What is a Gerber File?
A Gerber file is a standardized file format containing all the information needed to manufacture a PCB. Originally developed by Gerber Systems Corp. in the 1960s, it has evolved into the industry standard for PCB manufacturing data exchange. You can think of a Gerber file as a detailed construction plan that precisely indicates the placement of copper traces, drill holes, solder mask application, and component markings on each layer of the board.
Standard Gerber X2 or Extended Gerber (RS-274X) has replaced the older RS-274D format. The new versions are standalone, meaning each file contains all the information needed for proper interpretation, avoiding the confusion of requiring separate files common in older formats.
Gerber File Set Structure
A complete Gerber file package typically contains multiple individual files, each representing a specific layer or section of the PCB. For a standard two-layer board, you'll need at least six separate files, while complex multilayer designs may require dozens.
Copper layer files define the conductive paths for transmitting electrical signals. For a two-layer board, you need one top copper layer file and one bottom copper layer file. Each additional inner layer in a multilayer board requires its own Gerber file. These contain precise coordinates and shapes to tell the manufacturer where to etch away the copper, leaving the circuit traces and pads that form electrical connections.
Solder mask files specify where the protective coating should be applied. The solder mask prevents solder bridging during assembly and protects copper traces from oxidation and physical damage. These files indicate openings in the solder mask that need to expose component pads and test points for soldering or electrical testing.
Silkscreen layer files contain all text, logos, component outlines, and reference identifiers printed on the board surface. They aid in assembly, indicate component placement, and provide identification for troubleshooting and maintenance. While silkscreen markings themselves do not have electrical function, clear silkscreen markings are essential.
Drill files (strictly not Gerber files, but often categorized together) contain the coordinates and dimensions of all holes to be drilled on the circuit board. This includes through-holes for component leads, mounting holes for mechanical fastening, and vias for interlayer connections. Modern drill files typically use the Excellon format, which specifies precise X-Y coordinates and drill bit dimensions for each hole.
Applications of Gerber Files in Manufacturing
When PCB manufacturers receive your Gerber files, they use specialized CAM (Computer-Aided Manufacturing) software to process and verify the data. This software translates Gerber information into machine instructions to control lithography machines, drilling machines, and automated optical inspection systems.
The manufacturing process begins with substrate preparation. Copper layer Gerber files are used to create photomasks or to directly expose copper-clad laminates using laser lithography. Copper trace areas defined in the Gerber file are protected during etching, while unwanted copper is removed chemically. This photolithography process requires extremely high precision because modern PCB trace widths and spacing can be as small as 0.1 mm, or even smaller in advanced designs.
After copper patterning, the drilling file guides a CNC drilling machine to drill all necessary holes, typically with a precision within 0.075 mm. Then, the manufacturer applies the solder mask using screen printing or photolithography techniques, based on the solder mask Gerber file. Finally, the screen printing information is transferred to the board surface, and the exposed copper areas undergo special surface treatments, such as electroless nickel immersion gold (ENIG) or hot air leveling (HASL).
Common Gerber File Formats and Extensions
Although Gerber is a file format, manufacturers use various file extensions to organize and identify different layers. The most common naming conventions use extensions such as .GTL (top copper foil, Gerber top layer), .GBL (bottom copper foil, Gerber bottom layer), .GTS (top solder mask), .GBS (bottom solder mask), .GTO (top silkscreen layer, Gerber top overlay), and .GBO (bottom silkscreen layer). Internal layers in multilayer boards may use extensions such as .G1, .G2, and .G3.
However, these extensions are merely conventions and not part of the Gerber specification itself. Some CAD software uses different naming schemes, making the built-in layer attributes of Gerber X2 particularly important. They eliminate guesswork by directly embedding layer usage into the file content.
Generating and Verifying Gerber Files
Most PCB design software includes Gerber export functionality. When generating Gerber files, the coordinate format must be specified; designs typically use a 4.6 format (four decimal places before and six decimal places), with a resolution of up to 1 micrometer. Unit selection (imperial (inches) or metric (millimeters)) must be consistent across all files.
After generating the file, thoroughly verify it before sending it to the manufacturer. Use a free Gerber viewer to check its completeness and accuracy layer by layer. Check that all expected features are present, layer alignment is correct, and there are no unnecessary defects or missing elements. Pay special attention to small-pitch components, as even small errors in pad size or spacing can prevent assembly.
Many designers create manufacturing drawings that are released along with the Gerber file, detailing board dimensions, material requirements, copper foil thickness, surface finish, solder mask color, and any special manufacturing instructions. While the Gerber file contains geometric data, these additional documents ensure that the manufacturer fully understands your requirements and intentions.
Gerber files give you control over the manufacturing process and enable precise communication with manufacturers worldwide, regardless of their specific CAM systems or preferences.
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