Essential PCB Design File Types You Need to Know

Ordering printed circuit boards (PCBs) involves providing precise and detailed information about the design and specifications of the board. This is why specific PCB file formats are necessary. Manufacturing can only take place successfully with a completed PCB file. Designing a PCB and communicating those design decisions depends on files such as circuit schematic designs, Bill-of-Materials, PCB layouts, and stack-up design information.

To ensure interoperability among various electronics design software like Allegro, Altium Designer, KiCAD, and Eagle, a collection of files known as Gerber was developed.

Importance of Gerber Files in PCB Design:

Gerber files allow understanding PCB designs without being locked into specific design software. As a result, understanding and learning the Gerber file format has become a highly relevant topic today. Here are the three main reasons:

1- Standardization: PCB file formats standardize the way design data is communicated. This ensures that manufacturers receive the necessary information in a consistent and universally understood manner.

2- Detailed Information: PCB files contain detailed information about the board layout, including component placement, electrical connection routing, layer stack-up, drill holes, and other essential details. This level of detail is crucial for accurate manufacturing.

3- Compatibility: Different PCB design software tools generate files in specific formats. Manufacturers often work with various design tools, so having standard formats ensures compatibility and reduces the likelihood of errors.

Files Required For Ordering A PCB Online:

CAD aperture files are enough to order the PCB. This includes information on stack-up layers, drill holes, solder masks and silkscreen layers. BOM(Bill of Materials) is helpful for users while assembling the PCB.

CAD Aperture files:

In the format of RS-274X (Gerber x1), Gerber x2, IPC-2581, ODB++.

BOM (Bill of Materials):

The formats TXT, PDF, CSV, XLSV, and usually Excel files are widely adopted. For assembling the PCB from the manufacturer, a Centroid data file is required to ensure the manufacturer can assemble your boards.

Centroid file:

The formats TXT, PDF, CSV, XLSV, and usually Excel files are widely adopted and also known as CPL (Component placement) or PNP (Pick-And-Place) files.

The most common file formats adopted by designers, which are used to order PCBs online, are listed below. Let's know what these files are and what is inside them. We start our discussion with:

1. What is a PCB Gerber file?

2. What is the Bill of Materials (BOM)?

3. What is the Centroid file?

What is a PCB Gerber File?

A PCB Gerber file is a standardized format used to convey detailed information about a printed circuit board (PCB) layout to manufacturers. Gerber file, also known as "photo plot", generally represents the files in the format of 274D and 274X, which acts as a medium between graphics to data conversion and PCB manufacturing.

It includes data for each layer of the PCB, such as copper traces, solder masks, silkscreen printing, and drill holes, as shown above. Gerber files are essential for precise and accurate PCB fabrication, ensuring the design specifications are communicated and understood. They help to create photomasks used in manufacturing, making them a critical component of PCB production. To see how the PCBs are manufactured from these files, see the detailed guide of the PCB manufacturing process released by JLCPCB.

Types of Standard Gerber Format:

There are two main types of standard Gerber formats used in PCB design:

1- RS-274D: This is the older, more basic format. It requires a separate aperture file to define the shapes and sizes of various features, making it less convenient and more prone to errors compared to RS-274X. Because the growth of electronic goods has far outpaced current requirements, the original RS-274D format has progressively become obsolete, and an improved version of RS-274X has been created from it.

2- RS-274X: The extended Gerber format is widely used and considered the industry standard. It includes embedded aperture definitions and supports multiple layers, providing detailed and comprehensive PCB designs. RS-274X is the preferred format due to its improved capabilities and ease of use.

99% of PCB CAD software supports the conversion of PCB design into Gerber file format, either RS-274D or RS274X format. While drill files are typically not Gerber files, Drill files are also needed as part of the production data required to produce your PCBs. Drill files generally are in Excellon NC drill format and tell the manufacturer where holes should be drilled.

Generating drill files is often a separate process, and missing drill files is one of the most common order problems, so it is wise to assume that drill files are also required when a manufacturer requests Gerber files and gets used to exporting both. Gerber and drill files should be packaged into a single archive file (JLCPCB supports .rar or .zip files). In online software like EasyEDA, Gerber and Drill files are generated together, as shown in the image above.

What is the Bill of Materials (BOM)?

The Bill of Materials is a list of the raw materials that go into making a product or assembly. In the context of JLCPCB's turnkey PCB Assembly service, the BOM file includes all parts that should be purchased and soldered onto the boards by JLCPCB. BOM should be built and maintained during the PCB designing process, which increases accuracy at the end of the process.

JLCPCB's innovative quotation platform can read BOM files and match part numbers with significant distributors. With this approach, the platform can provide quotations in seconds. In order to achieve fast quotations, the BOM file needs to be organized according to JLCPCB's format, as shown above. It must include the Manufacturer's Part Number, Designator and Quantity. A typical JLCPCB BOM should be in .xls, xlsx or .csv format and includes:

1. Component Part Numbers: Manufacturer-specific identifiers for each part.

2. Descriptions: Briefly describe each component, such as resistors, capacitors, ICs, etc.

3. Quantities: The number of each component needed.

4. Reference Designators: Labels identifying where each component is placed on the PCB (e.g., R1, C2, U3).

5. Manufacturers info: Information on where to source each component.

Remove the parts from BOM that you do not want to place on PCB via JLCPCB assembly service or mark them with the letters' DNP' in the 'Customer Supply' column. The BOM is essential for procurement, assembly, and ensuring the final product meets design specifications.

What is the Centroid file?

Centroid is the particular file for assembly used to program the assembly machines quickly. This is also known as Pick-N-Place, XY Data or Component Placement(CPL) file. Some CAD tools will automatically generate this file, and some will not, but you may need to modify the file and then generate the Centroid file. The Centroid file describes the position and orientation of all the surface mount parts, which includes the reference designator, X and Y position, rotation and side of the Board (Top or Bottom). Both surface mount and through-hole parts are listed in the Centroid. It helps engineers locate the parts during the Design for Assembly (DFA) review and set up the pick & place machine for machine assembly. A typical JLCPCB Centroid file should be in .xls, xlsx or .csv format and include:

1. Designator: A reference designator (RefDes) that identifies the part. Unlike the BOM file, separate designators must have their line and coordinates.

2. XY Coordinate: Tells about the location of the part. X and Y coordinates should be in separate columns and center on the part origin if possible.

3. Rotation value: This tells about the component's orientation/rotation (0 to 360 degrees) as it is placed on the PCB. This value may not be accurate and should only be used as a reference.

Some software does not include through-hole parts in the centroid file since these are not placed by pick-and-place machines but can be hand-soldered by the assembly staff.

Other File Formats and their Advantages/ Disadvantages:

There are two more widely known PCB file formats(IPC-2581 and  ODB++). Stands out from Gerber file format and has its own advantages/disadvantages:

1. IPC-2581 File Format:

The IPC-2581 PCB file format is an industry-standard, vendor-neutral, open XML-based format. It was developed by the IPC (Association Connecting Electronics Industries) to provide a comprehensive and unified way to share PCB design information. This format streamlines the transition from design to manufacturing by providing a single, standardized file format that contains all necessary data. Some of the key features are listed below:

• Comprehensive Data: Includes detailed information about Design Data (PCB layouts, such as traces, pads, vias, and layer stack-up), Component Data (reference designators, part numbers, and placement coordinates), Netlist and Drill data.

• XML-Based: Being XML-based makes it human-readable and easy for machines to parse, enhancing the flexibility and adaptability of the format.

• Bidirectional Communication: Facilitates the hand-off of design data to manufacturers and feedback from manufacturers to designers, improving the overall design and manufacturing process.

• Support for Advanced Technologies: IPC-2581 supports advanced PCB technologies, including high-density interconnects (HDI), rigid-flex designs, and embedded components.

2. ODB++ File Format:

ODB++ is a comprehensive and widely used file format developed by Valor (now part of Mentor Graphics); ODB++ aims to consolidate all necessary information into a single file or dataset in a hierarchical format. Here's a detailed look at the ODB++ format:

• Comprehensive Data Integration: Includes detailed information about Design Data, Netlist, Component Information, Drill Data, Assembly Data, Fabrication Data and Test Data.
  

• Single File Format: ODB++ consolidates all necessary PCB design and manufacturing data into a single package, which simplifies data management and reduces the risk of errors associated with handling multiple files.

The Advantages of Conversion PCB Files to Gerber File Format:

The PCB file data is converted into Gerber and drill files to make the PCB manufacturing trouble-free. However, many electronic engineers will not convert the PCB file into drilling data and Gerber file before handing over the PCB file to the factory. Instead, they directly send the separate files to the PCB manufacturers. Because electronic and PCB engineers have a different understanding of PCB, the Gerber file converted by the PCB manufacturer may differ from what you want.

For example:

If you designed a PCB, send PCB files instead of Gerber. Then, there is the scope of special requirements like component name display and batch number; silkscreen settings are locked, and only the manufacturer can change/modify them. You can avoid such incidents if you convert the PCB file into a Gerber file.

General Comparison of all File Formats:

1. Gerber Files (.gbr or .ger):

Gerber files are the standard file format for PCB fabrication data. They contain information about individual PCB layers, including copper traces, pads, solder mask, and silkscreen elements.

Usage: Gerber files are essential for PCB fabrication and are typically generated and provided to manufacturers for the production of printed circuit boards.

2. Excellon Files (.drl):

Excellon files contain information about the locations and sizes of holes and other drilled features on the PCB. They are used to control the drilling machines during PCB fabrication.

Usage: Excellon files are crucial for accurately drilling holes and creating cutouts in the PCB during the manufacturing process.

3. ODB++ (Open Database Design++):

ODB++ is an intelligent, unified PCB manufacturing data format that includes comprehensive information about the PCB design, including layers, drill data, stack-up information, components, and assembly details.

Usage: ODB++ is used to streamline communication between PCB layout software and manufacturing processes, providing a comprehensive dataset for PCB fabrication and assembly.

4. IPC-2581:

IPC-2581 is a robust, XML-based PCB manufacturing file format that encompasses complete design and manufacturing information, including PCB stack-up, components, netlist, assembly, and fabrication details.

Usage: IPC-2581 offers a comprehensive, unified data format for PCB manufacturing, streamlining data exchange between design, fabrication, and assembly processes.

PCB Design Tool-Specific Formats (e.g., Altium Designer, Eagle, KiCad).

Conclusion:

Each PCB design software typically has its proprietary file format for storing project files, libraries, and designs. These formats may include .pcb, .sch, .lib, or other extensions specific to the software.

It's essential to note that while these file formats are crucial for PCB design and manufacturing, some formats, such as Gerber and Excellon, are primarily used for the fabrication phase. On the other hand, unified formats like ODB++ and IPC-2581 encapsulate comprehensive design and manufacturing data, facilitating seamless integration between design and fabrication processes.