PCB Files: what you shall know all

 

Introduction

PCB Files refer to the various data files used in the design, fabrication, and assembly of Printed Circuit Boards (PCBs). These files contain information necessary for different stages of the PCB lifecycle. Here are some common types of PCB files and their purposes and they are discussed in detail step by step.

Files generated by PCB Software

Schematic Files

Schematic files, also known as schematic diagrams or circuit diagrams, are graphical representations of electronic circuits. They use standardized symbols and notation to illustrate the connections between various electronic components and their functions within the circuit. The software’s generate the schematic file with extension “.sch”

Representation of Circuits

Schematic files visually represent the circuit’s components, such as resistors, capacitors, transistors, integrated circuits, and other electronic devices. Each component is represented by a unique symbol, making it easy to identify and understand the circuit’s layout.

Connectivity

Schematic files show how components are connected electrically in the circuit. Lines or wires connect the component symbols, indicating the flow of electrical current and signals between different parts of the circuit.

Design and Visualization

Schematic diagrams help design engineers visualize and plan the electronic circuit before its physical implementation on a PCB. They aid in identifying potential issues, ensuring proper connections, and optimizing the circuit’s functionality.

Documentation

Schematic files serve as crucial documentation of the circuit design. They are essential for communication between design teams, electronics manufacturers, and other stakeholders involved in the project.

Annotation and Labels

Schematic files include labels, reference designators, and other annotations to identify each component uniquely. This information corresponds to the components’ physical locations on the PCB and helps during the assembly process.

Design Validation

Engineers and designers can simulate the circuit behavior using specialized software based on the information provided in the schematic files. This allows for testing and validation of the circuit’s functionality before actual fabrication.

Compatibility with PCB Design

Schematic files are often used as the basis for creating PCB layouts. Many PCB design software tools allow the direct import of schematic files, making the transition from the circuit diagram to the PCB layout more streamlined.

A schematic file of an electronic circuit
A schematic file of an electronic circuit

 

Project Files

The Project file is created by software’s with extension “.pcb, .brd, .prj”: These files are the project files created by the PCB design software, containing all the information and settings specific to the PCB design. Different PCB design software tools use their proprietary formats for project files, and these files typically have a specific file extension corresponding to the software used. Some common examples include:

  • Altium Designer: .PrjPcb
  • KiCad: .pro
  • Eagle: .brd
  • OrCAD: .dsn
  • Allegro: .brd

The PCB project file organizes and manages all the design-related information and settings, allowing the designer to work on the design iteratively and maintain a coherent project structure. Here are the key components typically found within a PCB project file.

Schematic

The schematic section of the project file contains the circuit diagram, representing the components and their connections in the electronic circuit.

PCB Layout

The layout section contains the physical arrangement of components on the PCB, including their placement, routing, and any additional design elements.

Component Libraries

PCB design software allows users to create and import component libraries. The project file references these libraries to identify and use specific components in the design.

Design Rules

PCB design software allows users to set design rules, such as trace widths, clearances, and other constraints. These rules are stored in the project file to ensure consistent design integrity.

Preferences and Settings

Project files store various preferences and settings chosen by the designer, affecting the design environment and overall project behavior.

Design Variants

In more complex designs or projects with multiple versions, the project file may contain design variants or configurations.

Netlist and BOM Integration

Depending on the software, some project files may contain information related to the netlist and Bill of Materials (BOM) generation.

The PCB project file organizes and manages all the design-related information and settings, allowing the designer to work on the design iteratively and maintain a coherent project structure. Here are the key components typically found within a PCB project file:

3D view of components side generated by project file
3D view of components side generated by project file

Netlist File

A netlist file is a text-based or binary file that contains a list of connections and electrical connections between different components in an electronic circuit. It represents the logical connectivity of the circuit, specifying how each component is interconnected with others through their pins or terminals. The netlist serves as a critical intermediate step between the schematic capture and the PCB layout design phases of the PCB design process. The netlist is typically generated by the schematic capture software and used as input by the PCB layout software. It provides information about the following parameters: 

Connectivity Information

The netlist file identifies each component in the circuit and lists the connections between their pins or terminals. Each net (short for “network”) in the file represents a group of interconnected pins.

Unique Identifiers

Every pin in the netlist is associated with a unique identifier, which corresponds to the pins used in the component symbols within the schematic.

Net Names

Each net is assigned a name to represent its purpose or function in the circuit. These net names help identify and organize related connections.

Hierarchical Structure

The netlist may also include information about hierarchical design, where sub-circuits or blocks are interconnected to form the complete circuit.

Formats

Netlist files can be in various formats, such as plain text (.txt), EDIF (.edf), or other proprietary binary formats specific to the schematic capture software used.

PCB Layout Import

After the netlist is generated, it is imported into the PCB layout software, where it guides the placement and routing of traces to create the physical layout of the PCB.

Design Validation

Before PCB layout, the netlist can be used to perform a pre-layout simulation or verification, allowing designers to check the circuit’s functionality and detect potential errors or issues.

It is essential to keep the netlist synchronized with the schematic during the design process. Any changes to the schematic must be reflected in the netlist to ensure that the PCB layout accurately reflects the intended circuit connections. The netlist is a crucial component of the PCB design flow, enabling smooth communication between the schematic and layout stages and ensuring that the final PCB accurately represents the intended circuit design.

A Netlist generated by a software
A Netlist generated by a software

Gerber File

A Gerber file is a standard file format used in the PCB manufacturing process to represent the copper traces, solder masks, component pads, and other features of a printed circuit board (PCB) design. Gerber files are essential for communicating the PCB design information to PCB manufacturers and are widely accepted in the electronics industry as the standard format for PCB fabrication data.

 It contains the following information about the design circuit.

Format

Gerber files are ASCII text files with a .gbr extension. Each Gerber file corresponds to a specific layer of the PCB, such as the top copper layer, bottom copper layer, solder mask layer, silk screen layer, and more.

Layers

For complex PCBs, multiple Gerber files are generated, each representing a specific layer of the PCB design. Typical layers include the top and bottom copper layers, solder mask layers, silkscreen layers, and any additional layers required for specific design features.

Data Representation

In a Gerber file, the PCB features are represented using a series of drawing commands, also known as commands, that define the shapes and patterns of the features on the layer.

Aperture Definitions

Gerber files reference aperture definitions, which specify the shape and size of the drawing tools used to create the features on the PCB. Apertures can be circular, rectangular, or custom shapes.

Units

The units used in Gerber files are typically either inches or millimeters, and the format must be specified to the PCB manufacturer to ensure accurate fabrication.

Manufacturing Data

Gerber files provide the necessary manufacturing data for the fabrication process. PCB manufacturers use the Gerber files to create photomasks used in the PCB imaging process.

DFM (Design for Manufacturability)

It is crucial to ensure that the Gerber files are accurate and error-free to prevent issues during PCB fabrication. Designers must review and validate the Gerber files to verify that they correctly represent the intended PCB design.

A sample Gerber File 
A sample Gerber File

Gerber File Format

Standard Gerber RS274-D

It is the oldest format used and generated by software’s. It is NC(numerical control) compatible. It was compatible with plotters used in the late 70’s.

Extended Gerber RS274-X

It is based on ASCII commands that are human readable and contain graphical information on 2D plots. These formats were developed in the late 90’s. This improved format eliminates the error caused by plotting and data entry. Now almost PCB factories require this format for production.

Gerber X2

It is the latest format employed by the latest CAM software’s. It contains the function details of layers, functions like pad types, parts of layers etc.

Excellon Files

Excellon files, also known as Excellon Drill Files or NC Drill Files, are a standard format used in the PCB manufacturing process to provide information about the locations and sizes of drill holes on a printed circuit board (PCB). These files are generated by PCB design software and used by PCB manufacturers to create the holes necessary for component mounting, vias, and other purposes.

 The summary of excellon file is:

Format

Excellon files are ASCII text files with a .txt or .drl extension. The file contains a list of drill commands and coordinates in a specific format that the PCB manufacturer’s drilling machine can interpret.

Drill Information

Each line in the Excellon file corresponds to a drill hole to be made on the PCB. The line contains coordinates (X, Y) and information about the hole size (usually in inches or millimeters). 

Drill Tool Definition

The Excellon file may also include a drill tool table, which defines the drill sizes and corresponding codes used in the drill commands. The tool table helps the drilling machine select the appropriate drill bit for each hole size.

Layer-Specific

Like Gerber files, Excellon files are layer-specific. There is usually one file for each drill layer (e.g., top drill layer, bottom drill layer) of the PCB.

Registration Marks

Excellon files may also include registration marks or tool change commands to help align the drilling machine accurately during the manufacturing process.

A generated excellon file
A generated excellon file

Bill of Materials

The BOM file is not a specific format but rather a document that lists all the components required for the PCB assembly. It includes component part numbers, descriptions, quantities, and other relevant information. Generally, the BOM includes the following information:

Component Part Number and Description

The manufacturer’s part number or a unique identifier for each component. A brief description of the component, including its type, value, and package.

Quantity

The number of units of each component required for the assembly.

Reference Designator

A label or identifier that corresponds to a specific location on the PCB, associating each component with its place in the design.

Package Type

The package or footprint of the component, specifying its physical dimensions and shape.

Component Value

The electrical value or rating of the component, such as resistance, capacitance, or voltage.

Manufacturer

The name of the company that produces the component.

Supplier

The name of the company or vendor from which the component will be purchased.

Component Type

Indicates whether the component is a resistor, capacitor, integrated circuit, connector, etc.

Comment/Description

Additional information or notes about the component or its usage.

The BOM is typically generated automatically from the PCB design software or exported in a standardized format, making it easy to share with PCB manufacturers and suppliers. Properly managing the BOM is crucial to ensure the successful fabrication and assembly of the PCB, meeting quality standards and design specifications.

PCB Assembly Bill of Materials (BOM)
PCB Assembly Bill of Materials (BOM)

Centroid File

Centroid File (.csv, .txt, .xy): The Centroid file, also known as the Pick and Place file, contains the X, Y coordinates, rotation, and other data for each component on the PCB. It is used by the assembly machines to place components accurately during the PCB assembly process.

The key information about the centroid file is given below:

Format

The centroid file is usually in a plain text format with a .csv (comma-separated values) or .txt extension. Each line in the file represents a component to be placed on the PCB.

Component Information

For each component, the centroid file typically includes the following information:

Reference Designator

A unique identifier for each component, usually matching the designator used in the PCB design.

X and Y Coordinates

The precise X and Y positions of the component’s centroid (center point) on the PCB, often in millimeters or inches.

Rotation

The rotational orientation of the component, specified in degrees. This information helps the pick-and-place machine align the component correctly during placement.

Side Indication

The centroid file may also include information to specify whether the component is to be placed on the top (component side) or bottom (solder side) of the PCB.

Automated Assembly

The pick-and-place machine uses the information in the centroid file to precisely position and place each component onto the correct location on the PCB.

The centroid file streamlines the PCB assembly process, ensuring accurate and efficient placement of components. It eliminates the need for manual component placement, reduces the risk of human error, and increases the speed of PCB assembly. Properly generated and validated centroid files are crucial for achieving high-quality PCBA and minimizing production delays.

A centroid File
A centroid File

Assembly drawings for PCBs

Assembly drawings for PCBs are graphical representations that provide detailed information to aid in the assembly and soldering of electronic components onto the PCB. These drawings are created to guide assembly technicians or machines during the PCB assembly process and ensure correct component placement and orientation. Assembly drawings are typically generated by the PCB design software and may include the following information:

Component Placement

The assembly drawing shows the precise locations where each component is to be mounted on the PCB. It includes the X and Y coordinates for the centroid of each component to ensure accurate placement.

Reference Designators

Assembly drawings use reference designators to label each component’s location on the PCB. These reference designators correspond to the identifiers used in the Bill of Materials (BOM) and the Centroid file.

Component Orientation

The drawing provides information about the orientation or rotation of each component, ensuring that they are placed on the board in the correct alignment.

Polarities and Markings

For polarized components like diodes, LEDs, and electrolytic capacitors, the assembly drawing may include markings or symbols to indicate the correct polarity or orientation.

Mounting Instructions

Assembly drawings may include notes or instructions for specific components, specifying details such as the required mounting height or any special handling instructions.

Mechanical Data

In some cases, the assembly drawing may also include mechanical information, such as mounting hole locations or board dimensions, that aids in the assembly process.

Silkscreen and Solder Mask Information

The assembly drawing may include details about the PCB’s silkscreen layer, which displays component outlines, labels, and other markings. It may also show the solder mask layer, indicating areas where solder should or should not be applied.

Assembly Views

Assembly drawings may have different views, such as top, bottom, and detailed zoomed-in views, to provide a clear understanding of the component placements and connections.

Assembly drawings are essential for ensuring that the components are placed correctly during the PCB assembly process, reducing the likelihood of errors and facilitating a smooth manufacturing process. They are often shared with contract manufacturers or assembly houses to provide all the necessary information for successful PCB assembly.

Assembly file for a PCB
Assembly file for a PCB

 

Conclusion

In conclusion, PCB files play a pivotal role in the entire lifecycle of Printed Circuit Boards (PCBs). From the design phase to manufacturing and assembly, these files enable efficient communication and data transfer between different stakeholders. The Gerber files convey the physical layout of copper traces and solder masks, while the Excellon files provide crucial information about drill hole locations. The Bill of Materials (BOM) ensures accurate component procurement and inventory management. Finally, the Centroid file guarantees precise component placement during the PCB assembly process. By leveraging these essential PCB files, designers and manufacturers can achieve successful, reliable, and high-quality PCB production, contributing to the advancement of modern electronics and technology.

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