What is BOM | A Guide Before You Inquire PCB Assembly

Posted by

Introduction to PCB BOM

A PCB BOM, or Printed Circuit Board Bill of Materials, is a comprehensive list of all the components required to manufacture a printed circuit board assembly (PCBA). It serves as a crucial document for PCB Manufacturers, as it provides them with the necessary information to source components, plan the assembly process, and ensure the final product meets the desired specifications. In this guide, we will delve into the details of a PCB BOM, its importance, and how to create an effective one.

What is a PCB BOM?

A PCB BOM is a detailed list that includes all the components needed to assemble a printed circuit board. It typically contains information such as:

  • Component reference designators
  • Manufacturer part numbers
  • Descriptions
  • Quantities
  • Package types
  • Values (resistance, capacitance, etc.)
  • Suppliers

This information helps PCB manufacturers procure the correct components and ensure a smooth assembly process.

Why is a PCB BOM Important?

A well-organized and accurate PCB BOM is essential for several reasons:

  1. Procurement: The BOM helps manufacturers identify and source the required components from reliable suppliers.
  2. Assembly: It provides a roadmap for the assembly process, ensuring that the correct components are placed in the right locations on the PCB.
  3. Quality Control: An accurate BOM helps maintain consistency and quality throughout the manufacturing process.
  4. Cost Estimation: The BOM allows manufacturers to calculate the cost of components and estimate the overall production cost.
  5. Inventory Management: It enables efficient inventory management by providing a clear understanding of the required components and their quantities.

Creating a PCB BOM

Step 1: Gather Component Information

The first step in creating a PCB BOM is to gather all the necessary information about the components used in the design. This information can be obtained from the schematic diagram, PCB layout, and component datasheets. Ensure that you have the following details for each component:

  • Reference designator
  • Manufacturer part number
  • Description
  • Quantity
  • Package type
  • Value (if applicable)

Step 2: Organize the Data

Once you have collected all the component information, it’s time to organize the data into a structured format. A spreadsheet software like Microsoft Excel or Google Sheets is commonly used for this purpose. Create a table with the following columns:

Reference Designator Manufacturer Part Number Description Quantity Package Type Value
R1 ABC123 Resistor 10 0805 1k
C1 XYZ456 Capacitor 5 0603 10uF

Fill in the table with the corresponding information for each component.

Step 3: Review and Verify

After completing the PCB BOM, it’s crucial to review and verify the information for accuracy. Double-check the following:

  • Component quantities
  • Manufacturer part numbers
  • Package types
  • Values

Ensure that the BOM matches the schematic diagram and PCB layout. Any discrepancies should be resolved before proceeding with the manufacturing process.

Step 4: Include Additional Information

In addition to the basic component information, you may want to include additional details in the PCB BOM to facilitate the manufacturing process. Some examples include:

  • Supplier information
  • Lead time for components
  • Alternative part numbers
  • Special handling instructions

This extra information can help streamline the procurement and assembly processes.

Best Practices for PCB BOM Creation

To ensure an effective and error-free PCB BOM, consider the following best practices:

  1. Use a Consistent Format: Maintain a consistent format throughout the BOM, including column headers, units, and nomenclature. This makes it easier for manufacturers to understand and interpret the information.

  2. Include All Required Components: Make sure to include all the components needed for the assembly, including passive components, connectors, and mechanical parts. Don’t forget to include any custom or non-standard components as well.

  3. Provide Clear Descriptions: Use clear and concise descriptions for each component, avoiding ambiguity. Include relevant information such as tolerance, rating, and material, if applicable.

  4. Specify Package Types: Clearly specify the package type for each component, such as SMD (Surface Mount Device) or through-hole. This information is crucial for the assembly process.

  5. Use Unique Reference Designators: Assign unique reference designators to each component on the PCB. This helps in identifying and locating components during the assembly process.

  6. Collaborate with Manufacturers: Work closely with your PCB manufacturer and share the BOM early in the process. They can provide valuable feedback and suggestions to optimize the BOM for their manufacturing capabilities.

Common PCB BOM Mistakes to Avoid

To ensure a smooth and error-free PCB Assembly process, avoid these common mistakes when creating a PCB BOM:

  1. Incomplete or Inaccurate Information: Ensure that all the necessary information is included in the BOM and double-check for accuracy. Missing or incorrect information can lead to delays and assembly issues.

  2. Inconsistent Formatting: Inconsistent formatting, such as varying units or nomenclature, can cause confusion and errors during the manufacturing process. Maintain a consistent format throughout the BOM.

  3. Outdated or Obsolete Components: Verify that the components listed in the BOM are still available and not obsolete. Using outdated components can lead to sourcing issues and delays.

  4. Unclear or Ambiguous Descriptions: Provide clear and specific descriptions for each component to avoid misinterpretation. Ambiguous descriptions can result in the wrong components being sourced or assembled.

  5. Lack of Communication with Manufacturers: Failing to communicate effectively with your PCB manufacturer can lead to misunderstandings and issues during the assembly process. Share the BOM early and collaborate closely with the manufacturer.


1. What is the difference between a PCB BOM and a schematic?

A schematic is a graphical representation of the electrical connections and components in a circuit, while a PCB BOM is a list of all the components required to manufacture the physical PCB assembly.

2. Can I use a PCB design software to generate a BOM automatically?

Yes, most PCB design software tools have the capability to generate a BOM automatically based on the components used in the design. However, it’s important to review and verify the generated BOM for accuracy.

3. What should I do if a component becomes obsolete or unavailable?

If a component becomes obsolete or unavailable, you should work with your PCB manufacturer to find a suitable alternative. Update the BOM accordingly with the new component information.

4. How do I handle custom or non-standard components in the BOM?

For custom or non-standard components, provide as much information as possible in the BOM, including the manufacturer, part number, and any special handling instructions. It’s also a good idea to provide the manufacturer with datasheets or specifications for these components.

5. Can I use the same BOM for multiple PCB designs?

If the PCB designs share common components, you can create a master BOM that includes all the components used across the designs. However, it’s important to create separate BOMs for each specific design to ensure accuracy and avoid confusion.


A well-organized and accurate PCB BOM is essential for a successful PCB assembly process. It provides manufacturers with the necessary information to source components, plan the assembly, and ensure the final product meets the desired specifications. By following the steps outlined in this guide and adhering to best practices, you can create an effective PCB BOM that streamlines the manufacturing process and minimizes the risk of errors and delays.

Remember to gather all the required component information, organize the data in a structured format, review and verify the BOM for accuracy, and include any additional relevant information. Collaborate closely with your PCB manufacturer and communicate any changes or updates to the BOM in a timely manner.

By putting in the effort to create a comprehensive and accurate PCB BOM, you’ll be setting your PCB assembly project up for success and ensuring a smooth and efficient manufacturing process.