Just as chefs create recipes, hardware engineers create Bills of Materials (BOMs). The BOM serves as the “ingredients list” for building a product, outlining all the components required for assembly.

What is a Bill of Materials (BOM)?

A Bill of Materials (BOM) is a critical document that lists all the parts, assemblies, sub-assemblies, and materials required to manufacture a product. It also includes instructions on how to procure and use these materials. A well-organized BOM is essential for clear communication and successful product development.

From the BOM, key aspects of the manufacturing process can be determined, such as:

• Lead time to procure materials
• Cost of Goods Sold (COGS)
• Manufacturing processes
• Design for Assembly quality
• Supply chain reliability
• Cash flow requirements

Engineers and manufacturers often rely on BOMs to track parts, and they often work with two types: the Engineering Bill of Materials (EBOM) and the Manufacturing Bill of Materials (MBOM). The BOM is essential for sourcing parts, outsourcing, and ensuring the product is assembled correctly, making it crucial to maintain an organized, up-to-date document.

What to Include in an Effective Bill of Materials

An effective BOM doesn’t require complex software—it can be as simple as an Excel spreadsheet. However, it should be well-organized to meet the needs of different stakeholders, including design and engineering teams, purchasing, and manufacturing.

Categories within a BOM help simplify the search for specific parts. For example, a PCB assembly manufacturer would only focus on the PCBs and surface-mount components, not on packaging components used for final assembly.

For a BOM to be truly effective, it should contain detailed product data. Below is a high-level list of key information to include in your BOM:

• BOM Level — Assign a number to each part or assembly to indicate its place in the BOM hierarchy. This helps quickly interpret the structure of the BOM.
• Part Number — Each part or assembly should have a unique part number for easy reference and identification. Avoid duplicates to prevent confusion.
• Part Name — Clearly define the name of each part or assembly for better recognition.
• Phase — Record the lifecycle stage of each part (e.g., “In Production”, “Unreleased”, or “In Design”). This is especially useful for tracking progress during new product introductions (NPI).
• Description — Include a detailed description of each part to distinguish it from similar items and facilitate quick identification.
• Quantity — Indicate how many units of each part are needed for assembly, guiding purchasing and manufacturing decisions.
• Unit of Measure — Specify the measurement unit (e.g., “each”, “inches”, “ounces”) to ensure accuracy in procurement and production processes.
• Procurement Type — Specify whether parts are off-the-shelf or custom-made to streamline procurement and manufacturing planning.
• Reference Designators — If applicable (e.g., for PCBs), include reference designators to show the location of each part on the board. This helps avoid confusion during assembly.
• BOM Notes — Include any relevant notes to ensure everyone interacting with the BOM stays informed and aligned.

By documenting this information, you’ll ensure that your business activities, procurement, and manufacturing processes run smoothly and stay on track.

Approved Manufacturers

A proper factory requires the specified manufacturers for each part. This list is often referred to as the “AVL” (Approved Vendor List). A manufacturer is the actual company producing the part, not a distributor like Digikey, Mouser, or Avnet. For instance, capacitors may be produced by companies such as TDK, Murata, Taiyo Yuden, AVX, Panasonic, or Samsung. It’s surprising how many times I’ve seen BOMs listing distributors like Digikey as the manufacturer.

While it might seem trivial to specify the manufacturer of a capacitor, it can matter in certain cases—especially for components like capacitors used in switching regulators. Even if a substitute capacitor has the same rated capacitance and voltage, using the wrong manufacturer can lead to unstable performance, or in extreme cases, cause a fire.

Sometimes, it’s fine to leave the manufacturer open, particularly for parts like pull-up resistors. In this case, you can mark “any/open” on the BOM, allowing the factory to suggest their preferred supplier.

Tolerance, Composition, and Voltage Specifications

For components marked as “any/open”, certain key parameters should always be specified in the BOM to ensure the correct part is ordered:

• Resistors: The tolerance and wattage should be specified. A 1kΩ, 1% 1/4W carbon resistor is quite different from a 1kΩ, 5% 1W wire-wound resistor.
• Capacitors: The tolerance, voltage rating, and dielectric type should be included. For critical applications, parameters such as ESR or ripple current may also be important. A 10µF electrolytic, 10% 50V capacitor behaves very differently from a 10µF X7R ceramic, 20% 16V capacitor at high frequencies.
• Inductors: These should never be left as “any/open”. For power inductors, key parameters include core composition, DCR (DC resistance), saturation current, and temperature rise. Other parameters, like shielding and potting, significantly impact circuit performance. Always fully specify the inductor, including any special requirements for RF inductors.

Electronic Component Form Factor

It’s essential to fully specify the form factor, or “package type,” of each component. Inadequately specified package details can result in assembly errors. In addition to basic parameters like EIA or JEDEC package codes (e.g., 0402, 0805, TSSOP), consider the following:

• For SMT packages: Component height can vary, especially for packages larger than 1206 or inductors. Make sure the board fits in a tight case.
• For through-hole packages: Always specify lead pitch and component height.
• For ICs: Specify the common package name (e.g., TI “DW” package corresponds to SOIC), not just the manufacturer’s internal code. This helps with consistency and reduces errors.

Extended Part Numbers

Designers often use abbreviated part numbers, such as “7404” for a hex inverter. While the “7404” is widely recognized, a complete part number should specify additional details such as the package type, manufacturer, and logic family. For example, a fully specified part number might be “74VHCT04AMTC”, indicating an inverter by Fairchild Semiconductor in a TSSOP package.

These extra characters are crucial because small variations in part numbers can lead to serious issues. For example, confusing “VHC” with “VHCT” changes the input threshold from TTL to CMOS-compatible, causing performance issues. A similar issue can arise with switching regulators: an incomplete part number like LM3670MFX could be mistaken for a leaded version (if /NOPB is omitted), which may not meet RoHS compliance standards.

Even slight differences, such as the “X” in the part number, can affect the reel quantity (e.g., 3,000 pieces vs. 1,000 pieces). Pay attention to these details to avoid unnecessary costs or delays.

In short, every digit and character counts—small mistakes can lead to expensive consequences!

7 Key BOM Creation Tips

1. Document Consumables: Don’t overlook consumables like glue, wires, fasteners, labels, and packaging. Including these in the BOM ensures that all parts required for assembly are accounted for and available during production.
2. Attach Supporting Files: Include related documents such as CAD drawings, datasheets, and work instructions in your BOM. It’s best to associate these files with their corresponding BOM items for easy access.
3. Double-Check Information: Before submitting your BOM to a manufacturer, review it thoroughly to ensure all details are accurate. Even minor mistakes can lead to significant issues in PCB assembly.
4. Track Changes: Implement a system for tracking changes to your BOM. Keep a record of all versions, clearly labeling the most current one to avoid confusion and errors.
5. Tailor Your BOM: Customize the BOM to meet the needs of your project and your partner companies. Different companies and projects require different types of organization and information, so ensure your BOM fits your specific requirements.
6. Use Templates: Many PCB manufacturers provide BOM templates that you can use. These templates ensure compatibility and streamline the quoting and ordering process, reducing the chances of technical issues.
7. Stay Organized: Keep your BOM well-organized and ensure all necessary details are included to prevent delays and mistakes in the production process.

By following these tips, you’ll ensure that your BOM is comprehensive, accurate, and ready for efficient production.

 

Provide Sufficient Detail:
Ensure your document contains all the relevant information. Your Bill of Materials (BOM) is a vital tool for communicating how your PCB assembly should be constructed, especially for those who may be assembling it from scratch. These individuals might not be familiar with your project, or even with the same language you use. Therefore, clarity, precision, and detail are crucial in your BOM. While unnecessary information can cause confusion, strive to be as thorough as possible—generally, more detail is better.

 

An Accurate BOM Ensures Efficient Manufacturing

Creating a Bill of Materials is essential not only for product development but also for transforming your design into a tangible product. Before generating your BOM, consider who will use it and how you’ll manage additional product documentation, such as part datasheets and CAD files. By capturing detailed part information, you can improve manufacturing efficiency and reduce errors.

 

Sample BOM

Below is an example of a portion of a BOM. The sheet is neatly organized, consistently labeled, and easy to navigate. Although this example is simplified, your BOM will likely be longer and contain more detailed information.

In the far-left column of the BOM example above, you’ll find the manufacturer’s name, followed by the manufacturer’s part number, quantity, description (including units), package type, and placement method. This BOM also provides options specific to Wellcircuits’ services.

At Wellcircuits, we focus on delivering high-quality solutions with a comprehensive range of PCB services, including procurement, fabrication, and assembly. We simplify your PCB project with our all-in-one approach and useful tools. Explore our free resources, or contact our knowledgeable support team for assistance at any stage of your project.

We also provide instant online quoting, ordering, and reordering. If you’re ready with your PCB assembly specifications, circuit files, and BOM, submit them through the button below.

 

If you have any questions about PCBs or PCBA, feel free to reach out to me at info@wellcircuits.com.