What Is a Bill of Materials (BOM) File in PCB Assembly?

In any manufacturing-related industry, the core task is to assemble various components into a complete product. When dealing with a large quantity and wide variety of components, effective management becomes crucial. To ensure smooth project processes and cost control, it’s a valuable practice to use data sheets to list the components’ names, quantities, and other essential parameters, which is commonly known as a Bill of Materials file. This article takes the PCBA industry as an example, introducing the concept of a Bill of Materials, emphasizing its significance, and providing insights into considerations when creating a material list. Let’s get started!

Table of Contents

What Is a Bill of Materials?

The conventional workflow for electronic manufacturing projects follows this sequence: Planning→Design→Procurement→Production→Acceptance. The Bill of Materials is a crucial document that bridges the gap between the design and production phases and it is often referred to as a BOM file. It serves as the hub connecting manufacturers with clients, detailing the components required to manufacture a specific product. Once the circuit design is completed, designers can use design software to generate a BOM file. They then package and send it to the manufacturer. The manufacturer will, in turn, use this file for procurement and production.

A standard BOM file typically begins with information such as the product model, version number, creation date, document number, and total page count. The content includes part numbers, names, quantities, supplier names, and other annotations. This file can answer various questions that manufacturers may pose to facilitate production, such as:

  • Which components are needed?
  • How many of each component is required?
  • What is the estimated loss?
  • What is the production and assembly sequence?
  • Where will these be used?
  • Under what conditions?
  • Where will they be installed?
  • Are there any optional components?
  • Are there any constraints between optional sub-components?

The following diagram illustrates the workflow of a BOM file and its connections to other business processes.

What Does a BOM File Include?

Product Part Number

This is the unique identifier or number associated with the final product related to the specific batch, used to distinguish and avoid situations where different batches of products are shipped under the same product model. For example, if your company manufactures multiple products, each with several batches, without a product part number, you won’t be able to differentiate between the products in the first and fifth batches of product “C.” The product part number is typically associated with the product model and the BOM file revision. You can access information about different batch BOM file updates from the data group and find specific information about products in that batch through queries.

Hierarchy

Used to describe the subordination and assembly sequence of electronic components in electronic manufacturing, helping differentiate which components are directly involved in the final product assembly and which are used first to assemble semi-finished products. We refer to components directly involved in finished product assembly as “1st Hierarchy,” while those used in semi-finished product assembly are termed “2nd Hierarchy.”

Taking a mechanical keyboard as an example, the external outline, keys, connecting wires, and the internal PCBA board are part of the 1st Hierarchy because assembling a mechanical keyboard involves combining these parts. The PCBA board for the mechanical keyboard consists of resistors, control chips, LEDs, etc., making them part of the 2nd Hierarchy because they are not directly used in assembling the mechanical keyboard but in PCB assembly.

This division of hierarchy streamlines the entire production process, making it well-organized and facilitating coordination, management, and division of responsibilities among different departments. For example, the department responsible for finished product assembly only needs to look at 1st Hierarchy components, while the PCB assembly department focuses on 2nd Hierarchy components.

3rd Tier BOM

Material Number

A coding system used to label and identify each component, there are different types and specifications of components on a PCBA board. For example, resistors can be categorized as DIP or SMD based on their mounting method, and they come in various resistance values, sizes, and specifications. When the production batch is sufficiently large, and the density of components on a PCBA board is higher, managing different components and specifications can be challenging. Therefore, following specific coding rules is crucial. It’s worth noting that different companies may have different coding schemes.

For example, the code for a 7K-ohm resistor with a size of 0603 may follow a format that includes a category (301 represents resistor), a serial number (0001 represents the first number), and a version (001 represents the version). This method can help you quickly locate the target resistor among a large number of components.

However, some non-standard OEM contract manufacturers directly use the material numbers provided by the component manufacturers without creating their own codes. The result can be chaotic inventory or production delays.

For example, if a contract manufacturer is handling multiple engineering projects “A,” “B,” “C,” “D,” certain common components like LEDs and resistors may be used in all projects but in varying quantities. For ICs and other specific components, there might be overlaps between pairs of projects. When producing project “A,” if there is a shortage of the originally prepared materials, the manufacturer might need to source from “B,” “C,” or “D.” This cross-utilization can lead to supply chain confusion and production issues.

Product Name and Specification

Used to provide a detailed description of the components, including the component type (e.g., capacitor, resistor, etc.), numerical parameters (such as capacitance, voltage, resistance value, etc.), performance parameters (e.g., tolerance, operating temperature range, package type, etc.). While these details are less frequently used in the production process, they are valuable for incoming material inspection. Clearly specifying the component parameters that IQC inspectors need to check can help address issues related to product quality and incorrect deliveries.

Quantity

The quantity of each component specification used. This information is used for material procurement and supply-demand management. Whether you use MRP (Material Requirements Planning) or LRP (Limited Resource Planning) or other supply chain management tools, you can rely on accurate “quantity” information to ensure that you purchase enough components to meet production requirements.

Why BOM Is Essential for PCB Assembly?

For Customers:

  • Facilitates rapid and accurate project cost assessment.
  • Provides alternative solutions in cases of unusable or costly components.
  • Enables quick confirmation of product specifications and features.
  • Assists in swift identification and location of components during product maintenance and repairs.
  • Provides traceability during design upgrades and optimizations.
  • Aids in proper project planning and smooth operation.

For Processing Factories:

  • Guides material procurement to avoid errors or omissions.
  • Helps manage project materials to prevent production delays caused by component supply issues.
  • Estimates the number of assembly workers based on purchased parts.
  • Improves input efficiency of material requisition documents for batch picking.
  • Provides standards for incoming inspections to enhance production quality.

Methods of Generating PCB BOM Files

CAD Software Automated Generation

Most PCB design software today offer a built-in feature for generating BOM files (those that don’t should be considered outdated). After completing your design, you can simply locate the “Export BOM” button, configure the settings as needed, and then export the file. This method is highly accurate, provides data traceability, and significantly improves work efficiency, particularly for larger projects.

Manual Generation

In this approach, designers need to manually input information such as component types, package sizes, quantities, and more into an Excel spreadsheet or other spreadsheet software. This process can be time-consuming and prone to errors. Although it remains effective for some simple designs, it is not recommended.

What to Consider When Creating a Bill of Materials?

Versioning Concerns

Electronics products undergo rapid iteration and frequent updates. For each version or generation, it’s crucial to assign a distinct version number to the BOM. Whenever a design is modified or optimized, detailed change information should be documented in the previous version. If unexpected issues arise, you can revert to the older version, identify it as a different version, and restart from there.

For example, your initial design’s BOM is version A, and after an upgrade, it becomes version B, with detailed modifications documented in version A. If you encounter unexpected validation failures, you should revert to version A and define it as version C.

Why not define it as A again?

While versions A and C essentially contain the same content, redefining it as A would lead to version management confusion. If you define it as A, the question arises: should the fourth update be labeled C or B? If it’s B, then the errors in version B’s design information would be overwritten, compromising traceability. If it’s C, you’d essentially repeat the process of version B. Always ensure that the subsequent version after an update is accurately labeled.

Matching BOM to Product

The relationship between a BOM file and a product is not one-to-one. In some products using multiple PCBs, you can create several BOM files to correspond to those products. However, you should not use a single BOM file for multiple products, even if they only differ in color.

For example, if you’re producing mechanical keyboards for two customers, one in red (A) and the other in blue (B), with only a color difference and identical internal structures. As the products evolve, the requirements from both customers diverge: A may prioritize typing performance, while B may want vibrant lighting. If you initially use a single BOM file to manage both products, it becomes complicated, leading to a situation where assembly line workers may not know if you need feature A or B.

Model Replacement Version

For instance, a customer’s mechanical keyboard undergoes four iterations to address various issues. In the contract, it’s emphasized that 200 sets of the XXX model are needed, including 100 sets from version A, 50 sets from version B, 25 sets from version C, and 25 sets from version D. However, in practical production, we often emphasize the product model’s volume:

  • Emphasizing the version of the BOM file implies the need for additional internal work orders, increasing workloads.
  • Functional differences become evident only after production is completed, potentially leading to errors if the operator doesn’t pay attention to internal work orders.

For better collaboration, it’s recommended to name the products based on their model when communicating with manufacturers, rather than the version of the BOM file.

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