What is a BOM?

Posted on Aug 1, 2022.

In this post, we get back to basics. We'll cover what a BOM is and talk a bit about what BOM means in engineering and manufacturing. We assume that readers have very little prior knowledge about BOMs, but some familiarity with engineering and manufacturing.

BOM defined

The BOM acronym in manufacturing and engineering means "bill of materials," which refers to a document – usually a spreadsheet or something similar – that lists the components or parts of a product (i.e., "an assembly"). BOMs are used throughout the manufacturing process to communicate information about the parts that must be built, ordered, and assembled into a final product, as well as how that product is packaged, documented, and then shipped to retailers, distributors, or customers. The BOM, as you might imagine, is one of the most important documents needed to get right at each step in manufacturing your product.

Types of BOMs

People from different job roles (e.g., engineering, purchasing, manufacturing operations, management) consume, manage, and produce various types of BOM data that both address each of their roles’ objectives and provide information to those who are in the downstream roles. There are many types of BOMs, but the two most common types are engineering BOMs and manufacturing BOMs.

Engineering BOM versus manufacturing BOM

These BOMs are in principle very similar, but they are produced throughout different stages of manufacturing and serve distinct purposes. Depending on whether you are still in the design/engineering phase of manufacturing your product or you are trying to assemble finished goods, you will need either one or both. Let’s find out more about the BOM meaning in manufacturing and engineering in the sections below.

Engineering bill of materials (EBOM)

Engineers develop a BOM that represents the structure and critical-to-quality properties of their product's design. For complex electromechanical products, there may be two EBOMs: one from the electrical engineers and one from the mechanical engineers. In both cases, an EBOM serves to help engineers communicate their design and the parts that are contained therein.

The most common properties of an EBOM are:

  • Part number/name - a distinct name or number for a certain part. Organizations typically assign a unique number to each part they use in a design to ensure a common reference system.
  • Description - a human-readable name or description for a part (i.e., not just a number).
  • Quantity - the count of a certain part included in the design.
  • Quantity unit of measure - the unit used to express a quantity. This can be Each, i.e. the quantity of each product (e.g., quantity of 5 means 5 parts) or can be a mass unit (e.g., kilograms), length unit (e.g., meters), packages, etc. This property is more important for mechanical BOMs because ways of measuring material quantity are more varied than for electrical BOMs; electrical BOMs primarily contain discrete, individual parts, where the unit of measure can be assumed to be Each.

Screenshot of a mechanical BOM copied from the Hardware Document Toolkit

Other properties that may be included can vary based on the type of EBOM (e.g., electrical vs mechanical BOM): for electrical engineers, Reference Designator and Value are critical properties to ensure the correct part is used in the final product. For mechanical engineers, a Make/Buy indicator to specify if the part is made custom or bought “off the shelf”, the Material and Finish information about the part, and a Revision number all help the engineer ensure the procured parts will meet the expected design parameters.

Manufacturing bill of materials (MBOM)

The manufacturing BOM is produced using information from the EBOM, but it serves a different purpose: the MBOM indicates how a product is assembled and contains information that may not be in the EBOM, such as specific manufacturing processes; glues, adhesives, and paints that are specific to manufacturing; and kitting (which is the bundling of all finished products, accessories, documents, etc., into a package). The MBOM defines the "final word" of the parts/materials, costs, schedules, logistics, and other manufacturing operations.

In addition to the properties contained in the EBOM, the MBOM contains several obvious and not-so-obvious properties that should be considered:

  • Cost - the per-unit cost of a particular part.
  • Minimum package quantity/minimum order quantity - the minimum number of parts that must be ordered. This is either the count of parts that come in a package (bag, box, etc.) or the minimum quantity required by your factory to produce custom parts. This can lead to increased costs and waste. For example, if you only need 5 of a part, but you have to order 100, you now have to pay for 95 extra parts.
  • Extended cost - the total cost of a certain part to produce 1 assembled product. This can take into account the quantity, minimum package/order quantity, discounts, and other factors.
  • Labor - the cost of the labor required to assemble, package, kit, and ship your product from the factory.
  • Lead time - the amount of time required to get parts in hand. This will determine when you can start shipping the finished product.
  • Duty, taxes, and tariffs - any costs required to ship parts or products between countries. This can be significant and may affect purchasing decisions (e.g., the cheapest vendor may not be the cheapest if you have to pay high tariffs to import those parts).
  • Fulfillment - the cost to receive, hold, and ship inventory after it is manufactured.

Screenshot of a simplified manufacturing BOM copied from the Hardware Document Toolkit

Because the MBOM contains all details about how your product is manufactured, it can be very complex; you may even have multiple MBOMs if you are using multiple manufacturers or are manufacturing different configurations of your product in different ways.

Costed BOMs

Cost is listed as a common property in an MBOM and is important to ensure the cost targets for a product are met. It is not unusual for an EBOM to also contain cost and to either track the costs for prototype parts ordered during development or if the engineers are responsible for sourcing the final parts for assembly. Any BOM that contains cost information, such as cost/extended cost, lead time, and minimum package/order quantity, is called a costed BOM. A costed BOM can be used to monitor costs throughout the product development process and run cost-reduction exercises with your team(s).

Further readings

There are many resources available that can help you learn more about the types of BOMs and their uses. To support startup teams (or teams needing a refresher or just some good ideas), we helped produce the Bill of Materials module in the Hardware Document Toolkit (links.getbommer.com/toolkit).

You can also check out the following resources for more information on BOMs:

Spend less time on your BOM with Bommer

Our products plug into CAD to make it easy to populate and export your EBOM, saving engineering time and reducing potential for common errors like miscounting, omitting parts, or mistyping a critical-to-quality property. See www.getbommer.com for more info, or get in touch for a demo!

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