What is an Engineering Bill of Materials?

Posted on Nov 7, 2022.

A good, well specified bill of materials (BOM) is essential to make sure your product gets built and assembled exactly like you want; conversely, an incomplete or error-prone bill of materials is the source of innumerable errors that can cost you plenty. Having a complete, correct BOM is, as a result, really important at all stages in your product development process. We see frequently that it all starts with having a good engineering bill of materials (EBOM).

The EBOM is a document (typically a spreadsheet) created by the engineering team to describe the parts and their important attributes for a product or design. This document contains all components and their quantities, and can contain the assembly structure and additional metadata about the components. This is different than a manufacturing BOM, which will contain a subset of this information and also list additional costs and process details (you can read more about the difference in our post What is a BOM?).

The EBOM represents the engineering intent for an assembly. It is typically generated from engineering CAD data, and it's your foundation, so let's talk about how to make sure it's a good one.

Components of an effective EBOM

A good, effective EBOM will contain lines for each of the parts that are in the engineering design for your product. Every sheet, every bolt, every nut, every screw—if it's not listed or counted in the BOM, it could get missed when ordering parts or when doing the final assembly. Each row should also contain a Quantity property to indicate how much or many of these parts are needed. This can be in units of actual parts (also known as "Each"), or could be represented in mass (e.g. kg, lbs) or length (e.g. ft, meters) of raw material or packets or boxes (common with fasteners or hardware). Quantity can be shown in whatever measure makes the most sense for each line (and may vary from line to line); it is important, though, to make sure the unit of measure is also specified if it is not the count of individual parts to avoid any confusion when ordering or picking parts.

Additionally, it's a good idea to make sure some standard information exists for each line in the bill of materials.

Part number

This is a distinct identifier (usually a number, usually with some hyphenated sections, e.g. 12-3456) assigned to each part or component to establish a consistent reference system within your company or organization. There is a disagreement on whether part numbers can be "smart," meaning that each section in a part number means something (e.g. given the part number 12-34-56-01, the 12 means purchased part, 34 means motor, 56 means DC motor, etc.); whether they can ever be reused or reclaimed if a part is decommissioned; and whether they need to start at 1 or be sequential without any breaks. Whatever system you choose, the two rules we believe you must follow are these: every part has an assigned number and every number is unique (no duplicates) amongst parts actively in use.

Part name, Description, Thumbnail

Part name is a name, e.g. file name or vault name for the part or subassembly. It doesn't have to be as strict as part number, and therefore it can provide some "human readable" descriptor for the part. It's also not uncommon to eschew this descriptor and name files according to their part number, in which case this serves as a pointer back to the CAD or drawing document. The purpose is to give context, so that people who are using the BOM to perform a task can quickly identify parts, e.g. "LHS Hinge 40mm" is a lot easier to quickly understand than 213-235124.

Description is a long(ish) form text property that contains a description of the part. It can also contain work instructions, ordering instructions, finish notes, or any other information the engineer wants to convey (though these data can also be pulled into their own properties for clarity. We will discuss more on that below).

Including a thumbnail image can go a very long way in helping disambiguate parts or showcase some material properties (e.g. finish, grain direction, threading vs no threading) that may be otherwise hard to describe in a bill of materials Excel workbook or other spreadsheet. Similar to above, thumbnails are all about providing context, and as they say, a picture is worth 1000 words.

Other critical-to-quality properties

In addition to the above properties, a good EBOM will contain any other properties an engineer needs to communicate how they want the part purchased, procured, or produced. These are commonly called the "critical-to-quality" properties because, as the name suggests, they contain data critically important to producing a high-quality product. Examples of these properties are:

  • Color, material, finish (CMF) properties - these are properties that describe the material and appearance of a part, which can be important for both the industrial design and the mechanical performance of the product.
  • Manufacturer, Manufacturer Part Number - if the engineering team identified a manufacturer for a part, they can include the name and manufacturer part number as part of the EBOM.
  • Vendor, Vendor Part Number - similar to manufacturer, if the engineering team identified a vendor that sells the part in the right form and function, they can specify that as part of the EBOM.
  • Cost, Extended Cost - the per-unit cost and total cost (unit cost * quantity) for a part. Not all engineering teams track the cost of a part, but some do either as part of identifying the manufacturer and vendor or as a guide for the purchasers to ensure the project hits its cost target.

Different properties for mechanical vs electrical EBOMs

Until now, we have not talked much about different types of engineering BOMs, e.g. mechanical vs electrical. Our Introducing the Hardware Document Toolkit - Bill of Materials post covers this well, along with good bill of materials examples; here, we'll highlight some of the more common properties for each type of EBOM.

Mechanical EBOMs may also contain a Make/Buy indicator to communicate whether the part should be contracted out to a job shop or CM or sourced from a distributor. Mechanical engineering teams also may include part Revision to further remove the possibility for error introduced by purchasing or producing the wrong version of a part.

Electrical EBOMs will contain properties that are specific to PCB assembly, such as a Reference Designator that connects a part from schematic to the PCB layout, a Value that is component-type-specific and contains the value of that component such as the resistance and tolerance of a resistor and the capacitance and tolerance of a capacitor, and Alternates, which the engineer will have specified as replacements that can be used if this part is unavailable.

Getting it all right

Inaccurate, incomplete, or unmaintained EBOMs will cost you time and money, delay product launches, and possibly cause your company to miss opportunities. The transition from engineering to manufacturing in new product introduction (NPI) will go much more smoothly with a thorough and accurate engineering bill of materials. We’ve put together below some tips for creating and managing accurate and complete EBOMs to make sure your NPI goes as smoothly as possible:

Start with a standard

It can be as detailed or as pared down as you need (within reason, and in our opinion, with the guidelines we laid out above in mind), but engineering teams should start with a standard that describes which properties to specified, when in their workflow they need to be specified, and any quality gates for ensuring underspecified BOMs do not make it out into the wild.

Make sure it is detailed (but not too detailed)

More detail is generally better, but you want your EBOM process to be sustainable, and you don't want to constrain your purchasers or CM if you don't need to. In other words, specifying the required material is important, but if the finish doesn't matter, not including it means your purchasers can cast the net wide to get the best deal on these parts. This is a delicate balance to strike, but one that can be driven through sound engineering decisions.

Data accuracy is paramount

Even minor errors in your BOM can be costly, so make sure you have double-checked or thoroughly reviewed it before releasing the document. Creating a checklist or including the BOM in your part release review process is a good idea.

Track revisions

An EBOM needs to be updated and modified during the course of a project, usually in response to a design revision. The EBOM should similarly be revised, either on its own or with the same revision nomenclature as your CAD files. EBOMs can also be stored in your CAD vault alongside the CAD files they describe.

Control changes like any other design data

Giving edit access to all people involved in the project may not be necessary; some just require read-only access. Therefore, it is better to regulate the process of giving edit permissions to avoid errors, unnecessary changes, or even confusion.

Use a BOM automation tool

Many of the challenges with creating accurate, complete EBOMs can be solved with the right automation tools. Investing in an easy-to-use BOM tool like Bommer helps you build, manage, and track your BOM data efficiently. Bommer can hold your BOM standard as part of its configuration, ensuring every BOM looks the same. Spot checking for missing data, and filling in gaps, is easy with a spreadsheet like view on your properties. Most importantly, Bommer automates building your parts list, exporting property values, and counting quantity for your parts, eliminating an entire class of manual counting errors and "fat-finger" typos. It is designed to save you time when constructing this document so you can focus more on the engineering work in front of you.

Want to try Bommer out for yourself? Contact us for a demo!

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