Overview: BOMs and BOM management

By Paul Vizzio

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A bill of materials (BOM) is one of the most important documents for a hardware product. The BOM is a hierarchical document that lists every single part of your entire product including outer packaging, accessories, consumables, electronics, off the shelf parts, and anything else included in the final shipped unit. This document can be as simple as a spreadsheet, to something more exotic like a cloud based user interface. It is common to break the BOM into separate parts to have the different departments work on them and have the overall project manager collate the master BOM. For example, there can be a mechanical BOM, an electrical BOM and a packaging BOM, the details of each one is important for each department while the project manager is concerned about the big picture of all 3.

In this post we will go over the basics of a standard BOM using our in-house designed Vizlapse as an example. Note that there is no standard for BOMs, you can customize them however you would like and use whichever programs you would like to do so, we will just present how we've done it on this specific project.

What is it used for?

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This document is used in multiple aspects of the product design and lifecycle from internal engineering management to supply chain inventorying and fulfillment. The BOM is the document that factories will want to see in order to price out your product and give you estimates on how long it will take to build. When reaching out to investors, they will want to know what your BOM costs are and what you believe you can sell it for. Marketing will want to know how long the product will take to get to the shelves and how low they can sell the product for without incurring losses. Engineering will use the BOM to find the high priced components and try to design in cheaper improvements. Manufacturing will use the BOM to see where high assembly costs are and how they can be reduced, they'll also look at what parts take the longest and how they can optimize it. Fulfillment will use the BOM to see which configuration of the product needs to be built. Supply chain will use the BOM to track all of the inventory and document what/how/when parts are made. Basically, the BOM will touch virtually every department in your organization.

What does a BOM contain?

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Truncated Bill of Materials

At its simplest, a BOM is just a spreadsheet with a handful of columns. For larger BOM management systems you can expand the capabilities and power of it to include things like real time tracking of product parts and inventory management. At a minimum, the Bill of Materials should include:

Product name
BOM version number
BOM level of component
Part number
Component name
Part version/revision
If the part is custom, off the shelf, or an assembly
Quantity per assembly
Accepted supplier name and part number (if applicable)
Reference designator (specifically only for PCB components)

Other items that can be added onto the BOM are, but not limited to:

Sign offs and approvals
Price per unit at different buying intervals
Lead time
Country of manufacture
Contact information of POC
Target price
Target lead time
Certifications followed
BOM sign-off date and approvals
Part images

BOM title box

To highlight some of the above points, we'll go through a truncated BOM of the Vizlapse. First thing we will highlight is our title box area, which includes the Product Name, Version, Sign-off Date, and Approval. When a version of the BOM is completed, the owner will approve all changes and date it to lock it in as a standalone document. Any changes that take place after this will have to take place on a new document that has been versioned up; this is a crucial step in document management as this will be a hard copy that gets sent to all of your manufacturing partners.

BOM entries: level, part number and part name

We'll start examining what we have in the first row of data, which is the top level assembly, i.e. the box the product ships in and everything inside of it. You will notice under level we have this as 0; this is because it is the highest level of the assembly. Parts that comprise this assembly will be level 1, and parts that make up level 1 assemblies will be level 2 and so on. In our BOMs we color code the background color to show level number as well. For this top level assembly we assigned a part number and part name that we made up. There are several different strategies for coming up with part names/numbers, but it will depend on your needs. In our case, we assigned a 3-4 letter prefix that states what type of part it is, i.e. SKU is for a SKU and PAC is for packaging, followed by 4-6 numbers. Whatever you decide on, you must make sure that you only use part numbers once; you can always add revisions to a part number, but re-using the same part number for different parts will create logistics nightmares. Every single part should also have a 2D drawing that goes over all of the requirements of the part, often times there will also be 3D CAD files associated with the parts.

BOM entries: part type, version, supplier and supplier part number

Next you'll see we've got what type of component it is whether we make it, we buy it off the shelf, or it's an assembly/sub assembly. For each part we need to assign a version number. In the case of the first row, the version number (or revision) should match that at the top of your title box. This version will increase with every update to the BOM, whereas lower level assemblies that comprise the top level might change minimally or not at all. Only if something changes do you need to update the version, and that includes if a subassembly gets changed. The supplier and supplier part number are important for vendor management. You will have your own part names and numbers, but that does not mean they will match what your supplier has for a part name and number especially if you are buying the part off the shelf. In the case of our SKU, we are the supplier so they match.

BOM entries: quantity, costs and lead time

Since the BOM lists all of the components that make up your product, it is important to have the quantity of each used in the assembly. This becomes especially apparent in items like screws and nuts; it is common to overlook the cost of 10 screws as just one. The next three columns have data that those outside of engineering are most interested in: cost and lead time. As we went over in the Manufacturing Options post, you will usually be able to lower your costs by increasing your quantities. It is therefore beneficial to quote out several different quantities to see how you can reduce costs with scale, in our case we picked 100 and 1000 units. Lead time refers to how long something will take to get to you from the moment you place the order. The Top Level assembly will have the longest lead time as it needs everything that makes it up to come in first. The BOM comes in handy for reducing overall lead times as you can scroll down to the long lead items and see if there are ways to reduce them.

Electrical BOM

Lastly we'll highlight a snippet of the electrical BOM. You'll see that our PCBA is a level 2 component being that it is part of the level 1 Vizlapse Assembly, which itself is a sub-assembly of the level 0 Vizlapse SKU. The majority of the components in your PCBA will be bought off the shelf either from a distributor or directly from the company itself. Electrical BOMs include reference designators for every component, where a reference designator is a label for a component that gets written (via a silkscreen) directly onto the PCB. The labeling of a reference designator is standard per IEEE and consists of 1-2 letters stating what the component is followed by a series of numbers stating which number of that component it is. For example, U1, U2, and U3 refer to integrated circuit chips (because of the "U") and the numbers state which of the integrated circuit chips it is (1 is the microcontroller, 2 is the H-bridge and 3 is the bluetooth chip). The designator is important for populating the PCB during assembly. The PCB will have the names of the designators printed on the board, and the BOM lists which of the components that corresponds to so the right parts get assembled in the right spots.

BOM Management

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We hinted earlier at some forms of BOM management, like having approval processes and including revisions, but properly managing this document is vital to the success of your products. It is easy to manage one product with a small team, but as your company starts scaling and has multiple teams with multiple projects, managing everything can get to be complicated and cumbersome. BOMs form the central part of many large management procedures such as Enterprise Resource Planning (ERP), Material Requirements Planning (MRP) and Product Lifecycle Management (PLM) to name a few. What these programs all have in common is that they are using the BOM to track items and products for different purposes, for instance ERP will track how many screws are currently in inventory and which products they can be used in, while the PLM will be keeping track of the current 2D drawing of that particular screw to make sure all parts in inventory are up to date. While it is possible to manage this all internally via spreadsheets, there exist several enterprise level management solutions with the three most notable paid versions being Arena PLM, Dragon Innovation, and Duro. A great resource for a free version is GrabCAD, which also acts as a cloud based solution for your team to collaboratively work on CAD models.