Building Your Next Multi-Level Bill of Materials

Created: January 4, 2019
Updated: September 25, 2020

 Automated machine soldering components on a PCB

My wife and I always argue about what goes on the grocery list each week. I’m a total carnivore, while she prefers I get everything we need from local farmers. Planning out our shopping trips in multiple lists does more than just allow us to plan our Saturdays coherently, it helps ensure marital bliss.

Any PCB that you intend to manufacture will need an associated bill of materials. If you’re building a complex product that includes multiple sub-assemblies, multiple boards, and components that do not actually appear on any of your PCBs, you’ll want to use a multi-level bill of materials to delineate all of this information. Using the right bill of materials format can help you avoid production delays and improve collaboration throughout your organization.

Single-level and Multi-level Bills of Materials

Simple bills of materials are like a shopping list: they contain a list of components and the quantities required. These are called single-level bills of materials. This document lists the quantities of all the parts used to assemble a PCB, where the components are typically listed in order of part numbers. This type of bill of material is fine if you are preparing a single board for assembly, but it becomes unsuitable for more complex electronic systems that involve multiple boards of electrical/mechanical systems required to support the entire product.

In contrast, a multi-level bill of materials contains all the information required to take multiple components and build an entire system. Even folks that write bills of materials for a living may not know that they have been writing multi-level bills of materials all along. If you’re preparing an electronic system with multiple PCBs, you’ll need to prepare a multi-level bill of materials.

A single-level bill of materials can be prepared using information from a multi-level bill of materials, but not the other way round. The single-level bill of materials does not distinguish sub-assemblies that are required for an overall system and is really only appropriate for a single board. If you’re preparing a complex system with multiple PCBs, each PCB will appear in the multi-level bill of materials as its own sub-assembly.

If you’re part of a smaller enterprise, you might find that it is common to use word processing software or Excel to write your bills of materials. Excel is more useful than word processing software because you can, at minimum, segment sub-assemblies into their own spreadsheets. This helps you enforce strict organization in a multi-level bill of materials.

The problem with using a Excel is that many of the required tasks must be performed manually. Unless you are a VBA guru and you are using other databases to support your component and sourcing information, you’ll spend a significant amount of time compiling information by hand. The complex parent-child relationships between sub-assemblies makes this more difficult and creates the potential for errors, especially in multi-level bills of materials.

PCB with densely packed components

PCB with densely packed components

Using Bills of Materials for Design and Collaboration

Defining the right bill of material structure requires consider who will be using it and product you intend to build. Bills of materials have an in-house use, sometimes called an engineering bill of materials. This document helps product designers get a full view of everything that will appear in the product and its sub-assemblies.

Mechanical designers often prefer a multi-level bill of materials as it allows them to easily drill down to the information they need for their particular design, as well as see the relationship between their sub-assembly and the entire product. PCB designers and electrical engineers, on the other hand, tend to prefer single-level bills of materials, as they generally use off-the-shelf components to build their sub-assemblies.

When people refer to a bills of materials, they are probably referencing a manufacturing bill of materials. The manufacturing bill of materials can be written as a single-level document as it includes all the information required to build a single PCB. This includes your shopping list of components, tolerances, orientation for each component, and any other specifications that your manufacturer needs to assemble your PCB properly.

If you need to update a sub-assembly within your product, this is much easier if you use a multi-level bill of materials. Any ECO would only need to refer to a sub-assembly within the product. This helps your design team focus only on revising the relevant sub-assembly.

PCB designers need the information in a bill of materials at the outset if they are to avoid time-consuming redesigns due to obsolete or unsourceable components. You can use a multi-level engineering bill of materials to give a PCB designer suitable component replacement options before they start designing and prepare for manufacturing. The supply chain can change quickly, and PCB designers that are armed with sourcing information can anticipate changes to their boards and plan ahead.

Assembler soldering components on a PCB

Assembler soldering components on a PCB

What Does Your Manufacturer Need?

If you want to get your PCB off the fabrication line, your manufacturer needs more than just a simple list of components and quantities. You’ll need to provide sourcing information for your components, which is preferable to allowing your manufacturer to source your components. Since your PCB will include many components, you’ll also need to provide assembly instructions and information.

Your manufacturer will also need Gerber files for your board as it includes all solder mask and copper pour information for your board. If you want to increase throughput, you’ll want to panelize your boards. Make sure to obtain the proper panel size from your manufacturer so you can optimize your panelization arrangement. This arrangement will need to be placed in a CAD file for your fabricator.

When you use an integrated software package, all of these features are accessible from a single interface. Your deliverable generation tools grab information directly from your design data, allowing you to design top-notch PCBs and generate complete bills of materials in a single program. All your product and component information is present in one location, easing collaboration across your organization.

A PCB design software package like Altium Designer 18.1 includes all the tools you need to generate your bill of materials for your manufacturer. The ActiveBOM tool is integrated into your design tools, allowing you to generate your bill of materials directly from your layout. You can manage your components, view sourcing information, and manage obsolescence. These features facilitate collaboration between mechanical and electronic designers.

To learn more about how Altium Designer can help you build the best bills of materials for your manufacturer and your design teams, talk to an Altium expert today.

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