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Collecting All the Pieces You Need: What is a Bill of Materials?

Created: March 30, 2018
Updated: September 25, 2020

A pen and a laptop on top of blueprints with two rolled schematics

On a cold and rainy Saturday afternoon with small children, of course you’ll start to panic a bit to figure out what kind of antics you can get them into without causing too much trouble. So, I turned to drawing. We arranged crayons and paper on the dining room table and I began to draft a simple periscope. I outlined each part of the periscope and placed a letter alongside—my engineer was showing. Completed, I showed it to the kids and asked if they’d like to build it.

What had been faces full of boredom immediately perked up and, pretty soon, we were putting together a list of materials, including letter designators and material description. A Bill of Materials for a printed circuit assembly works much the same way. Schematics are drawn, components are designated, and materials are chosen.

Common information listed on the Bill of Materials for each component contains not only the location on the Schematic but descriptions of each component, amounts used in the design, enterprise part numbers, vendor part numbers, and any alternate vendors. The Bill of Materials is used in a variety of ways to catalog the components, to purchase the components, and to kit the components for use at the assembly house.

Bill of Materials Starts With the Schematic

You’ll start with a schematic illustrating a circuit and that circuit will be made with the components that you need for your design. The circuits become printed circuit assemblies used in products. Common symbols will define the type of circuit elements that an engineer uses to realize their design such as resistors and designator symbols like letters. These symbols eventually will grow up to become unique components within your enterprise library.

It’s not entirely different from what the kids did with the periscope drawing: they took letter designators from the drawing and made a list containing each piece. From their list, they researched hardware stores for material and pricing options.

Electronics Schematic)
Components Have Reference Designators for Tracking

Materials Chosen and Enterprise Part Number Assigned

Once the schematic is complete, the design engineer is ready to choose hardware for their circuit symbols. Getting a list of symbols and their designators happens by running a simple BOM from the PCB software. Working from the simple BOM, the engineer selects vendors who make the part.

The whole process will require you to collect datasheets, assign enterprise part numbers, load vendor information into the designated part element within the design tool. Each component symbol, together with its enterprise part number, becomes a unique part of the PCB library, associating back to the schematic designator and forward into the enterprise database.

Knowing the vendor, dimensional information is available for PCB layout. 3D representations are available to mechanical engineers for verification in the product. By enabling system engineers to have a forward look, you can give confidence to order parts for manufacturing and assembly.

Real Hardware, Listed on the Bill of Materials, is Procured

As the design develops and parts are needed, a first-look BOM containing comprehensive information is made available for procurement. Enterprise purchasing and supply chain departments begin their work using the BOM.

Similar to my kids who used their list to shop around for materials and keep track of costs, purchasing departments shop around for component availability and costing. Supply chain departments use the BOM to take inventory of supplies and create space for new parts.

Vegetables shopping list
A PCA Bill of Materials is a shopping list for electrical components

Making a list is simple. Adding information to the list makes it useful for designing and building printed circuit assemblies. In its basic form, a Bill of Material lists each and every piece of material that makes a printed circuit assembly. The BOM contains the printed circuit board and components that make the assembly. By including vendor information, you allow sourcing to make it useful across the enterprise.

These days, the kids are making lists of everything in the house. For now, I’m able to fund their building projects. Their curiosity is infectious. This last weekend, they found a website with instructions on how to build a photosensitive alarm clock. I’m going to download CircuitStudio and get them going. Imagine their faces when they find out about ActiveBOM!

Using Schematic Capture and ActiveBOM in Altium allows compiling a Bill of Material listing circuit components in the early stages of a design for material selection. ActiveBOM enables real-time updates within the tool throughout design, development, and production of the printed circuit assembly. Adding information organically in the tool provides flexibility when vendors change availability or new vendors become available throughout the lifecycle of the product.

If you’re even more curious about how you can manage your design and assembly data to have the most accurate and carefree process to move into production, consider talking to an expert at Altium today.

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