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Powerful Prototypes: The Ideal Bill of Materials

A good portion of a quality electronics build is simply the result of clear information. Not long ago, I wrote about the set of files containing the information required by your manufacturing partner to ensure a quality build. All of the files discussed in that column are important, but one of them—the bill of materials (BOM)—is deserving of extra attention.

Those of us who spend much of our day inside of an assembly house sometimes have an overly simplified picture of this file, which can cause some confusion. At the design location, there may be several levels of BOM. The master BOM will contain mechanical enclosures, PC boards as subassemblies, mounting hardware, labels, and even the packaging and shipping materials.

We don’t care about most of that here at the assembly house unless we’re dealing with a complete box build. However, the engineers and purchasing folks ordering from us do care about that. If you are an assembly provider like me, please remember that. If you’re a designer, check with your manufacturing partner to make sure you only send them the level of BOM needed for the job being asked of them. Too much information is not much better than not enough information.

The assembly BOM used to build just the PCB needs to have all of the components being placed on the board—including nuts and bolts if you are asking for that—and nothing more. The assembly BOM is a list of all of the components to be placed on the PCB, and only the components to be placed on the PCB. The file typically includes an index number for each type of component by part number, the number of times a specific component will be used on the board, the reference designator from the schematic and PC board silkscreen, the component manufacturer, and the manufacturer's part number.

The most common BOM file format is the Excel spreadsheet. Almost everyone can take that format. If your CAD system uses a different BOM file format—or if the BOM is embedded in an intelligent CAD file format, like IPC-2581—double check with your manufacturer before sending it in to make sure they can accommodate it. Avoid PDF files as they make it more difficult to access the actual data.

Then, each component type gets its own line in the BOM spreadsheet—not each individual component, but each type. If a specific component is used more than once—such as a common bypass capacitor, for example—it will still only take up one line in the BOM. One field in the BOM will list the number of times the component is used, and another field will list all of the reference designators for that part number. Figure 1 shows a sample BOM file.

Figure 1: Sample BOM file.

For example, line 5 in Figure 1 is a 0.1-microfarad, 10-volt capacitor. The first field in the table has a line item index 5 because this is the fifth unique part number in the BOM. The next field has a quantity of this component used on the board, which is 5. Field three holds reference designators C1, C2, C3, C4, and C5. The next field has the manufacturer, and the final field has the manufacturer’s part number.

You will likely have additional fields—such as a distributor part number, a description, the package type, and other tidbits, as shown in Figure 1. But the first five columns in the sample show what is generally considered to be the minimum data set needed for a good bill of materials.

Note that at the bottom of Figure 1, three lines are highlighted in red with the label “DNS” in the type column. DNS means “do not stuff.” That’s an instruction to your manufacturer to not install that component during the assembly phase. Some people use DNP for “do not place” or DNI for “do not insert.” It’s always best to consult with your manufacturer to get their preferred labeling.

You may also want to include alternate parts for components that are likely to go out of stock. Passives—like capacitors and resistors—are notorious for going out of stock without notice. Invariably, though, there will be a half dozen nearly identical parts that will fit the bill just as well. Create an alternates list so that your purchasing folks or manufacturer won’t get stuck not knowing if a substitute is valid or not.

A good bill of materials is the first step toward getting a quality product. Much of the assembly process is anchored around this humble little file. Give it some extra care and feeding and you will greatly increase the odds of an easy build process and a perfect product. 

Duane Benson is marketing manager and CTO at Screaming Circuits.