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.

Sample_BOM.jpg 

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.

 

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2019

Powerful Prototypes: The Ideal Bill of Materials

07-10-2019

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.

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Powerful Prototypes: Moisture Sensitivity—What’s the Risk, and What Can You Do About It?

04-18-2019

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02-11-2019

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01-17-2019

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2018

What Is Your Supply Chain Telling You About Components?

12-24-2018

Right now, many, many parts are in short supply, or unavailable with extraordinarily long lead times. Allocation is the word of the day and substitutions are your friend. Sure, electronics components shortage happens every now and then in this industry. It's a periodic nuisance, but what should you do for the long term? Read on.

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Electronic Manufacturing Files: What We Need for PCB Assembly

12-07-2018

As PCB assemblers, manufacturing is all about taking data and delivering good working circuit boards. It can be just data, as in full turn-key, data plus some parts, or a partial turn-key or a kitted job. Regardless of whether you're sending parts and boards or having us buy everything, PCB assemblers need good data, and a lot of it.

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The Future of PCB Designs

11-28-2018

Duane Benson designed his first PCB using tape and etch-resist pens from RadioShack. He penciled the schematic on graph paper, drew the layout directly onto the single-sided copper-plated board, and then etched it. At the time, commercial PCB design wasn’t too different. In his column, he talks about the advancements in PCB design and the key considerations when designing boards.

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Top 5 Things to Know When Moving from Hand Assembly to Robotic Assembly

11-14-2018

A lot of factors go into the decision to hand-build or outsource circuit boards. When the decision is to outsource, there are a few important things to consider. Some things that work fine when hand soldering may stand in the way of quality, repeatability, and reliability when machine assembling. Here are some of the most important considerations when changing from hand-build to outsourced.

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Top 5 Ways to Mitigate PCB Component Availability Problems

11-07-2018

The electronics design world is by now aware that we're in a very serious period of components shortages. Allocation and shortages hit every few years, but this one seems to be the worst in recent memory. It could be a problem until 2020 and the supply chain and world of components manufactures will likely be a different animal coming out of it. Here are five things you can do to minimize the effects.

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