PCB Assembly Services - Screaming Circuits: The Ideal Bill of Materials


The Ideal Bill of Materials

A good portion of a quality build is simply the result of clear information. One of the more important pieces of information we deal with is the Bill of materials, called “the BOM.”

The BOM is a list of all of the components to be placed on the PCB. The file typically includes; an index number, the number of times a specific component will be used on the board, the reference designator from the schematic, the component manufacturer, and the manufacturer's part number.

If a specific component is used more than once, 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.

Sample BOM

For example, line 5 in my BOM on this slide, is a .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 my 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 manufacture, 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 I have here.

But the first five columns in this example shows what is generally considered to be the minimum data set needed for a good bill of materials.

Note that, down at the bottom, I three lines that are highlighted in red, with the label “DNS”, over 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 prefered 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 your purchasing folks or manufacturer won’t get stuck not knowing if a substitute is valid or not.

Duane Benson
In the 90's, when people said good things were "the bom", this is what they were talking about


Hi Jerry - It's not 100% necessary, but given that we buy the lion's share of our parts from Digikey, it's a good thing. If we do buy from Digikey, having their part number is one fewer transcription, and thus one fewer opportunity for error.

Is the "Digikey Part number" column necessary? Or, do you include it, because Digikey is your favorite distributor? I'm a big fan of Digikey, but with at least a dozen other distributors to buy parts from, just curious why illustrate this company & column?

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