Do Bolts Go on Your PCB Bill of Materials?

Duane Benson

The short answer. Yes. If you want prototype assemblers to install it, it must go in the bill of materials.

For the most part, we solder thru-hole and surface mount components on the PC boards. As pretty much everyone knows, all of those parts need to be put in the bill of materials (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.

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.

But what about things that aren't soldered? Like nuts and bolts, double sticky tape, or display panels and such? Where do they go? The quick answer is that they go in the BOM like all of the other parts. Manufacturers build from the BOM. That means that if it's not in the BOM, they won’t know to install it.

Some of these parts are non-standard and can't easily be quoted online, but they still need to be in the BOM. If you have such things, give your manufacturer a call to see how much it will cost and they we can assemble it. Then either put the reference designator in silk screen or offer an assembly drawing with a reference designator for whatever it is.

That means a set of bolts might be BT1, BT2, BT3... The washers could be W1, and nuts N1. A glue dot could be G1. It doesn't matter that much. Just make sure the reference designator in the BOM matches that on silk screen or in an assembly drawing.

If it requires hand operations like double-stick tape under a display, again check with your customer service rep first, but then put the display and tape in the BOM and provide any non-obvious information in an assembly drawing or special instructions.

Duane Benson is Chief Technology Champion at Screaming Circuits.