Is Your BoM a Smart BoM?

Please be aware that there is no “B” at the end of these BoMs. Still, without the “B”, you can have a smart BoM or a dumb BoM. It is important to note, however, that a dumb BoM may not be a bad thing. It just depends on what you want to do with it.

No matter what you’re doing, there are a few necessities. Let’s start at the very minimum, for someone designing something to be self-built from mostly already owned parts; maybe just a few from a dealer.

Reference designator: R1, R2, R3, C1, C2, U1… You have to have this information.

Quantity is important so know know how many to pull. Although, with a small garage-built project, you can probably just as easily count how many you need for a given value. And what about the value? Actually, the value isn’t always all that necessary if you have the correct part number information. A line item number is hand for big bills to keep things straight.

This is actually too basic and kind of pointless, so I’m going to jump ahead. Take an assembly house like Screaming Circuits. Screaming Circuits will either build your boards from your kit of parts or purchase them from your BoM (or a combination thereof).

Once you have the item number, quantity and reference designator, you need to tell your assembler or purchaser what it is. If you already have the parts kit, just add in the manufacturer’s part number and a description / value. That should do it. Some assemblers, like Screaming Circuits, will take part numbers from a distributor in place of or in addition to the manufacturer’s part number (e.g., Element14, Digikey, etc.).

If the assembly house is going to buy the parts, then add in the manufacturer and double check that the part numbers are accurate with all suffixes and things of that sort. The distributor part number can be added, but when the assembler is going to build the boards, you really should include the manufacturer and manufacturer’s part number to cover all basis.

BoM Sample

That’s cool, but your circumstances might require just a little more. You might need to list an approved substitute or two for parts that come in and out of stock frequently. You could also list multiple distributor part numbers for the same specific component, again, in case of lead-time or stock issues.

Sometimes I get myself into a bit of trouble by not specifying some part values at design time. I might just throw in things like bypass caps, RS232 driver charge pump caps or LED current limiting resistors assuming that I just know what the value is. It’s not a big issue, but it would probably be less work to just do it at the start.

Duane Benson
What the Bureau of Meteorology has to do with your parts kit, I’m not sure.

http://blog.screamingcircuits.com/

Ambiguous PCB Markings

Here’s a little issue we run into now and then. Which reference designator goes with which part? Quick. I need to know. Now. Now. Now!

It’s less of an issue with SMT parts because we machine place them and use your centroid file to do the programming. Still though, It’s always good to have things marked clearly in case rework is needed and for visual inspection.

For through-hole, though, it is definitely an issue because a real human being is putting the parts in and the visual markings are the programming for the human type person.

For best results, take a little extra time and make sure all of your ref designators are clearly associated with the correct part. It’s also always a good idea, when possible, to have all the designators in the same position relative to their part. Consistency is a good thing here. Consistency is a good thing with mashed potatoes too. Who likes lumpy mashed potatoes?

Duane Benson
You say poe-ta-toe, I say ugly brown tuber

http://blog.screamingcircuits.com/