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 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 the reference designators for that part number.

For example, line 5 in my BOM on this slide, is a 0.1 microfarad, 10V 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 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 I have here.

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

Note the three lines at the bottom highlighted in red with the label “DNS” in the Type column.

DNS means “do not stuff.” That’s an instruction to the 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 likely to go out of stock. Passives, such as capacitors and resistors, are notorious for going out of stock without notice. Invariably, though, a half dozen nearly identical parts will fit the bill just as well.

Create an alternates list so the 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

 

BoM Process

All of this talk about BoMs these days, (all of my talk, that is), kind of begs the question of how BoMs are put together. An Excel spreadsheet seems to be the most common “BoM management” tool in use today. Large organizations often have more formal systems, but for everyone else, it seems to be largely a manual spreadsheet process.

When I’m putting together a new design, I first look at the big parts, like MCUs and other “big” chips. Sometimes big is 3 X 3mm, so “big” is a relative term. Then I’ll put in active discreet components and connectors. Next will be specific passives (like for a crystal that requires a certain value of capacitor) and finally, the more generic components like pull-up resistors and bypass caps. This may not be the best, or even a typical process, but it seems to work for me and it more or less follows my circuit design process. There are some traps that I leave myself open to.

I pick the big components based on what the board is going to do and how much real estate I have. The next set of components are largely used to make the big components work. The specific passives attach to parts with specific requirements and the generics just get thrown in as needed. Often I don’t even define the generics until after the design is complete. Take a look at this schematic clip:

U3, the MCP73833 battery charger and U4, the ADM3101E RS232 line driver will both get specific part numbers right off the bat. I may change packages as I get into the layout, but I will usually fill the BoM for those items when I put them in the schematic.

I’ve put U3, the charger chip in my BoM twice, each with a different package, because my preferred package isn’t available at the moment but might be soon. It’s not best-practice, but you can do that as long as the reference designator differs in some way and the part is labeled “DNS” (Do Not Stuff). I simply gave my alternate the part number “U3alt.”

R6 and R8 get specific values because the battery charger chip calls out for specific values. Q1 will be a small P-channel MSOFET, but the specific part number can come later. All of the rest of the resistors and caps will also be defined later.

The biggest trap I have to watch for when filling out the BoM in this order is forgetting any design decisions or leaving fields blank when I go back and fill in the rest of the part number information.

Passive part of BOM

This is typical of a BoM of mine just after I’ve finished the layout. If I were building this by hand out of my own parts bins, this would be fine. But when sending it out for assembly, it’s not. I personally know that C3 and C4 could be any of a dozen different part numbers. The only things that matter in that case are that it’s .01uf, it’s an 0402 and the voltage rating is 10V or higher. I know that R2, R3, R4 and R5 are just LED current limiting resistors and can be anything between 220 and 680 ohms for this particular circuit.

I know those things, but the assembly house doesn’t know that those parts have pretty loose specs. It will just cause delays if I don’t find an exact part number before sending it out. It may seem obvious, but just because it’s obvious to me doesn’t mean that it’s obvious to anyone else. That ambiguity has to be gone before anyone else sees it.

Duane Benson
Knock three times
on the ceiling if you want p-channel.
Twice on the pipe If the answer is n-channel.

http://blog.screamingcircuits.com/

BOMS Away

Yes, I’m talking about BoMs (bills of materials), not bombs. That would be silly and irrelevant. At least mostly irrelevant. If you make bombs, it wouldn’t be, but it would probably be all secret so we couldn’t talk about it.

The question of the day is: “What makes a good BoM?” There are a lot of BoM formats in use. It’s one area that the standards train more or less left behind. Well, there are standards. For example, IPC-2581 covers not only BoM standards, but a replacement for Gerbers and the whole manufacturing data package. One of these days, we’ll all be using the IPC-2581 formats for our data and life will be beautiful all of the time.

However, those standards aren’t really in common use today. And, they are complex enough that they can’t really be used in spreadsheet form. There’s a lot of nesting and hierarchy that makes it more difficult to deal with without a BoM management software package. Still there is good data in there. A lot of good data. So much good data that my head is still swimming.

But until that day, there is a set of data and data labels that will help ensure accuracy. The headers are important too. If this seems quite rudimentary, that’s because it is. But it’s important.

BOM snippet

  • “BomItem” or “Item #”: This is just the line number. Each type of part gets an item line, not each part. If the pat number is the same, you just put it down once and give the quantity.
  • “quantity” or “Qty”: How many of this specific part you need per board
  • “RefDes”: The reference designators used by the parts on the PCB silk screen. All of the same part number should be in the same excel spreadsheet cell: i.e., “R3, R4, R5, R6”. You can also indicate a contiguous range with a dash: “R3-R6” or “R3-R6, R10, R15”
  • “Manufacturer” or “Manf”: The name of the component manufacturer. It’s best to spell out the full name, e.g., “Texas Instruments”, but common abbreviations such as “TI” generally work too. The less ambiguity, the better.
  • “Mfg Part #” or “Manufacturer Part #”: The part number that you would use if you were buying this exact part from the manufacturer or a distributor. All of the suffixes are important too. For example, “PIC16F88” is not enough when you really need a “PIC16F88-I/P”.
  • “Dist. Part #” or “Distributor Part #”:Not strictly necessary, but can help in cases with a bit of ambiguity. Again, this would need to be the exact part numer as you would order it from that distributor.
  • “Description”or “Desc”: This is the component description as given by the manufacturer. Again, this isn’t strictly required, just a good idea.
  • “Package”: This is the standard package type, e.g., “SOT-23”, “TO-92”, “0201”. Again, not strictly necessary but can be a good redundant check.
  • “Type”: Optional indicator of the generic type. e.g., “fine pitch”, “smt”, “thru-hole”, “Leadless”. Not required but can help with assembly quoting.

That’s not IPC-2581, but it is a good set of usual requirements. It’s also best to put your final BoM on the first tab in your excel spreadsheet. That will make it easier for buyers to know exactly what you want.

Duane Benson
So long mom, I’m off to drop the bill of materials
So, don’t wait up for me

http://blog.screamingcircuits.com/

Counting Once, Counting Twice…

Panel single scLet’s say you have two options: First, you could send in your boards for assembly as individuals. Second, you could send them in a panel. That’s all fine and dandy. For a few, send individuals. For a bunch, panels might make more sense. But, when you do go to quote and order, how do you count the parts?

Let’s take this example. As a single, this board has 32 line items on it’s bill of materials. That’s 32 unique parts. Counting all of the individual part placements, there are 56 total parts: 42 SMT and 14 through-hole. So, naturally, if you quoted the assembly of 20 of this board at Screaming Circuits, you would enter your desired board quantity as 20, 32 total unique parts, 42 SMT and 14 through-hole.

But what do you do if you send it in panel form? How do you count? It’s actually not as difficult as it seems. In this example, it’s in a panel of four. There are still only 32 BOM line items, but there are four times as many placements. That means that if you quoted this, as a panel, you would enter 32 total unique parts, 168 SMT and 56 through-hole parts. If you still need 20 of the final boards assembled, you would enter 5 as your desired board quantity.

In the end, you will have 20 assembled boards. In case you are wondering about the cost, there won’t be a difference. As long as the final number of boards (after the panel is broken apart) are the same, your cost will be exactly the same for panel vs. one up. You don’t save any money by sending in singles. However, if your board is panelized and all of your parts on on reels, full or partial, you can save money by ordering Short-Run production.

Duane Benson
50 Years ago today
Robert Rushworth flew the X-15 to Mach 5.03 at 100,400 feet altitude

http://blog.screamingcircuits.com/

Design Costs, But How Much?

Lots of studies find that most of the cost (75%-plus) of an electronics product is determined in the design phase.

Here’s my question: Do those studies take the BoM costs into account as part of the design cost? Or are all the “costs” in this sense value-added (design, fab, assembly, rework, etc.)?

Design Costs, But How Much?

Lots of studies find that most of the cost (75%-plus) of an electronics product is determined in the design phase.

Here’s my question: Do those studies take the BoM costs into account as part of the design cost? Or are all the “costs” in this sense value-added (design, fab, assembly, rework, etc.)?

My Mistake — Naturally

I received my PCBs for this project from Sunstone yesterday at about 10:10. I quickly dropped them into my box of parts and delivered it to the receiving pile-of-boxes in our shipping.receiving department. At 11:40 a.m. yesterday, I received an email from our auditing department informing me of a BoM mismatch.

BOM mismatch Yes. I had made a mistake in my bill of materials. The board has a bunch of yellow LEDs and one red LED. I had mistakenly only packed up yellows. Our audit department caught my mistake and sent me a quick email. If I hadn’t have responded yesterday, I would have received another email at midnight. I could have just told the Downsized_0421110938a manufacturing folks to put a yellow LED in that spot, but the yellows are for a display and the red is a power-on indicator so I got my red LEDs and delivered them to receiving.

This morning I got my assembled boards all nicely wrapped in anti-static bags along with all my leftover parts in their original packaging. Next step, get some batteries and power them up.

Duane Benson
Thor, Dog of Thunder, is not allowed

http://blog.screamingcircuits.com/