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How to Build a BOM (Bill Of Materials)

by: Jun 07,2018 10878 Views 1 Comments Posted in PCB Assembly

BOM Bill of materials PCB Assembly BOM Example

Chefs create recipes. Hardware Engineers create BOMs (Bill of Materials). The BOM is an ingredients list for building your product.

What is a Bill of Materials (BOM)?

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a Bill Of Materials (BOM), which serves as the foundation for building the product. It’s a fundamental set of data, used throughout the process. A well constructed BOM clearly communicates all of the technical ingredients required to build the product.

From this information, it’s possible to determine:

  • Lead-time to procure the materials

  • Cost of Goods Sold (COGS)

  • Manufacturing processes

  • Quality of the Design for Assembly

  • Supply chain robustness

  • Cash flow requirements

A bill of materials (also known as a BOM or bill of material) is a comprehensive list of parts, items, assemblies and other materials required to create a product, as well as instructions required for gathering and using the required materials. All manufacturers building products, regardless of their industry, get started by creating a bill of materials (BOM).


Because the bill of materials pulls together all sorts of product information, it is common that several disciplines (design and engineering, document control, operations, manufacturing, purchasing, contract manufacturers and more) will consume data contained within the BOM record to get the job done right. In fact, engineers and manufactures rely so heavily on BOMs they their own special subsets called the engineering bill or materials (EBOM) and the manufacturing bill of materials (MBOM). The BOM guides positive results from business activities like parts sourcing, outsourcing and manufacturing, so it is important to create a BOM that is well organized, correct and up-to-date.

 

What to include in an effective bill of materials

 

An effective BOM doesn’t have to be generated by sophisticated software — it can be as simple as an Excel spreadsheet.

BOMs are usually broken up into several categories .

Categorization makes it easy for you, your colleagues, and vendors to find what they need in your BOM. For example, the PCBA (printed circuit board assembly) manufacturer only cares about the PCBs and the components that need to be surface mounted onto the boards — the packaging components usually don’t need to be procured for the early proto-builds.


Because one of the main functions of the BOM is to ensure that the product is built right, it is best to include specific pieces of product data in the BOM record. Whether you are creating your first bill of materials or are looking for ways to improve how you create a bill of materials, here is a high level list of information to include in your BOM record:

  • BOM Level—Assign each part or assembly a number to detail where it fits in the hierarchy of the BOM. This allows anyone with an understanding of the BOM structure to quickly decipher the BOM.  

  • Part Number—Assign a part number to each part or assembly in order to reference and identify parts quickly. It is common for manufacturers to choose either an intelligent or non-intelligent part numbering scheme. Whichever scheme you use, make sure you avoid creating multiple part numbers for the same part.  

  • Part Name—Record the unique name of each part or assembly. This will help you identify parts more easily.  

  • Phase—Record what stage each part is at in its lifecycle. For parts in production, it is common to use a term like ‘In Production’ to indicate the stage of the part. New parts that have not yet been approved can be classified as 'Unreleased' or 'In Design'. This is helpful during new product introduction (NPI) because it allows you to easily track progress and create realistic project timelines.  

  • Description—Provide a detailed description of each part that will help you and others distinguish between similar parts and identify specific parts more easily.  

  • Quantity—Record the number of parts to be used in each assembly or subassembly to help guide purchasing and manufacturing decisions and activities.  

  • Unit of Measure—Classify the measurement in which a part will be used or purchased. It is common to use ‘each’, but standard measures like inches, feet, ounces and drops are also suitable classifications. Be consistent across all similar part types because the information will help make sure the right quantities are procured and delivered to the production line.  

  • Procurement Type—Document how each part is purchased or made (i.e. off-the-shelf or made-to-specification) to create efficiencies in manufacturing, planning and procurement activities.  

  • Reference Designators—If your product contains printed circuit board assemblies (PCBAs), you should include reference designators that detail where the part fits on the board in your BOM. Capturing this information in the BOM can save time and help you avoid confusion down the road.  

  • BOM Notes—Capture other relevant notes to keep everyone who interacts with your BOM on the same page.

Documenting all this information in your BOM will keep business activities and manufacturing tasks on target.

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Approved manufacturers

A proper factory will require the allowed manufacture(s) to be specified for every part. This is frequently referred to as the “AVL”, or Approved Vendor List. A manufacturer is not a distributor (i.e., Digikey, Mouser, Avnet); a manufacturer is the actual company that makes the part. A capacitor, for example, could be made by TDK, Murata, Taiyo Yuden, AVX, Panasonic, Samsung, etc. You’ll be surprised how many times I’ve reviewed a BOM listing “Digikey” or some other distributor as the manufacturer for a part.

While it may seem silly to trifle over who makes a capacitor, there are definitely situations in which the maker of a component matters – even for the humble capacitor. For example, blindly substituting the filter capacitors on a switching regulator, even if the substitute has the same rated capacitance and voltage, can lead to unstable operation and even boards catching fire.

Of course, there are times when one is truly insensitive to the manufacturer, in which case I would mark on the BOM “any/open” for the AVL (particularly true for things like pull-up resistors). This invites the factory to suggest their preferred supplier on your behalf.


Tolerance, composition, and voltage spec

For passive components that are marked as “any/open”, there are some key parameters that should always be specified in a BOM to ensure the right part is purchased:

  • For resistors, at a minimum the tolerance and wattage should be specified. A 1k, 1% 1/4W carbon resistor is very different beast from a 1k, 5% 1W wirewound resistor!

  • For capacitors, at a minimum the tolerance, voltage rating, and dielectric type should be specified. For special applications, certain parameters such as ESR or ripple current tolerance also need to be specified. A 10uF, electrolytic, 10% 50V capacitor has vastly different performance at high frequencies compared to a 10uF, X7R [ceramic], 20% 16V capacitor.

  • Inductors are sufficiently specialized that it’s not recommended to ever leave them as “any/open”. For power inductors, core composition, DCR, saturation, temperature rise current, are the basic parameters, but there is also no standard for casing like there is for resistors and capacitors. Furthermore, important parameters such as shielding and potting, which can have material impacts on the performance of a circuit, are often implicit in a part number; hence, it’s best to simply fully specify the inductor and not leave it any/open. The same goes for RF inductors.

Electronic component form factor

It’s always important to fully specify the form factor, or “package type”, of a component. Poorly specified or under-specified package parameters can lead to assembly errors. Beyond the basic parameters such as the EIA or JEDEC package code (0402, 0805, TSSOP, etc.), here are some other things to consider:

  • For SMT packages, the height of a component can vary, particularly for packages larger than 1206, or inductors. Pay attention if the board is slotting into a tight case.

  • For through-hole packages, lead pitch and component height should always be specified.

  • For ICs, try to specify the common name that corresponds to the package, not just the manufacturer’s internal code (for example, a TI “DW” type package code corresponds to SOIC). It’s a good consistency check that can guard against errors.


Extended part numbers

Designers often think using abbreviated part numbers. A great example of this is the 7404. The venerable 7404 is a hex inverter, and has been in service for decades. Because of its ubiquity, the term “7404” can be used as a generic term for an inverter. However, when going to production, things like the package type, manufacturer and logic family must be specified. A complete part number might be 74VHCT04AMTC, which specifies an inverter made by Fairchild Semiconductor, from the “VHCT” series, in a TSSOP package, shipped in tubes. The extra characters are very important, because small variations can lead to big problems, such as quoting and ordering the wrong packaged device (and subsequently being stuck with a reel of unusable parts), or subtle reliability problems. In fact, I encountered a problem once due to a mistaken substitution of a “VHC” for the “VHCT” logic family part. This switched the input thresholds of the inverter from TTL to CMOS logic-compatible, and resulted in some units having an asymmetric response to input signals. Fortunately, I caught this problem before production ramped, avoiding a whole lot of potential rework or worse yet, returns.

Here’s another example of how missing a couple of characters can cost thousands of dollars. A fully specified part number for the LM3670 switching regulator might be LM3670MFX-3.3/NOPB. Significantly, if the /NOPB is omitted, the part number is still valid and orderable – but for a version that uses leaded solder. This could be disastrous for products exporting to a region, such as the EU, that requires RoHS compliance (meaning lead-free, among other things). A more subtle issue is the “X” in the part number. Part numbers with an “X” come with 3,000 pieces to a reel, and ones lacking an “X” come in 1,000 pieces to a reel. While many factories will question the /NOPB omission (since factories typically assemble RoHS documentation as they purchase parts), they will rarely flag the reel quantity as an issue. However, you care about the reel quantity because if you only wanted 1,000 pieces, including the X in the part number means you’ll be paying for 2,000 extra pieces you don’t need. Or, if you’re doing a much larger production run and you omit the X, you could be paying a premium for shipping three times the volume of reels for the same purchase quantity. Either way, the factory will quote the part exactly as specified, and you could be missing out on a cost savings if you’re not paying attention to the reel quantities.

The bottom line is that every digit and character counts, and lack of attention to detail can cost real money!

 

7 key BOM Creation Tips

  1. Will you document consumables in your BOM record? Many manufacturers second-guess the decision to include glue, wires, fasteners and other non-modeled parts like labels and boxes in their BOM record. But if the part does not make it into your BOM, it might not make it into your product. So take the time to document these parts.  

  2. How will you attach files to your BOM record? As you create your BOM, keep records of supporting documentation like CAD drawings, part datasheets and work instructions. It is best to also associate these files with their specific BOM level items.  

  3. Double-check all information: Before uploading or sending your BOM to a manufacturer, check it meticulously to make sure all the included information is correct and you can navigate the document the way you intended. Even relatively small mistakes can be costly when it comes to PCB assembly.

  4. ? Keep track of changes: Ensure you have a system in place to keep track of all changes to the document. You may want to keep a record of all previous versions in case you want to refer back to them. Just make sure each version is clearly labeled and that the most up-to-date one is always the one in use for the project.

 

? Tailor your BOM to your needs: Before you create your BOM, decide what you and your partner companies need from it and design it to meet those needs. Different companies do BOMs in different ways, and different projects require different organization, information and other features. Customize your BOM according to the unique demands of your project.


? Use a template: Each PCB manufacturer and assembler has BOM templates you can obtain from their sales departments or websites before you request a quotation. Using a template from the company with which you're working helps avoid problems with accessing and opening the BOM document and technical issues caused by incompatible file types.


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? Include the right amount of details: Include as much helpful information as you can in your document. Remember, your BOM plays a crucial role in explaining your PCB assembly to people who have to build it entirely from scratch. These people may have never spoken to you in person, and they might not even speak the same language. For this reason, you need to make your BOM as clear, detailed and precise as possible. Some types of information aren't helpful and will only create confusion, but use your best judgment on this. In general, the more details, the better.

 

An accurate BOM supports efficient manufacturing processes

Creating a bill of materials is not only a necessary step in the product development process, it is also what makes your product design a reality. Before you create a BOM record, it is important to consider who will utilize the information and how you will maintain and manage all associated product documentation like part datasheets and CAD files. Develop more efficient manufacturing practices by capturing detailed part information when creating a bill of materials.

 

 

BOM Example

 

Here's an example of part of a BOM that includes some of the information mentioned above. You'll notice the sheet is clean, organized and clearly labeled, and that information is noted consistently throughout it. This is just an example BOM list, and yours will likely be longer and contain more information.

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In the far left column of the bill of materials example above, you'll see the name of the manufacturer. Next to that is the manufacturer's part number, the quantity and a description of the item with units. After that is the case or package type. Next is the placement method. This BOM provides the option of  PCBWay
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