Building a BOM : The 7 Optional Fields

In the previous article in this BOM Building series, I listed the 5 bare minimum fields required for a proper BOM (Bill of Materials). The goal of any BOM is to accurately represent the discrete parts of your product with sufficient detail to procure them - no more, no less. While only 5 fields are required to accomplish this, the addition of the following optional fields helps make your BOM more robust and efficient for a procurement team to do their job. 



  1. Distributor
  2. Distributor Part Number
  3. Manufacturer Standard Lead Time
  4. Manufacturer Part Datasheet URL
  5. Reference Designator (Required for PCBA electronic parts)
  6. Alternates Allowed
  7. Alternates Criteria


Not all parts can be purchased directly from the original manufacturer. In fact, even at modest volumes, most manufacturers prefer you purchase through their distribution channels. This may include several different Distributors to choose from. For these fields, select your preferred Distributor and their respective Distributor Part Number (DPN). 

Distributor Name

Most off-the-shelf parts are sold through distributors, and in some cases, several competing distributors at once. This field lists the name of your preferred distributor from whom to procure the associated part. If the part is custom, listing the Manufacturer as the Distributor is common. I encourage you to consolidate your list of unique distributors to a minimum to make ordering parts much simpler.

Distributor Part Number (DPN)

Much like how a company assigns a CPN (Company Part Number) to a part to uniquely identify it in their master parts library, a distributor may assign their own part number to uniquely identify it in their part catalog. Including this field streamlines the process by saving the procurement team time from having to search for places to purchase the part and requesting approval. 



In the previous blog post I cautioned about why you should not list part Pricing and Lead times in your BOM. The one exception is to include the Manufacturer Standard Lead Time. This field represents the worst-case amount of time the Manufacturer promises to deliver you parts upon order when Distributors don’t have sufficient stock. This field is valuable for planning purposes once you get to higher volume Mass Production as you may be purchasing more parts than any Distributor normally carries in stock. It also gives you a drop dead deadline of when to place parts orders to meet your production schedule.

But, this value is to be taken as a worst case scenario, not an actual lead time (i.e. it’s a variable). I'm still sticking to my guns about not including Pricing in your BOM, though.



A manufacturer's datasheet has all the detailed specifications for a part, well beyond what is stored in the BOM. The datasheet has value to the procurement team to assist in validating if an alternate part is parametrically identical to the original and can be approved as a viable alternate. I’ve found it actually has the most value to an engineer or QA staff to assist with testing and debugging a product. It’s convenient to link datasheets to all parts in the BOM, as it’s in one location and easy to search.



This is a unique label for each instance of a part in a specific BOM assembly. This is actually a required field for any Printed Circuit Board Assembly BOM. This Ref Des field can also be used for mechanical parts to support assembly documentation, but it is not common, nor required.



This is rarely seen, but one I highly recommend incorporating into your engineering teams’ workflow. It’s a Boolean field ({True, False} or {Yes, No}) to inform the procurement team if they can substitute an alternate part for the current one listed in the Source Details. 

The power of this field comes to play when you are enlisting a Contract Manufacturer (CM) to procure your parts. The CM has a much richer Roledex than you of sources for alternate parts with potentially better pricing and lead times. You need to help them help you by empowering them to do their job and find alternates to reduce production costs and lead times. Without this field in your BOM you will undoubtedly get a series of emails or phone calls from your CM asking questions and approval for replacing individual parts. 

What it means:

  • True/Yes: The CM is allowed to unilaterally find equivalent alternate parts, referencing the Alternates Criteria you specify
  • False/No: The CM is not allowed to use any other part. Only the specific Manufacturer Part Number is allowed to be purchased. This may be because it’s sole-sourced, or you have separately negotiated favorable terms with the Source, or your engineering team has already done the homework and there is no viable alternate that will meet the requirements.



If you do elect to allow the Procurement team to use alternates for a part, you need to let them know which parametrics are important to match, and which are ok to deviate from. 

For example, if you originally picked a resistor with 10k resistance, 0402 package, and 1% tolerance, an experienced procurement team will already know they must match the 10k, 0402 parameters, but is the 1% tolerance important? Can they select a 10k, 0402 with 5% tolerance part? Or perhaps the operating temperature is what’s important to match. Without listing the specific criteria to match to the original Source, they will be too constrained and may be limited in alternate part options. 

I've mentioned several times that pricing should never be included in a spreadsheet BOM. I'll explain why in the next article in this Building a BOM series.

What fields do you include in your BOMs? I'd love to hear your thoughts...