The above schematic includes component values, reference designators, part numbers (where appropriate), signal names, and connector pin numbers. However, to actually buy the parts and build the circuit (particularly if someone else is going to buy and build), you need more information. A bill of material (BOM) is a complete list of parts in your project. For example, for the above circuit (again, click on the table for a larger (more readable) version).
As shown here, the BOM must include
- Line number (for reference)
- Manufacturer's part number (complete with all suffixes)
- Description (standardized, see below)
- Package (form factor or layout-footprint cell name)
- Reference designator(s)
- URL or filename of datasheet (or a copy in the documentation zip file; the PDF version of this BOM includes links to datasheets)
Optional, but helpful for small production runs, are columns including retailer, retailer stock number, and cost (for example, item number 4, Digikey, 478-1751-1-ND, $0.77 each).
Getting the manufacturer’s part number correct (complete with all suffixes) is really important. Here's the part-numbering table out of a Texas Instruments data sheet.
Not only does part number determine the package of the component (like SOIC or TSSOP), but it also determines the number of parts in a shipping package (tube of 25 or reel of 2500). Note the embarrassment that you would suffer if you needed ten tubes of SN74HCT00D, but accidentally ordered ten of SN74HCT00DR (or vice versa).
For volume production, you also may want to include information such as
- Is this part substitutable?
- Minimum order quantity
- Lead time for delivery
- Internal tracking number
- Part 1: The Quotation (or, How to Make a BOM)
- Part 2: On Design for Manufacturing
- Part 3: Industrial Design for Startups
- Part 4: Picking (and Maintaining) a Partner