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How to Compile a Bill of Materials What, exactly, goes into your assembly?

If your company manufactures something, whether it is cameras or coffee makers, satellites or slot machines, chances are it uses a bill of materials or parts list of some kind along the way. A bill of materials (often referred to by the acronym BOM) is simply a very specific shopping list that tells somebody elsewhere in the company exactly what to purchase and how much.

Steps

1. Understand what it is you're building. While you can start to compile a BOM as you go during the early phases of a design, its completion will probably be one of the last steps toward releasing the documentation.
2. Even the little things count. List the parts in your assembly. Be thorough and complete, even if you think it is obvious what goes in or boring.
3. Assign part numbers to each item in your assembly. If your company keeps a database of part numbers, it is often worthwhile to reuse standard parts such as fasteners, connectors, resistors and other small, purchased items. Avoid creating multiple part numbers for the same part.
4. Make sure each of the part numbers you will use has some specification, drawing, data sheet, etc. that allows it to be purchased or fabricated.
5. Assign item numbers. Many database programs require you to assign numbers to the items in a BOM, or they may assign item numbers for you.
6. List exact quantities of each item on your BOM. Be sure the quantity is consistent with the unit of measure in which an item is purchased (each, feet, meters, ounces, etc.)
7. Include reference designators if appropriate. The L201 alongside the inductor in this photo is an example of a reference designator.
8. Make sure the BOM corresponds exactly to the assembly documentation, whether that is an assembly drawing, schematic, etc.
9. Include the assembly documentation or a link to it in the bill of materials. How this happens, exactly, should depend on your database and document control system.

Tips

• Consider over-kitting small parts. If you don't know exactly how many wire ties an assembly will require, you could specify a few extras. Ask somebody in purchasing, document control, or manufacturing how best to handle your extras. Your company may prefer to add a note, add extras directly to the quantity, specify an over-kitting percentage, or simply keep spares in stock. If an assembler can't figure out where to put the three extra screws you specified, she may get very confused.
• While it is not the direct purpose of a BOM, a BOM is also a good way to track the weight, materials cost, and other attributes of an assembly. Just remember that you get out information that is no more accurate than what you put in.
• Depending on your system, it may not be necessary to list individually the parts of an assembly that is purchased. That is, if you send somebody a weldment drawing and get back a welded frame, you may need to list the raw materials for the frame only on the drawing and only in such detail as is necessary to weld the frame.
• Don't forget such items as labels, packaging, and silkscreens.

Warnings

• Don't expect the people purchasing or making parts for you to read minds. The more clearly you can document your assembly, the more likely you are to get all the right parts ordered and assembled correctly.