Wow,

What an interesting, enlightening and lively subject.

I too used to recommend putting "mechanical" components on the schematic because it was the most straightforward way of ensuring necessary other parts - screws, washers, nuts, heatsink pads and grease, and for some products even the enclosure and the blank board itself got into the BOM.

There are some very good points raised here about this - the BoM really *should* be considered a design SOURCE but most CAD (all that I have seen, at least) treat it as a generated OUTPUT.

That's changing now as we realize you may need to account for many other costs and materials that go into the assembled board (or whole finished product!). Previously, we were even getting to the point of including graphical components on our schematics representing the final packaging (i.e. the printed cardboard box and foam) that our development boards were being shipped to customers in. Partly this is because we needed to cost-engineer the design from the beginning. While this actually worked surprisingly well, managing the data integrity is compromised somewhat by this approach, because fixing a package design flaw meant a whole new revision to an otherwise unchanged schematic.

So, it's best to add the BOM as a design document (at least in principle) perhaps near the beginning of the design cycle. It's probably wise to treat the final assembled board as just that, not including other packaging or mech assembly - which noted should be driven by MRP systems and mechanical CAD tools (well, unless you're a cottage manufacturer and wish to reduce the number of tools in use).

I believe most CAD packages should allow you to at least link to an Excel spreadsheet if that's what you're using, or as the case with Altium there's a dedicated Active BOM which is a source document and can include non-board items such as test and tooling costs.

- Ben.

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