4.1 System BOM and Mechanical BOM (MBOM)

A missing $0.02 screw stops a production line just as effectively as a missing $50.00 processor. While Engineering teams obsess over the PCBA Bill of Materials, the Mechanical BOM (MBOM) is often relegated to drawing notes or loose spreadsheets. This negligence creates the "99% complete" trap: the electronics work, but the product cannot be assembled, sealed, or shipped. Treat the MBOM with the same data integrity and revision control rigor as the PCBA BOM.

The System BOM Scope

Do not hide parts in drawing notes. If a component is required to build the final SKUs, it must exist as a discrete line item in the System BOM.

If the item is physically present in the shipping box ⭢ Then it requires a unique Part Number (PN) and Quantity.

Include the following categories:

1. Enclosure & Structural: Plastics, machined metal, die-cast parts, brackets, chassis.
2. Fasteners & Hardware: Screws, nuts, washers, standoffs, inserts.
   • Requirement: Define material (e.g., SS304), finish (e.g., Black Oxide), and locking features (e.g., Nylon patch).
3. Interconnects: Internal cables, flat flex cables (FFC), wire harnesses, antennas.
4. Thermal & Sealing: Thermal pads, heatsinks, O-rings, gaskets, EMI shields.
5. Chemicals & Consumables: Adhesives, thermal paste, threadlockers, tape.
   • Control: Assign a PN to the dispensable unit (e.g., 50ml cartridge) and estimate usage per unit (e.g., 0.2ml) for costing, or kit as "shop supplies."
6. User Interface: Light pipes, buttons, overlays, display windows.
7. Packaging & Accessories: Unit box, foam inserts, ESD bags, user manuals, warranty cards, labels, master cartons.

Pro-Tip: Create specific Part Numbers for "kitting consumables" like zip-ties or kapton tape. If they are not on the BOM, the line will run out, and operators will improvise with non-compliant tape.

Alternates Policy: The "Looks Similar" Trap

Mechanical alternates pose a higher risk than electronic passives because parameters are rarely standardized. A "compatible" screw from a different vendor may have a slightly different head height that cracks a display glass.

Rule: Do not apply generic "Form-Fit-Function" rules to custom mechanical parts.

Approval Logic

• If component is Custom Mechanical (Molded/Machined):
  • Action: No alternates allowed without First Article Inspection (FAI) and physical fit-check.
  • Reason: Tooling tolerances vary. A 0.1 mm variance can cause interference.
• If component is Commodity Hardware (Screws/Washers):
  • Action: Alternates permitted only if Material, Finish, Thread Class, and Drive Type match exactly.
  • Risk: A Zinc-plated screw will corrode where a Stainless screw was specified.
• If component is Chemical (Adhesives/Grease):
  • Action: No alternates without Engineering approval.
  • Reason: Cure times, outgassing properties, and thermal conductivity are specific to the formulation.

Output Requirement: The Unified MBOM

Generate a single, flat, or hierarchical BOM file that the EMS can import into their ERP system. Do not rely on CAD assembly trees or drawing balloons.

• Format: CSV or Excel.
• Structure: Level (indentation), Part Number, Description, Manufacturer, MPN, Quantity Per Unit, Reference Designator (if applicable).
• Audit: The MBOM must match the exploded view drawings 1:1.

Pro-Tip: For packaging materials (manuals, labels, boxes), set the revision strictly. A regulatory change in the manual text requires a new Part Number or Revision, otherwise, the factory will pack old manuals.

Final Checklist