Bill of Materials (BOM)

1. What is a BOM?

A Bill of Materials defines:

• All components (raw materials, sub‑assemblies) needed to make a product.
• Quantities of each component (e.g., 4 wheels per car).
• Hierarchical relationships (if multi‑level).
• Reference designators (where components go).

The BOM is used by MRP to calculate material requirements, by costing to determine product cost, and by production to guide assembly.

2. Types of BOM

Type	Description	Use case
Single‑level	Lists only components directly needed for a product, without sub‑assemblies.	Simple products, high‑level planning.
Multi‑level	Shows hierarchical structure with sub‑assemblies and their components.	Complex products (e.g., cars, electronics).
Engineering BOM	Created by engineering, based on CAD/design. Often different from manufacturing BOM.	Product development.
Manufacturing BOM	Used on shop floor – includes all items needed for production.	Production execution.
Phantom BOM	Transient sub‑assembly that is consumed immediately.	Intermediate steps not stocked.
Template BOM	Generic BOM for configurable products (e.g., with options).	Configure‑to‑order.

3. Single‑level BOM

A single‑level BOM lists only the components directly needed for a product. Example for a "Desk Lamp":

This is simple but doesn't show that the wiring kit itself has components (wire, plug, switch).

4. Multi‑level BOM

A multi‑level BOM shows the hierarchical structure. The same desk lamp with a wiring kit sub‑assembly:

MRP "explodes" the multi‑level BOM to calculate requirements for all levels.

5. Engineering vs Manufacturing BOM

Engineering BOM (EBOM): Created by product design teams, often from CAD systems. It reflects how the product is designed, which may differ from how it's built.

Manufacturing BOM (MBOM): Used on the shop floor. It includes all items needed for production, such as consumables, packaging, and instructions. It may also reflect assembly sequence.

ERP systems often need to manage both and synchronize changes (engineering change orders).

6. Phantom BOM

A phantom BOM represents a sub‑assembly that is consumed immediately in the next production step and never goes to stock. Example: In furniture manufacturing, you might assemble a drawer (sub‑assembly) and immediately attach it to a desk. The drawer is never stored as inventory.

Benefits:

• Simplifies planning – MRP plans the components directly.
• Avoids unnecessary inventory movements.
• Common in repetitive manufacturing.

7. BOM in MRP

Material Requirements Planning (MRP) uses the BOM to calculate component requirements. The process is called BOM explosion:

1. MRP starts with demand for finished goods (sales orders, forecasts).
2. It looks at the BOM for each finished good.
3. It multiplies demand by component quantities to get gross requirements.
4. It subtracts inventory and scheduled receipts to get net requirements.
5. It repeats for each level of multi‑level BOM.

8. Best practices

• Maintain accuracy: BOM errors cause production delays and stock issues.
• Use consistent units of measure: Ensure component quantities are in the right UOM (each, kg, meter).
• Manage engineering changes: Have a formal process for BOM updates.
• Structure for planning: Design BOM levels to match your manufacturing process.
• Include all components: Don't forget packaging, labels, consumables.
• Regular audits: Cycle‑count BOM accuracy by comparing to physical builds.