Multi-Level BOM (Bill of Materials) in Manufacturing
In a manufacturing environment where final products are built using intermediate sub-assemblies, a structured BOM hierarchy is required to manage production efficiently. This approach is known as a Multi-Level BOM, where sub-assembly BOMs are first defined and then linked to the BOM of the final finished product.
This structure ensures that complex products are broken down into manageable manufacturing stages, improving planning accuracy and material visibility across production levels.
Understanding Sub-Assembly BOM Structure
In a multi-level BOM system, each sub-assembly is created as an independent BOM before being linked to the final product BOM. These sub-assembly BOMs are then referenced within the main BOM under the Materials table.
For example, a Brush Bristles sub-assembly BOM is linked inside the BOM of a Shaving Brush final product. This linkage is visible in the Materials section of the Bill of Materials master.
The Materials table displays only sub-assembly items, while ERPNext maintains the hierarchy internally to support full production breakdown and material planning.
Materials Table vs Exploded View
ERPNext provides two different views of BOM structure to simplify manufacturing analysis:
- Materials Table: Displays only sub-assembly items directly used in the current BOM level.
- Materials Required (Exploded): Expands all sub-assemblies to show complete raw material requirements.
This distinction helps planners understand both the hierarchical structure and the total raw material consumption required for production.
Materials Table View (Sub-Assemblies Only)
The Materials table shows only immediate sub-assembly components that belong to the current BOM level.
Example view:
Exploded BOM View (Raw Materials Only)
The exploded view expands all nested BOMs and displays only raw materials required to manufacture the final product.
Enabling Multi-Level BOM in Work Orders
To utilize multi-level BOM functionality during production planning, the Use Multi-Level BOM option must be enabled in the Work Order.
The “Use Multi-Level BOM” checkbox is enabled by default. It ensures that sub-assembly planning is automatically included when generating material requirements for production.
If sub-assemblies are manufactured separately, this option can be disabled to prevent automatic explosion of nested BOM structures.
Example: Computer Assembly with Sub-Assemblies
A real-world example of multi-level BOM can be seen in computer manufacturing, where multiple sub-assemblies are produced independently before final assembly.
- Personal Computer (Finished Goods)
- Motherboard
- SMPS
- Accessories and Wires
- Hard Disk (Sub-Assembly)
- Item A
- Item B
- Item C
- DVD Drive (Sub-Assembly)
- Item X
- Item Y
- Item Z
This structure allows each sub-assembly to be independently manufactured, tested, and updated without disrupting the entire production process.
Caution: Impact of BOM Changes in Sub-Assemblies
Changes in sub-assembly BOMs can have a cascading effect on higher-level BOM structures. Any modification in a low-level component may require updates across multiple dependent BOMs.
Updating a sub-assembly BOM does not automatically update parent BOMs. Each dependent BOM must be manually revised to reflect the latest changes.
For example, if a component in a Hard Disk sub-assembly is changed from Item C to Item D, a new BOM version must be created and linked appropriately. Parent BOMs such as the Personal Computer BOM must also be updated separately to reflect this change.
Version Control in Multi-Level BOM
To manage changes effectively, new BOM versions should be created whenever sub-assemblies are updated. This ensures production accuracy and prevents inconsistencies in exploded material calculations.
- BOM-HDD-001 → Original Hard Disk structure (Item A, B, C)
- BOM-HDD-002 → Updated Hard Disk structure (Item A, B, D)
- BOM-PC-002 → Updated PC BOM referencing BOM-HDD-002
Key Summary
Multi-Level BOM enables structured manufacturing by linking sub-assembly BOMs within a final product BOM. It provides clear separation between hierarchical manufacturing stages and raw material requirements. However, changes in sub-assemblies require careful version control to ensure consistency across all dependent BOM structures.
