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Bill of Materials (BOM): Types, Structure, ERP and MES
By Christian Fieg · Last updated: March 2026
What Is a Bill of Materials (BOM)?
A Bill of Materials (BOM) is a structured list of all materials, components, sub-assemblies, and quantities required to manufacture one unit of a finished product. It defines what goes into a product and in what quantity. In manufacturing, the BOM is the single source of truth for what needs to be procured, what needs to be assembled, and what the finished product consists of.
The BOM connects three core systems in a manufacturing company: The ERP system uses the BOM for material planning, procurement, and costing. The PLM/PDM system manages the BOM through its lifecycle from design to production release. The MES uses the BOM to control production execution: which components to install at which station, in which sequence, with which process parameters.
Types of BOM
| BOM type | Created by | Purpose | Content |
|---|---|---|---|
| Engineering BOM (eBOM) | Product engineering / R&D in the PLM or CAD system. | Defines the product as designed. Represents the functional structure of the product. | All components and sub-assemblies as designed. Part numbers, revision levels, material specifications. May include components that are purchased as assemblies. |
| Manufacturing BOM (mBOM) | Manufacturing engineering / process engineering in the ERP or MES. | Defines the product as manufactured. Represents the production structure: what is built where, in what sequence, on which machine. | All components in the order they are assembled. Work centers and production steps. Phantom assemblies (sub-assemblies that are not stocked but built inline). Process parameters per step. |
| Service BOM (sBOM) | After-sales / service engineering. | Defines the product as serviced. Lists replaceable parts and service kits. | Serviceable components, spare part numbers, replacement intervals, service instructions. |
| Sales BOM / Configurable BOM | Sales / order management in the ERP or configurator. | Defines the product as ordered by the customer. Maps customer options and variants to specific components. | Base product + optional components. Option codes mapped to component lists. Used in configure-to-order (CTO) and JIS/JIT production. |
The critical transition in manufacturing is from eBOM to mBOM. The engineering BOM says "the product contains these parts." The manufacturing BOM says "build this product at these stations in this sequence using these parts with these tools and these parameters." This translation is where many production problems originate: a design that looks simple on paper may require complex tooling changes, specific assembly sequences, or critical process parameter windows.
BOM Structure: Single-Level vs. Multi-Level
| Structure | Description | Example | Use case |
|---|---|---|---|
| Single-level BOM | Lists only the direct components of the finished product. One level deep. Does not show what goes into sub-assemblies. | Finished product: Door panel. Components: Carrier (1x), Trim cover (1x), Armrest (1x), Speaker grille (1x), Window switch panel (1x), Clips (12x). | Simple products with few components. Final assembly where sub-assemblies arrive pre-built. Procurement planning for top-level components. |
| Multi-level BOM (indented BOM) | Shows the complete hierarchy: finished product, sub-assemblies, sub-sub-assemblies, down to raw materials. Multiple levels deep. | Finished product: Door panel. Level 1: Carrier (1x). Level 2: PP granulate (0.8 kg), Insert molding (1x). Level 1: Armrest (1x). Level 2: Foam core (1x), Leather cover (1x), Stitching thread (2.5 m). | Complex products with multiple assembly stages. MRP (Material Requirements Planning) explosion. Complete cost calculation including all levels. Full traceability from finished product to raw material. |
| Flattened BOM | All components across all levels listed in a single flat list. No hierarchy shown. Quantities are calculated to the finished product level. | Door panel: PP granulate (0.8 kg), Insert molding (1x), Trim cover (1x), Foam core (1x), Leather cover (1x), Stitching thread (2.5 m), Speaker grille (1x), Window switch panel (1x), Clips (12x). | Procurement planning (total material requirements). Cost summaries. Weight calculations. |
What a BOM Contains
| BOM field | Description | Example |
|---|---|---|
| Item / Part number | Unique identifier for each component. Defined in the ERP master data. | 10045823-A (part number with revision). |
| Description | Human-readable name of the component. | Carrier, Door Panel LH, PP-GF30. |
| Quantity per unit | How many of this component are needed per finished product. | 1x (carrier), 12x (clips), 0.8 kg (granulate). |
| Unit of measure | Pieces, kilograms, meters, liters, etc. | PC (pieces), KG (kilograms), M (meters). |
| Level | Position in the BOM hierarchy. Level 0 = finished product. Level 1 = direct components. Level 2 = components of sub-assemblies. | 0 (door panel), 1 (carrier, armrest, trim), 2 (PP granulate, foam core). |
| Make or buy | Whether the component is manufactured in-house or purchased from a supplier. | Make (carrier is injection molded in-house). Buy (window switch panel is purchased). |
| Reference designator | Where the component is installed in the product. Used in assembly instructions. | Position A1 (top left clip), Position B3 (speaker grille mounting). |
| Revision / Version | Current revision level of the component. Critical for engineering change management. | Rev. A (initial), Rev. B (after design change), Rev. C (current production). |
BOM in ERP vs. BOM in MES
The ERP system manages the BOM as a planning and costing tool. The MES uses the BOM (or a derivative of it) as a production execution tool. These are different perspectives on the same product:
| Dimension | BOM in ERP | BOM in MES (Product Production Rule) |
|---|---|---|
| Purpose | Material planning (MRP), procurement, costing, inventory management. | Production execution: assembly instructions, process control, quality validation, traceability. |
| Structure | What: Components, quantities, and their hierarchical relationship. Routing: Work centers and operation times. | How: Station-by-station assembly sequence. Per-station instructions. Process parameters per operation. Dependency checks (Poka Yoke). Tooling and fixture requirements. |
| Granularity | Component level. "This product contains 12 clips." | Operation level. "At station 7, install clip in position A1. Scan barcode. Verify torque 3.5 Nm. Green light = proceed." |
| Variant handling | Variant BOM or configurable BOM with option codes. "If option X, add component Y." | Product production rule per variant. Complete assembly instruction set per variant. MES selects correct instructions automatically based on order/option code. |
| Update frequency | Updated with engineering changes (ECN/ECO). Weeks to months between revisions. | Updated whenever the production process changes. New tool, new station, new parameter limit, new inspection step. |
| Data flow | ERP sends production order with BOM data to MES. | MES maps the production order to the product production rule and executes it. MES reports back: quantities produced, material consumed, scrap, downtimes. |
In SYMESTIC, this separation is explicitly architected: The Product & Process Configurator, Segment Configurator, and Material Configurator define the MES-side representation of the BOM. The ERP sends the production order (with BOM/work plan reference) to SYMESTIC via interface (SAP R3 ABAP IDoc at Meleghy and Carcoustics, Navision file interface at Klocke, InforCOM bidirectional at Schmiedetechnik Plettenberg). SYMESTIC maps each production order to the corresponding product production rule and controls execution station by station.
BOM and Production Execution
In JIS/JIT (Just-in-Sequence / Just-in-Time) production, the BOM drives everything. An EDI order from the OEM contains a BOM code or transfer code that specifies the exact variant. The MES translates this code into a complete set of assembly instructions for every station on the line.
| Production step | Role of BOM | MES function |
|---|---|---|
| Order receipt | EDI order contains BOM code / option codes that define the specific product variant. "Build door panel variant LHD-Premium-Black-SpeakerUpgrade." | MES receives the order, maps the BOM code to the product production rule, and generates the station-by-station work plan for this specific variant. |
| Assembly | BOM defines which components are installed at each station. Different variants require different components, different torque values, different inspection criteria. | Worker instructions per station: text instructions, scanning instructions (verify correct component), torque instructions, picking instructions, pick-by-light control. All variant-specific, all BOM-driven. |
| Component validation (Poka Yoke) | BOM specifies the correct component for each position. If the wrong component is scanned, the station blocks. | Request/release control: MES checks scanned component against expected BOM component. Match = release. Mismatch = rejection. No wrong parts in the product. |
| Material consumption | BOM defines the planned material consumption per unit. Actual consumption is recorded during production. | MES records which material batches and serial numbers were actually consumed. Traceability links BOM (planned) to actual (consumed). Deviation = alert. |
| Traceability | BOM provides the reference structure: what should be in the product. Traceability records what is actually in the product. | Traceability per product: quality status (Poka Yoke), processing information, process data per segment, linked parts and material consumed, operator and authorization level. |
| ERP backflush | BOM determines the material backflush calculation: when a finished product is reported, the BOM quantities are deducted from inventory. | MES reports completed quantities per order back to ERP. ERP uses BOM to calculate material consumption and update inventory. |
BOM Challenges in Manufacturing
| Challenge | Impact | Solution |
|---|---|---|
| BOM accuracy | If the BOM does not match the actual product, material planning is wrong, assembly instructions are wrong, and traceability is unreliable. Estimated cost of BOM errors in manufacturing: 1% to 5% of production cost. | Single source of truth for BOM data (PLM/ERP). Automated BOM transfer to MES via interface. No manual BOM entry on the shopfloor. |
| Engineering changes (ECN/ECO) | A component is redesigned, but the old BOM version is still in production. Old and new components are mixed. Quality problems, customer complaints, potential recalls. | Revision-controlled BOM with effectivity dates. MES enforces the correct BOM revision per production order. Poka Yoke validates correct component at the station. |
| Variant explosion | Products with many options create thousands of BOM variants. Managing thousands of unique BOMs is impractical. | Configurable BOM with option logic (if option X, then component Y). MES product production rules that are dynamically assembled based on order option codes. |
| eBOM to mBOM translation | Engineering designs a product without considering manufacturing constraints. The mBOM requires phantom assemblies, alternative processes, and production-specific adjustments that the eBOM does not contain. | Dedicated manufacturing engineering that translates eBOM to mBOM. MES product/process configurator that adds station assignments, tooling, and parameters to the BOM structure. |
| Multi-plant BOMs | The same product is manufactured in multiple plants, but each plant has different machines, different tools, different material suppliers. The BOM (what) is the same, but the process (how) differs. | Global eBOM with plant-specific mBOMs. MES product production rules that are plant-specific. Central BOM management with local process adaptation. |
Frequently Asked Questions About Bill of Materials
What is the difference between a BOM and a routing?
The BOM defines what: which components, in what quantity, at what level. The routing defines how: which work centers, in what sequence, with what operation times. Together they form the complete production instruction. In ERP, BOM and routing are typically separate master data objects that are linked via the work plan. In MES, both are combined into a single execution instruction (product production rule) that tells the operator: at this station, install this component, with this tool, in this time, and verify with this check.
Who is responsible for maintaining the BOM?
The eBOM is maintained by product engineering (R&D). The mBOM is maintained by manufacturing engineering (process engineering / industrial engineering). The MES product production rule is maintained jointly by manufacturing engineering and the MES team. In practice, BOM accuracy problems almost always originate from poor handoff between these three groups. A clear change management process (ECN/ECO with approval workflow) is essential.
How does the BOM get from ERP to MES?
Via a standardized interface. The ERP sends the production order including BOM/work plan references to the MES. The MES maps the order to its internal product production rule and executes it. At Meleghy and Carcoustics, SYMESTIC receives production orders from SAP R3 via bidirectional ABAP IDoc interface. At Klocke, Navision ERP sends order data via file interface. At Schmiedetechnik Plettenberg, InforCOM sends production orders including work operations and machine assignments bidirectionally to SYMESTIC.
How many BOM variants can a product have?
In automotive JIS/JIT production, a single product (e.g., instrument panel) can have hundreds to thousands of variants based on customer options: left-hand drive vs. right-hand drive, navigation vs. no navigation, premium vs. standard, leather vs. fabric, speaker upgrade vs. standard. Each combination requires a different set of components. Without a configurable BOM and MES-based variant control, managing this complexity is impossible. SYMESTIC handles this through product production rules that are dynamically assembled from option codes in the EDI order.
Can BOM errors cause OEE losses?
Yes, directly. A wrong BOM causes wrong material at the station (line stop to correct), missing components (line stop to procure), wrong assembly sequence (rework or scrap), and wrong process parameters (quality defects). Each of these reduces availability (stops), performance (rework time), or quality (scrap). Accurate BOM data that flows automatically from ERP to MES without manual re-entry is a prerequisite for stable OEE.
About the author:
Christian Fieg
Head of Sales at SYMESTIC. Six Sigma Black Belt. Over 25 years in the manufacturing industry. Global MES and traceability responsibility for 900+ machines at Johnson Controls.
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