Bill of Materials (BOM)

BOMs can be structured as single-level or multi-level. A single-level BOM lists only the direct components of an assembly — for example, a bicycle frame assembly BOM would list the frame tube, headset, bottom bracket, and seat post. A multi-level BOM (also called an indented BOM) shows the complete product structure from finished product down to purchased raw materials, with each sub-assembly exploded into its own components. Multi-level BOMs are essential for MRP because they allow the system to calculate requirements at every level and net them against available inventory. Common BOM types include: Engineering BOM (EBOM) — created by product design, reflecting how the product is designed; Manufacturing BOM (MBOM) — reflecting how the product is actually built (which may differ from the EBOM due to manufacturing process requirements); Service BOM — listing replaceable parts for after-sales support; and Configurable BOM — used for products with options and variants, where rules define which components are included based on customer selections.

The BOM directly influences production scheduling by defining the dependency structure of the manufacturing process. A multi-level BOM shows which sub-assemblies must be completed before final assembly can begin, creating the scheduling logic that determines operation sequencing. If a finished product requires three sub-assemblies, and Sub-assembly A takes 2 days while Sub-assembly B takes 5 days, the scheduler knows that B must start at least 3 days before A to ensure both are ready for final assembly simultaneously. This relationship between BOM structure and scheduling is called the bill of materials explosion in time — MRP uses it to calculate planned order release dates. In visual scheduling tools like LinePlanner, planners can represent these dependencies by scheduling upstream operations on earlier shifts or days, ensuring material flow aligns with assembly requirements. The BOM also determines changeover requirements: if consecutive scheduled products share most of their BOM components, changeover between them will be faster than between products with completely different BOMs.

BOM accuracy is critical and requires ongoing discipline. Best practices include: Engineering Change Control — all BOM changes go through a formal review process to prevent unauthorized modifications. Cycle counting — regularly verifying that physical inventory matches BOM-based records to catch discrepancies early. BOM audits — periodically building a product while independently verifying every component against the BOM. Version control — maintaining revision history so that you can trace which BOM version was used for any production run. Cross-functional ownership — engineering owns the design intent, manufacturing owns the build sequence, and procurement owns supplier-related fields. Many manufacturers target 98%+ BOM accuracy as measured by the ratio of correct component records to total component records. Even 2% error rate means that for a product with 50 components, on average one component will be wrong — potentially causing a line stoppage, quality issue, or delivery delay.