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Manufacturing Bill of Materials (MBOM) vs Engineering BOM
A manufacturing bill of materials lists how a product is built, not how it was designed. MBOM vs EBOM explained with a worked example from a real BOM.
by the HardwareMap editors4 min read
Contents
A manufacturing bill of materials (MBOM) lists everything required to build a product, organized by how the factory builds it. An engineering BOM (EBOM) lists the same product organized by how it was designed. Same parts, different structure, different owner. This note converts the EBOM from our bill of materials example into an MBOM, line by line, so you can see what changes.
EBOM vs MBOM at a glance
Siemens defines the MBOM as the list of materials and components needed to manufacture a product, organized around how it is assembled rather than how it is designed (Siemens on MBOM). PTC describes the EBOM as the structure created by engineering at the start of design and updated as the design changes (PTC on EBOM).
PTC's own Windchill documentation treats the two as separate structures, the as-designed and the as-planned, and maintains links between them so quantities and changes stay tracked (Windchill help). That link is the whole game. Two BOMs that drift apart produce boards built to the wrong revision.
From EBOM to manufacturing bill of materials: a worked example
The sensor node in our BOM example has a three-level EBOM: finished good, PCBA, components. Here is what a contract manufacturer typically turns it into. Op numbers are operations on the line; each one consumes the lines beneath it.
Four kinds of change happened. Lines were added (paste, firmware, carton). Lines were regrouped (the PCB became a panel, the PCBA became an output of Op 40). Process steps became first-class rows. Quantities picked up scrap and pack factors. Nothing in the design changed.
ERP systems model exactly this. Odoo's manufacturing docs, for example, describe a BOM with a Components tab, an Operations tab bound to work centers, and by-products (Odoo BoM configuration). Your CM's system will have the same shape under different names.
When a startup needs a separate MBOM
You need one the moment someone other than you builds the product. Before that, a single sheet with process lines (as in our example) is enough. The triggers:
- You hand off to a contract manufacturer. The CM builds the MBOM in its ERP whether you ask or not. Ask to see it. Reviewing it is how you catch a wrong revision, a missing alternate or a padded attrition factor. See choosing a contract manufacturer for what to ask before signing.
- You build in two places. A pilot line in-house and volume at a CM means two MBOMs from one EBOM.
- Firmware ships with hardware. A firmware version that is not on a BOM cannot be traced to a serial number when something fails in the field.
- You sell kits or variants. Colorways, regional plugs and bundles are MBOM and packaging changes on one EBOM.
When you review a CM's MBOM, check three columns first: the revision of every custom part against your EBOM, the attrition factor on each reeled part, and any line you do not recognize. An unrecognized line is either a consumable you forgot or a cost you did not agree to. Both are worth a question before the purchase order goes out.
The prototype to production guide places this handoff in the wider sequence, and the PCB assembly guide covers what the CM needs for the board itself.
Keeping EBOM and MBOM in sync
Most BOM errors at a startup are sync errors, not design errors. Four habits prevent them.
- Same part numbers in both. The MBOM adds lines. It never renames EBOM parts.
- Every engineering change order names its MBOM impact. A field on the ECO that says which operations and kits change, even if the answer is none.
- Effectivity by serial or date. State which units get the change. "Next build" is not an effectivity.
- One owner per BOM. Design owns the EBOM. The CM or your manufacturing lead owns the MBOM. Both sign the ECO.
PLM tools automate the link between the two structures, which is the main reason teams buy them. PTC frames the progression as EBOM to MBOM to service BOM along one digital thread (PTC blog). For which tools do this at startup budgets, see bill of materials software.
Continue with Bill of Materials Management: Process, Versions, and the Mistakes That Cost Builds.
Frequently asked questions
What is a manufacturing bill of materials?
A manufacturing bill of materials, or MBOM, is the list of everything needed to build a product, organized the way the factory builds it. It includes the parts from the engineering BOM plus consumables, packaging, firmware images and process steps, grouped into the subassemblies and operations of the actual line. It is what the factory's ERP uses to kit, schedule and cost a build.
Who owns the MBOM, engineering or manufacturing?
Manufacturing engineering owns the MBOM, and design engineering owns the EBOM. At a startup using a contract manufacturer, the CM usually builds the MBOM inside its own ERP from your EBOM, drawings and assembly instructions. You should still review it, because the CM's version decides what gets bought, in what quantity, and what you are invoiced for.
Can a startup use one BOM for both?
For prototypes, yes. One spreadsheet with process lines added works until you hand the build to a factory. After that, keep the EBOM as your controlled design record and let the MBOM live where the build happens. What you cannot skip is the link between them: every EBOM change needs a check that the MBOM changed to match.
What goes in an MBOM that is not in an EBOM?
Typical additions are solder paste, adhesives, thermal paste, labels, protective film, packaging, the firmware image as a part number, test and programming steps, and attrition allowances for small parts. The MBOM also regroups parts into the subassemblies the line actually builds, which may not match the design hierarchy in CAD.
HardwareMap tracks the startups that made the jump from EBOM to production line. Building one? Submit it and get a part number.
Building one?
Submit it to the index and get a part number. Every entry is reviewed by hand.