PLM, product structure and multi-level BOM. Let’s sort it out…

PLM, product structure and multi-level BOM. Let’s sort it out…


One of the complex topics in product development and manufacturing is multi-level BOM. Very often, multi-level BOM is presented as an important functionality supported by PLM systems in comparison to management of BOM using Excel spreadsheets or sometimes even ERP systems.

Here is Wikipedia definition in Bill of Materials article:

A Multi-Level Bill of Materials (BOM), or referred as an indented BOM, is a bill of materials that lists the components, assemblies, and parts required to make a product. It provides a display of all items that are in parent-children relationships. When an item is a sub-component, unfinished part, etc., all of its components, including finished parts and raw materials, are also exhibited. A multi-level structure can be illustrated by a tree with several levels. In contrast, a single-level structure only consists of one level of children in components, assemblies and material.

Autodesk PLM blog – What should be included on the Bill of Materials brings the following passage explaining if you should manage BOM as a flat of hierarchical structure.

Will the BOM be flat or hierarchical? Sometimes, companies like to manage the BOM so that parts within a certain assembly are grouped together under their parent, which creates a hierarchical BOM. This can make certain management tasks easier, but adds to the overall complexity, particularly if a company is using Excel for their BOM management strategy. A flat BOM, where everything is given the same priority, might be good for electrical engineers and for simpler management, but it doesn’t allow for complex relationships between products and makes working with contract manufacturers slightly more complex.

Another article from Arena PLM – Managing multi-level BOMs brings some additional definition points about managing of multiple level BOMs and Arena’s capability to stand for such requirement.

Choosing which BOM Structure to Use. To define the BOM structure that best suits your needs, it is important to consider who will be using the BOM as well as the type of product you build using the BOM. Ask yourself the following questions. Will your BOM be used in-house for engineering purposes? Will your BOM be shared with a contract manufacturer, partner or other collaborator? What types of products do you make? What is the complexity and configurability of your products? These are just a few important factors to consider when structuring your bill of materials

How Arena PLM Helps Manage Multi-Level BOMs. Arena PLM, bill of materials and change management, offers a more efficient approach to managing multi-level BOMs than Excel. Because the software is built on a relational database, you can create an unlimited number of one-to-many and many-to-many relationships, which prevents data entry mistakes and eliminates product data ambiguity. You can also mass replace a single part in several locations within a BOM or in multiple disparate BOMs—saving hours of time and eliminating BOM discrepancies.

Multi-level BOM is often confused with product structure. In the past, I’ve been asked a lot about what is a difference between these two things, in what situations to single BOM vs multi-level BOMs and How many levels should be in a BOM? Here is my take… But I also want to hear your definitions and opinions as well. So, don’t be shy if you disagree :).

Multi-level BOM is a way to organize multiple single level BOMs together based on a specific breakdown structure. There are different reasons why organizations of BOM into multiple-level can be very helpful. Multiple BOM levels can represent a certain way of managing BOM. The way levels (hierarchies) are organized can reflect structure of product, configurable modules, supply packages, manufacturing processes and many other aspects. It is important to remember that each “level” is a separate (single-level) BOM identified in the manufacturing system of record by Part Number and sometimes additional parameters (e.g. revision).

Product structure is a hierarchy of product (parent-child relationships). Product structure is a three (or graph) of relationships between parts and assemblies representing product itself. Some multi-level BOMs (especially engineering BOM) can reflect product structure  when it comes to relationships between assemblies, sub-assemblies and parts. But multi-level BOM is a broader definition and can represent other data as well in manufacturing, supply chain, etc.

What is my conclusion? Terminology is a complex thing in engineering and manufacturing. You ask 2 engineers the same question and end up with 3 answers. So, here is my take on multi-level BOM and product structure now. Multi-level BOM and product structure are related, but this is not the same. Muti-level BOM can represent processes in multiple areas of product development – engineering, manufacturing, supply chain, support, etc. I can be used to create product configuration modules, assembly instructions as well as to structure supplier collaboration. Product structure is a representation of part-child relationships in the way assembly and components are coming together in a product. It can represent design or physical product. Just my thoughts…

Best, Oleg

Want to learn more about PLM? Check out my new PLM Book website.

Disclaimer: I’m co-founder and CEO of openBoM developing cloud based bill of materials and inventory management tool for manufacturing companies, hardware startups and supply chain. My opinion can be unintentionally biased.


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  • Simon Hailstone

    Oleg, you are right about terminology and it has been the bane of the PLM world for, let’s face it, forever.

    You say that “multi-level BOM is a broader definition”, however, for 30 years I have always regarded the term ‘product structure’ as the most generic definition of whatever we want to call a ‘product’. The BOM I feel is something quite historically manufacturing oriented – just the words “bill of material” conjure up the image of paper based supply and make environments going way back. The term BOM is in many ways not helpful in a 21st century digital, multi-faceted connected environment. Software is not a ‘material’, neither are services, and future additive and molecular production techniques will result in less separation of discrete materials in final products.

    Having said that, we as engineers, software creators and especially industry analysts, need to communicate clearly in a common language in the short term. The term BOM is embedded in our DNA for now and so it probably makes sense to use it in the ways you have defined here. Perhaps our successor generation will find a more appropriate term for these complex graph/network definitions.

  • beyondplm

    Simon, thank you for sharing your insight! I agree, product structure used as a “generic term” in PLM systems, but after many years I found people outside of engineering department have hard time to adopt to that. BOM is clearly in DNA of product development and manufacturing. It is especially true if you want to connect people outside of engineering departement, contractors, suppliers, manufacturing planners, procurement. Appreciate your support! Best, Oleg

  • Sumeyye

    Hi Oleg,

    My thesis topic is very confusing. I must search different laptop case then draw sample of it in Solid or Catia. There are many laptop case components measurement. Then I’m going to collect all data. My teacher says: You collect all data and if customer want different type of laptop you wont draw it from the beginning. You enter data(for example I will change fan measurement) and your system will revise your laptop according to this. (Oyher components measurement will change and system make it automatically)(?) But how can I use plm for this? Could you help me? I am mechanical student and I dont know plm or sap.

  • Ian Holmes

    Hi Oleg,

    My take on it is that historically we have tended to structure data in the ways that we plan to use it, however over time the data sets have become more complex and the use cases for the data have become ever more sophisticated that we are quickly outstripping those assumptions, we find we are wanting to restructure the data but we have made this task more difficult and we don’t know how it should be structured to support our

    Going forward we need to recognise that how you want to use data in the future is unpredictable therefore we need systems that can cope with creating multiple views of data “on-the-fly” based on existing and newly formulated relationships. So capture data as richly as you can afford to, make few assumptions regarding the structure when you capture and store it and implement technological systems that are capable of generating dynamic views on this data based on sophisticated algorithms or simple relationships.

    Ian Holmes

  • beyondplm

    Amen! +1 and Like!

  • beyondplm

    You need to create a configurable view. Different systems do it differently. But it is a possible task.

  • Loïc M.

    Hi Oleg,
    I think the wording BOM is more appropriated to ERP (previously MRP) systems, as they do work with Materials for manufacturing or logistic purpose. But now, quite everybody is speaking about eBOM (for engineering or electical), manufacturing BOM, service BOM, etc.

    If you consider the CAD integration, you can also consider a structure of document (CAD-Structure), what can/is synchronized with the part structure (certainly a subset of your product breakdown structure).

    While using configuration management, it could be more confusing for the ‘novices’: configured BOM, 150% BOM…
    In most cases, the (PLM) system is storing lots of links, and deliver dynamically a result to the user depending of the resolution rule (configuration rule) used.


  • beyondplm

    Loïc, thank you for sharing your insight. Yes, BOM originally was MRP word. But PLM and CAD systems are using it very extensively these days. Some of these terms (150% BOM) are very confusing, but in my view it is better than various “structures” that are not structures, but links in the graph. PLM did a very poor job exposing its guts to customers and it is still very confusing terminology. Best, Oleg

  • beyondplm

    Francois, sorry- your comment was caught by spam filter because of the link. Thank you for sharing your insight !

    The question is NOT about “single flat BOM”. Although I can see your point. Many people are considering BOM flat and use ‘product structure’ as soon as things are getting hierarchical. This is why I brought the question about ‘multi-level’ BOM.

    In my view, multi-level BOM is a wider category that can fit things beyond product structure, which is assembly-component relationships. But again… just my thoughts 🙂

    Thanks for sharing the link!
    Best, Oleg