BoM Product Structure
Students will analyse BoMs, derive various types of BoMs (Single Level, Indented and Summarised) and learn about implosions and explosions. They will learn key phrases and definitions used throughout the BoM process.
What is the Bill of Materials (BoM)? 
A Bill of Material is an inclusive definition of a final product that includes a list of the items, ingredients and/or materials and the quantities of each needed to manufacture, assemble or mix an end product.
A BoM for a house would include the following sub-assemblies: cement block, lumber, shingles, doors, windows, plumbing, electric, heating etc. Each subassembly also contains a BoM; the heating system is made up of the furnace, ducts etc.
There are several different forms of the BoM, which include:
- Single Level Bill of Material (Figure 1)
- Indented Bill of Material (Figure 2)
- Summarised Bill of Material (Figure 3)
These are basic examples, a BoM generally contains more information, such as, an item's effective date, item number, description etc.
A summarised BoM is useful as it shows the total usage of each item needed for the product in a single list, but results in the duplication of the product's assemblies. For every product BoM that uses a duplicated assembly, it must be changed whenever there is a change in the assembly.
The indented BoM is useful as it helps one to understand the structure of the product, but all the parts belonging to an assembly are repeated(look at B, E and F). This can be a problem: lets imagine that the BoM is much bigger than the one provided in Figure 2, it will lead to a massive duplication of data.
As a solution to the duplication problem, each assembly is held once in a ‘single level’ BoM. In Figure 1, level one shows the single level BoM of Product 1, it then has pointers to each single level BoM of its assemblies. This allows changes to an assembly to occur in one place.
A single level BoM (Level 1 of Figure 1) is sufficient when a product is assembled at one time from a set of purchased parts and raw materials, but it does not adequately describe a product that has sub assemblies. The BoM is also known as the Product Structure, and the Product Structure helps to describe a product with sub assemblies. Although, the Product Structure does not specify the quantity of items needed for a product, therefore, the Bill of Materials (BoM) and the BoM Product Structure are used together.
BoM Product Structure 
To illustrate the Product Structure; a tree like structure can be drawn (Figure 4). The Product Structure provides a hierarchical classification of the items which form a product. Items can take many forms: raw materials; intermediate items; items; assemblies; parts; components.
Raw materials appear at the bottom of the structure. In the example opposite; E, F, D and C are raw materials.
B is a component of A,
Intermediate Items are those in between. There are 2 intermediate items in the example; B and A.
The Product Structure helps to display and describe the relationships between one item and another, it does not specify the sequence of assembly. The assembly sequence is important to determine what operations should be performed by a worker, and what kind of work stations are needed. So, the assembly sequence is provided in an assembly chart.
BoM Implosion and Explosion 
A BoM explosion (Figure 5) helps to break an assembly or sub-assembly into its components or raw materials. For example, an engineer needs to see what is required to construct Assembly A, using the explosion, they can see a list of all Assembly A's components.
A BoM implosion (Figure 6) helps to link a component to an assembly, sub-assembly or product. For example, an engineer needs to see where Raw Material E is used, using the implosion, they can see the range of assemblies, sub-assemblies or products where Raw Material E is used.
Where is the BoM used? 
The BoM is stored in a database and it is used in Materials Requirement Planning systems (Figure 7) in conjunction with the MPS and Inventory file to determine exactly how many components are needed in order to produce the quantity of finished products stated in a MPS and to determine the items for which purchase requisitions and production orders must be released. MRP systems help to determine: what to order; how much to order; when to order; when to schedule delivery. They also help to plan and control inventory, support detailed capacity planning and support priority planning on the shop floor.
Accounting use the BoM to cost the product and it is also used for kitting production orders, order entry and engineering purposes.
Test yourself 
Consider a plug. The Product Structure for the plug is as follows:
1. Create a single level explosion for the 13 AMP Square Pin Plug
2. Pick the odd one out: Negative Pin Assembly, Earth Pin Assembly, Final Assembly Cover Screw or Fuse Holder Assembly
3. List the Components of 'Positive Lead Hold Assembly'
4. List all Raw Materials
5. True or False:
- Assembly has many components
- Assembly has many sub-assemblies
- Part has many Assemblies
- Part has many components
6. Where do raw materials tend to appear in the Product Structure?
7. What is meant by each of the following:
- a) A single-level explosion?
- b) A indented implosion?
- c) A summarised explosion?
8. For the Product Structure below, derive the:
- a) Summarised BoM
- b) Indented BoM
- c) Single-level BoM
Test yourself - Answers 
Provide answers to the questions!
1. 13 AMP Square Pin Plug
- Wire Retainer Screws
- Negative Pin Assembly
- 13 amp Top Cover
- 13 amp Bottom Cover
- Earth Pin Assembly
- Final Assembly Cover Screw
- Fuse Holder Assembly
2. Final Assembly Cover Screw because it is a raw material. All the others as assemblies.
3. Positive Lead/Fuse Holder Assembly, Screw
4. There are 14 Raw Materials used; Positive Lead Holder, Fuse Holder, Screw (3 per), Positive Pin, 13 amp Fuse, Earth Pin, Negative Pin, Retainer, Final Assembly Cover Screw, 13 amp Bottom Cover, 13 amp Top Cover, Wire Retainer Screws.
5. a) True
- b) True
- c) True
- d) True
6. They tend to be children
7. a) A single-level explosion: A list of what components are needed to build the Parent item.
- b) An indented implosion: A listing of all components used in an assembly. There are not totals provided. When each component is used it is repeated..
- c) Summarised explosion: The total quantity of each component required to build the Parent part. The main difference from the single-level explosion is the need to aggregate the quantities. The first example indicates the quantity of sub-parts required for the part whenever it is required. It does not indicate how many of the subparts are needed to build the Parent item.
- a) Summarised BoM
- b) Indented BoM
- c) Single Level BoM
Assembly: A group of components which are put together in manufacturing (e.g. cooking).
Component: A raw material, ingredient, part or sub assembly, but not a product, that is used in the manufacture of a higher-level assembly, compound, or other item.
Intermediate Item: Any material which is neither a product nor a raw material but is consumed in the making of products.
Item: Any unique manufactured or purchased part or assembly. May be a product, raw material, intermediate item, but not in process.
MPS: A Master Production Schedule is a plan for production, staffing, inventory, etc.
MRP: Material Requirements Planning, a type of application software system used to schedule and monitor the use of components and other materials in a manufacturing operation.
Part: Any one-piece item which may be manufactured or purchased. Typically manufactured at Fabrication.
Raw Material: The material which is bought from a supplier and used directly or indirectly in producing a product.
Resource: Any asset which is scarce or limited and therefore needs to be planned and managed(Materials, Human Resources, production facilities).
Evans, J. R. (1993) Applied Production Operations Management, 4th ed. Minnesota: West Publishing Company.
Fogarty, D. W., Hoffman, T. R. (1983) Production and Inventory Management. Cincinnati: South-Western Publishing Co.
Power, N. (2011) CS4008: Enterprise Resource Planning , 22 April, University of Limerick, unpublished.