For any complex electronics project, effectively tracking all the components and parts required is critical. Rather than a simple list, advanced systems need bill of materials (BOM) structures with hierarchy and organization. Multi-level BOMs provide the capabilities to manage BOMs for intricate systems in a structured manner.
In this article, we’ll look at the benefits of multi-level BOMs, recommended practices for organizing your BOM tree, and examples of effective multi-level BOM usage for complex system designs.
Why Use a Hierarchical BOM?
For a simple circuit board, a flat BOM listing all components may be sufficient. But when designing an entire system encompassing PCBs, wiring, harnesses, assemblies, enclosure parts, and more, tracking everything in one list becomes unwieldy.
A hierarchical, multi-level BOM provides advantages such as:
- Organizing components, sub-assemblies, and parts logically
- Structuring the BOM based on physical design relationships
- Allowing different views for specific sub-systems
- Handling BOM data efficiently for large projects
- Enabling design reuse at the sub-assembly level
So rather than one long parts list, a properly structured, multi-tiered BOM helps you architect, assemble, and manage extensive system projects.
Multi-Level BOM Guidelines
To effectively utilize multi-level BOM capabilities, you should follow some general hierarchical structuring guidelines:
- Plan BOM hierarchy upfront - Like a schematic, don’t let the BOM hierarchy emerge randomly. Architect it deliberately.
- Top-down BOM assembly - Work top-down to construct the BOM, defining systems then expanding into sub-assemblies.
- Organize by physical relationships - Group components into sub-assemblies based on how parts physically connect and integrate together, not just electrical connectivity.
- Standardize naming conventions- Use consistent naming for BOM levels to clarify the tier relationships and identifiers, like “Top-Level System”, “Sub Assembly A1”, “Circuit Board B2”, etc.
- Limit BOM levels - Don’t go too deep. Try to keep assembly levels to 3-5 tiers maximum for easiest usability. Deeper can get convoluted.
- Minimize repetition - Design sub-assemblies for reuse so shared parts don’t have redundancies repeated under multiple parents.
These practices will help yield clean, streamlined multi-level BOM organization.
BOM Software Capabilities
Many BOM management software tools now support multi-level, hierarchical BOM capabilities that align with the guidelines above, such as:
- User-defined BOM levels and tiered parent-child relationships
- Tree-view navigation of the full BOM structure
- Visibility controls by BOM level (collapse, expand, hide)
- Part repetition avoidance using sub-assemblies
- Parameterized sub-assembly designators
- Cross-level part change propagation
- BOM data imports from mechanical and electrical CAD tools
This enables proper configuration and usage of multi-tier BOMs for complete system designs.
BOM Organization Examples
To understand multi-level BOM principles better, let’s walk through some examples of how to structure BOMs for different types of system designs. We’ll use simple placeholder names for illustration.
Electronics System BOM
For a physical electronics system like an AV receiver, multi-level BOM organization could follow the physical design relationships:
Level 1 - Complete System
- System Enclosure
- Main Board Assembly
- Power Supply Assembly
- Front Panel Assembly
Level 2 - Board Assemblies
- Main Board
- Processor
- Memory Modules
- RF Transceiver
- Interface Connectors
- Power Supply Board
- AC/DC Converter IC
- Filtering Components
- Output Capacitors
- Cable Harness
Level 3 - Components
- Processor
- Part # XYZ1212, Rev 2
- Qty: 1
- Memory Module
- Part # ABC3412, Rev 1
- Qty: 2
So the BOM follows the actual integration of physical pieces from complete system down to individual components.
Software Application BOM
For a software deliverable like an app, the multi-level BOM shifts to logical organization:
Level 1 - Complete Application
- Server Application
- Database Backend
- Mobile App
- Cloud Services
Level 2 - Sub-systems
- Server Application
- Application Codebase
- 3rd Party Libraries
- Server Middleware
- Database Backend
- Database Software
- Database Schema
- Stored Procedures
Level 3 - Deliverable Components
- Application Codebase
- Repository: http://code.myapp.com
- Version: 2.1.4
- Release Date: 2022-05-16
Here the multi-tier BOM captures software components and deliverables organized into the deployed sub-systems and services.
BOM Synchronization
A key benefit of intelligent BOM software is keeping assembly levels synchronized. When changes occur, they can automatically ripple upward through the BOM structure.
For example, swapping out a new power supply sub-assembly for an improved design automatically updates the higher level main board assembly in the BOM. Manual BOM manipulation is not required.
This allows efficiently evolving and reconfiguring systems while minimizing errors and omissions in the BOM data. Changes are reflected wherever relevant across the assembly hierarchy.
Frequently Asked Questions
What are some tips for deciding how many levels to use in a multi-level BOM hierarchy?
Aim to keep your overall BOM structure relatively flat, with 3 - 5 levels maximum. Too many deep nested levels can become confusing and hard to maintain. A good rule of thumb is to not allow more than 3-4 levels below any parent assembly.
Should mechanical and electrical BOM data be integrated in a single hierarchy, or kept separate?
It's generally best to maintain a single master multi-level BOM encompassing both mechanical and electrical components for the full system view. But you can also generate filtered BOM subsets - for example, just the PCB parts list for the fabrication shop.
What happens if the same common component is used in multiple sub-assemblies in a hierarchical BOM?
The best practice is to define that reused component as its own sub-assembly, then instance it under each higher level as needed. This avoids redundant entries for the same part scattered throughout a BOM.
How can I track where each component fits in the overall multi-level assembly hierarchy?
Intelligent BOM software will include capabilities such as displaying the full parent-child relationship tree for a part, highlighting its usage within different levels of the BOM structure.
How do I handle variant configurations in a multi-level BOM structure?
Parameters can be defined for sub-assemblies to support variants. For example, Power_Supply_Assembly(Type=A) vs. Power_Supply_Assembly(Type=B) where the configuration drives the actual contents.
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