Reference Designators for Components in BOM but Not Mounted on PCB

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Understanding Unmounted Components and Their Significance in PCB Design

When designing a printed circuit board (PCB), engineers often include components in the Bill of Materials (BOM) that are not physically mounted on the board. These unmounted components, also known as “ghost components” or “virtual components,” serve various purposes and play a crucial role in the overall design and functionality of the PCB. In this article, we will explore the concept of unmounted components, their reference designators, and their significance in PCB design.

What are Unmounted Components?

Unmounted components are components that are listed in the BOM but are not physically soldered or mounted on the PCB. These components are usually represented on the schematic diagram and have a designated reference designator, just like any other component on the board. However, they do not occupy any physical space on the PCB layout.

There are several reasons why designers include unmounted components in their PCB designs:

  1. Placeholders for Future Upgrades: Unmounted components can serve as placeholders for future upgrades or modifications. By including these components in the BOM and schematic, designers can easily add them to the board later without having to redesign the entire PCB.

  2. Alternative Component Options: Sometimes, designers include unmounted components as alternative options for different configurations or versions of the same PCB. This allows for flexibility in the manufacturing process and enables the creation of multiple product variants using the same base design.

  3. Testing and Debugging: Unmounted components can be used for testing and debugging purposes. By including test points or connectors as unmounted components, engineers can easily access specific signals or voltages during the testing phase without having to modify the physical board.

Reference Designators for Unmounted Components

Reference designators are alphanumeric codes used to uniquely identify each component on a PCB. They typically consist of a letter prefix followed by a number, such as R1 for a resistor or C5 for a capacitor. Unmounted components also receive reference designators, even though they are not physically present on the board.

The assignment of reference designators for unmounted components follows the same guidelines as for mounted components. The prefix letter indicates the component type, while the number distinguishes between multiple components of the same type. For example, if a design includes three unmounted capacitors, they might be labeled as C10, C11, and C12.

It is important to maintain consistency and clarity when assigning reference designators to unmounted components. This helps in communication between team members, reduces confusion during the design review process, and ensures accurate documentation.

Representing Unmounted Components in Schematics and BOMs

In schematic diagrams, unmounted components are represented just like any other component. They are drawn using the appropriate symbols and labeled with their respective reference designators. However, to indicate that a component is unmounted, designers often use special symbols or annotations.

One common method is to enclose the unmounted component in a dashed box or circle. This visual cue helps distinguish unmounted components from their mounted counterparts. Another approach is to add a text label or attribute next to the component, such as “DNP” (Do Not Populate) or “NM” (Not Mounted).

In the BOM, unmounted components are listed along with the mounted components. However, they are usually marked with a special designation or placed in a separate section to clearly identify them as unmounted. This distinction is important for the procurement and assembly processes, as it prevents unnecessary ordering or placement of components that are not meant to be physically installed on the PCB.

Reference Designator Component Description Mounted/Unmounted
C1 10µF Ceramic Capacitor Mounted
C2 100nF Ceramic Capacitor Mounted
C3 1µF Tantalum Capacitor (DNP) Unmounted
R1 1kΩ Resistor Mounted
R2 10kΩ Resistor (NM) Unmounted

In this example BOM table, the unmounted components C3 and R2 are clearly marked with “DNP” and “NM” designations, respectively, to indicate their unmounted status.

Best Practices for Managing Unmounted Components

To effectively manage unmounted components in your PCB design process, consider the following best practices:

1. Clear and Consistent Documentation

Ensure that your schematic diagrams and BOMs clearly identify unmounted components using consistent symbols, labels, or annotations. This helps avoid confusion and errors during the design review, procurement, and assembly stages.

2. Communication with Stakeholders

Communicate the presence and purpose of unmounted components to all relevant stakeholders, including designers, engineers, procurement teams, and manufacturers. Clear communication prevents misinterpretations and ensures everyone is on the same page regarding the intended use of unmounted components.

3. Design for Manufacturability

When including unmounted components in your design, consider the impact on manufacturability. Ensure that the placement of unmounted components does not interfere with the assembly process or cause confusion for the manufacturing team. Provide clear instructions and guidelines for handling unmounted components during PCB assembly.

4. Version Control and Change Management

Implement robust version control and change management processes to track modifications related to unmounted components. This is particularly important when unmounted components are used as placeholders for future upgrades or when multiple product variants are derived from the same base design.

5. Testing and Validation

Include unmounted components in your testing and validation plans. Ensure that the absence of these components does not affect the overall functionality or performance of the PCB. If unmounted components are used for testing or debugging purposes, establish clear procedures for their utilization and document the results accordingly.

Frequently Asked Questions (FAQ)

1. Can unmounted components have the same reference designator as mounted components?

No, each component on a PCB, whether mounted or unmounted, must have a unique reference designator. Assigning the same reference designator to multiple components can lead to confusion and errors during the design, assembly, and testing processes.

2. Are unmounted components included in the PCB assembly process?

No, unmounted components are not physically installed on the PCB during the assembly process. They are listed in the BOM for reference purposes, but the assembly team will skip these components during the actual PCB population.

3. How do I indicate that a component is unmounted in the schematic diagram?

There are several ways to indicate that a component is unmounted in the schematic diagram. One common method is to enclose the component symbol in a dashed box or circle. Another approach is to add a text label or attribute next to the component, such as “DNP” (Do Not Populate) or “NM” (Not Mounted).

4. Can I assign reference designators to unmounted components in a different sequence than mounted components?

Yes, you can assign reference designators to unmounted components in a different sequence than mounted components. The important thing is to maintain uniqueness and clarity in the reference designator assignments. Some designers prefer to assign reference designators to unmounted components in a separate range or with a distinct prefix to differentiate them from mounted components.

5. What happens if an unmounted component is accidentally populated on the PCB?

If an unmounted component is accidentally populated on the PCB, it can potentially affect the functionality, performance, or reliability of the board. In such cases, manual rework may be necessary to remove the unwanted component. To prevent accidental population, it is crucial to have clear communication and documentation regarding unmounted components throughout the design and manufacturing process.


Unmounted components play a significant role in PCB design, offering flexibility, scalability, and testability. By understanding the concept of unmounted components, their reference designators, and best practices for managing them, you can create more efficient and adaptable PCB designs.

Remember to maintain clear and consistent documentation, communicate effectively with stakeholders, consider manufacturability aspects, implement version control and change management, and include unmounted components in your testing and validation plans.

By following these guidelines and leveraging the benefits of unmounted components, you can streamline your PCB design process, reduce errors, and create high-quality electronic products that meet your intended specifications.