Anatomy of a PCB Design > Projects > Schematics

Symbol No-Connects (often represented as “NC” or a crossed line) are indications on a schematic symbol that signify certain pins or terminals of a component are intentionally left unconnected. They tell the designer and the schematic capture software that those specific pins do not require a connection to any other part of the circuit.

Purpose and Role of Symbol No-Connects

•Symbol No-Connects(often represented as “NC” or a crossed line) are indications on a schematic symbol that signify certain pins or terminals of a component are intentionally left unconnected. They tell the designer and the schematic capture software that those specific pins do not require a connection to any other part of the circuit.

•Indicating Unused Pins: Many components, especially complex ones like micro-controllers, ICs, or connectors, have multiple pins that might not be used in a particular design. Symbol No-Connects indicate that these pins are intentionally left disconnected.

•Clarifying Design Intent: No-Connect symbols clearly show the designer’s intent that certain pins should remain unconnected. This prevents confusion during design reviews and makes it clear that these pins were not accidentally overlooked.

•Error Checking and Design Rule Compliance: In schematic capture tools, No-Connect symbols prevent the software from flagging these unconnected pins as errors during design rule checks (DRCs). This helps ensure the schematic passes verification without unnecessary warnings or errors.

•Documentation and Communication: No-Connect symbols serve as a form of documentation, letting other engineers and technicians know that these pins do not need to be wired or tested during the assembly and testing phases.

Characteristics of Symbol No-Connects

•Appearance: Symbol No-Connects are typically represented as a small cross (`X`), circle, or a specific “NC” label placed near or on the unconnected pin. The exact symbol varies depending on the schematic capture tool or design standard being used.

•Location: The No-Connect indicator is placed directly on or near the pin in question, making it clear which pin is being referenced.

•Behavior in Design Tools: When a pin is marked as a No-Connect in schematic design software, the tool understands that this pin is not supposed to be connected to anything else, and it won’t show an error during net-list generation or design rule checks.

When to Use Symbol No-Connects

•Unused Component Pins: Many components have multiple functions or configurations and, therefore, more pins than may be used in a particular design. For example, a micro-controller might have GPIO pins or special function pins that aren’t needed. These would be marked as No-Connects.

•Spare Pins on Connectors: Connectors often have extra pins for future expansion or alternative connections. These pins may be left unconnected in the current design and should be marked as such.

•Reserved Pins: Some ICs have reserved or no-connect (NC) pins as per the data-sheet specifications. These pins should be explicitly marked as No-Connects to show they are intentionally left unused.

•Testing and Debugging: During design iterations or testing, some pins may temporarily be left unconnected. Using No-Connect symbols ensures the design tools don’t treat these as errors.

How to Use Symbol No-Connects in Schematic Design

•Select the Correct No-Connect Symbol: Choose the standard No-Connect symbol used in your design tool or organization (e.g., cross, circle, or “NC” text).

•Place No-Connect on Relevant Pins: Place the No-Connect symbol directly on or adjacent to the pin that is intentionally left unconnected. Make sure it is clearly visible and unambiguous.

•Document the No-Connects: If possible, add comments or notes to explain why certain pins are marked as No-Connects, especially if they are reserved or have specific requirements from the component’s data-sheet.

•Double-Check During Design Reviews: Verify that all pins marked as No-Connects are intentional and do not require any further connections in the design.

Common Use Cases and Examples

•Micro-controller Unused GPIO Pins: A micro-controller might have numerous GPIO pins that are not used in a particular design. These can be marked as No-Connects to indicate that they should be left floating or tied to a specific voltage level as per the data-sheet’s recommendation.

•Unused Address Lines on Memory Chips: If a memory chip is used in a configuration where some address lines are not needed, those pins can be marked as No-Connects.

•Unconnected Power Supply Pins: Some components have multiple power supply options or ground pins. If not all of these are used, the unconnected ones can be marked as No-Connects.

•No-Connect Pins Specified in the Data-sheet: Many IC data-sheets specify certain pins as “No-Connect” (NC) or “Do Not Connect” (DNC). These pins should be marked as No-Connects in the schematic to prevent accidental connections that could interfere with the IC’s operation.

Impact of Symbol No-Connects on PCB Design

In PCB design, Symbol No-Connects ensure that the net-list generated from the schematic correctly represents the intended connections and does not include any unintended connections or errors. During PCB layout and routing, No-Connect pins will not be routed or connected to other nets, helping avoid shorts or unwanted connections.

Design Guidelines for Using Symbol No-Connects

•Follow the Data-sheet Guidelines: If the component data-sheet specifies pins as “NC” or “DNC,” always mark these pins as No-Connects in the schematic.

•Avoid Unnecessary No-Connects: Only use No-Connect symbols for pins that are truly meant to be left unconnected. Unnecessarily marking pins as No-Connects can lead to confusion or missed connections.

•Review During Schematic Validation: Always check the No-Connects during schematic validation and design reviews to ensure they align with the intended design and do not cause issues during PCB layout or testing.

Symbol No-Connects vs. Unconnected Pins

•Symbol No-Connects: Explicitly indicate the designer’s intent to leave a pin unconnected and are not considered errors during design rule checks.

•Unconnected Pins: If a pin is simply left without a connection and not marked as No-Connect, it might be flagged as an error by the schematic capture tool, as it could indicate a missing or forgotten connection.

In summary, Symbol No-Connects are critical for clarifying which component pins are intentionally left unconnected, ensuring that the schematic design, net-list, and final PCB layout reflect the designer’s intent accurately. They prevent design errors and improve communication between design, assembly, and testing teams.