Keep-Out Areas

Keep-out areas are designated regions on a Printed Circuit Board (PCB) where specific elements, such as components, traces, vias, or cutouts, are prohibited. These areas are essential for ensuring the proper functionality, reliability, and manufacturability of the PCB. This documentation provides a detailed examination of keep-out areas, their applications, and their configuration in CAD software, with a focus on KiCad. The content is tailored for advanced users, emphasizing technical precision and practical insights.

Definition and Purpose

A keep-out area is a restricted zone on a PCB where certain elements are not allowed. The primary purposes of keep-out areas include:

1. Electrical Performance:
   Preventing interference with sensitive components, such as antennas or high-frequency circuits, by ensuring no conductive elements are placed in proximity.

2. Mechanical Clearance:
   Ensuring adequate space for moving parts, such as switches or connectors, to operate without obstruction.

3. Thermal Management:
   Avoiding the placement of heat-sensitive components near heat-generating elements.

4. Manufacturing Constraints:
   Complying with fabrication and assembly requirements, such as avoiding components near board edges or drill holes.

Applications of Keep-Out Areas

Example 1: Antenna Clearance (ESP32 Dev Kit)

In the provided example of an ESP32 development kit, the integrated antenna requires a keep-out area to ensure optimal performance. Key considerations include:

1. Location:
   The keep-out area is marked by an orange box around the antenna section on the PCB.

2. Layer Restrictions:
   The keep-out area must encompass all copper layers, not just the top layer where the antenna is located. This prevents any conductive traces or components on inner layers from interfering with the antenna's radiation pattern.

3. Prohibited Elements:
   No components, traces, or vias are allowed within the keep-out area to avoid detuning the antenna or causing signal degradation.

Example 2: TFT Screen (Arduino Projects)

In the case of a TFT screen used in Arduino projects, the keep-out area is configured to allow components on one side of the PCB while restricting placement on the other side. Key details include:

1. Front-Side Restriction:
   The front side of the PCB, where the TFT screen is mounted, is designated as a keep-out area to prevent any components from obstructing the screen or its ribbon connector.

2. Back-Side Allowance:
   The back side of the PCB is not restricted, allowing for the placement of components and routing of traces. This dual-sided approach maximizes the use of available space while maintaining functionality.

Example 3: Slide Switch (UART Interface)

For a UART interface with a voltage slide switch, the keep-out area ensures mechanical clearance for the switch's operation. Key aspects include:

1. Switch Travel Path:
   The area beneath the slide switch is marked as a keep-out area to prevent any components from obstructing the switch's movement.

2. Trace and Via Allowance:
   While components are prohibited, traces and vias may still be routed through the keep-out area, provided they do not interfere with the switch's operation.

Configuration in KiCad

KiCad provides robust tools for defining and configuring keep-out areas. The following steps outline the process:

1. Creating a Keep-Out Area:

   • Use the "Add Keepout" tool in the PCB editor to define the boundaries of the keep-out area.
   • The area can be drawn as a polygon of any shape to match the specific requirements of the design.

2. Configuring Restrictions:

   • Specify which elements are prohibited within the keep-out area (e.g., footprints, tracks, vias, cutouts).
   • Apply the restrictions to specific copper layers or all layers, depending on the design requirements.

3. Layer-Specific Rules:

   • For designs requiring different restrictions on different layers (e.g., TFT screen example), configure the keep-out area to apply only to the relevant layers.

4. Validation:

   • Use KiCad's Design Rule Check (DRC) to verify that the keep-out areas are respected and that no prohibited elements are placed within them.

Advanced Considerations

Dynamic Keep-Out Areas

In complex designs, keep-out areas may need to adapt to changes in component placement or routing. KiCad allows for dynamic adjustment of keep-out areas, ensuring they remain effective as the design evolves.

Signal Integrity and EMI

Keep-out areas play a critical role in maintaining signal integrity and minimizing electromagnetic interference (EMI). For high-frequency circuits, ensure that keep-out areas are sufficiently large to prevent coupling between sensitive traces and other components.

Thermal Management

In designs with significant heat generation, keep-out areas can be used to prevent the placement of heat-sensitive components near hot spots. This is particularly important for power electronics and high-density designs.

Conclusion

Keep-out areas are a vital aspect of PCB design, ensuring the proper functionality, reliability, and manufacturability of electronic circuits. By understanding their applications and configuring them effectively in CAD software such as KiCad, designers can optimize their layouts for performance and compliance with design constraints. Advanced users must consider factors such as layer-specific restrictions, dynamic adjustments, and thermal management when implementing keep-out areas in complex designs. Mastery of keep-out area configuration is essential for creating high-quality PCBs that meet the demands of modern electronics.