What is DRC in Kicad?

DRC stands for Design Rule Check. It is an automated check performed by the Kicad PCB design software to verify that the PCB layout meets certain design rules and constraints. The purpose of running DRC is to catch potential issues and errors in the PCB design before sending it out for manufacturing.

Kicad has a built-in DRC tool that checks the PCB layout against a set of predefined design rules. These rules specify things like:

• Minimum trace width and spacing
• Minimum drill size and Annular Ring
• Clearance between copper features
• Solder mask and silkscreen constraints
• And many other properties

Running DRC on your PCB layout in Kicad will flag any violations of these design rules so you can fix them. This helps ensure your board is manufacturable and will function properly.

How to Run DRC in Kicad

To run a Design Rule Check in Kicad:

1. Open the PCB layout (.kicad_pcb file) in the Pcbnew editor
2. Click the “DRC” button in the top toolbar (or go to Tools > DRC)
3. In the DRC dialog, select which tests you want to run
4. Click “Run DRC”
5. The DRC results will appear in the “Problems/Markers” tab at the bottom of the screen

You can then review the error markers, make necessary changes to your layout to resolve them, and re-run DRC until the layout passes all checks.

It’s important to configure the DRC settings (Design Rules) appropriately for your PCB design and fabrication process. The default settings may need to be adjusted.

Issues with Ground Planes and DRC

One common issue that can come up when running DRC on a Kicad PCB layout is errors related to the ground plane. A ground plane is a large copper pour used to provide a low-impedance return path for signals and shielding. They are often used on PCBs with sensitive analog or high-speed digital circuits.

However, ground planes can sometimes cause DRC errors if not set up properly. Some typical problems include:

Clearance Violations

DRC checks the spacing (clearance) between different copper features on the PCB to make sure they are not too close together. This includes the spacing between traces, pads, vias, and copper pours like ground planes.

If the clearance between the ground plane and other copper features is less than the minimum specified in the design rules, DRC will flag it as an error. This often happens if the ground plane is too close to component pads or traces.

To fix this, you can either:
– Move the components/traces farther away from the ground plane
– Adjust the ground plane outline to provide more clearance
– Modify the design rules for copper-to-copper clearance

Isolated Islands

Another issue is when the ground plane becomes disconnected or has “islands” that are not electrically connected to the main ground. This can happen if:

• The ground plane does not completely surround pads/vias
• Traces or other copper features overlap and cut off part of the ground plane
• The ground plane outline is not a single contiguous shape

Isolated sections of the ground plane cause issues because components that should be grounded may not have a proper return path. It can also make soldering and assembly difficult.

DRC will usually flag disconnected areas of the ground plane. To resolve this:

• Make sure there are no traces or other copper breaking up the ground plane
• Route the ground plane outline around pads/vias instead of having cutouts
• Stitch isolated ground sections together with vias or traces
• Enable the “Remove dead copper” option when pouring the ground plane

Thermal Relief Problems

Thermal relief is a technique used to improve soldering of through-hole component leads or vias that connect to a ground plane. It involves creating a pattern of small traces or spokes that connect the pad to the plane.

This is necessary because large ground planes can sink a lot of heat away from the pad during soldering, making it difficult to get a good solder joint. The thermal relief pattern reduces the heat sinking while still providing electrical connection.

Issues with thermal relief can trigger DRC errors in some cases:

• If the thermal relief openings are too small, it may flag them as starved thermal connections
• If the spokes are too narrow, it can cause Acid Traps that eat away at the traces over time
• Having too many (or too few) spokes can also be a problem for reliable soldering

Configuring the thermal relief properly in the DRC rules and copper pour settings is important. Kicad provides options to customize the spoke width, angle, and count.

Slivers and Acute Angles

Ground planes with very thin sections (slivers) or sharp acute angles can also be flagged by DRC. These geometries may not meet minimum width requirements and can cause manufacturing issues.

For example, if the ground plane has a very narrow area between two pads, acid from the etching process can become trapped and cause over-etching. Sharp inward angles are also prone to over-etching.

Some strategies to avoid sliver and angle problems:
– Move components to provide more space for the ground plane
– Adjust the shape of the ground plane to eliminate thin sections and sharp angles
– Use ground plane cutouts (edge cuts) to remove problem areas entirely
– Increase the minimum width and acute angle limits in the DRC settings

Tips for Preventing Ground Plane DRC Errors

Here are some general tips to avoid running into DRC errors with ground planes in Kicad:

1. Make sure to set appropriate clearance constraints in the design rules. The ground plane needs adequate space from other copper features.

2. Route the ground plane outline carefully. Avoid creating isolated islands or slivers.

3. Provide proper thermal relief for pads/vias connected to the ground plane. Customize the thermal relief settings as needed for your design.

4. Use ground plane cutouts and keep-out areas to maintain required clearances. Consider using the “Edit Zone Outline” tool to fine-tune the shape.

5. Pour the ground plane after the components are placed and routed. This allows Kicad to calculate the proper outline based on your clearance rules.

6. Run DRC often during layout. Don’t wait until the end to check for errors. Fixing problems is easier if caught early.

7. Review the ground plane geometry closely. Look out for unintended narrow sections or angles that may be flagged by DRC.

With proper care and attention to the design rules, you can create robust, error-free ground planes in your Kicad PCB layouts. Taking the time to resolve any DRC issues with ground planes (and other aspects of the layout) is crucial to ensuring your board is manufacturable and will function as intended.

FAQ

What is the purpose of a ground plane on a PCB?

A ground plane serves several important functions on a PCB:

1. It provides a low-impedance return path for electrical currents. This is especially important for high-frequency signals, as the return current follows the path of least inductance which is directly under the signal trace.

2. It helps to reduce electromagnetic interference (EMI) by providing shielding. The ground plane essentially acts as a barrier that reflects or absorbs stray electromagnetic fields.

3. It improves power supply distribution and regulation by lowering the impedance between the power source and components. This helps to maintain signal integrity.

4. It provides a common reference point for signals, which is necessary for proper circuit operation. Components that need to be at the same potential (i.e. ground) are all connected to the ground plane.

5. In some cases, it can also help with Heat Dissipation by providing a large area for thermal spreading.

Properly designing the ground plane is crucial to the performance and reliability of the PCB.

Why does the ground plane need thermal relief?

Through-hole component leads or vias that connect to a ground plane need thermal relief for soldering. The ground plane is a large area of copper that can sink a lot of heat away from the pad during soldering. This makes it difficult to heat the pad up sufficiently to melt the solder and get a good joint.

Thermal relief is a pattern of small traces or spokes that connect the pad to the ground plane. These thin connecting traces have higher Thermal Resistance, which limits the heat sinking effect of the ground plane during soldering. This allows the pad to reach a high enough temperature for the solder to flow properly.

Without thermal relief, you may end up with poor solder joints that are unreliable or prone to failure. The component may not be electrically connected to the ground plane, or the joint may crack over time due to mechanical stress.

Kicad allows you to customize the thermal relief pattern in the copper pour settings. You can adjust the number, width, and angle of the thermal spokes to suit your manufacturing process and reliability needs.

How do I resolve DRC errors related to the ground plane?

There are several strategies to resolve DRC errors caused by the ground plane in Kicad:

1. Clearance violations: If the DRC reports that the ground plane is too close to other copper features, you can either move the components/traces farther away, adjust the ground plane outline to provide more clearance, or modify the copper-to-copper clearance constraint in the design rules.

2. Isolated islands: Ground plane areas that are disconnected from the main plane can be fixed by removing any traces or copper that is breaking up the ground plane, routing the plane outline around obstacles instead of creating holes, stitching isolated sections together with traces or vias, and using the “Remove dead copper” option when pouring the plane.

3. Thermal relief problems: Issues with thermal relief can be addressed by configuring the thermal settings correctly in the DRC rules and zone properties. Make sure the thermal spoke width, count, and angle are appropriate for your soldering process.

4. Slivers and acute angles: Ground plane areas that are too thin or have sharp inward angles can cause acid trapping and over-etching. To avoid this, move components to give more room for the ground plane, adjust the plane shape to remove problem areas, use edge cuts to remove slivers, and increase the minimum width and acute angle limits in DRC.

5. Insufficient copper: If the ground plane is not providing enough copper coverage, you may need to adjust the “minimum width” setting in the copper pour properties to make it larger. You can also use the “Add filled zones” tool to add more copper areas.

The key is to review the DRC error markers carefully, understand the root cause of each violation, and take appropriate action to modify the ground plane or update the design rules. It may take some iteration to fully resolve all the issues.

Can I have multiple ground planes in my PCB design?

Yes, it is possible and sometimes necessary to have multiple ground planes in a PCB design. Here are a few scenarios where you might use more than one ground plane:

1. Separate analog and digital grounds: If your PCB has both analog and digital circuitry, it’s often recommended to have separate ground planes for each domain. This helps to isolate the sensitive analog signals from the noisy digital ones. The analog and digital ground planes are usually connected at a single point, often near the power supply.

2. Multi-layer boards: On PCBs with four or more layers, it’s common to have dedicated ground planes on one or more of the inner layers. For example, a 4-layer board might have the following stackup: Top signal layer, Ground plane layer, Power plane layer, Bottom signal layer. The ground and power planes provide low-impedance paths for return currents and power distribution.

3. High-frequency designs: In some high-speed or RF designs, you may have multiple ground planes to provide shielding and controlled impedance for the signal layers. The ground planes are often stitched together with vias to ensure good electrical connection.

4. Split grounds: In certain applications, you may intentionally split the ground plane into separate sections for safety, noise isolation, or other reasons. For example, in a mixed-voltage design with both high and low voltage components, you would want to keep their grounds separated to prevent faults.

When using multiple ground planes, it’s important to manage their connections carefully. Improper grounding can lead to ground loops, noise coupling, and other issues. The DRC in Kicad can help catch some potential problems, like isolated ground plane sections or insufficient clearance between planes.

What are some best practices for ground plane design in Kicad?

Here are some general guidelines and best practices to follow when designing ground planes in Kicad:

1. Use copper pours: The easiest way to create a ground plane is to use the “Add filled zones” tool to pour a solid copper area on the desired layer. This automatically creates a plane that follows the outline of your board and fills in the open areas.

2. Set appropriate clearance rules: Make sure to specify adequate clearance between the ground plane and other copper features in the design rules. The exact values depend on your PCB manufacturer’s capabilities and the electrical requirements of your design.

3. Provide thermal relief: Through-hole pads and vias that connect to the ground plane should have thermal relief spokes to aid in soldering. Kicad allows you to customize the thermal relief settings in the zone properties dialog.

4. Stitch multiple planes together: If you have ground planes on multiple layers, use stitching vias to electrically connect them. This ensures a low-impedance path for return currents. The via size and spacing should be chosen based on the frequency and current requirements.

5. Avoid narrow areas and sharp angles: Ground plane sections that are too thin or have acute angles can cause manufacturing issues like acid trapping or over-etching. Try to keep the ground plane geometry as wide and smooth as possible.

6. Consider split planes carefully: If you need to split the ground plane for noise isolation or other reasons, be sure to manage the return paths and connections properly. Use a single point of connection between planes if needed to avoid ground loops.

7. Run DRC frequently: The Design Rule Check tool in Kicad is your friend. Run it often during layout to catch any ground plane issues early. Don’t wait until the end of the design process to check for errors.

8. Review the ground plane: Take the time to visually inspect the ground plane geometry and connections. Look for any areas that seem problematic or violate good design practices.

Remember, the goal is to create a ground plane that provides a low-impedance return path, good shielding, and reliable connections while also being manufacturable. It may take some trial and error to find the right balance of design rules and layout practices for your particular project.

By following these guidelines and carefully considering the requirements of your PCB, you can create robust, error-free ground planes in Kicad. Properly designed ground planes are essential to the overall performance and reliability of your electronic devices.