How to Run a Design Rule Check (DRC) for Your PCBs

Any time you design a PCB, and you want to turn it into a real product, you will have to make sure the design obeys all the constraints within the standard PCB manufacturing process. These constraints in the PCB design are known as Design rules and to check these rules in a CAD software we have to run the DRC(Design Rule Check) algorithm. The ever-increasing complexity of boards has made it nearly impossible to carry out the design rule verification process manually, so designers have to program the right design rules into their CAD software to move easily through the PCB design workflow.

DRC is a process that checks your PCB design against a set of predetermined rules to ensure manufacturability and functionality. Running a DRC early and often in the design process can save you time, reduce errors, and minimize costly rework. Here's a step-by-step guide on how to effectively run a DRC for your PCBs.

Rule Checking in the PCB Design Workflow

DRCs should be integrated at various stages of the PCB design process, but maintaining productivity depends on how they are initiated. ECAD software offers rules-checking tools that can be fully or partially automated, and it's important for designers to set up these tools at the beginning of a project. Before starting the PCB layout, designers should configure rules in categories such as:

1. Minimum clearances between all copper elements and components

2. Trace constraints, such as total length, width and routing

3. Net-specific rules, for example rules on differential pairs and impedance control.

4. DFA rules related to solder mask, pad-to-pad clearances, and component spacing

5. Board, via, and mechanical edge clearances

While these design rules help ensure compliance with most DFM requirements, manually managing all of them can be overwhelming and error-prone. Since human involvement always carries the risk of mistakes, it's crucial to complement a manual design review with automated rules checks throughout the PCB design process. This way, errors can be identified and corrected early on. All the DRC rules are given in a brief in the next section.

7 Common DRC Checks in a PCB Design

1. Spacing Constraints

• Via-to-Via Spacing: Specifies the minimum allowable spacing between vias.
• Net Spacing Types: Defines spacing rules for different types of nets.
• Copper Spacing Constraints: Defines spacing rules for copper features.
• Etch Text to Copper Shapes: Checks for spacing between etch text and copper shapes.
• Package-to-Package Spacing: Defines the minimum spacing required between different packages.

2. Line Width Constraints

• Line Width Constraints: Constraints related to the width of lines in the design.
• Minimum Line Width: Specifies the minimum width allowable for traces.
• Maximum Line Width: Specifies the maximum width allowable for traces.

3. Via Constraints

• Stacked Vias: Rules for creating and managing stacked vias in designs.
• Via List Constraints: Constraints related to the allowable via types and their usage.
• Via Under Component: Rules prohibiting vias under specified components.

For more information on via constraints and the different types of vias in PCB design, check out our comprehensive guide on PCB vias.

4. Net Constraints

• Net Physical Types: Defines physical properties and constraints of different nets.
• Differential Pair Constraints: Rules for handling differential pairs in designs.

5. Annular Ring

• Annular Ring Constraints: Constraints related to the annular ring dimensions.
• Through-Hole Pins Under SMD Components: Checks for incorrect through-hole pin placements under SMD components.
• Component Lead Checks: Checks related to the leads of components.

Check out our comprehensive guide for more information on annular rings in PCB design.

6. Fiducial and Outline Constraints

• Fiducial Checks: Checks for proper fiducial placements.
• Outline Constraints: Constraints related to the outline of the design.

7.Cutout Constraints

Cutout Constraints: Rules for design cutouts and their placements.

Design for Manufacturing Constraints

Automating Your DRC Process

In general, there are three ways to streamline rule checking in a typical PCB design workflow:

1. Manual Design Review: The designer manually reviews the PCB to ensure it meets physical and functional requirements. Teams usually discuss and verify specific aspects like mechanical and electrical constraints.

2. Online DRC: Always keep this enabled during layout and routing. It helps catch rule violations in real-time as you're working, with any issues highlighted visually so you can fix them on the spot.

3. Comprehensive DRC: Once the design is done and you're ready to generate outputs, run a comprehensive DRC. It will check all rules and create a detailed report of any violations that might have been missed during layout. After that these highlighted errors can be manually addressed by the designer.

How to Run a Design Rule Check (DRC) for Your Custom PCBs

Step 1: Set Up Your Design Rules

As mentioned earlier, before running a DRC, you need to define your design rules. Most PCB design software, such as EasyEDA, KiCad, or Eagle, allows you to customize these rules based on your specific needs and your manufacturer’s recommendations. Make sure to obtain the design rule parameters from your PCB manufacturer, as they often have specific guidelines. The rules are generally listed in the capabilities section of the manufacturer.

Once you've determined required clearances, trace geometry and routing limits, assembly constraints, and any custom design constraints, you'll want to define these inside the PCB Rules and Constraints Editor window before you begin the PCB layout.

Step 2: Run the DRC in Your PCB Design Software

After setting the design rules, it's time to run the DRC within your PCB design software. Most tools have an integrated DRC function that will automatically analyze the entire design for any rule violations. Here's how to typically execute the DRC:

Open the DRC feature which is usually available under the "Tools" or "Validation" section in the software. Then initiate the DRC, and the software will generate a list of potential violations. Here is a blog article covering all the DRC errors in detail.

Step 3: Review and Resolve Violations

Once the DRC is complete, review the list of violations generated by the software. Each violation will be flagged in your PCB layout, showing the location and nature of the issue.

Resolve each violation by modifying the layout according to the flagged issues, adjusting trace widths, spacing, or repositioning components as necessary.

Step 4: Re-run the DRC

After making adjustments to your design, re-run the DRC to ensure all violations have been addressed. It’s common to go through multiple iterations before the design is entirely error-free.

Best Practices for Running DRC

• Run DRC Frequently: Don't wait until the design is complete to run the DRC. Run checks frequently as you design to catch errors early.

• Understand Manufacturer Capabilities: Make sure your design rules align with your manufacturer’s capabilities to prevent redesigns.

• Prioritize Critical Errors: Some DRC errors might be non-critical, but others could lead to design failure. Prioritize resolving critical errors that can affect functionality.

Conclusion

Running a Design Rule Check (DRC) is a crucial part of the PCB design process. It ensures that your layout meets manufacturing standards and prevents costly mistakes down the line. By understanding your design rules, using your PCB design software effectively, and collaborating with your manufacturer, you can ensure a successful DRC process and a reliable, manufacturable PCB.