Printed Circuit Boards (PCBs) form the backbone of modern electronics, enabling intricate designs and high-speed signal transmission. Among the numerous techniques employed in PCB design, mitred traces are particularly noteworthy for improving signal integrity and manufacturability in specific scenarios.
This article delves into what mitred traces are, their advantages, design considerations, and common use cases.
What Are Mitred Traces?
In PCB design, mitred traces (also called beveled or chamfered corners) refer to traces where sharp right-angle corners are replaced by angled cuts or segments. Instead of a trace making a 90° turn, it transitions via one or more intermediate angles, such as two 45° segments or a rounded curve.
For example:
- Right-angle corner: A direct 90° turn.
- Mitred corner: Two 45° bends creating a smoother path.
- Curved corner: A continuous arc replacing the corner.
Why Use Mitred Traces?
1. Signal Integrity
Mitred traces help mitigate issues with high-frequency signal integrity by reducing impedance discontinuities and electromagnetic interference (EMI). In high-speed PCBs, sharp corners can cause:
- Signal reflections
- Increased radiated noise
- Crosstalk
By smoothing out the trace path, mitred corners minimize these effects, ensuring more consistent signal transmission.
2. Manufacturability
Sharp corners in traces are prone to acid traps during PCB etching, where etching chemicals accumulate, potentially causing incomplete etching or short circuits. Mitred traces eliminate these sharp corners, improving the etching process’s reliability.
3. Mechanical Durability
Corners with a sharp 90° angle concentrate stress, which can lead to cracking or delamination under mechanical strain or thermal cycling. Mitring spreads this stress over a larger area, enhancing the board’s structural integrity.
Design Considerations for Mitred Traces
When incorporating mitred traces in your PCB design, consider the following:
1. Frequency and Signal Type
Mitring is most beneficial for high-frequency signals (e.g., GHz-range RF or digital signals). For low-frequency or DC traces, sharp corners generally don’t cause significant issues.
2. Trace Width
Wider traces have more significant impedance mismatches at sharp corners. For power delivery or high-current traces, use mitred or curved transitions to reduce these effects.
3. Mitring Angles
Commonly, two 45° angles are used to replace a 90° corner. For even smoother transitions, you can opt for rounded traces, which are particularly beneficial at ultra-high frequencies.
4. Manufacturing Tolerances
Ensure that the mitred angles and segment lengths comply with your manufacturer’s capabilities. Overly complex mitring or tight tolerances may increase manufacturing costs.
How to Implement Mitred Traces in PCB Design Software
Most modern PCB CAD tools, such as Altium Designer, KiCad, or Eagle, allow you to configure trace corners easily. Here’s how:
- Corner Configuration: Check for corner options in the trace routing settings.
- Miter Ratio: Specify the miter ratio (e.g., 50%) to control the length of the beveled edge.
- Arc Routing: If a rounded corner is preferred, enable the arc routing mode in your software.
Common Applications
- High-Speed Digital Circuits: To reduce reflections in high-speed buses (e.g., DDR, USB, PCIe).
- RF Circuits: To maintain impedance matching in RF paths.
- Power Distribution Networks (PDNs): To improve reliability in wide power traces.
- Military and Aerospace: Where mechanical stresses and thermal cycling are more demanding.
Conclusion
Mitred traces are a simple yet effective technique to enhance PCB performance and reliability, particularly in high-speed, high-frequency, or high-current applications. While they may not always be necessary for lower-frequency designs, their inclusion demonstrates good engineering practices and forward-looking design considerations.
By adopting mitred traces where appropriate, designers can create more robust, high-performance PCBs that meet the demands of modern electronics.
