- Ben Seaman
High speed digital PCB design tips/tricks
Designing high-speed digital PCB traces involves several factors that need to be considered that can seriously impact the performance of your signal integrity (SI). Setting up design rules (constraints) during the capture/layout process will help the engineer create a solid foundation for the high speed lines. We at Convergence always recommend post layout simulation to validate the constraints/board impedance/reflections to ensure manufacturing guidance for reliable signals quality.
Here are some guidelines/tips for designing high-speed digital PCB traces:
1. Trace Length and Impedance Matching: The trace length should be kept as short as possible to reduce signal distortion and crosstalk. The trace impedance should be matched to the driver and receiver impedances to minimize reflections.
2. Trace Width and Space: Trace width affects the trace impedance and should be adjusted accordingly. Always work with your PCB board shop of choice to verify stackup/trace width. The distance between adjacent traces (trace spacing) should be kept as large as possible to reduce crosstalk. Usually 2-3 track width apart min.
3. Plane Layer Usage: Power and ground planes should be used to reduce noise and provide a low-impedance return path for the signals. Saturn PCB tools offers a fantastic free tool to help you calculate different type of tracking types like microstrip, stripeline, coplanar wave, etc.
4. Controlled Impedance: Controlled impedance is important in high-speed designs to ensure consistent impedance along the entire trace length. Some designs require tighter tolerances in controlled impedance. You can also look at high speed dielectric materials with better properties for higher speed signals, again talk to your board shops for guidance.
5. Differential Pair Routing: Differential pair routing should be used for high-speed signals to reduce noise and crosstalk. The trace length of the differential pair should be kept as matched as possible to maintain the differential characteristic. Typically manufacture datasheets will give you information on max allowable delta length. You can tune/trombone to match accordingly without using sharp 90deg bends.
6. Ground Bounce and Decoupling Capacitors: Ground bounce can be a major issue in high-speed designs. Decoupling capacitors should be placed as close as possible to the power pins of the high-speed components to reduce the ground bounce.
7. Signal Integrity Analysis: Signal integrity analysis should be performed to validate the design and identify any issues such as reflections, crosstalk, and termination problems. At Convergence we have all the latest tools for SI analysis and DDR validation (Cadence or Mentor)
8. Via Structure: You can utilize advanced via structure such as micro vias (laser) and back-drilling of vias to reduce stub/reflection points in the signal chain. This will add costing to the PCB price but improve SI performance significantly.
It is important to use proper PCB design software and simulation tools to verify the design and ensure that it meets the high-speed performance requirements. Let our Convergence SI experts help through the high speed design process for additional confidence before printing PCBs.