<h1>High-Speed Impedance Control</h1> <h2>Overview</h2> <p>Impedance control is critical for high-speed PCB design to ensure signal integrity and minimize reflections.</p> <h2>Key Concepts</h2> <h3>1. Controlled Impedance Basics</h3> <p><strong>Why Impedance Matters:</strong></p> <ul> <li>Prevents signal reflections</li> <li>Maintains signal integrity</li> <li>Reduces EMI and crosstalk</li> <li>Ensures proper signal timing</li> </ul> <p><strong>Target Impedance Values:</strong></p> <ul> <li>Single-ended: 50Ω (common), 75Ω (video)</li> <li>Differential: 90Ω, 100Ω (USB), 120Ω (PCIe)</li> </ul> <h3>2. Trace Geometry Control</h3> <p><strong>Critical Parameters:</strong></p> <ul> <li>Trace width</li> <li>Dielectric thickness</li> <li>Copper weight</li> <li>Dielectric constant (Dk)</li> </ul> <p><strong>Calculations:</strong></p> <ul> <li>Use field solvers for accurate results</li> <li>Online calculators for estimates</li> <li>Manufacturer impedance calculators</li> </ul> <h3>3. Routing Best Practices</h3> <p><strong>High-Speed Routing Rules:</strong></p> <ol> <li>Solid reference planes under traces</li> <li>Avoid crossing split planes</li> <li>Maintain consistent spacing</li> <li>Minimize via transitions</li> <li>Use 45° or curved bends (not 90°)</li> </ol> <p><strong>Differential Pair Routing:</strong></p> <ul> <li>Match lengths within 5-10 mil</li> <li>Maintain consistent spacing</li> <li>No components between pairs</li> <li>Minimize layer transitions</li> </ul> <h3>4. Via Considerations</h3> <p><strong>Via Effects on Impedance:</strong></p> <ul> <li>Vias create impedance discontinuities</li> <li>Stub lengths cause reflections</li> <li>Use via-in-pad for high-speed</li> <li>Consider back-drilling for long stubs</li> </ul> <h3>5. Material Selection</h3> <p><strong>Dielectric Properties:</strong></p> <ul> <li>FR-4: Dk ≈ 4.0-4.5 (general use)</li> <li>High-speed materials: Dk ≈ 3.0-4.0</li> <li>Low-loss materials for &gt;5GHz</li> </ul> <h3>6. Testing and Validation</h3> <p><strong>TDR Testing:</strong></p> <ul> <li>Time Domain Reflectometry</li> <li>Verify impedance values</li> <li>Locate discontinuities</li> <li>Validate differential pairs</li> </ul> <h2>Design Checklist</h2> <ul> <li><input disabled="" type="checkbox"> Calculate required impedance</li> <li><input disabled="" type="checkbox"> Select appropriate dielectric</li> <li><input disabled="" type="checkbox"> Determine trace width/spacing</li> <li><input disabled="" type="checkbox"> Plan reference planes</li> <li><input disabled="" type="checkbox"> Specify in fabrication notes</li> <li><input disabled="" type="checkbox"> Include impedance test coupon</li> </ul> <hr> <p><em>References: <a href="https://www.protoexpress.com/blog/best-high-speed-pcb-routing-practices/">High-Speed PCB Routing Practices</a>, <a href="https://www.ti.com/lit/ml/slyp173/slyp173.pdf">TI High-Speed Layout Guide</a>, <a href="https://s3vi.ndc.nasa.gov/ssri-kb/static/resources/High-Speed%20PCB%20Design%20Guide.pdf">NASA High-Speed Design</a></em></p>