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Parameters
DFM Check
Characteristic Impedance
Engineering Cheat Sheets
Common Protocols
Material Selection Guide
- Standard FR-4 (Tg 130-150)
Low cost. Good for digital <1GHz. High loss tangent (Df ~0.02).
- High Tg FR-4 (Tg 170+)
Reliable for multi-layer (>6L). Isola 370HR.
- Low Loss / High Speed
Required for 10Gbps+. Megtron 6, Rogers 4350B. Low Df (~0.002).
Fabrication Limits (DFM)
Impedance Basics for PCB Design
Essential knowledge for signal integrity engineers. Master these concepts to design reliable high-speed circuits.
What is Characteristic Impedance (Z₀)?
Characteristic impedance is the ratio of voltage to current for a wave traveling along a transmission line. It depends on the physical geometry of the trace (width, thickness, height above ground plane) and the dielectric constant (Dk) of the PCB material. For a lossless line, Z₀ = √(L/C), where L is inductance per unit length and C is capacitance per unit length.
Key Impedance Formulas
Microstrip Impedance
Approximation for outer layer traces. Valid when W/H > 0.1 and εᵣ < 16.
εᵣDielectric constant (Dk)HHeight to ground planeWTrace widthTTrace thicknessStripline Impedance
For inner layer traces between two ground planes. Better EMI shielding.
εᵣDielectric constantHTotal dielectric heightWTrace widthTTrace thicknessPropagation Delay
Time for signal to travel one inch. Critical for timing analysis.
tpdPropagation delayεᵣEffective dielectric constantDifferential Impedance
For differential pairs. k is the coupling coefficient between traces.
ZdiffDifferential impedanceZ₀Single-ended impedancekCoupling coefficient (0-1)Skin Depth
Depth at which current density drops to 37%. Affects high-frequency loss.
δSkin depthρResistivity (copper: 1.68×10⁻⁸ Ω·m)fFrequency in HzVia Inductance
Johnson formula for via inductance. Critical for power integrity.
hVia height in inchesdVia diameter in inchesLInductance in nHQuick Reference Tables
Common PCB Materials
| Material | Dk | Df | Use Case |
|---|---|---|---|
| FR-4 Standard | 4.2-4.5 | 0.02 | General purpose, <3Gbps |
| FR-4 High Tg | 4.2-4.4 | 0.018 | Lead-free, high temp |
| Isola 370HR | 4.04 | 0.021 | High reliability |
| Megtron 6 | 3.4 | 0.002 | High-speed, 25Gbps+ |
| Rogers 4350B | 3.48 | 0.0037 | RF/Microwave to 10GHz |
| Rogers 4003C | 3.55 | 0.0027 | Low-cost RF |
Standard Impedance Targets
| Interface | Z₀ (SE) | Zdiff | Notes |
|---|---|---|---|
| DDR4/DDR5 | 40Ω | 80Ω | ±10% tolerance |
| USB 2.0 | 45Ω | 90Ω | ±10% |
| USB 3.x/4 | 45Ω | 85Ω | ±10% |
| PCIe Gen3/4/5 | 50Ω | 85Ω | ±10% |
| HDMI 2.x | 50Ω | 100Ω | ±10% |
| Ethernet 1G | 50Ω | 100Ω | ±10% |
| SATA | 50Ω | 100Ω | ±15% |
Copper Weight Conversion
| Weight (oz) | Thickness (mil) | Thickness (μm) | Current (A/mm) |
|---|---|---|---|
| 0.5 oz | 0.7 mil | 17.5 μm | ~3A |
| 1 oz | 1.4 mil | 35 μm | ~6A |
| 2 oz | 2.8 mil | 70 μm | ~12A |
| 3 oz | 4.2 mil | 105 μm | ~18A |
Skin Depth vs Frequency
| Frequency | Skin Depth | Effect |
|---|---|---|
| 100 MHz | 6.6 μm | Minimal impact |
| 1 GHz | 2.1 μm | Starts affecting 0.5oz |
| 5 GHz | 0.93 μm | Significant loss |
| 10 GHz | 0.66 μm | Use smooth copper |
| 25 GHz | 0.42 μm | Critical - HVLP required |
Microstrip vs Stripline Comparison
Microstrip
Outer layer trace
- Faster propagation (≈6.4 in/ns for FR-4)
- Easier to probe and debug
- Lower manufacturing cost
- Higher EMI radiation
- More susceptible to crosstalk
Stripline
Inner layer trace
- Excellent EMI shielding
- Lower crosstalk between traces
- More consistent impedance
- Slower propagation (≈5.8 in/ns)
- Harder to access for testing
Pro Tips for Impedance Control
Keep trace spacing ≥3× trace width to minimize crosstalk. For critical signals, use 5W.
Always ensure a continuous ground plane beneath high-speed traces. Avoid splits and slots.
For DDR, match data lines within ±10mil. Use serpentine routing on shorter traces.
Back-drill vias for >10Gbps signals. Stubs cause reflections at λ/4 frequency.
Why Engineers Trust ImpedanceCalculator
High-fidelity physics engines combined with AI to solve signal integrity problems in seconds.
Real-Time Precision Physics
IPC-2141 compliant solver gives instant feedback on impedance, inductance, and capacitance.
- Instant Feedback
- IPC-2141 Compliant
- Microstrip & Stripline
AI-Powered Analysis
Integrated AI analyzes geometry for manufacturing risks and physics limitations.
- Detects Acid Traps
- Warns of High Loss
- Optimizes Stackup
Frequency Dependent Loss
Calculate insertion loss across your target frequency range for signal integrity.
- Dielectric Loss (Df)
- Skin Effect Loss
- Roughness Modeling
Frequently Asked Questions
Why is 50Ω the standard impedance?
What is the difference between Dk (Dielectric Constant) and Df (Loss Tangent)?
How does 'Skin Effect' impact my design?
What impedance tolerance should I specify?
How do vias affect signal integrity?
When should I use differential signaling?
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