Microstrip vs Stripline vs CPW Guide
When routing high-speed signals on a PCB, choosing the right transmission line structure is critical. Microstrip, stripline, and coplanar waveguide (CPW) are the three main options, each with distinct characteristics that affect impedance, crosstalk, and signal integrity.
This guide explains the differences, shows when to use each, and provides practical impedance calculations. Whether you're designing USB, HDMI, PCIe, or RF circuits, understanding these structures is essential.
Transmission Line Overview
Microstrip
Trace on an outer layer with a ground plane below. Partially exposed to air.
Stripline
Trace sandwiched between two ground planes. Completely enclosed in dielectric.
Coplanar (CPWG)
Trace on outer layer with ground on both sides and below. Best shielding.
Quick Comparison
| Feature | Microstrip | Stripline | CPWG |
|---|---|---|---|
| Layer Position | Outer layer | Inner layer | Outer layer |
| Reference Planes | 1 (below) | 2 (above & below) | 3 (sides & below) |
| Dielectric | Air + PCB | PCB only | Air + PCB |
| Eff. Dielectric Constant | ~2.8-3.2 | ~4.0-4.5 | ~2.5-3.0 |
| Typical Impedance | 50-100Ω | 40-60Ω | 50-75Ω |
| EMI Shielding | Fair | Excellent | Good |
| Crosstalk | Moderate | Low | Low |
| Attenuation | Lower | Higher | Medium |
| Routing Density | High | Medium | Lower |
Microstrip: The Workhorse
Microstrip is the most common transmission line structure because it's on an outer layer, making it easy to route and debug. The trace sits on top of the dielectric with a ground plane below.
Impedance Formula
Z₀ = (87 / √(εᵣ + 1.41)) × ln(5.98h / (0.8w + t))
Where: h = dielectric height, w = trace width, t = trace thickness, εᵣ = dielectric constant
Typical Values for 50Ω
| Dielectric Height | Trace Width | Ratio (w/h) |
|---|---|---|
| 4 mil (0.1mm) | 7.5 mil (0.19mm) | 1.9:1 |
| 5 mil (0.127mm) | 9.5 mil (0.24mm) | 1.9:1 |
| 8 mil (0.2mm) | 15 mil (0.38mm) | 1.9:1 |
| 10 mil (0.254mm) | 19 mil (0.48mm) | 1.9:1 |
| 12 mil (0.3mm) | 23 mil (0.58mm) | 1.9:1 |
Advantages
- Easiest to route and manufacture
- Components can be placed directly
- Lower dielectric loss
- Easy to probe and debug
- More forgiving tolerances
Disadvantages
- Susceptible to EMI pickup
- Higher crosstalk than stripline
- Signal radiates more
- Impedance varies with solder mask
Stripline: The Shielded Champion
Stripline places the signal trace between two ground planes, providing natural shielding. It's ideal for sensitive signals that need protection from EMI and minimal crosstalk.
Impedance Formula
Z₀ = (60 / √εᵣ) × ln(4b / (0.67π(0.8w + t)))
Where: b = distance between ground planes, w = trace width, t = trace thickness
Typical Values for 50Ω
| Ground Spacing (b) | Trace Width | Notes |
|---|---|---|
| 8 mil | 4 mil | HDI stackup |
| 10 mil | 5 mil | Standard 6-layer |
| 12 mil | 6 mil | Standard 4-layer |
| 16 mil | 8 mil | Thicker stackup |
| 20 mil | 10 mil | Power/RF boards |
Note: Stripline traces are narrower than microstrip for the same impedance because the full dielectric constant applies (not reduced by air interface).
Advantages
- Excellent EMI shielding
- Very low crosstalk
- Consistent impedance
- No solder mask impact
- Better for high-frequency signals
Disadvantages
- Requires vias for component access
- Higher manufacturing cost
- More dielectric loss
- Harder to debug
- Uses inner layer routing resources
Coplanar Waveguide with Ground (CPWG)
CPWG adds ground traces on either side of the signal trace, plus a ground plane below. This provides excellent shielding while keeping the signal on an outer layer for easy component access.
When to Use CPWG
- RF/Microwave circuits where impedance control is critical
- High-density designs needing outer-layer controlled impedance
- Mixed-signal boards with analog and digital sections
- Differential pairs requiring tight coupling
Typical Values for 50Ω
| Trace Width | Gap to Ground | Impedance |
|---|---|---|
| 8 mil | 5 mil | 50Ω |
| 10 mil | 6 mil | 50Ω |
| 12 mil | 8 mil | 50Ω |
| 15 mil | 10 mil | 50Ω |
Advantages
- Outer layer accessibility
- Good EMI shielding
- Low crosstalk
- Excellent for RF circuits
- Ground readily available
Disadvantages
- Uses more routing space (3× minimum)
- Complex impedance calculation
- Sensitive to gap width variations
- Higher manufacturing tolerance needed
Differential Pairs: Microstrip vs Stripline
Differential signals (USB, HDMI, PCIe, Ethernet) can use either edge-coupled microstrip or edge-coupled stripline. Here's how they compare:
| Parameter | Edge-Coupled Microstrip | Edge-Coupled Stripline |
|---|---|---|
| Typical Trace Width | 5-8 mil | 4-6 mil |
| Typical Spacing | 5-10 mil | 5-8 mil |
| Crosstalk Isolation | -20 to -30 dB | -30 to -40 dB |
| Length Matching | ±5 mil typical | ±5 mil typical |
| Common Use | USB, HDMI | PCIe, DDR |
Tip: For differential impedance (Zdiff), the traces are coupled, so: Zdiff ≈ 2 × Z₀ × (1 - k), where k is the coupling coefficient (typically 0.1-0.3).
Selection Guide: Which One to Use?
| Application | Recommended | Reason |
|---|---|---|
| USB 2.0 (480 Mbps) | Microstrip | Simple, forgiving tolerances |
| USB 3.0/3.1 (5-10 Gbps) | Microstrip or Stripline | Depends on EMI requirements |
| HDMI 1.4/2.0 | Microstrip | Outer layer for connector access |
| HDMI 2.1 (48 Gbps) | Stripline | Better signal integrity at 12 GHz |
| PCIe Gen3/4 | Stripline | Low crosstalk critical |
| PCIe Gen5/6 | Stripline (required) | 32+ Gbps needs shielding |
| DDR4/DDR5 | Microstrip | Short traces, component access |
| Ethernet 1G | Microstrip | Simple, proven design |
| Ethernet 10G+ | Stripline | EMI and crosstalk critical |
| RF (< 6 GHz) | CPWG | Best impedance control |
| RF (> 6 GHz) | CPWG/Stripline | Loss and radiation concerns |
Impedance Calculation Tools
Manual impedance calculations are tedious and error-prone. Use our free PCB Impedance Calculator to quickly determine trace width for your desired impedance. It supports:
- Microstrip (single-ended and differential)
- Stripline (symmetric and asymmetric)
- Coplanar waveguide with ground (CPWG)
- Custom dielectric constants and stackups
Manufacturing Tolerances
Your calculated impedance is only as good as your manufacturing accuracy. Here's what affects each structure:
| Variation | Microstrip | Stripline | CPWG |
|---|---|---|---|
| Trace width ±1 mil | ±3-5Ω | ±4-6Ω | ±2-4Ω |
| Dielectric height ±10% | ±4-5Ω | ±3-4Ω | ±3-4Ω |
| εᵣ ±5% | ±2Ω | ±3Ω | ±2Ω |
| Copper thickness ±20% | ±1-2Ω | ±1-2Ω | ±1-2Ω |
Practical Tip: Request impedance-controlled manufacturing and specify your target impedance with tolerance (e.g., 50Ω ±10%). The fab will adjust trace widths based on their actual material properties.
Summary
Use Microstrip When:
You need easy routing, component access, and lower manufacturing cost. Good for most digital interfaces up to ~5 GHz where EMI isn't critical.
Use Stripline When:
You need maximum EMI shielding, low crosstalk, or are routing high-speed signals (>10 GHz). Essential for PCIe Gen4+, 10G+ Ethernet, and sensitive RF.
Use CPWG When:
You need shielded signals on outer layers, RF/microwave design, or tight impedance control with component access. Common in wireless and antenna circuits.
Ready to calculate your trace dimensions? Try our Impedance Calculator for quick, accurate results.
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