Stripline configurations are widely used in high-frequency circuit designs due to their excellent shielding and predictable impedance characteristics. The determination of line widths for achieving specific characteristic impedances in RT-duroid® laminates requires accurate calculations based on material properties and geometry.

1. Parameters Affecting Characteristic Impedance

The characteristic impedance (Z0Z_0Z0​) of a stripline depends on several factors:

1. Substrate Thickness (hhh)
   • The distance between the ground planes and the signal conductor.
2. Dielectric Constant (ϵr\epsilon_rϵr​)
   • The relative permittivity of the RT-duroid® material (e.g., 5870: ϵr=2.33\epsilon_r = 2.33ϵr​=2.33, 5880: ϵr=2.20\epsilon_r = 2.20ϵr​=2.20).
3. Conductor Width (www)
   • The width of the signal trace.
4. Conductor Thickness (ttt)
   • The thickness of the signal trace, typically 17 µm (0.5 oz copper) or 35 µm (1 oz copper).
5. Ground Plane Separation (2h2h2h)
   • The total spacing between the two parallel ground planes.

2. Design Equations for Stripline Impedance

For a symmetric stripline, the characteristic impedance can be calculated using:

Equation 1: Simplified Formula (Approximations)

For w/h≤1w/h \leq 1w/h≤1:Z0=60ϵrln⁡(4hw+1.393+23wh)Z_0 = \frac{60}{\sqrt{\epsilon_r}} \ln \left( \frac{4h}{w} + 1.393 + \frac{2}{3} \frac{w}{h} \right)Z0​=ϵr​​60​ln(w4h​+1.393+32​hw​)

Equation 2: Precise Formula (Including Conductor Thickness)

Z0=87ϵr+1.41ln⁡(5.98h0.8w+t)Z_0 = \frac{87}{\sqrt{\epsilon_r + 1.41}} \ln \left( \frac{5.98h}{0.8w + t} \right)Z0​=ϵr​+1.41​87​ln(0.8w+t5.98h​)

Where:

• hhh: Distance from the signal trace to the nearest ground plane.
• ttt: Trace thickness.
• www: Trace width.

3. Impedance vs. Line Width Table for RT-Duroid® 5870 (Example)

For a substrate with h=0.508 mmh = 0.508 \, \text{mm}h=0.508mm (20 mil), t=0.017 mmt = 0.017 \, \text{mm}t=0.017mm (0.5 oz copper), and ϵr=2.33\epsilon_r = 2.33ϵr​=2.33:

Characteristic Impedance (Z0Z_0Z0​)	Line Width (www) [mm]
50 Ω	0.61
75 Ω	0.39
100 Ω	0.29
120 Ω	0.23

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4. General Design Guidelines

1. Impedance Matching:
   • For RF circuits, ensure that the characteristic impedance of the stripline matches the system impedance (e.g., 50 Ω).
2. Dielectric Constant Consideration:
   • Use the specific dielectric constant (ϵr\epsilon_rϵr​) of the chosen RT-duroid® laminate for precise calculations.
3. Signal Integrity:
   • Minimize signal losses by optimizing the trace width for the desired impedance while considering manufacturing tolerances.
4. Manufacturing Constraints:
   • Ensure the line width is manufacturable given the PCB fabrication process (e.g., minimum trace width).

5. Tools for Line Width Calculations

1. CAD Software:
   • Utilize PCB design tools (e.g., Altium Designer, KiCad, or Cadence) with built-in impedance calculators.
2. Online Calculators:
   • Tools like Rogers Corporation’s Impedance Calculator or other RF/microwave design platforms.
3. Simulation:
   • Perform electromagnetic simulations (e.g., using Ansys HFSS or CST Studio Suite) for high-precision designs.

6. Example Calculation

Given:

• ϵr=2.33\epsilon_r = 2.33ϵr​=2.33
• h=0.508 mmh = 0.508 \, \text{mm}h=0.508mm (20 mil)
• Z0=50 ΩZ_0 = 50 \, \OmegaZ0​=50Ω

Using Equation 2:

Z0=872.33+1.41ln⁡(5.98×0.5080.8w+0.017)Z_0 = \frac{87}{\sqrt{2.33 + 1.41}} \ln \left( \frac{5.98 \times 0.508}{0.8w + 0.017} \right)Z0​=2.33+1.41​87​ln(0.8w+0.0175.98×0.508​)

Solving for www, we find:w≈0.61 mmw \approx 0.61 \, \text{mm}w≈0.61mm

7. Summary

• Precise control over line width is critical for achieving the desired impedance in stripline designs.
• Use the provided equations and impedance calculators to design traces tailored to your specific RT-duroid® laminate and application requirements.
• Ensure thorough testing to verify impedance performance, especially for high-frequency circuits.