The industry-standard tool for RF and High-Speed Digital engineers. Calculate precise characteristic impedance (Z₀), signal delay, and inductance for microstrip and stripline transmission lines based on IPC-2141 recommendations.
1 oz ≈ 1.37 mil
Everything engineers need to know about designing high-speed transmission lines, from dielectric constants to manufacturing tolerances.
In modern electronics, PCB traces are no longer just point-to-point connections; they are transmission lines. When digital switching speeds increase (rising edge times drop below 1ns), the trace geometry begins to significantly impact signal quality.
Controlled Impedance is the maintenance of a consistent signal reflection characteristic along the length of a trace. Without it, your circuit may suffer from:
Energy bouncing back from impedance discontinuities, causing ringing and data errors.
Impedance mismatches act as antennas, radiating noise and failing EMC compliance.
Loss of signal strength over long traces due to dielectric absorption and skin effect.
Unpredictable delays that close the data eye diagram, leading to bit errors.
Understanding the difference between transmission line structures is critical for stackup planning.
| Feature | Microstrip (Outer Layers) | Stripline (Inner Layers) |
|---|---|---|
| Propagation Speed | Faster (~140-150 ps/in) | Slower (~170-180 ps/in) |
| EMI / Radiation | High (Radiates outwards) | Low (Shielded by planes) |
| Impedance Control | Good (Subject to plating var) | Excellent (Better consistency) |
| Layer Count | Uses 1 signal layer | Requires 2 ref planes |
Microstrip lines are located on the outer layers of the PCB (Top or Bottom) and consist of a conductor separated from a single ground plane by a dielectric. Due to one side being exposed to air (Er ≈ 1.0), the effective dielectric constant is lower, resulting in faster signal propagation velocity.
Stripline traces are embedded within the PCB, sandwiched between two ground planes. This structure provides complete shielding, eliminating external radiation and reducing susceptibility to noise, making it ideal for high-speed clocks and sensitive RF signals.
This calculator utilizes the equations defined in IPC-2141 "Design Guide for High-Speed Controlled Impedance Circuit Boards". While 2D field solvers provide the highest accuracy by solving Maxwell's equations numerically, the closed-form IPC equations provide a standard approximation suitable for most FR-4 rigid PCB designs below 2GHz.
IPC-2141 Surface Microstrip Equation
Z₀ = [87 / √(Er + 1.41)] × ln[(5.98 × H) / (0.8 × W + T)]
Key Takeaway: Key Takeaway: Impedance is inversely proportional to trace width (W) and trace thickness (T), and directly proportional to dielectric height (H). To increase impedance, you must either narrow the trace or increase the dielectric thickness.
The Dielectric Constant (Dk or Er) of your PCB substrate is a critical variable. While standard FR-4 (woven glass reinforced epoxy) is cost-effective, its Dk can vary significantly (ranging from 3.8 to 4.8) depending on the resin content and glass weave style (e.g., 1080, 2116, 7628 styles).
For applications exceeding 5GHz, engineers often switch to specialized materials like Rogers RO4350B or Isola Tachyon. These materials offer:
Designing the perfect 50Ω trace on screen is only half the battle. You must account for real-world manufacturing tolerances.
PCB traces are trapezoidal, not rectangular. During the etching process, the top of the trace is etched more than the bottom. Most impedance calculators (including this one) assume a rectangular cross-section for simplicity, but for high-precision, always consult your fab house's field solver which accounts for the Etch Factor.
Common Manufacturing Tolerances:
PCB trace impedance (characteristic impedance or Z₀) is the ratio of voltage to current for a signal traveling along a transmission line. It depends on trace geometry (width, thickness), dielectric properties, and layer stackup. Proper impedance control is critical for high-speed digital and RF circuits.
Microstrip traces are on the outer PCB layers with air on one side and dielectric on the other. Stripline traces are embedded between two ground planes. Microstrip is easier to manufacture but has more EMI radiation. Stripline offers better shielding and consistent impedance but requires more layers.
50Ω is a compromise between minimum signal attenuation (around 77Ω for coax) and maximum power handling (around 30Ω). It became the industry standard for RF and high-speed digital applications. 75Ω is used for video/cable TV applications, and 100Ω differential is common for high-speed serial links.
This calculator uses IPC-2141 closed-form equations which are accurate to within ±5% for typical PCB geometries. For the highest accuracy, especially above 5GHz or for unusual geometries, use a 2D field solver. Always verify with your PCB manufacturer's impedance calculations.
The main factors are: trace width (narrower = higher Z₀), dielectric height (thicker = higher Z₀), dielectric constant (higher Er = lower Z₀), and trace thickness (thicker = slightly lower Z₀). Dielectric height and trace width have the largest impact.
Use our full suite of professional impedance calculation tools for RLC circuits, capacitors, and inductors.