| εr | h (mm) | t (mm) | w (mm) | Estimated Z0 (Ω) |
|---|---|---|---|---|
| 4.20 | 0.180 | 0.035 | 0.200 | 39.46 |
| 3.60 | 0.150 | 0.018 | 0.160 | 44.91 |
| 4.40 | 0.200 | 0.035 | 0.260 | 35.02 |
This calculator uses a common closed-form estimate for a symmetric stripline (trace centered between two reference planes). The characteristic impedance is:
- εr: dielectric constant of the insulating material
- h: distance from trace to each reference plane
- w: trace width
- t: trace thickness
This form is often associated with IPC-2141 style guidance and is intended for quick estimates.
- Select Compute impedance or Solve width.
- Pick units (mm, mil, or µm) for all dimensions.
- Enter dielectric constant and geometry values.
- Press Calculate to show results above the form.
- Download CSV or PDF after calculation, if needed.
- Impedance is sensitive to εr and dielectric thickness tolerance.
- Plating, solder mask, and weave effects can shift results.
- For controlled impedance fabrication, use your board house stackup.
- Validate critical designs with a field solver and test coupons.
Impedance drivers in symmetric striplines
Characteristic impedance responds strongly to dielectric constant and spacing. With εr near 4.2 and plane spacing b near 0.395 mm, small changes in width can shift Z0 by several ohms. In a thin stackup, moving width by 0.02 mm may change impedance by 2–5 Ω. Keeping geometry consistent across layers improves repeatability and simplifies constraints. For repeatable builds keep trace widths within your board house’s stated etch tolerance.
Width, thickness, and the w/b ratio
The calculator reports w/b because it summarizes how “wide” the conductor is relative to the reference plane spacing. In many practical stackups, w/b between 0.40 and 0.80 lands near 40–70 Ω, depending on εr and copper thickness. Changing t from 18 µm to 35 µm can reduce Z0 by roughly 1–3 Ω for common geometries.
Dielectric constant sensitivity
If εr drifts from 3.9 to 4.5, impedance changes through the 1/√εr scaling term, typically several percent. For a 50 Ω target, that can be a 2–4 Ω swing before you account for geometry tolerance. When available, use datasheet values measured at microwave frequencies rather than low-frequency catalog numbers.
Spacing tolerance and fabrication reality
The spacing parameter b = 2h + t is frequently controlled by prepreg and core thickness tolerance. A 10–20 µm shift in h on a thin dielectric is not unusual and can be material to impedance. Copper plating and etch compensation also move effective width, especially on fine lines. Treat computed values as nominal and plan for coupon-based verification.
Using solve-for-width effectively
When you enter a target impedance, the width solver searches for a stable solution using bisection, which is robust for monotonic curves. After you get w, sweep the plotted curve to see how quickly Z0 changes around that point. If the curve is steep, consider increasing b or adjusting constraints to match fabrication capability.
When to move beyond closed-form estimates
Closed-form models are best for early routing and trade studies. For high-speed interfaces, dense via fields, or unusual copper profiles, move to a 2D/3D field solver and align assumptions on copper roughness and plating. If your stackup includes resin-rich regions or very thin dielectrics, measured coupons can differ from estimates by 5–10%. Combine the calculator with fab notes, stackup drawings, and coupon data to close the loop.
1) What stripline type does this calculator model?
It models a symmetric stripline where the trace is centered between two reference planes. Inputs assume equal spacing to each plane and use a fast closed-form estimate for Z0.
2) Can I use it for asymmetric stripline or microstrip?
Not directly. Asymmetric stripline needs different equations or a solver. Microstrip includes air, solder mask, and surface effects. Use a dedicated model for those cases.
3) Why does Z0 decrease when width increases?
A wider trace increases capacitance per unit length to the reference planes, which lowers characteristic impedance. The curve helps visualize how quickly Z0 changes around your nominal width.
4) What εr value should I enter for FR-4?
Use the value provided by your board material datasheet at the frequency of interest. Many FR-4 materials fall roughly between 3.7 and 4.6, but high-speed laminates can differ.
5) Are the CSV and PDF exports suitable for documentation?
Yes for quick traceability. The exports capture inputs, derived spacing, computed impedance, and the equation used. For signoff, include stackup drawings and coupon measurements from fabrication.
6) Why might measured impedance differ from the estimate?
Manufacturing tolerances, resin content, copper roughness, plating, and frequency-dependent dielectric behavior can shift impedance. Always validate controlled-impedance nets with your fabricator’s stackup and test coupons.