FAQ
TL;DR: One stackup returned 36.3 Ω, but the correct microstrip calculator gave 50.6 Ω; the issue was “used the wrong calculator.” [Elektroda, Cody Miller, post #21661135]
Why it matters: Picking the wrong field‑solver or geometry model can misroute budgets and ruin impedance on finished PCBs.
Quick Facts
- Correct tool/inputs for 2.08 mil T, 4.06 mil H, 5.4 mil W, εr=3.84 → 50.6 Ω. [Elektroda, Cody Miller, post #21661135]
- IPC‑2141A equations underpin many web tools; a Polar check showed 87.91 Ω vs ~90 Ω from two calculators. [Elektroda, Cody Miller, post #21661134]
- Using a stripline calculator instead of microstrip led to 33.1 Ω earlier; user confirmed resolution after switching. [Elektroda, Rick Santarpio, post #21661136]
- Embedded‑microstrip example: surface ≈54.7 Ω, embedded ≈34.6 Ω for similar stackup—flag for validation. [Elektroda, Ian Lewis, post #21661137]
- Field report: EEWeb 47.9 Ω vs Mantaro 29.967 Ω; measured board matched Mantaro’s prediction. [Elektroda, Shen Zhao, post #21661142]
Why did I get 36.3 Ω when others show about 50.6 Ω for the same stackup?
You likely chose the wrong calculator geometry. Re‑running as a microstrip with the same dimensions produced 50.6 Ω. The poster who flagged the mismatch confirmed it after using the microstrip tool rather than a stripline calculator. “Use the wrong calculator” was the root cause. [Elektroda, Cody Miller, post #21661135]
Which formulas do many online impedance calculators implement?
Several sites implement approximation formulas collected in IPC‑2141A, a design guide for controlled‑impedance boards. One contributor noted their calculator used IPC‑2141A, aligning with common industry practice for quick estimates rather than full 2D/3D solvers. [Elektroda, Cody Miller, post #21661134]
Are IPC‑2141A‑based results close to field solvers?
They can be close but not exact. A Polar impedance solver returned 87.91 Ω, while two web calculators gave ~90.2 Ω and ~90.23 Ω for the same case. Expect a few ohms difference without detailed field solutions. [Elektroda, Cody Miller, post #21661134]
How does a nearby copper plane change microstrip impedance?
A nearby plane increases capacitance, which lowers characteristic impedance. As one expert said, “The additional copper layer increases the capacitance of the trace which decreases the impedance.” That’s consistent with Z0 ∝ √(L/C). [Elektroda, Cody Miller, post #21661139]
Why did an embedded‑microstrip calculator give ~34.6 Ω while the surface version gave ~54.7 Ω?
The embedded geometry includes dielectric above the trace, raising capacitance and lowering Z0. A user observed 34.6 Ω embedded versus 54.7 Ω surface for similar inputs, prompting a validation check of that tool. [Elektroda, Ian Lewis, post #21661137]
Can an embedded microstrip ever show lower Z0 than a symmetric stripline with equal plane spacing?
One report showed an embedded microstrip calculated lower Z0 than an equivalent stripline, which contradicts physical intuition. The user highlighted this as a calculator anomaly needing review. Treat such outputs as red flags. [Elektroda, Ian Lewis, post #21661138]
What real‑world risk comes from trusting one calculator?
A team built a PCB using one web calculator predicting 47.9 Ω. The finished board measured closer to another tool’s 29.967 Ω prediction. Cross‑check before fabrication to avoid expensive respins. [Elektroda, Shen Zhao, post #21661142]
Quick 3‑step: how do I sanity‑check a microstrip impedance number?
- Run two independent calculators (e.g., EEWeb, Mantaro, Saturn) using identical inputs.
- Compare with a reference from a field solver or vendor stackup note.
- Reconfirm geometry (microstrip vs stripline vs embedded) and copper thickness units.
[Elektroda, Chris Lee, post #21661133]
Which inputs dominate microstrip impedance?
Conductor width (W), dielectric height (H), copper thickness (T), and dielectric constant (εr) dominate. A shared comparison table varied W, spacing, and thickness, showing Z0 near 100 Ω when tuned across those parameters. [Elektroda, Chris Lee, post #21661133]
What’s the difference between microstrip, stripline, and embedded microstrip?
Microstrip runs over a reference plane with air above. Stripline sits between two planes inside the dielectric. Embedded microstrip runs over a plane but is buried under a dielectric cover. The thread discusses all three geometries and their calculators. [Elektroda, Ian Lewis, post #21661137]
Why do different tools disagree by several ohms?
Most web tools use simplified approximations, and some implementations deviate from published formulas. One poster found several formula differences from IPC‑2141A in an online explanation, suggesting possible source errors. [Elektroda, Rainer Kordmaa, post #21661141]
Did the calculator’s output change over time for the original case?
Yes. A later screenshot showed 58.0 Ω for the same scenario discussed years prior. This variability underscores why you should verify results across tools and time. [Elektroda, Shen Zhao, post #21661142]
How do I avoid the stripline vs microstrip mix‑up?
Match the geometry to your stackup. If your trace is on an outer layer above a single plane, use microstrip. One user resolved a 36.3 Ω vs 50.6 Ω discrepancy by switching to the microstrip tool. [Elektroda, Rick Santarpio, post #21661136]
Do calculators handle copper thickness consistently (e.g., 0.5 oz)?
Not always. One comparison used “0.5 oz” and “0.7 mil” entries across tools that still converged near 100 Ω. Always confirm units and thickness mapping in each interface. [Elektroda, Chris Lee, post #21661133]
