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Embedded Microstrip Calculator (Soldermask / Cover Dielectric)

Microstrip with a cover dielectric above the trace (soldermask, prepreg, conformal coat). Wadell 3.5.4 cover-correction. Substrate and cover er are independent inputs.

Embedded Microstrip Calculator

Microstrip with a cover dielectric (soldermask, prepreg, conformal) above the trace. The cover raises effective permittivity and shifts Z0 down by 1-4 ohm versus an exposed trace.

Units
Length
Frequency

Inputs

mm
mm
mm
GHz
W = 0.5 mmh = 0.4 mmh₂ = 0.2 mmcover | εr = 4.40εr = 4.40
Embedded (buried) microstrip
Z0 (embedded)
62.56Ω

More

eeff3.316
Guided lambda_g68.59 mm

Analytical calculation

Open-microstrip baseline
Start from plain (no-cover) microstrip via Hammerstad-Jensen.
Cover correction
alpha is the fraction of field pulled into the cover dielectric.
Characteristic impedance
Z0 scales as sqrt(eeff_open / eeff).

Frequently asked

How much does a cover dielectric shift Z0?

The Wadell exponential blend used here scales as 1 - exp(-1.55 h2/h), so for a typical 25 um soldermask (h2/h about 0.03 on a 0.8-1.6 mm substrate) the closed form returns a Z0 shift well under 0.5 ohm. Real measurements and full-wave EM typically show 0.5 to 1.5 ohm of drop on a 50 ohm trace because near-field coupling into the cover extends further than h sets the scale for; treat the Wadell number as a lower bound. As h2 grows toward h, the closed form becomes a good predictor and the drop can reach several ohm. When you need better than ~2 percent on Z0 with a thin cover, run the geometry through RayRF.

References

  • PrimaryWadell, B. C. Transmission Line Design Handbook, Artech House 1991 (embedded / buried microstrip with cover dielectric). The exponential blend factor = 1 - exp(-1.55 h2 / h) is from this reference.
  • BaselineHammerstad, E. & Jensen, O. 1980 (open microstrip Z0 and eeff used as the no-cover baseline).
  • NoteThe Wadell exponential blend is conservative for thin covers (h2/h << 1): it returns a smaller Z0 shift than full-wave EM typically reports for soldermask-class covers. Use the er,cover input when the cover material differs from the substrate, and cross-check in a 3D solver when high accuracy matters.

Closed-form is just the start.

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