Transmission Lines

Covered Microstrip

Q: How high should the cover be so it does not affect microstrip impedance?

As a practical rule the cover (or package lid) should sit at least five times the substrate thickness above the ground plane, and ten times is safer for tight impedance tolerances. On a 0.254 mm (10 mil) alumina substrate, a lid 1.3 mm above the trace keeps the impedance shift under about one percent, while a lid only 0.5 mm high can raise the effective permittivity enough to drop a nominal 50 ohm line to roughly 47 to 48 ohms. The cover couples to the fringing fields above the strip, so the closer it is, the more it loads the line. When a metal housing or shielding can must sit closer than 5h, the line width should be widened during synthesis to recover the target impedance.

Q: Why does adding a metal cover lower the characteristic impedance of microstrip?

The cover acts as a second ground plane above the conductor. Some of the fringing electric field that would otherwise terminate in air now terminates on the lid, increasing the line's shunt capacitance per unit length. Since characteristic impedance is proportional to the square root of L over C, raising C lowers Z0. The cover also pulls more field into the air region versus the substrate in some geometries, but the dominant effect at practical lid heights is added capacitance, which both lowers Z0 and slightly raises the effective dielectric constant and therefore the propagation delay.

Q: Does a covered microstrip behave like a stripline?

Only in the limit of a very low, symmetric cover. Standard microstrip is an open inhomogeneous line where fields straddle substrate and air, so it supports a quasi-TEM mode with frequency-dependent dispersion. As the cover descends toward the strip and the air gap shrinks to match the substrate height, the structure approaches a stripline with its conductor sandwiched between two grounds, the mode becomes more nearly pure TEM, and the effective permittivity moves toward the bulk substrate value. At normal package lid heights of several substrate thicknesses, however, covered microstrip is still firmly in the microstrip regime and retains dispersion, just with modestly perturbed Z0 and effective permittivity.

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Placing a grounded metal lid a finite distance above an ordinary microstrip line creates this shielded variant, in which the cover intercepts part of the fringing field and adds shunt capacitance per unit length. The result is a higher effective dielectric constant and a lower characteristic impedance than the equivalent open line. The perturbation is strong when the cover is within a few substrate thicknesses of the trace and fades to under 1% once the lid height exceeds roughly 5h to 10h. Engineers must account for it whenever a microstrip circuit lives inside a metal package, shielding can, or connectorized housing, since the same trace that measures 50 Ω in the open can drop to 47 to 48 Ω under a low lid.

Category: Transmission Lines

Negligible cover height: > 5h to 10h

Typical Z₀ shift: −2 to −6%

How a Top Cover Reshapes the Microstrip Field

Open microstrip is an inhomogeneous, quasi-TEM line: the electric field above the strip arcs through air while the field beneath it sits in the substrate, so the mode sees a blend of the two media. Lowering a grounded conductor over the top of the structure forces some of those air-side field lines to terminate on the cover instead of looping back to the ground plane below. That extra termination path increases the capacitance per unit length C while leaving the inductance per unit length L almost unchanged, and because Z₀ ≈ √(L/C), the impedance falls. The same added capacitance raises the effective permittivity, slowing the phase velocity and increasing propagation delay along the line.

The magnitude of the effect is governed by the dimensionless ratio of cover height to substrate thickness, h_cover/h. When the lid is far away (h_cover/h above about 10) the air-side fields close almost entirely on the lower ground and the line behaves like conventional open microstrip. As the lid descends, the perturbation grows monotonically; in the extreme of a symmetric, very low cover the geometry degenerates toward a stripline with a nearly pure TEM mode. Most real designs operate in the middle of this range, where the cover is a package lid or shield positioned a few substrate thicknesses above the board.

Conformal-mapping and variational solutions, such as those tabulated by Bahl, Garg, and the classic Hammerstad-Jensen formulation extended to the covered case, let designers compute the corrected Z₀ and effective permittivity from h_cover/h, the width-to-height ratio W/h, and the substrate ε_r. In practice a field solver is used for tight tolerances, but the closed-form corrections are accurate to a few percent and are excellent for first-pass synthesis.

Governing Relationships

Characteristic impedance (lumped form):
Z₀ = √(L / C), so increasing C lowers Z₀

Effective permittivity vs. phase velocity:
ε_eff = (c / v_p)², with 1 < ε_eff < ε_r

Cover correction (capacitive loading):
C_covered ≈ C_open × [1 + Δ(W/h, h_cover/h)]

Where L, C = inductance and capacitance per unit length, c = speed of light, v_p = phase velocity, ε_r = substrate relative permittivity, h = substrate thickness, h_cover = lid height above the ground plane, and Δ → 0 as h_cover/h grows beyond ≈ 10. Example: a 50 Ω open line on ε_r = 9.8 alumina (h = 0.254 mm) with a cover at h_cover/h ≈ 2 drops to roughly 47 to 48 Ω.

Cover Height vs. Impedance Perturbation

Cover ratio h_cover/h	Approx. Z₀ shift	ε_eff change	Behavior	Design guidance
> 10	< 0.5%	Negligible	Essentially open microstrip	No correction needed
5 to 10	−0.5 to −2%	Small rise	Mild capacitive loading	Add margin in synthesis
2 to 5	−2 to −6%	Moderate rise	Clear covered-microstrip regime	Widen trace or model lid
1 to 2	−6 to −15%	Large rise	Strong cover coupling	Full-wave solve required
≈ 1 (symmetric)	Approaches stripline	Toward ε_r	Near-TEM, low dispersion	Treat as stripline

Common Questions

Frequently Asked Questions

How high should the cover be so it does not affect microstrip impedance?

Keep the lid at least 5 times the substrate thickness above the ground plane, and 10 times for tight tolerances. On 0.254 mm (10 mil) alumina, a lid 1.3 mm high keeps the Z₀ shift under about 1%, while a 0.5 mm lid can pull a 50 Ω line down to 47 to 48 Ω. If a housing or shield must sit closer than 5h, widen the trace during synthesis to recover the target impedance.

Why does adding a metal cover lower the characteristic impedance of microstrip?

The cover acts as a second ground above the strip, so fringing field lines that once terminated in air now land on the lid. That raises the shunt capacitance per unit length C, and since Z₀ ≈ √(L/C), increasing C lowers Z₀. The added capacitance also nudges the effective permittivity upward, slightly increasing propagation delay.

Does a covered microstrip behave like a stripline?

Only when the cover is very low and roughly symmetric. At normal package-lid heights of several substrate thicknesses the line stays in the microstrip regime, retaining quasi-TEM dispersion with a modestly perturbed Z₀ and ε_eff. As the air gap shrinks to match the substrate height, the mode approaches pure TEM and the effective permittivity migrates toward the bulk substrate value, the stripline limit.

Related Terms

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Packaged RF Assemblies

Hold Your Impedance Inside the Housing

RF Essentials designs and characterizes microstrip and stripline circuits where lid height, shielding, and packaging all shift the line impedance. Talk to our engineers about controlled-impedance assemblies through millimeter-wave bands.

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