Transmission Lines

Conductor-Backed CPW

Q: How does adding a backside ground change CPW impedance and dispersion?

The lower ground plane introduces a parallel-plate capacitance in series with the coplanar slot capacitance, so the effective capacitance per unit length rises and the characteristic impedance drops compared with ungrounded CPW of identical top-side geometry. On a 254 micron (10 mil) alumina substrate, a 50 ohm ungrounded CPW with a 200 micron center conductor may fall to 38 to 42 ohms once a backside ground is added, requiring a wider center conductor or narrower gaps to recover 50 ohms. As the substrate thickness approaches the slot dimensions, the field distribution shifts from pure coplanar toward microstrip-like, raising the effective dielectric constant and increasing dispersion at millimeter-wave frequencies.

Q: Why do via fences suppress the parallel-plate mode in CBCPW?

The backside ground and the two top coplanar grounds form a parallel-plate region that supports a parasitic TM parallel-plate mode and can leak energy as a substrate or microstrip mode. Stitching the top grounds to the bottom ground with closely spaced vias short-circuits this parallel-plate region near the line and forces the top and bottom grounds to the same potential. The via pitch must stay between one twentieth and one tenth of a guided wavelength (lambda_g/20 to lambda_g/10) at the highest operating frequency so the via fence behaves as a solid wall rather than a leaky slot. On a thin alumina substrate at 40 GHz (effective permittivity near 6, guided wavelength near 3 mm), that means a via pitch of about 150 to 300 microns flanking each ground.

Q: When should I choose CBCPW over microstrip for a millimeter-wave circuit?

Choose CBCPW when you need series and shunt surface-mount components, wire-bond pads, or air-bridge crossovers that benefit from top-side ground access, and when the backside ground is needed for heat sinking from power devices or for mechanical stiffness on a thin substrate. CBCPW also keeps the line width more independent of substrate thickness than microstrip, which simplifies thin-substrate layouts at 60 to 110 GHz. Microstrip is preferable when you want the lowest radiation loss, the simplest single-ground stackup, and no via fencing, and when the design is dominated by long low-loss feed runs rather than densely packed shunt elements.

/kuhn-DUK-ter bakt C-P-W/ (CBCPW)

Adding a continuous ground plane to the back of the substrate of a coplanar waveguide produces this variant, often abbreviated CBCPW or GCPW, in which a center conductor sits between two top-side coplanar grounds while a third ground sheet covers the underside. The backside conductor stiffens thin substrates, provides a direct thermal path for power devices, and lowers the characteristic impedance relative to ungrounded CPW of the same top geometry. Its drawback is a parasitic parallel-plate region between the top and bottom grounds that supports leaky substrate modes; designers suppress these with dense grounding via fences. CBCPW is widely used in MMIC packaging, millimeter-wave modules, and surface-mount RF boards from 1 GHz to beyond 110 GHz.

Category: Transmission Lines

Typical Z₀: 30 to 60 Ω

Via pitch: < λ_g/10

Why a Backside Ground Changes the Picture

Standard coplanar waveguide confines its quasi-TEM field in the two slots between a center conductor and the coplanar grounds on a single metal layer, which makes it convenient for shunt components and probe measurement because both signal and ground are reachable from the top. Many practical builds, however, place that CPW pattern on a thin substrate bonded to a metal carrier, or fabricate it on a board whose bottom layer is already a solid ground for other circuitry. The presence of that lower conductor turns the structure into conductor-backed CPW, and the field is no longer purely coplanar: a fraction of it now terminates downward on the backside ground, mixing CPW behavior with microstrip behavior.

The immediate electrical consequence is added shunt capacitance. The backside ground forms a parallel-plate capacitance with the top metal, in parallel with the coplanar slot capacitance, so the total capacitance per unit length rises and the line impedance falls. The strength of the coupling scales with the ratio of substrate thickness h to the slot and conductor dimensions. When h is large compared with the slot width, the structure behaves almost like ungrounded CPW; as h shrinks toward the slot width, the backside ground dominates and the line approaches microstrip, with a higher effective dielectric constant and stronger frequency dispersion at the upper end of the band.

The second consequence is parasitic modes. The two top grounds and the backside ground bound a parallel-plate cavity, which supports a TM parallel-plate mode and can radiate into substrate or surface-wave modes if left open. Energy that couples into these modes shows up as excess loss, ripple, and crosstalk between adjacent lines. The standard remedy is a fence of plated grounding vias stitching the top grounds to the bottom ground along both sides of the line, spaced closely enough that the fence acts as a solid wall at the highest operating frequency.

Impedance and Mode Suppression Equations

Effective dielectric constant (parallel-plate limit):
ε_eff ≈ (ε_r + 1)/2 (wide-slot CPW) → trending toward ε_r as h decreases

Characteristic impedance (quasi-static):
Z₀ = √ε_eff / (c × C) = 1 / (c × √(C × C_air)) where C is the total (coplanar + parallel-plate) capacitance per unit length and ε_eff = C / C_air

Via fence pitch (mode suppression):
p < λ_g/10 = c / (10 × f_max × √ε_eff)

Where ε_r = substrate relative permittivity, h = substrate thickness, c = speed of light, f_max = highest operating frequency, p = via pitch. Example: alumina ε_r=9.8, ε_eff≈6 at 40 GHz → λ_g≈3.06 mm, so p < 0.3 mm.

CBCPW vs. CPW vs. Microstrip

Property	Conductor-Backed CPW	Ungrounded CPW	Microstrip
Ground access	Top + backside	Top only	Backside only
Shunt component mounting	Easy (top grounds)	Easy (top grounds)	Requires via to ground
Heat sinking	Good (backside metal)	Poor	Good (backside metal)
Parasitic parallel-plate mode	Yes (needs via fence)	No	No
Dispersion at mmWave	Moderate to high	Low to moderate	Moderate
Sensitivity to substrate thickness	Moderate	Low	High
Typical Z₀ range	30 to 60 Ω	40 to 75 Ω	20 to 90 Ω

Common Questions

Frequently Asked Questions

How does adding a backside ground change CPW impedance and dispersion?

The lower ground adds parallel-plate capacitance in parallel with the slot capacitance, raising capacitance per unit length and lowering Z₀ versus ungrounded CPW of the same top geometry. On 254 micron alumina, a 50 Ω CPW can drop to 38 to 42 Ω once backed, so the center conductor must widen or the gaps narrow. As substrate thickness approaches the slot width, the field turns microstrip-like, raising ε_eff and dispersion at mmWave.

Why do via fences suppress the parallel-plate mode in CBCPW?

The top and bottom grounds bound a parallel-plate region that supports a parasitic TM mode and leaks into substrate modes. Stitching the top grounds to the bottom ground with closely spaced vias shorts that region and ties both grounds to one potential. Keep the via pitch between λ_g/20 and λ_g/10 at f_max so the fence acts as a wall; on a thin alumina substrate at 40 GHz (ε_eff≈6, λ_g≈3 mm) that is roughly a 150 to 300 micron pitch.

When should I choose CBCPW over microstrip for a millimeter-wave circuit?

Pick CBCPW when you need top-side ground access for shunt components, wire-bond pads, or air-bridge crossovers, and when the backside metal is needed for heat sinking from power devices or for stiffening a thin substrate. It also keeps line width less dependent on substrate thickness at 60 to 110 GHz. Microstrip wins for lowest radiation loss, the simplest single-ground stackup, no via fencing, and long low-loss feed runs.

Related Terms

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