🔬 applied physics

Terahertz Channel Transmission in Dielectric Waveguide Near PCB Substrate

This study investigates the impact of nearby PCB substrates on terahertz dielectric waveguide transmission, revealing that direct contact with bare substrates causes severe frequency-selective loss while copper cladding or increased separation effectively suppresses leakage, thereby offering critical guidance for packaging-aware THz interconnect design.

Original authors: Wenbo Liu, Jiabiao Zhao, Kefeng Huang, Peian Li, Baiquan Xu, Yang Cao, Weidong Hu, Jianjun Ma

Published 2026-03-27

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CC BY 4.0

Original authors: Wenbo Liu, Jiabiao Zhao, Kefeng Huang, Peian Li, Baiquan Xu, Yang Cao, Weidong Hu, Jianjun Ma

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are trying to send a secret message using a beam of invisible light (Terahertz waves) through a clear plastic straw. This straw is designed to keep the light trapped inside so it can travel fast and far without losing energy. This is essentially what a dielectric waveguide is: a high-tech "plastic pipe" for data in the future of super-fast electronics.

However, in the real world, these pipes don't exist in a vacuum. They are usually packed tightly next to computer circuit boards (PCBs), which are covered in copper wires and layers of plastic. The big question this paper asks is: What happens to our secret light beam when it has to travel right next to a messy, copper-covered circuit board?

Here is the breakdown of their findings using simple analogies:

1. The "Leaky Pipe" Problem

Think of the light inside the plastic straw not as a solid beam, but as a glowing fluid that slightly "leaks" out the sides of the straw. This leakage is called an evanescent field. It's like a ghostly aura surrounding the pipe.

The Scenario: If you place this glowing pipe directly against a bare, plastic circuit board (with no metal), the "ghostly aura" touches the board.
The Result: The circuit board acts like a sponge. It soaks up the leaking light, turning it into heat. The paper found that when the pipe touches the bare board, the signal gets eaten alive. At certain frequencies (around 240-260 GHz), the signal loss was so massive (over 45 dB) that the message was completely lost. It's like trying to shout a secret across a room, but the walls are made of sound-absorbing foam.

2. The "Metal Shield" Solution

Now, imagine the circuit board has a layer of copper foil on the side facing the pipe.

The Analogy: Copper is like a shiny, reflective mirror for this type of light. Instead of the light leaking out and hitting the sponge (the plastic board), it hits the mirror (the copper) and bounces back into the pipe.
The Result: The signal stays strong! The copper layer acts as a shield, blocking the "sponge" from stealing the energy. The loss dropped from a catastrophic 45 dB down to a manageable 8 dB. It's the difference between shouting into a pillow versus shouting at a wall of mirrors.

3. The "Gap" Matters

The researchers also tested what happens if you lift the pipe slightly off the board.

The Analogy: Think of the "ghostly aura" as a short-range radio signal. If you move the receiver (the board) just a tiny bit away, the signal strength drops off very quickly.
The Result: Even a tiny gap of 0.5 mm or 1 mm made a huge difference. The further the pipe is from the board, the less the board can "steal" the signal. The interference fades away exponentially, like a whisper that can't be heard if you step back a few feet.

4. The "Fence" vs. The "Solid Wall"

Real circuit boards aren't just solid sheets of copper; they have patterns of thin copper lines (traces) with gaps in between. The researchers tested how the pipe behaves when it runs parallel (alongside) or perpendicular (crossing over) these lines.

Parallel (Running alongside):
- If the pipe runs right over a copper line, the metal acts like a shield, keeping the signal safe.
- If the pipe runs over the gap between lines, the signal leaks out into the plastic board underneath, causing a loss.
- Lesson: It's a tug-of-war between the metal protecting you and the gaps letting you leak.
Perpendicular (Crossing over):
- If the pipe crosses over many lines like a ladder, the dense metal acts like a solid fence, blocking the leak.
- But if the lines are far apart (sparse), the gaps are huge, and the signal leaks out easily into the board.
- Lesson: A dense "fence" of copper is better than a sparse one at protecting the signal.

The Big Picture Takeaway

This paper is a guide for engineers building the next generation of super-fast computers and 6G networks.

The main lesson is: You cannot just design a high-speed data pipe and ignore what's next to it.

Don't let your data pipe touch bare plastic circuit boards; it will kill the signal.
Do put a copper layer between them, or keep a small air gap.
Do design your copper patterns carefully, because the shape and spacing of the wires can either act as a shield or a trap for your data.

By understanding these "near-field" interactions, engineers can pack computers tighter and faster without the data getting lost in the noise.

1. Problem Statement

As electronic systems become more integrated, there is a critical need for high-capacity, low-loss short-reach interconnects to overcome the bandwidth limitations of traditional electrical links. Terahertz (THz) dielectric waveguides have emerged as a promising solution due to their low loss and broadband characteristics. However, a significant gap exists in understanding how these waveguides perform in realistic packaging environments.

In practical systems, dielectric waveguides are rarely isolated in free space; they are routed in close proximity to Printed Circuit Board (PCB) substrates, copper traces, and solder masks. The evanescent field of the guided mode extends into the surrounding air, making the transmission highly sensitive to nearby dielectric and metallic structures. This proximity can cause:

Perturbation of modal boundary conditions.
Redistribution of electromagnetic fields.
Excess transmission loss due to leakage into lossy substrates or parasitic coupling.

The specific impact of PCB metallization (bare vs. copper-clad vs. periodic traces) and waveguide separation on THz transmission remains insufficiently understood, posing a risk to signal integrity in compact 6G and chiplet-based systems.

2. Methodology

The authors employed a combination of experimental measurements, numerical simulations, and analytical modeling to investigate these effects over the 220–325 GHz frequency band.

Waveguide Fabrication:
- Material: Polypropylene (PP), chosen for its low loss and weak dispersion in the THz regime.
- Geometry: A square cross-section ( $1 \times 1$ mm²) 3D-printed via Fused Deposition Modeling (FDM). This size balances single-mode operation with mechanical rigidity.
- Configuration: A U-shaped routed path (approx. 523 mm total length) with 90° bends to ensure end-fire coupling and minimize free-space line-of-sight interference.
Experimental Setup:
- Source/Receiver: Continuous-wave (CW) THz system using a signal generator, frequency multiplier, and horn antennas.
- PCB Samples: Three types were tested:
  1. Bare PCB: FR4 substrate with solder mask (no copper).
  2. Fully Copper-Clad PCB: FR4 with a continuous 20 $\mu$ m copper layer on the waveguide-facing side.
  3. Periodic Copper-Trace PCBs: Four variations with trace widths of 1 mm and gaps of 1–4 mm (copper coverage ratios from 49% down to 20%).
- Variables: Measurements were taken at waveguide-PCB separation distances ( $g$ ) of 0 mm, 0.5 mm, and 1 mm.
- Characterization: THz Time-Domain Spectroscopy (THz-TDS) was used to characterize the dielectric properties (refractive index and loss) of the FR4 substrate.
Modeling & Simulation:
- Simulations: Full-wave finite-element simulations (2D) were performed to validate experimental data.
- Analytical Model: An equivalent radiation-channel model was developed. It treats the PCB-induced loss as an exponential decay of the evanescent field ( $\exp(-2\kappa_{air}g)$ ) modulated by a spectral weighting function to account for frequency-selective leakage.

3. Key Contributions

Quantification of PCB-Induced Loss: The study provides the first quantitative analysis of how different PCB metallization states (bare, solid copper, periodic traces) affect THz dielectric waveguide transmission.
Mechanism Elucidation: It distinguishes between substrate-assisted leakage (dominant in bare/dielectric gaps) and metallic shielding (dominant in copper-clad areas), explaining the physical mechanisms behind excess attenuation.
Design Guidelines: The paper establishes practical rules for the co-design of THz waveguides and PCBs, specifically regarding clearance distances and trace orientation.
Validation of a Simplified Model: The proposed equivalent radiation-channel model successfully reproduces the complex frequency and distance-dependent loss behaviors observed in experiments.

4. Key Results

A. Impact of PCB Metallization (Bare vs. Copper)

Bare PCB (Direct Contact, $g=0$ ): Causes severe, frequency-selective excess loss. In the 240–260 GHz range, loss exceeds 45 dB (hitting the system's dynamic range limit). This is attributed to efficient evanescent coupling and leakage into the lossy FR4 substrate.
Fully Copper-Clad PCB: The copper layer acts as an electromagnetic barrier, suppressing substrate leakage. Excess loss is reduced to approximately 8 dB and the spectral response becomes smooth.
Distance Dependence: For both cases, excess loss decreases exponentially as the separation ( $g$ ) increases from 0 to 1 mm, confirming the near-field nature of the interaction.

B. Impact of Periodic Copper Traces

The transmission behavior depends heavily on the alignment between the waveguide and the traces:

Parallel Alignment (Waveguide || Traces):
- On-Trace: Loss is moderate; the copper suppresses leakage but induces some conductor loss.
- Between-Traces: Loss is high. The waveguide couples directly to the lossy FR4 substrate through the dielectric gaps. As gaps widen, this leakage increases.
Perpendicular Alignment (Waveguide $\perp$ Traces):
- Dense Traces (Small gaps): Acts as an effective shield, minimizing loss by blocking the evanescent field from the substrate.
- Sparse Traces (Large gaps): Shielding fails; the field penetrates the gaps into the substrate, causing high loss.
- Trend: Unlike the parallel case, increasing trace spacing in the perpendicular configuration generally increases loss because the shielding effect weakens.

C. Dispersion

Measurements of Group Velocity Dispersion (GVD) showed that PCB proximity introduces negligible perturbation to the intrinsic modal dispersion. The degradation is primarily due to attenuation (leakage), not dispersion penalties.

5. Significance and Implications

This work is critical for the deployment of THz technology in next-generation electronic systems (e.g., 6G, chiplet interconnects, and antenna-to-chip links).

Packaging Awareness: It demonstrates that "packaging-aware" design is not optional; ignoring nearby PCB structures can lead to catastrophic signal loss (>45 dB).
Design Rules:
- Clearance: A separation of >0.5–1 mm is recommended to minimize coupling effects.
- Shielding: If close proximity is unavoidable, a solid copper ground plane facing the waveguide is superior to bare FR4 or sparse traces.
- Trace Layout: For periodic traces, the orientation relative to the waveguide is a critical design parameter. Dense traces perpendicular to the waveguide offer the best shielding.
Future Applications: The findings provide a foundation for designing robust, high-capacity THz interconnects in compact, heterogeneous integration scenarios where space constraints force waveguides to run near complex PCB layouts.