Terahertz PCB | Sub-THz Substrate & Interconnect

What Is a Terahertz PCB?

A terahertz PCB is a printed circuit board engineered to carry signals in the 100 GHz to 1 THz frequency range, often called the sub-THz or terahertz gap. At these frequencies, free-space wavelengths shrink below 3 mm and conductor skin depth falls under 0.2 µm, so every material property, trace dimension, and surface finish directly limits usable bandwidth and link budget. Terahertz boards are emerging in 6G research front-ends, D-band (110–170 GHz) backhaul radios, sub-THz imaging systems, and spectroscopy instruments.

Compared with 5G millimeter-wave PCBs that top out around 40–50 GHz, terahertz designs face roughly an order-of-magnitude increase in dielectric and conductor loss per unit length. That shift changes which substrates, interconnects, and fabrication processes are viable.

Why Conventional RF Laminates Fail Above 100 GHz

Standard PTFE-woven-glass laminates such as RO4350B (Dk 3.48, Df 0.0037 at 10 GHz) perform well through Ka band. But dissipation factor rises with frequency: by 140 GHz a laminate with Df of 0.004 at 10 GHz may exhibit Df above 0.008, and conductor loss on copper roughened to Rz 3 µm can exceed 1 dB/mm in microstrip. Two loss mechanisms dominate:

• Dielectric loss — scales linearly with frequency for most thermoset and thermoplastic substrates. A 0.001 increase in Df at 300 GHz adds roughly 0.15 dB/mm to a 50 Ω microstrip on 100 µm thick substrate with Dk of 3.0.
• Conductor loss — skin depth at 300 GHz is about 0.12 µm in copper. Surface roughness profiles (Rz) that exceed skin depth scatter current and can double ohmic loss. Electrodeposited copper foils with Rz of 1.5–6 µm are unacceptable; rolled-annealed or sputtered copper with Rz below 0.5 µm is required.

These realities push designers toward exotic substrates and non-traditional fabrication, topics explored in depth in our look at future RF materials toward 2030.

Substrate Materials for Terahertz PCBs

Fused Quartz and Borosilicate Glass

Fused quartz offers Dk of approximately 3.8 and Df below 0.0001 up to 1 THz, making it the gold-standard dielectric for sub-THz passive circuits. Wafer-level processing allows 25–100 µm thick substrates. The drawback is brittleness and limited panel size — typically 150 mm or 200 mm wafers, not standard PCB panels.

Liquid Crystal Polymer (LCP)

LCP films (Dk ≈ 2.9, Df ≈ 0.002 at 110 GHz) can be laminated in thicknesses from 25 µm to 100 µm, are compatible with standard PCB drilling and plating, and absorb very little moisture (< 0.04 %). LCP is the most practical organic substrate for D-band and G-band (140–220 GHz) work when full PCB-style integration is needed.

Modified PTFE and Low-Loss Ceramic-Filled Laminates

Materials like Rogers RO3003G2 (Dk 3.00, Df 0.0010 at 10 GHz) represent the lowest-loss class of bondable PTFE laminates. They remain usable through W band (75–110 GHz) but become marginal above 140 GHz unless trace lengths are kept very short (< 2 mm).

Thin-Film on Silicon or Glass Interposers

For frequencies above 200 GHz, thin-film redistribution layers (BCB, polyimide, or SiO₂) deposited on high-resistivity silicon (> 10 kΩ·cm) or glass interposers offer the finest line/space (down to 5 µm) and the smoothest metal (sputtered gold, Rz < 0.1 µm). These processes borrow from semiconductor back-end-of-line fabrication rather than PCB manufacturing.

Material Comparison Table

Transmission-Line Topologies at Sub-THz Frequencies

Microstrip, the workhorse of lower-frequency RF boards, becomes increasingly lossy and radiative above 100 GHz because the substrate height needed for 50 Ω impedance shrinks to tens of micrometers, tightening fabrication tolerance to single-micron levels. Alternative guided-wave structures are preferred:

• Grounded coplanar waveguide (GCPW) — signal and ground conductors on the same layer, backed by a ground plane. The gap dimension controls impedance. At 140 GHz on 50 µm LCP, a typical 50 Ω GCPW has a signal width of ~60 µm and gaps of ~25 µm. Via fences spaced at λ/10 suppress parallel-plate modes.
• Substrate-integrated waveguide (SIW) — two rows of plated vias form the sidewalls of a dielectric-filled rectangular waveguide. Useful from 60 GHz through D band. Loss is lower than microstrip but the structure is physically wider.
• Dielectric waveguide / image line — a dielectric strip on a ground plane guides the wave with minimal metal loss. Attractive above 200 GHz but difficult to couple to active devices.
• Air-filled waveguide transitions — machined or 3D-printed metallic waveguide sections interface with planar lines through E-plane probes or antipodal finline tapers. This hybrid approach keeps the lowest-loss path in air and uses the PCB only for the active-device neighborhood.

Choosing a topology often depends on how much of the circuit must be planar. Phased arrays at D band, for instance, require hundreds of radiating elements on a single board, making SIW or GCPW the practical options — similar trade-offs arise in phased-array AESA board design at lower bands.

Interconnect Challenges and Solutions

Die-to-Board Transitions

At 140 GHz a wire bond as short as 200 µm introduces enough parasitic inductance (roughly 0.2 nH) to create a significant impedance discontinuity. Flip-chip bumping with gold or copper pillar bumps at 80–150 µm pitch reduces the transition to a near-lumped capacitance of a few femtofarads, keeping return loss below −15 dB through D band when properly compensated.

Board-to-Board and Board-to-Waveguide

Coaxial connectors rated beyond 110 GHz (1.0 mm interface, DC–110 GHz per IEEE 287) exist but are expensive and bulky relative to the circuits they serve. For production systems, waveguide flanges (WR-6.5 for 110–170 GHz, WR-3.4 for 220–330 GHz) with PCB-to-waveguide probe transitions are more common. Alignment tolerances tighten to ±25 µm at D band.

Via Transitions

Through-board vias must be modeled as transmission-line stubs at terahertz frequencies. A 100 µm diameter via through 200 µm of substrate resonates near 300 GHz. Back-drilled or blind microvias with diameters of 50–75 µm and stub lengths under 30 µm are essential. Laser-drilled microvias on LCP or glass achieve these dimensions routinely in advanced packaging lines.

Fabrication Constraints and Process Requirements

Building a terahertz PCB blurs the line between PCB fabrication and semiconductor packaging. Key process requirements include:

1. Line/space resolution — GCPW gaps of 15–25 µm demand semi-additive patterning (SAP) or modified semi-additive process (mSAP). Conventional subtractive etching cannot hold these tolerances reliably.
2. Copper smoothness — rolled-annealed foil (Rz < 0.5 µm) or sputtered/plated copper on smooth dielectric. Surface treatment must not re-roughen the conductor.
3. Dielectric thickness tolerance — ±2 µm on a 50 µm core, because impedance sensitivity to height is extreme when the substrate is thin. Press-cycle uniformity and resin flow must be tightly controlled.
4. Registration — layer-to-layer alignment within ±10 µm for via-fence placement and ground-plane overlap. This is at the limit of advanced PCB lithography and more naturally achieved with wafer-level steppers.
5. Surface finish — ENEPIG or electrolytic soft gold for flip-chip assembly. HASL is not viable because solder topology is too uneven at these dimensions.

These requirements place terahertz PCBs firmly in the domain of specialty fabricators equipped with SAP lines, laser direct imaging at sub-5 µm resolution, and plasma-assisted surface cleaning.

Application Domains Driving Terahertz PCB Development

6G and Beyond-5G Communications

Pre-6G research targets the 140 GHz and 220–330 GHz windows for short-range, high-capacity links exceeding 100 Gbit/s. Antenna-in-package modules on LCP or glass substrates are the leading integration approach, with the PCB serving as a carrier and DC/IF distribution layer. Understanding current 5G PCB requirements provides a useful baseline for appreciating how much further terahertz designs must go.

Automotive and Security Imaging

Sub-THz imaging at 220–330 GHz can see through clothing, packaging, and some building materials. Focal-plane arrays with hundreds of receiver pixels need dense, low-loss feed networks — a natural fit for SIW on thin LCP or quartz.

Spectroscopy and Scientific Instruments

Rotational spectroscopy of molecules in the 100–1000 GHz range uses heterodyne receivers whose local-oscillator and IF distribution circuits can benefit from planar integration on low-loss substrates, replacing bulky split-block waveguide assemblies.

Radar and Sensing

D-band radar (120–170 GHz) offers finer range resolution than W-band radar PCBs and is being explored for gesture recognition, industrial gauging, and drone collision avoidance. SiGe and InP MMICs at these frequencies are commercially available; the PCB and package are now the integration bottleneck.

Design Guidelines and Best Practices

• Keep planar runs short. Even on LCP, loss at 140 GHz is roughly 0.3–0.5 dB/mm in GCPW. A 10 mm feed line costs 3–5 dB. Place the MMIC as close to the antenna as possible.
• Simulate with frequency-dependent material models. Use wideband Debye or multipole Debye dielectric models rather than single-frequency Dk/Df values. Extrapolating 10 GHz data to 300 GHz introduces large errors.
• Model every discontinuity in 3-D EM. Bends, tees, via transitions, and mode launchers must be individually optimized. At 200 GHz, a 50 µm misalignment can shift a resonance by several gigahertz.
• Plan thermal paths early. Sub-THz MMICs in InP or SiGe BiCMOS dissipate modest power (< 1 W typical), but the thin substrates and small die pads concentrate heat. Thermal vias under the die and metal coin inserts in the PCB are standard remedies.
• Co-design antenna and feed. At D band, a patch antenna is only about 0.7 × 0.7 mm. The feed network, matching structure, and radiator are inseparable; treat them as one EM simulation domain.

Looking Ahead: From Lab to Production

Terahertz PCB technology today sits where millimeter-wave PCBs were a decade ago — proven in research, emerging in prototypes, and approaching the cost and yield thresholds needed for volume products. Key enablers moving forward include panel-level glass-core substrates (300 × 300 mm panels with ±5 µm registration), advances in SAP copper patterning on LCP, and heterogeneous integration that embeds InP or SiGe dies directly into the substrate stackup. As these processes mature, the boundary between a “PCB” and a “package” will continue to dissolve — especially above 200 GHz, where the substrate is the package.

For teams entering this space, the practical advice is to start with the loss budget. Work backward from the system noise figure and link margin to determine how many millimeters of planar transmission line you can afford, then choose the substrate and topology that fit within that budget. Everything else — fabrication process, interconnect method, surface finish — follows from that single constraint.

FAQ

Can FR-4 be used for any terahertz circuit?

No. FR-4’s dissipation factor exceeds 0.02 above 10 GHz and climbs further at higher frequencies. It is unsuitable for any signal path above roughly 10–15 GHz, let alone the 100+ GHz terahertz range. It may still serve as a mechanical carrier for DC and low-frequency control signals in a hybrid assembly.

What is the lowest-loss PCB-compatible substrate at 140 GHz?

Liquid crystal polymer (LCP) film, with Df around 0.002 at 110 GHz, is the lowest-loss organic substrate that can be processed on standard PCB-class equipment. Fused quartz is lower loss but requires wafer-level processing.

How narrow must traces be for a 50 Ω line at 300 GHz?

On a 25 µm thick LCP substrate (Dk 2.9), a grounded coplanar waveguide at 50 Ω needs a signal conductor roughly 30–40 µm wide with gaps of 10–15 µm. These dimensions require semi-additive patterning, not conventional etching.

Are connectors available above 110 GHz?

Coaxial connectors are standardized to 110 GHz (1.0 mm interface). Above that, waveguide flanges (WR-6.5, WR-5.1, WR-3.4) are the standard test and interconnect interface, per IEEE Std 287 and MIL-DTL-3922 waveguide specifications.

Is terahertz PCB fabrication available commercially?

A small number of specialty fabricators and advanced-packaging houses offer sub-THz circuit fabrication on LCP, quartz, or glass interposers. Volumes are currently low (prototype to low-rate production), and lead times are longer than for conventional RF boards.