Materials and Substrates Dielectric Materials Informational

How do I select a PCB laminate material for a circuit operating above 30 GHz?

Selecting a PCB laminate for operation above 30 GHz requires evaluating dielectric constant stability across frequency, low loss tangent (typically below 0.002), controlled thickness tolerance for impedance accuracy, and low moisture absorption. At millimeter-wave frequencies, substrate losses dominate the link budget, making material selection one of the most critical early design decisions. PTFE-based laminates like Rogers RT/duroid 5880 offer extremely low loss, while ceramic-filled hydrocarbon materials like Rogers RO3003 provide better dimensional stability and easier processing. For frequencies above 60 GHz, fused silica and quartz substrates may be necessary to achieve acceptable performance.

Category: Materials and Substrates

Updated: April 2026

Product Tie-In: PCB Laminates, Substrates

Selecting High-Frequency PCB Laminates for mmWave Circuits

At frequencies above 30 GHz, the choice of PCB laminate becomes a defining factor in circuit performance. Conductor losses increase with the square root of frequency, and dielectric losses increase linearly, making low-loss substrates essential for any design with meaningful transmission line length.

Key Material Properties

The dielectric constant (Dk) determines transmission line dimensions and must remain stable across the operating frequency range. A Dk variation of just 2% can shift a filter center frequency by 1%, which is unacceptable at mmWave. The loss tangent (Df) directly determines the dielectric contribution to insertion loss. For circuits above 30 GHz, target Df values below 0.002 at the operating frequency, not at 10 GHz where most datasheets specify.

Material Categories

PTFE-based (Rogers RT/duroid 5880, 5870): Lowest loss (Df ~ 0.0009), excellent for passive structures, but soft material complicates multilayer fabrication
Ceramic-filled hydrocarbon (Rogers RO3003, RO3006): Low loss (Df ~ 0.0013), better mechanical stability, compatible with standard FR-4 processing
Liquid crystal polymer (LCP): Very low moisture absorption, good for hermetic-free packaging, Df ~ 0.002
Fused silica/quartz: Ultra-low loss, excellent for thin-film circuits above 60 GHz, but requires specialized processing
Alumina (Al2O3): High Dk (~9.8), very stable, used extensively in hybrid MICs and MMICs

Practical Selection Criteria

Beyond electrical properties, consider CTE match to copper for thermal cycling reliability, availability in the required thickness (thinner substrates reduce radiation losses at mmWave), and compatibility with your assembly process including soldering temperatures and via formation methods. Cost is also a factor: PTFE substrates can be 10-50x more expensive than FR-4, so use them only where performance demands it.

Substrate Loss Equations

Dielectric Loss: α_d = (π × f × √ε_r × tan δ) / c [Np/m]
Conductor Loss: α_c ∝ √f (skin effect)
Total Loss: α_total = α_d + α_c + α_radiation

Common Questions

Frequently Asked Questions

What is the best PCB laminate for 77 GHz automotive radar?

Rogers RO3003 and Isola Astra MT77 are popular choices, offering Df below 0.0017 at 77 GHz with good processability. These materials balance electrical performance with the volume manufacturing requirements of automotive applications.

Can I use FR-4 above 30 GHz?

Standard FR-4 is generally unsuitable above 10 GHz due to its high loss tangent (Df ~ 0.02) and variable dielectric constant. Some modified FR-4 variants exist for lower mmWave frequencies, but dedicated low-loss laminates are strongly recommended above 30 GHz.

How does substrate thickness affect mmWave circuit performance?

Thinner substrates (5-10 mil) reduce surface wave excitation and radiation losses at mmWave frequencies. However, thinner substrates also narrow the microstrip line width, increasing conductor loss. The optimal thickness balances these competing effects for your specific frequency and circuit topology.

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