Materials

Substrate

/sub-strayt/

A substrate is the base material on which RF circuits are fabricated. The substrate's dielectric constant (er), loss tangent (tan delta), thickness, and thermal conductivity directly determine circuit impedance, loss, size, and thermal performance. Common RF substrates include Rogers (er 2-10, low loss), FR-4 (er 4.4, high loss), alumina (er 9.8, very low loss), and GaAs/GaN/InP (semiconductor substrates for MMICs).

Category: Materials

Related to: PCB, Microstrip, Dielectric, MMIC, Dielectric Loss

Units: er, tan delta, mils

Understanding RF Substrates

Substrate selection is one of the most important decisions in microwave circuit design. The wrong substrate can make a design impossible or prohibitively lossy. Each substrate material presents different trade-offs between cost, loss, dielectric constant, thermal performance, and manufacturability.

Common RF Substrates

Material	er	tan delta	Cost
PTFE/Rogers	2-10.8	0.001-0.003
FR-4	4.4	0.02	$
Alumina	9.8	0.0001	$
Duroid 5880	2.2	0.0009
GaAs	12.85	0.0004
Silicon	11.7	0.015	$

Common Questions

Frequently Asked Questions

What substrate should I use?

Below 1 GHz: FR-4 is acceptable. 1-10 GHz: Rogers RO4003/4350 (standard) or PTFE-based (lowest loss). Above 10 GHz: Rogers RT/Duroid 5880 or alumina for lowest loss. MMIC: GaAs, InP, or SiGe semiconductor substrates.

Why is FR-4 bad for microwave?

FR-4 has high loss tangent (0.02), causing significant attenuation above 1-2 GHz. It also has inconsistent er (varies with frequency and lot), making impedance control difficult. Use FR-4 only for low-frequency RF or non-critical digital sections.

What is the effect of dielectric constant on circuit size?

Higher er shrinks circuit dimensions proportionally to sqrt(er). A 50-ohm microstrip line on er=10 substrate is ~3x narrower and the quarter-wave sections ~3x shorter than on er=1 (air). This enables smaller circuits but makes fabrication tolerances tighter.

Related Terms

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