What Is a High-Frequency Substrate?
High-frequency substrates are used for transmitting signals in the range of 500 MHz to 5 GHz. In high-frequency circuits, substrate materials are selected based on signal frequency, size, cost, and other factors, considering dielectric constant, dielectric loss tangent, thickness, and circuit accuracy.
For large-capacity, high-speed data communications, signal speed is crucial. Materials must have a low dielectric constant and minimal transmission loss. Common materials include glass cloth substrate epoxy resins, ceramics, and fluoropolymers.
Uses of High-Frequency Substrates
These substrates are utilized in mobile devices, aerospace equipment, and more, demanding higher noise countermeasures than general circuits. They help prevent equipment from radiating and being affected by noise.
Applications range from TV broadcasting and communication devices to GPS, weather sensors, and human body sensors.
Principle of High-Frequency Substrates
High-frequency substrates require materials with low dielectric constants and signal losses to increase transmission speed, approaching the speed of light. Lower dielectric loss tangent materials, like PPE and PTFE, minimize signal loss.
The skin effect, where current flows at the maximum skin depth in higher frequencies, increases conductor resistance, resulting in higher transmission losses.
Materials’ coefficient of thermal expansion should match copper foil to prevent delamination due to temperature changes.
High-Frequency Substrate Materials
Materials include FR-4, PPE, ceramic filler resin (Rogers), fluoropolymer (PTFE), and hybrid materials, each offering distinct benefits for specific applications.
1. FR-4
FR-4 is cost-effective and widely used in double-sided and multilayer substrates, generally suitable up to about 1 GHz.
2. PPE
PPE-based substrates offer higher glass transition temperature and lower moisture absorption, improving frequency characteristics in the high-frequency band. They are used in large computers, high-frequency devices, and antennas.
3. Ceramic Filler Resin
These substrates are excellent for high-frequency characteristics and can be processed using the same equipment as FR-4. They are used in mobile device base stations and automotive radars.
4. PTFE
Fluoropolymers, like Teflon, feature low dielectric constants and are used for high-speed communications in antennas, radar, and aerospace.
5. Hybrid Materials
Hybrid materials combine high-frequency and general materials (FR-4) to create cost-effective, multi-layered substrates with improved wiring flexibility and strength.