When selecting substrates for high-frequency PCBs, special attention must be paid to the material’s dielectric constant (DK).
Dielectric Constant (DK): Also known as relative permittivity, it affects the signal transmission speed and phase delay. A lower DK value helps improve signal speed and reduce transmission delay. The DK can vary depending on frequency, and its variation characteristics need to be considered.
For designs focused on high-speed signal transmission or impedance control, the Dissipation Factor (DF) and its performance under varying conditions (such as frequency, temperature, and humidity) are crucial.
Dissipation Factor (DF): Represents the energy loss of a material under high-frequency electric fields. A lower DF indicates that the material experiences less loss under high-frequency signals, which helps preserve signal integrity.
Standard PCB substrate materials show significant variations in their DK and DF values when frequency changes. This is particularly noticeable in the frequency range of 1 MHz to 1 GHz. For example, a typical FR-4 (epoxy resin-glass fiber) substrate material exhibits a DK of 4.7 at 1 MHz, which reduces to 4.19 at 1 GHz. Beyond 1 GHz, the DK value variation tends to flatten. With higher frequencies (e.g., 10 GHz), the DK for FR-4 is approximately 4.15, showing a small decrease with frequency increase (though the change is not large).
In contrast, high-speed, high-frequency PCB materials show a much smaller variation in DK values across the frequency range from 1 MHz to 1 GHz. Their DK generally stays within a 0.02 variation range. The DK tends to slightly decrease with increasing frequency.
The DF of standard substrate materials, especially under high-frequency conditions, changes more significantly than DK. This increase in DF is important when evaluating a material’s high-frequency performance. High-speed and high-frequency materials often exhibit one of two behaviors:
A minimal change in DF with frequency.
A similar change in DF as standard materials but with a lower base DF.
Other important factors in selecting high-frequency PCB materials include:
Coefficient of Thermal Expansion (CTE): Influences the dimensional stability of the PCB under temperature changes. A lower CTE helps maintain stable PCB dimensions and avoids performance changes due to temperature fluctuations.
Glass Transition Temperature (Tg): The temperature at which the material transitions from a glassy state to a rubbery state. A higher Tg means the material retains good physical properties at high temperatures.
Thermal Conductivity: The material’s ability to conduct heat, which impacts the PCB’s heat dissipation performance.
Flammability Rating: For example, UL94 V-0, which indicates the material’s flame-retardant properties.
Chemical Resistance: The material’s ability to withstand chemical corrosion.
Mechanical Properties: Including toughness and tensile strength.
Surface Roughness: Affects soldering performance and signal transmission quality.
Important Considerations in Selecting High-Frequency PCB Materials:
Material Consistency: Ensure the material’s electrical characteristics remain consistent during manufacturing.
Processability: The material should be suitable for the required processing techniques, such as drilling, lamination, and copper plating.
Cost: High-performance materials may come at a higher cost, so a balance between cost and performance should be considered.
Supply Chain Stability: Ensure a steady supply of materials to avoid production disruptions.
Environmental Regulations: Ensure the material complies with environmental standards such as RoHS.
Material Availability: Consider procurement lead times and inventory levels.
Design and Material Compatibility: Ensure the selected material matches the design requirements, including impedance control and layer structure.
Long-Term Reliability: Evaluate the material’s performance stability over extended use.
Testing and Verification: Ensure the material’s electrical and physical properties are thoroughly tested and validated.
Design and Simulation: Conduct simulations during the design phase to predict how the material will perform in actual applications.
Our company specializes in manufacturing 2-36 layer high-frequency PCBs. We offer fast production for high-frequency boards, mixed-press high-frequency boards, specialty PCBs, microwave RF boards, and microwave antenna boards. We stock high-frequency materials such as Rogers, Taconic, F4B, TP-2, FR-4, and more. Custom designs are welcome.