When choosing substrates for high-frequency PCBs, special attention must be paid to the dielectric constant (Dk) of the material.
Dielectric Constant (Dk): Also known as relative permittivity, it affects signal transmission speed and phase delay. A low Dk value helps improve signal transmission speed and reduce transmission delay. Its variation characteristics at different frequencies should also be considered.
For requirements focused on high-speed signal transmission or controlled impedance characteristics, the dissipation factor (Df) and its performance under conditions such as frequency, temperature, and humidity should be emphasized.
Dissipation Factor (Df): Represents the energy loss of the material under high-frequency electric fields. A low Df value indicates lower loss at high frequencies, which helps improve signal integrity.
Conventional substrate materials exhibit significant changes in Dk and Df values under varying frequencies. This is particularly evident within the frequency range of 1 MHz to 1 GHz.
For example, conventional epoxy resin-fiberglass cloth substrates (standard FR-4) have a Dk value of 4.7 at 1 MHz, which changes to 4.19 at 1 GHz. Beyond 1 GHz, the change in Dk tends to stabilize, gradually decreasing with increasing frequency (though the change is minimal). For instance, at 10 GHz, the Dk value of standard FR-4 is 4.15.
High-speed, high-frequency substrate materials exhibit minimal changes in Dk under varying frequencies. From 1 MHz to 1 GHz, the Dk value typically remains within a variation range of 0.02. The Dk value tends to slightly decrease as the frequency increases.
For conventional substrate materials, the dissipation factor (Df) is more significantly affected by frequency changes, especially in the high-frequency range. The general trend is that Df increases with frequency. Therefore, when evaluating the high-frequency performance of a substrate material, the focus should be on its Df variation.
High-speed, high-frequency substrate materials exhibit two distinct types of behavior under high-frequency variations compared to conventional materials:
One type shows minimal change in Df with frequency variations.
The other type, while exhibiting a similar magnitude of change to conventional materials, has inherently lower Df values.
Coefficient of Thermal Expansion (CTE): Affects the dimensional stability of the PCB under temperature changes. A low CTE value helps maintain the dimensional stability of the circuit board, preventing performance changes caused by temperature variations.
Glass Transition Temperature (Tg): The temperature at which the material transitions from a glassy state to a rubbery state. A high Tg value indicates that the material can maintain good physical properties even at high temperatures.
Thermal Conductivity: The thermal conductivity of the material, which affects the heat dissipation performance of the PCB.
Flame Retardant Rating: Such as UL94 V-0, indicating the material’s flame retardancy.
Chemical Resistance: The material’s ability to resist chemical corrosion.
Mechanical Properties: Including toughness, tensile strength, etc.
Surface Roughness: Affects soldering performance and signal transmission quality.
Key considerations when selecting high-frequency PCB materials include:
Material Consistency: Ensure the electrical properties of the material remain consistent during production.
Processability: Whether the material is suitable for 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 must be considered.
Supply Chain: Ensure a stable supply of materials to avoid production interruptions.
Environmental Regulations: Compliance with environmental regulations such as RoHS.
Material Availability: Procurement lead times and inventory status.
Design-Material Compatibility: Ensure the selected material matches design requirements, such as impedance control and layer stack-up.
Long-Term Reliability: The stability of the material’s performance over extended use.
Testing and Validation: Whether the electrical and physical properties of the material have been thoroughly tested and verified.
Design and Simulation: Conduct sufficient simulations during the design phase to predict the material’s performance in practical applications.
Our company is a professional manufacturer of 2-36 layer high-frequency PCBs, offering expedited production of high-frequency PCB boards, mixed-pressure high-frequency boards, special circuit boards, microwave RF boards, microwave antenna boards, and other products. We regularly stock high-frequency materials such as Rogers, Taconic, F4B, TP-2, and FR-4. Custom orders are welcome based on provided designs.