Substrate Parameters

Dielectric Constant (Dk)
The substrate of high-frequency PCB needs to have a low dielectric constant to reduce signal transmission delay. Generally, the lower the dielectric constant, the better, typically ranging from 2.2 to 10.8, depending on the type of substrate.
The dielectric constant directly affects the signal transmission rate, and a high dielectric constant can easily cause signal transmission delay.
Common substrates such as Polytetrafluoroethylene (PTFE) have an extremely low dielectric constant (around 2.0), making them very suitable for high-speed digital and high-frequency applications.

Dielectric Loss (Df)
Dielectric loss is an indicator that measures the loss of signal during transmission and must be kept low to reduce signal loss and ensure signal quality.
The lower the dielectric loss, the less attenuation during signal transmission and the higher the transmission efficiency.
High-frequency PCBs typically require dielectric loss to be in the very low or ultra-low loss range.

Water Absorption
The substrate should have low water absorption to avoid increases in dielectric constant and dielectric loss due to water absorption.
High water absorption can affect signal transmission performance and the stability of the circuit board.

Electrical Performance Parameters

Impedance Control
High-frequency PCBs need to have good impedance control capabilities to meet the quality requirements of signal transmission.
Impedance control includes parameters such as resistance and reactance, and the lower the impedance, the better, to reduce reflection and loss during signal transmission.

Frequency Response
High-frequency PCBs should maintain stable electrical performance within the required operating frequency range to ensure the integrity and accuracy of signal transmission.
The operating frequency range typically covers the microwave frequency band (>1 GHz) and even the millimeter-wave range (such as 77 GHz).

Coefficient of Thermal Expansion (CTE)
The coefficient of thermal expansion of the substrate should match that of metal materials such as copper foil to reduce the impact of temperature changes on the performance of the circuit board.

Physical Parameters

Board Thickness
The thickness of high-frequency PCB substrates typically ranges from 0.3 mm to 2.0 mm, depending on the application requirements.
Consistency in board thickness is crucial for ensuring the electrical performance and mechanical strength of the circuit board.

Copper Foil Thickness
The thickness of the copper foil affects the conductivity of the circuit board and the efficiency of signal transmission.
High-frequency PCBs typically use thinner copper foil (such as 1 ounce per square foot or thinner) to reduce signal loss and improve frequency response.

Minimum Line Width and Spacing
The design of high-frequency PCBs needs to consider the minimum line width and spacing to ensure complete signal transmission and reduce interference.
The minimum line width and spacing are usually determined based on the manufacturing process of the circuit board and the signal frequency.

Other Parameters

Glass Transition Temperature (Tg Value)
The Tg value is an important indicator for measuring the heat resistance of a material, and for high-frequency PCBs, a higher Tg value helps improve the stability and service life of the circuit board.

Flame Retardancy
High-frequency PCBs need to have good flame retardancy to ensure the safety of the circuit board in extreme situations such as fires.

Processability
The manufacturing process of high-frequency PCBs may require special treatment methods, such as plasma treatment, sodium naphthalene activation treatment, etc., to improve the adhesion of the copper layer on the hole walls and the processing accuracy of the circuits.