The important indicators for selecting high-frequency and high-speed PCB (printed circuit board) materials

The important indicators for selecting high-frequency and high-speed PCB (printed circuit board) materials include the following aspects:

1. Frequency stability and thermal stability of Dk (dielectric constant);

In addition to pursuing a low Dk, it is even more crucial that Dk remains as constant as possible across the entire operating frequency range (such as 10GHz to 100GHz) and operating temperature range. Excessive fluctuations in Dk with frequency or temperature can lead to impedance loss of control and signal distortion.

2. The measured value of Df (dielectric loss/tangent of loss angle) at the target frequency;

Many materials exhibit low Df at 1MHz, but their Df significantly increases above 10GHz. When selecting materials for high-frequency and high-speed applications, it is essential to consider the Df at the target application frequency (such as 10GHz, 28GHz, 77GHz), rather than relying on typical low-frequency values.

3. Surface roughness of copper foil (conductor loss)

At high frequencies, the skin effect becomes significant, and the rougher the copper foil, the greater the conductor loss. Therefore, ultra-low profile copper foil (VLP, HVLP), or even rolled copper foil, is often chosen, and attention needs to be paid to its bonding strength with the resin interface.

4. Glass Weave Effect

For high-speed digital signals (differential pairs), uneven dielectric constant of ordinary E-glass cloth can cause differential mode to common mode conversion and skew deviation. When selecting materials, consideration can be given to open fiber cloth, flat cloth, non-glass cloth (such as LCP, PTFE + ceramic filler), or avoidance through wiring design.

5. Thermal decomposition temperature (Td) and glass transition temperature (Tg)

High-frequency and high-speed materials often utilize PTFE, carbon-hydrogen, or modified PPO resins due to their low Df. The Tg and Td of these materials determine their thermal reliability under lead-free soldering and long-term operation. For example, PTFE-based materials require attention to their high-temperature dimensional stability.

6. Moisture absorption and Dk/Df stability in a wet state;

Moisture (Dk ≈ 80, Df is relatively high) can severely degrade electrical properties. Materials such as PTFE and hydrocarbon resins have low moisture absorption, but some low Dk materials may exhibit a significant increase in Dk/Df after absorbing moisture. Therefore, it is important to pay attention to performance drift under humid conditions.

7. Anisotropy;

Some boards exhibit different Dk values in the X/Y direction (plane) and Z direction (thickness direction), or the dielectric constant is influenced by the weaving direction of the glass cloth, which affects high-speed wiring (such as the consistency of differential pair directions).

The core of high-frequency and high-speed material selection lies in balancing a low and stable Dk/Df ratio, low-roughness copper foil, sufficient thermal/mechanical reliability, and processability (especially considering the difficulty of hole metallization for PTFE-based materials) under the target frequency and operating conditions (temperature/humidity). 

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