Materials Suitable For High-speed PCBs Need To Meet These 5 Major Requirements

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The general requirements for high-speed PCB materials are as follows:

1. Low loss, CAF/heat resistance, and mechanical toughness (good reliability).
2. Stable Dk/Df parameters (low variance with frequency and environmental changes).
3. Small tolerance for material thickness and resin content (good impedance control).
4. Low copper foil surface roughness (reduce loss).
5. Use flat, small window glass fabric as much as possible (reduce skew and loss).

high-speed PCB

Next, let’s provide a detailed explanation of the five requirements for high-speed PCB materials.

The integrity of high-speed signals mainly relates to impedance, transmission line loss, and time delay consistency. Receiving a proper waveform and eye diagram at the receiving end ensures signal integrity. Therefore, the main parameter indicators for choosing high-speed digital circuit PCB materials are Dk, Df, and loss.

Whether in analog or digital circuits, the dielectric constant Dk of PCB materials is a crucial parameter because it is closely related to the actual circuit impedance values applied to the material. When the Dk value of PCB material changes, whether with frequency or temperature, unexpected changes in transmission line impedance occur, adversely affecting the signal transmission performance of high-speed digital circuits. If the Dk value of the PCB material shows different values for different frequency harmonic components, the impedance will also vary at different frequencies. The unexpected changes in Dk value and impedance will cause certain degrees of loss and frequency offset in harmonic components, leading to signal distortion and a decrease in signal integrity.

Dispersion, closely related to Dk value, is also a characteristic of the material. The smaller the change in Dk value with frequency, the smaller the dispersion, and the better the performance for high-speed digital circuit applications. Various factors, such as the polarization of dielectric material, material loss, and surface roughness of high-frequency copper conductors, contribute to circuit dispersion. Therefore, high-speed material Dk values should remain stable, with minimal fluctuations across different frequency bands and temperatures.

Transmission line loss generally includes dielectric loss, conductor loss, and radiation loss.

Dielectric loss, also known as insulation loss, increases with frequency, particularly with the frequency of high-order harmonics in high-speed digital signals, causing severe amplitude attenuation and resulting in signal distortion. Dielectric loss is proportional to the signal frequency, the square root of the dielectric constant Dk of the insulating layer, and the dielectric loss factor Df of the insulating layer.

Conductor loss is related to the type of conductor (different types have different resistances), the physical dimensions of the insulating layer and conductor, and is proportional to the square root of the frequency. In PCB manufacturing, using different substrates mainly affects conductor loss through skin effect and surface roughness. The roughness of signal lines varies with different copper foils. Due to the influence of the skin effect/depth, the length of copper teeth directly affects the transmission quality of high-speed signals; the shorter the copper teeth, the better the high-speed signal transmission quality.

Radiation loss is related to dielectric properties and is proportional to the dielectric constant Dk, the dielectric loss factor Df, and the square root of the frequency.

Reminder: Higher quality materials also come with higher costs. Engineers should balance meeting design requirements, manufacturability, and cost to achieve the best cost-performance ratio for the product.

Materials suitable for high-speed PCBs need to meet these 5 major requirements

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