PCB Material for High Speed Design

The properties listed above are not exhaustive. For example, if heavy components like transformers are to be mounted, strength needs to be factored into your consideration. However, it is important to understand why dielectric constant, loss tangent, and thermal considerations are always critical for high-speed applications.    

Critical Material Properties for High-Speed Design

Dielectric Constant and Its Impact

The dielectric constant, also known as relative permittivity (εr), fundamentally determines signal propagation velocity along traces and the characteristic impedance of layers in the stackup of your PCB. For high-speed signal routing, it is essential to maintain constant impedance. Maintaining or matching impedance between layers requires careful Dk selection and control.

Lower Dk materials offer several advantages for high-speed designs. They enable faster signal propagation, reduce board thickness requirements, and simplify via design by allowing lower aspect ratios. Materials with Dk values between 2.2 and 3.5 are typically preferred for frequencies above 1 GHz, as they provide better signal integrity while reducing the overall PCB stackup complexity. It is often wrongly assumed that standard FR4 is the right choice for high-speed RF PCB design. However, the frequency-dependent nature of Dk can create significant issues, such as signal dispersion and timing skew in high-speed applications.

Loss Tangent and Signal Attenuation

Loss tangent (tan δ or Df) directly correlates with signal attenuation in transmission lines. The dielectric loss in dB per inch follows the relationship: αd = 2.32 × f × tan(δ) × √εr, where f represents frequency in GHz. Materials with lower loss tangent values significantly reduce signal attenuation, especially at higher frequencies. For example, Rogers RO4350B with a loss tangent of 0.0037 provides substantially lower losses compared to standard FR4 materials with loss tangent values ranging from 0.018 to 0.025. 

Thermal Considerations in Material Selection

Thermal properties become increasingly critical as power densities and operating frequencies increase. For example, Tg defines the temperature at which the material loses structural integrity and transitions from a rigid to a softened state. High-speed applications typically require materials with Tg values above 170°C (this may be elevated further when high-power devices like microprocessors or other devices are present) to ensure dimensional stability during manufacturing and operation. Another impactful parameter for multi-layer PCBs is the coefficient of thermal expansion (CTE), which can lead to physical separation of the layer substrate from the attached copper surface. 

Important Manufacturing Considerations

Obviously, the material you select affects the fabrication of your circuit board. Consequently, following good PCB stackup guidelines and implementing your CM’s DFM rules are very important. Additionally, mitigation strategies should be employed to address certain attributes, such as inherent material inhomogeneity from the construction of woven glass laminates and varying effective dielectric constant due to traces that traverse resin-rich areas or fiber bundles. 

High Speed Mitigation Strategies for Inherent PCB Material Attributes

• Route traces at angles relative to the fiber weave pattern
• Select materials with more uniform glass distribution
• Select drill holes with aspect ratios between 6:1 and 8:1

By striving to optimize the balance between electrical performance and manufacturing considerations, you can select PCB material for high speed design that will ensure your boards can be reliably built and achieve design objectives. 

How to Choose the Best PCB Materials for High-Speed Design

Choosing the best material for your high-speed design requires following best practices, as listed below.