Hybrid PCB Stackups: Why they matter and how to get them right

As Digital and RF systems reach multi-gigahertz speeds, traditional PCB materials struggle with signal loss, thermal instability, and poor impedance control. Hybrid PCB stackups solve this by combining high-performance materials (like Rogers) with cost-effective ones (like FR-4), optimizing both performance and budget. However, this approach adds complexity as different materials behave differently during fabrication, making early collaboration with your PCB manufacturer and smart material selection essential.

What is a hybrid stackup?

A Hybrid stackup is a multilayer PCB using multiple dielectric materials. For instance, Rogers 4350B may be used for high-speed signal layers, while FR-4 handles power and ground planes. This selective use of low-loss materials ensures signal integrity where it matters most, especially in mixed RF/digital designs.

Real-World Example: 6-Layer Hybrid Stackup

Here’s a simplified example of a hybrid stackup for a mixed-signal board:

This setup keeps the high-speed and RF signals on low-loss layers while using FR-4 for the rest to keep costs down.

To accurately model such a stackup, especially when combining materials with different electrical properties, tools like the Cadence Allegro Cross Section Editor can be quite useful. It enables designers to choose from a library of predefined dielectric materials or input custom ones by specifying parameters such as Dielectric constant (Dk), Loss tangent (Df), Conductivity, Density, and Specific heat. If the required material isn’t available in the default list, you can define it through Setup > Materials > Material Editor and incorporate it into your stackup.

This ensures that impedance calculations are based on realistic material behavior, which is critical for maintaining signal integrity in high-speed and RF designs.

Why go hybrid?

Here’s where hybrid stackups shine:

• Signal Integrity: Materials like Rogers or Megtron have lower dielectric loss, which means cleaner signals at high frequencies.
• Thermal Stability: High-Tg materials can handle more heat without warping or delaminating.
• Cost Control: You don’t need to use expensive materials across the entire board, you’ll use it just where they’re needed.
• Layer-Level Optimization: Different materials can be assigned to specific layers (e.g., RF, digital, power) based on performance needs, enabling targeted electrical and thermal tuning.

In Part 2, we’ll dive into the real-world challenges of hybrid stackup design like material compatibility, impedance control, and why your fabricator should be your best friend.

Stay tuned! In the meantime, feel free to drop your comments about the challenges you have faced while designing stackup.