Materials and Substrates Dielectric Materials Informational

How do I select between a bonded and an unbonded multilayer PCB construction for RF?

Bonded multilayer RF PCBs use adhesive prepreg or fusion bonding to permanently join multiple substrate layers into a single laminate stackup, while unbonded constructions use separate substrate layers connected only at specific via locations or RF transitions. Bonded construction is the standard approach for most multilayer RF designs, offering mechanical integrity and reliable via connections between layers. The choice of bonding material is critical: low-loss bond plies like Rogers RO4450F maintain the RF performance of the core materials. Unbonded or partially bonded approaches are used in specialized applications where the bonding layer's dielectric properties or thickness tolerance cannot meet RF performance requirements, such as in air-cavity antenna designs or suspended stripline configurations.

Category: Materials and Substrates

Updated: April 2026

Product Tie-In: PCB Laminates, Substrates

Multilayer PCB Construction Methods for RF Circuits

Modern RF systems increasingly require multilayer PCB stackups to integrate digital control, DC bias, and RF signal layers in a compact package. The method used to join these layers significantly impacts RF performance, particularly at millimeter-wave frequencies where bond ply properties become electrically significant.

Bonded Construction

Standard bonded multilayer fabrication uses prepreg materials heated under pressure to bond core layers together. For RF applications, the bond ply material must have low loss tangent and well-controlled dielectric constant. Rogers RO4450F (Df 0.004) and Taconic FastRise 27 are designed specifically as RF-compatible prepregs. The bond layer thickness (typically 2-4 mil) must be included in electromagnetic simulations as it affects coupled-line spacing and vertical transition performance.

Fusion Bonding

Some PTFE-based substrates can be fusion-bonded by heating above the PTFE melting point (327°C) under pressure. This eliminates the need for a separate bond ply, maintaining consistent dielectric properties throughout the stackup. However, fusion bonding requires careful process control to prevent delamination and is limited to compatible PTFE materials.

Unbonded and Air-Cavity Construction

Air-cavity constructions use machined cavities in the PCB stackup to create air-dielectric regions for antennas or transmission lines. These achieve the lowest possible loss but require specialized fabrication capabilities and careful mechanical design to maintain structural integrity. Suspended stripline, where a thin substrate is held in an air gap between ground planes, combines low loss with excellent shielding.

Design Guidelines

Include bond plies in EM simulation: Model the actual stackup including bond layer thickness and properties
Minimize bond ply in signal path: Route RF traces on core layers, use bond plies only as prepreg between ground layers where possible
Via transitions: Design via anti-pads and ground via fences accounting for the complete stackup
Thermal management: Bonded constructions typically provide better thermal conductivity between layers than air-gap designs

Multilayer Stackup Design

Bond ply effective Dk: epsilon_eff(stack) = Sum(t_i x epsilon_i) / Sum(t_i)
Vertical transition loss: IL_via approximately 0.03 x f_GHz [dB per via]
Stripline Z0 in bonded stackup: Z0 = (60 / sqrt(epsilon_r)) x ln(4b / (pi x d x 0.67))

Common Questions

Frequently Asked Questions

What prepreg material should I use with Rogers RO4350B cores?

Rogers RO4450F is the recommended bonding prepreg for RO4350B stackups. It has compatible CTE, a Dk of 3.52, and Df of 0.004, closely matching the RO4350B core properties. This combination is widely supported by RF PCB fabricators.

How many RF signal layers can a multilayer RF PCB support?

Most practical RF multilayer PCBs use 4-8 layers, with 2-3 dedicated RF signal layers and the remaining layers for ground planes, DC bias, and digital signals. Above 8 layers, registration accuracy and via drill quality become challenging for RF-critical designs, particularly at mmWave frequencies.

Does the bonding process affect substrate dielectric properties?

The lamination cycle's heat and pressure can slightly alter dielectric properties of some substrates, typically by less than 1-2%. More significant is the dimensional change (shrinkage) during lamination, which can affect transmission line impedance. Fabricators typically compensate for known shrinkage factors by adjusting artwork dimensions.

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