High-Frequency Hybrid Stack-Up: Engineering an 8-Layer Board with RO4350B, FR-4, and Sequential Lamination

High-Frequency Hybrid Stack-Up: Engineering an 8-Layer Board with RO4350B, FR-4, and Sequential Lamination

As wireless communications, automotive radar, and high-speed digital systems push into multi-gigahertz frequencies, the choice of circuit board material becomes critical. However, cost constraints often prevent the use of high-performance laminates across an entire multilayer board. The solution? A hybrid stack-up that combines Rogers RO4350B for high-frequency layers with standard FR-4 for the rest.

This article examines a sophisticated 8-layer design that leverages the best of both worlds: low-loss RO4350B on the outer layers for signal integrity, and FR-4 in the middle for structural and cost efficiency. We will also explore the unique properties of RO4350B, as well as the implementation of blind vias, buried vias, and resin-plugged vias.

Product Snapshot: The 8-Layer Hybrid Board

Structure: 8-layer multilayer PCB

Finished Board Thickness: 1.553 mm

Copper Weight: Inner layer 1 oz, Outer layer 1 oz

Surface Finish: ENIG (Electroless Nickel Immersion Gold)

Solder Mask: Green with white lettering

Panel Size: 120 mm x 30 mm = 1 piece

Plated Through-Hole (PTH) Copper Thickness: 25 μm (meeting IPC Class 3)

Edge Plating: Metal edging (edge plating / castellated holes)

Stack-up

Extended Learning 1: RO4350B Laminate – Properties, Parameters, and Processing

RO4350B is a hydrocarbon ceramic laminate from Rogers Corporation’s RO4000® series, designed to deliver high-frequency performance at a cost compatible with standard FR-4 fabrication processes.

Key Parameters (from Rogers RO4000 Series Datasheet)

Application Areas

Due to its stable Dk over frequency and temperature, and low dissipation factor, RO4350B is widely used in:

Automotive radar (77 GHz and 24 GHz)

5G infrastructure (massive MIMO, small cells)

RF microwave circuits (power amplifiers, LNAs)

Satellite communications (Ku-band, Ka-band)

High-speed digital (backplanes, test equipment)

Key Processing Points for RO4350B

Unlike PTFE-based materials (e.g., RO3000 series), RO4350B offers significant fabrication advantages:

No Sodium Etch Required: RO4350B is a thermoset hydrocarbon material, not PTFE. Standard FR-4 desmear (alkaline permanganate) or plasma (CF₄/O₂) processes are sufficient.

Drilling: Standard carbide drills are compatible. Recommended surface speed: 300-500 SFM (90-150 m/min). Chip load: 0.002-0.004 inches/rev. Avoid speeds >500 SFM to prevent excessive tool wear. Expected hole wall roughness: 8-25 µm.

PTH Processing: Desmear is typically not required for double-sided boards due to the high Tg (>280°C). For multilayers, a single or double pass through alkaline permanganate may be used. Rogers recommends against etchback as it can loosen filler particles.

Routing: Use carbide multi-flute spiral chip breakers. Surface speed below 500 SFM, chip load 0.0010-0.0015 inches/rev.

Oxidation Consideration: Prolonged exposure to high-temperature oxidative environments can change the dielectric properties of hydrocarbon-based materials. Rogers recommends evaluating each design for long-term reliability.

Extended Learning 2: Blind Vias and Buried Vias – Why Use Them?

In complex multilayer PCBs like this 8-layer board, not all holes need to go through the entire thickness. This is where blind and buried vias become essential.

Definitions

In This Product: 1-2 Blind Vias and 5-6 Buried Vias

1-2 Blind Via: Connects Layer 1 (top RO4350B) to Layer 2 (first FR-4 inner layer). This allows a high-frequency signal to enter the board from the top layer and immediately go to an inner layer without stubbing through the entire stack-up.

5-6 Buried Via: Connects Layer 5 to Layer 6 entirely within the FR-4 core. This inner connection is completely hidden after final lamination.

Why Use Blind and Buried Vias?

Higher Routing Density: By freeing up surface area on outer layers, blind vias allow more components and traces to be placed. Buried vias create internal "interposer" connections without consuming outer layer real estate.

Improved Signal Integrity (SI): A through via creates a long stub (unused portion of the via barrel) that acts as an impedance discontinuity and a resonant antenna at high frequencies. Blind vias eliminate or significantly reduce stubs, preserving signal quality for multi-gigabit or RF signals.

Better Power Integrity (PI): Buried vias can be used to create low-inductance connections between power and ground planes deep inside the board, reducing noise.

Reduced Layer Count (Potential): Efficient use of blind/buried vias can sometimes reduce the total number of layers needed for a complex routing scheme.

Miniaturization: Smaller pad sizes are possible with blind vias (especially laser-drilled microvias), allowing finer-pitch component breakout.

Additional Features in This Design

Resin-Plugged Vias: Vias (likely the buried vias or certain blind vias) are filled with resin and then capped. This prevents solder wicking, provides a flat surface for stacking vias, and enhances reliability in sequential lamination processes.

25 μm Copper in Holes (IPC Class 3): This exceeds the typical IPC Class 2 requirement (20 μm). Class 3 demands higher reliability for harsh environments (aerospace, medical, automotive safety). The thicker copper ensures robust mechanical and thermal fatigue resistance.

Metal Edging: Copper plating on the board outline (edge plating) provides EMI shielding, allows board-to-board soldering (castellations), or improves thermal conductivity along the edge.

Conclusion: A Purpose-Built Hybrid Design

This 8-layer board exemplifies modern PCB engineering for mixed-signal applications. By placing 10 mil RO4350B on the outer layers, the designer achieves low loss and stable Dk for high-frequency or fast-edge signals. The FR-4 Tg180 core provides a cost-effective, mechanically rigid middle section for power distribution and low-speed signals.

The use of 1-2 blind vias and 5-6 buried vias demonstrates a commitment to signal integrity and routing density, while IPC Class 3 plating (25 μm copper) and resin-plugged vias ensure long-term reliability. This hybrid approach is an increasingly common strategy in automotive radar, 5G infrastructure, and aerospace telemetry—where performance, cost, and reliability must all be balanced.