RO4450B Bondply: Multilayer RF Stackup Guide

What Is Rogers RO4450B Bondply?

RO4450B is a hydrocarbon-ceramic thermoset bondply (prepreg) manufactured by Rogers Corporation for bonding RO4000-series core laminates into multilayer RF and microwave PCBs. It has a dielectric constant (Dk) of 3.54 ± 0.04 at 10 GHz and a dissipation factor (Df) of 0.004, making it electrically compatible with RO4350B and RO4003C cores. Unlike PTFE-based bonding films that demand specialized high-temperature sintering cycles, RO4450B processes on conventional FR-4 lamination presses, which significantly reduces fabrication cost and cycle time for high-frequency multilayer boards.

Key Electrical and Mechanical Properties

The table below summarizes the published datasheet values that drive stackup modeling and process decisions. All electrical values are at 10 GHz per IPC-TM-650 2.5.5.5 (clamped stripline resonator) unless noted.

Two properties deserve special attention. First, the Dk of 3.54 is deliberately close to RO4350B’s 3.48 (design Dk at 10 GHz), so bondply layers do not create large impedance steps in stripline structures. Second, the Tg above 280 °C and low moisture absorption mean the material survives lead-free reflow without delamination—a practical requirement for any board that will go through SMT assembly.

Why RO4450B Instead of PTFE Bonding Films?

Engineers working with PTFE laminates such as RT/duroid or CuClad often bond multilayer stacks with CuClad 6250 bonding film or similar fluoropolymer prepregs. Those materials require sintering temperatures above 370 °C and extremely long dwell times, often 2–4 hours. RO4450B cures at roughly 375 °F (190 °C) for 90 minutes under 300–500 psi—parameters that any shop with a multi-opening press for FR-4 can handle. The practical benefits are:

• Lower fabrication cost. No high-temperature PTFE press, no sodium-etch surface preparation, no sequential lamination.
• Faster turns. Standard cure cycle means the bondply step fits into normal production scheduling.
• Better dimensional stability. The thermoset resin cross-links fully during cure, yielding tighter X-Y registration than thermoplastic PTFE bonds—critical for fine-pitch via alignment in dense multilayer RF boards.
• Lead-free compatible. The high Tg means no re-softening during 260 °C reflow peaks.

The trade-off is that RO4450B’s Df of 0.004 is higher than the best PTFE bonding films (Df ≈ 0.001–0.002). For frequencies above roughly 30 GHz where every hundredth of a dB matters, PTFE bonds may still be justified. Below 20 GHz—covering the vast majority of radar, 5G sub-6 GHz, and avionics bands—the difference is negligible.

Stackup Design with RO4450B

Symmetric Four-Layer RF Stackup

The most common starting point is a symmetric four-layer board with two signal layers and two ground planes. A typical construction looks like this:

• Layer 1 — signal (microstrip), ½ oz Cu
• RO4350B core, 10 mil (0.254 mm)
• Layer 2 — ground plane, 1 oz Cu
• RO4450B bondply, 4 mil (0.102 mm) cured
• Layer 3 — ground plane, 1 oz Cu
• RO4350B core, 10 mil (0.254 mm)
• Layer 4 — signal (microstrip), ½ oz Cu

Because the Dk of the bondply (3.54) is close to the core (3.48), a stripline trace routed between Layers 2 and 3 through the bondply region sees a nearly uniform dielectric environment. The small Dk offset—about 1.7 %—causes less than 0.3 Ω impedance deviation on a 50 Ω stripline, well within manufacturing tolerance for most designs.

Six-Layer and Eight-Layer Hybrid Stackups

For designs that need both RF and digital sections—common in phased-array radar modules and 5G radio units—a hybrid stackup pairs RO4350B RF cores with FR-4 or RO4835 oxidation-resistant cores in the digital section, all bonded with RO4450B. The bondply’s processing temperature is compatible with standard epoxy-glass prepregs, so mixed-material lamination in a single press cycle is feasible. Key rules:

• Place RF signal layers on RO4350B cores adjacent to solid ground planes.
• Use RO4450B as the bonding layer between every pair of cores, keeping the dielectric predictable.
• Maintain stackup symmetry about the mid-plane to minimize bow and twist after lamination; IPC-6012 Class 3 limits bow and twist to 0.75 %.
• Account for copper balancing: if one side has a dense ground pour and the other a sparse signal layer, add non-functional pads or thieving to equalize copper density.

Dk Matching: Why It Matters for Stripline

In a stripline structure, the trace is sandwiched between two reference planes with dielectric above and below. If the core Dk and bondply Dk differ significantly, the effective Dk becomes a weighted average that depends on the trace geometry and the thickness ratio. With RO4350B (Dk 3.48) and RO4450B (Dk 3.54), the mismatch is small enough that most field solvers converge on a stable impedance with minimal iteration. Compare this to bonding with a generic FR-4 prepreg (Dk ≈ 4.2–4.5): the Dk mismatch would shift a 50 Ω stripline by 3–5 Ω, requiring trace width compensation that complicates routing and increases coupling risk.

Lamination Process Parameters

The following process window is based on Rogers’ published fabrication guidelines for RO4450B. Always verify against the latest revision before production.

Vacuum-assisted lamination is strongly recommended. Air trapped between the bondply and the core foil can create voids that act as localized low-Dk regions, distorting impedance and degrading insertion loss. A vacuum level below 5 Torr during the initial heat-up phase virtually eliminates this risk.

Drill and Via Considerations

RO4450B’s thermoset resin drills cleanly with standard carbide bits, unlike PTFE materials that tend to smear and require plasma desmear. Recommended drill parameters are similar to FR-4: entry speed 150–200 SFM with a chip load of 1.0–1.5 mil per revolution. After drilling, a standard permanganate or plasma desmear cycle ensures good copper-to-barrel adhesion in plated through-holes.

For high-density designs, laser-drilled microvias through a single RO4450B bondply layer (3.5 or 4 mil) are achievable with UV or CO₂ lasers. The ceramic filler content in the resin can accelerate laser drill-bit wear slightly compared to pure resin prepregs, so test coupons should be run to verify hole quality before committing to production.

Compatibility with Other Rogers Laminates

RO4450B is part of the RO4000 family and is specifically designed to bond with:

• RO4350B — the workhorse RF laminate (Dk 3.48, Df 0.0037). The most common pairing for multilayer RF boards up to 20 GHz.
• RO4003C — a lower-Dk option (Dk 3.38, Df 0.0027) used when slightly lower loss or a different trace width is preferred.
• RO4360G2 — a higher-Dk laminate (Dk 6.15) for miniaturized filters and couplers; RO4450B bonds it without the thermal-expansion mismatch problems seen with PTFE adhesives.

It can also be used to bond RO4835IND LoPro laminates in industrial and automotive radar applications where the LoPro copper profile reduces conductor loss at millimeter-wave frequencies. For entirely different material families—such as the thermoset TMM4 ceramic laminates—Rogers recommends its own dedicated bonding solutions (e.g., RO4450F or CuClad 6700), so always check material compatibility before specifying RO4450B in a mixed-family stackup.

Design Tips and Common Pitfalls

Resin Flow and Copper Fill

RO4450B has controlled resin flow, but areas with very low copper density on inner layers can see excessive resin flow into the etched regions, thinning the bondply locally and shifting the effective dielectric thickness. To mitigate this, add copper thieving or hatched ground fill to inner layers, keeping copper density above 60 % everywhere.

Impedance Modeling

When entering RO4450B into a field solver, use the cured thickness (not the uncured prepreg thickness, which is typically 0.5–1 mil thicker) and the design Dk of 3.54. Rogers recommends the “design Dk” values rather than the raw resonator-test Dk for impedance modeling, because they account for typical copper roughness effects. If you are using a different copper profile (e.g., LoPro or VLP foil), adjust the roughness model in your solver accordingly.

Moisture Sensitivity

Although cured RO4450B absorbs only 0.06 % moisture, the uncured prepreg is hygroscopic. Store rolls sealed in the original moisture-barrier bag with desiccant, and pre-bake sheets at 250 °F (120 °C) for 2 hours if they have been exposed to ambient humidity for more than 48 hours. Failure to do so can cause micro-voiding during lamination.

When to Choose RO4450B vs. Alternatives

Relevant Standards

Multilayer RF boards bonded with RO4450B should be qualified to IPC-6018 (Qualification and Performance Specification for High-Frequency Printed Boards), which extends IPC-6012 with RF-specific requirements for Dk tolerance, insertion loss, and impedance control. Class 3 boards (high-reliability, military/aerospace) require tighter Dk uniformity and via reliability testing, both of which RO4450B supports given its low CTE and high Tg.

Frequently Asked Questions

What is the dielectric constant of RO4450B?

RO4450B has a Dk of 3.54 ± 0.04 at 10 GHz, measured per IPC-TM-650 2.5.5.5. This is intentionally close to RO4350B (Dk 3.48) to minimize impedance discontinuities in stripline structures.

Can RO4450B be laminated on a standard FR-4 press?

Yes. RO4450B cures at 190 °C (375 °F) and 300–500 psi for 90 minutes—parameters well within the capability of any multi-opening hydraulic press used for FR-4 production. Vacuum assist is recommended but not a specialized requirement.

Is RO4450B compatible with lead-free assembly?

Yes. With a Tg above 280 °C and moisture absorption of only 0.06 %, RO4450B withstands multiple lead-free reflow cycles at 260 °C peak without delamination or measles.

What is the difference between RO4450B and RO4450F?

RO4450F uses a spread-glass reinforcement that reduces the localized Dk variation caused by the glass-resin weave pattern. This makes RO4450F preferable for frequencies above roughly 25–30 GHz or for tightly coupled differential pairs where Dk uniformity across the trace width is critical. RO4450B uses a standard woven glass and is the lower-cost option for applications up to about 20 GHz.

How thick is RO4450B after lamination?

Standard cured thicknesses are 3.5 mil (0.089 mm) and 4 mil (0.102 mm). The uncured prepreg is slightly thicker; always use the cured dimension in impedance calculations and stackup drawings.