6-Layer 10mil RO4350B + IT180A Hybrid PCB with ENIG Blind Via Edge Plating

Product Overview

This6-layer hybrid PCB integrates Rogers RO4350B high-frequency laminates on the outer layer pairs with ITEQ IT-180A high-Tg FR-4 in the central core, delivering controlled-impedance RF performance on critical outer signal layers while maintaining cost efficiency and proven FR-4 processability for inner routing and power distribution. At 1.6 mm finished thickness with 10 mil (0.254 mm) RO4350B cores, the board targets applications from sub-GHz through millimeter-wave bands where low insertion loss, stable Dk over temperature, and reliable plated through-holes are essential. The hybrid architecture places low-loss dielectric precisely where RF signals travel, while the central FR-4 core provides structural rigidity and a cost-effective platform for digital and power-plane layers.

PCB Construction Details

The table below summarizes the complete construction specifications of this hybrid PCB, covering base material, layer count, via architecture, copper weights, surface finish, and quality assurance measures.

PCB Stackup

The layer stackup places RO4350B dielectric directly beneath the top and bottom signal layers, ensuring that high-frequency traces reference a low-loss, temperature-stable substrate, while the IT-180A central core provides structural rigidity and a stable platform for inner-layer power and ground planes. The total pressed thickness is 1.57 mm, reaching 1.6 mm after solder mask and plating.

Hybrid Architecture: Balancing RF Performance and Cost

• The defining feature of this board is its asymmetric hybrid dielectric construction. RO4350B, a hydrocarbon ceramic-reinforced laminate with a process dielectric constant (Dk) of 3.48 ± 0.05 at 10 GHz and a dissipation factor (Df) of 0.0037, occupies the outer two layer pairs (L1–L2 and L5–L6). This placement is deliberate: high-frequency traces routed on L1 and L6 see the RO4350B dielectric immediately below, minimizing insertion loss and ensuring stable effective Dk across the operating range. The thermal coefficient of Dk for RO4350B is only +50 ppm/°C from −50 °C to +150 °C, among the lowest of any circuit material, so filter cutoff frequencies and antenna matching networks drift minimally over temperature extremes.

• The central core (L3–L4) uses IT-180A, ahigh-Tg (~180 °C) FR-4 epoxy laminate offering excellent dimensional stability, CAF resistance, and full compatibility with standard multilayer lamination. By reserving the more expensive RO4350B for only the RF-critical layers, the hybrid approach reduces overall material cost compared to an all-RO4350B 6-layer constructionwhile still delivering the required high-frequency behavior. The 1080-style prepreg (64% resin content) between the RO4350B and IT-180A sections ensures reliable resin fill and interlayer adhesion across the dissimilar dielectric interface.

Copper Foil Engineering for High-Frequency Performance

• The choice of copper foil is as critical to high-frequency performance as the dielectric itself. At microwave and millimeter-wave frequencies, conductor surface roughness contributes measurably to insertion loss—a phenomenon first quantified by S.P. Morgan in 1949, later modeled by Hammerstad and Jensen, and more accurately described by the Hall-Huray model now adopted in modern EM simulation tools such as Rogers' MWI calculator.

• RO4350B laminates are supplied with electrodeposited (ED) copper foil as standard. In the ED process, copper is electroplated onto a slowly rotating titanium drum from a copper sulfate electrolyte; the drum-side surface is smooth (the "shiny side"), while the opposite surface is naturally rough (the "matte side"). The matte side receives treatment coatings—copper dendrites and a zinc-chromate conversion layer—that enhance mechanical adhesion to the dielectric and act as anti-tarnish agents. For 1 oz (35 μm) ED foil on RO4350B, the treated (dielectric) side exhibits a root-mean-square roughness Sq of approximately 3.2 μm by white light interferometry, while the top side measures only 0.4 μm.

• For applications pushing into higher frequency bands where conductor loss dominates, Rogers offers LoPro® reverse-treated foil as an option on RO4350B. In reverse-treated foil, the smooth drum side receives the adhesion treatment instead of the matte side, with a thin adhesive bond-enhancement layer applied. This reduces the dielectric-side Sq to approximately 0.9 μm for 1 oz foil—a ~72% reduction versus standard ED foil. Studies on 4 mil LCP laminates show that at 90 GHz, a 50 Ω line clad with ED foil (Sq = 2.0 μm) exhibits insertion loss of 3.7 dB/inch, compared to 2.2 dB/inch for rolled annealed foil (Sq = 0.4 μm)—a 1.5 dB/inch difference. Higher-profile foils can also increase effective Dk by nearly 10%, affecting phase response and impedance beyond what traditional loss models capture.

• This PCB uses 1 oz (35 μm) base copper on all layers, with a finished weight of 1.42 oz on outer layers after plating and ENIG. The 1 oz inner copper provides sufficient current capacity for power planes while maintaining fine-line etching. The outer-layer 1.42 oz finished weight ensures robust via barrel plating and adequate copper for high-current RF traces. Mechanically, ED copper offers 40 kpsi tensile strength and 3% elongation for 1 oz foil, resisting thermal stress cracking in narrow conductors under rapid cycling.

Via Technology and Signal Integrity

• The hybrid circuit board employs a mixed-via architecture: blind vias from L1–L2 and L3–L6, complemented by through-hole vias for full-depth interconnects. The L1–L2 blind vias connect the top RF signal layer directly to the first reference plane within the RO4350B core, minimizing stub length and reducing high-frequency reflections. The L3–L6 vias connect bottom-side RF circuitry through the lower RO4350B core to the inner IT-180A layers, enabling controlled-impedance transitions between the high-frequency outer domain and the digital/power inner domain. This strategy eliminates excess via stubs that would degrade signal integrity at multi-GHz frequencies, while RO4350B's low Z-axis CTE (32 ppm/°C) ensures reliable plated through-hole quality under thermal shock.

Surface Finish, Solder Mask, and Reliability

• The board is finished with Electroless Nickel Immersion Gold (ENIG), providing a flat, coplanar surface ideal for fine-pitch RF components and BGA packages. ENIG offers excellent solderability, oxidation resistance, and consistent contact resistance—properties essential for RF connectors and high-frequency signal pads. Both sides feature green solder mask with white silkscreen for clear component markings and standard industry aesthetics.
• Edge plating is included for perimeter EMI shielding, grounding, or mechanical mounting. Every board undergoes 100% electrical testing prior to shipment, ensuring continuity and isolation integrity across all nets. RO4350B's decomposition temperature of 390 °C and Tg above 280 °C provide ample margin above lead-free reflow temperatures (245–260 °C peak), minimizing delamination risk during assembly.

Target Applications

This RO4350B 6-layer hybrid PCB construction is well-suited for the following applications:

· Cellular base station antenna feed networks and power amplifiers

· Automotive radar (77 GHz) and ADAS sensor modules

· Satellite communication LNBs and phased array antennas

· RF identification (RFID) readers operating at UHF and microwave bands

· Test and measurement equipment requiring controlled-impedance multilayer routing

· Millimeter-wave sensors and 5G mmWave front-end modules

Conclusion

The 6-layer 10mil RO4350B + IT180A hybrid PCB represents a pragmatic engineering solution for high-frequency designs demanding both performance and cost consciousness. By placingRogers RO4350B low-loss dielectricprecisely where RF signals travel—on the outer layers—and using provenIT-180A high-Tg FR-4 for the structural core, the board achieves stable Dk over temperature, low insertion loss, and reliable multilayer fabrication without the premium of an all-high-frequency laminate. Combined with a blind-via architecture that minimizes signal stubs, ENIG surface finish for fine-pitch assembly, edge plating for EMI control, and 100% electrical test, this PCB delivers the signal integrity, reliability, and manufacturability required for demanding RF and microwave applications.

CCL Material Knowledge: Rogers RO4350B High-Frequency Copper Clad Laminate

Introduction

Rogers RO4350B is a hydrocarbon ceramic-reinforced copper clad laminate (CCL) engineered specifically for high-frequency circuit applications from 500 MHz through millimeter-wave bands. Unlike PTFE-based microwave substrates, RO4350B is a rigid thermoset material that can be fabricated using standard FR-4 processing techniques, eliminating the need for specialized via preparation such as sodium etch. This manufacturability advantage, combined with its electrical performance and RoHS-compliant UL 94 V-0 flame retardancy, has made RO4350B one of the most widely adopted high-frequency laminates in the RF and microwave industry.

Figure 1. Rogers RO4350B copper clad laminate sheets with electrodeposited copper foil.

Complete Datasheet

The following table presents the complete electrical, thermal, and mechanical properties of RO4350B as specified in the official Rogers datasheet (PUB# 92-004, Rev. 1592 080322).

Manufacturing Advantages

Unlike PTFE-based laminates, RO4350B does not require sodium naphthalene etch or plasma treatment for via wall preparation before electroless copper deposition. It is fully compatible with standard FR-4 drilling, desmear, plating, etching, and lamination processes. The material is rigid enough for automated handling systems and scrubbing equipment, and it is lead-free process compatible with a UL 94 V-0 flammability rating using RoHS-compliant flame retardant technology. These characteristics mean that PCB fabricators can produce RO4350B-based boards on existing FR-4 lines without capital investment in specialized equipment, reducing both cost and lead time compared to PTFE-based alternatives.

Standard Configurations

• RO4350B is available in standard thicknesses ranging from 0.004" (0.10 mm) to 0.060" (1.52 mm), with the 0.010" (0.25 mm / 10 mil) thickness used in this PCB being one of the most commonly specified for microstrip and grounded coplanar waveguide designs.
• Standard panel sizes include 24" × 18" (610 × 457 mm), 24" × 21" (610 × 533 mm), 24" × 36" (610 × 915 mm), and 48" × 36" (1219 × 915 mm).
• Standard cladding options are ½ oz (18 μm) or 1 oz (35 μm) electrodeposited copper foil, with LoPro® low-profile foil available for reduced conductor loss in higher-frequency applications.
• Additional non-standard thicknesses from 0.0066" to 0.060" and alternative cladding weights are available upon request.

Figure 2. Cross-section of a high-frequency multilayer PCB showing layered dielectric and copper structure.

Conclusion

Rogers RO4350B occupies a unique position in the high-frequency CCL market: it delivers PTFE-level electrical stability and low loss while retaining the processability, mechanical robustness, and cost structure of an FR-4-compatible thermoset laminate. 

Its process Dk of 3.48 ± 0.05, Df of 0.0037 at 10 GHz, TCDk of +50 ppm/°C, and Z-axis CTE of 32 ppm/°C collectively address the most critical requirements of RF and microwave circuit designers—stable impedance, low insertion loss, temperature resilience, and reliable plated through-holes. For the 6-layer hybrid PCB described in this document, RO4350B provides the high-frequency foundation on the outer layers, while its FR-4-compatible processing ensures seamless integration with the IT-180A central core in a single, manufacturable multilayer stackup. Designers seeking to push into higher frequency bands may further optimize conductor loss by specifying LoPro® foil, which reduces dielectric-side surface roughness by approximately 72% compared to standard electrodeposited copper.