8-layer Blind Vias PCB RO4350B and MT40 material Impedance Controlled

This custom 8-layer hybrid dielectric PCB uses a symmetrical stack-up: 0.168mm RO4350B on the top and bottom sides, paired with a 0.381mm I-Tera MT40 core and 0.1245mm MT40 prepreg. The finished pressed thickness reaches 1.636mm. Outer layers carry 1oz finished copper, while inner layers use 0.5oz copper. The board is finished with green solder mask and white silkscreen, and palladium gold surface treatment. Board dimension is 179mm × 121mm (1PCS). The design includes blind vias L1-L2 and L7-L8. The top layer features 6mil traces built for 50-ohm single-ended impedance.

PCB Specification

The table below summarises all core manufacturing parameters for this hybrid PCB.

PCB Stack-up

RO4350B & I-Tera MT40 Laminate Introduction

I-Tera MT40 is designed for high-speed digital and RF/microwave PCB applications. It delivers stable dielectric constant (Dk) across -55°C to +125°C up to W-band frequencies, with a low dissipation factor (Df) of 0.0031. MT40 acts as a cost-effective alternative to PTFE microwave materials. MT40 is offered as laminate and prepreg, compatible with standard FR-4 fabrication processes and requires no specialised PTFE through-hole processing. It supports multiple reflow and repeated lamination cycles, features CAF resistance and meets RoHS requirements.

RO4350B is a well-known ceramic filled hydrocarbon laminate for RF and microwave circuits. It maintains a low dissipation factor across microwave frequencies, delivering consistent electrical performance for impedance-controlled RF traces. Its stable Dk and low Df minimise signal attenuation, which is essential for high-frequency transmission lines. RO4350B has a defined CTE profile different from standard FR4 materials, a key factor to consider during hybrid lamination.

Design Rationality of This 8-Layer Hybrid Dielectric Stack-up

1. High-Frequency Design Logic for Dual-Sided RO4350B with Middle I-Tera MT40

The top and bottom outer layers adopt 0.168mm RO4350B. Thanks to RO4350B’s inherently low Df and stable dielectric constant, it keeps microwave signal attenuation low. These material properties make it ideal for the 50Ω single-ended RF routing on the top layer, preserving signal integrity as RF signals travel along the 6mil traces.

The central 0.381mm I-Tera MT40 core leverages MT40’s high Tg of 215°C and high decomposition temperature Td at 360°C. These thermal properties deliver robust mechanical support and stable thermal performance for the 8-layer stack. Using MT40 for inner layers avoids the high material cost that comes with building an all-RO4350B board.

The 0.1245mm MT40 prepreg is selected for its consistent resin flow and well-defined dielectric properties. With MT40’s stable Dk characteristics, this prepreg precisely tunes interlayer dielectric thickness. When combined with the outer RO4350B cores, the stack assembly consistently hits the target 1.636mm finished thickness with manageable thickness tolerance.

2. Manufacturing Challenges and Improvement Strategies for This 8-Layer Three-Material Stack

RO4350B and I-Tera MT40 have dissimilar CTE properties. RO4350B’s thermal expansion behaviour differs from MT40, which has a high 215°C Tg epoxy matrix. When exposed to heat during lamination, mismatched expansion rates create interfacial stress, which can lead to delamination or board warpage. To counteract this material property difference, fabricators implement segmented gradient heating and slow cooling cycles to release thermal stress gradually.

This 8-layer stack reaches 1.636mm after pressing, and layer offset is a known risk. While MT40 itself supports multiple lamination cycles, thick multi-layer structures still face layer shift risk. Adding registration targets and equalising pressure distribution across press plates helps maintain layer alignment during lamination.

Impedance is directly governed by dielectric constant and dielectric thickness. RO4350B and MT40 each hold their own fixed Dk values. Any variation in incoming substrate thickness will alter the physical distance between copper planes and shift impedance readings. Incoming material inspection is critical to lock in laminate thickness tolerance before production.

3. Cost-Performance Advantages of This 8-Layer Hybrid Stack versus Full RO4350B Boards

If the entire stack used RO4350B, the total material expense would be substantial. Replacing inner core and prepreg layers with I-Tera MT40 cuts material costs significantly, making this stack suitable for medium-to-large RF module mass production.

MT40’s high Tg (215°C) and high Td (360°C), paired with its ability to survive multiple reflow cycles, outperforms pure ceramic high-frequency substrates in thermal reliability. These thermal properties allow the board to endure repeated SMT assembly without degradation.

Only the top and bottom signal layers use RO4350B, whose low Df is required for RF transmission. Inner power and ground layers rely on MT40, which has stable Dk for digital and power functions. This selective material deployment matches material properties to circuit function and eliminates unnecessary high-frequency material waste.