8-Layer RO4350B + Isola I-Tera® MT40 multilayer Hybrid RF PCB with blind vias (L1–L2, L7–L8) and 50Ω single-ended impedance control withstand multiple SMT reflow soldering cycles

Product Description

 

This article presents a detailed design strategy for an 8-layer hybrid RF PCB that combines the superior high-frequency performance of Rogers RO4350B™ on the outer layers with the high thermal reliability and cost-effectiveness of Isola I-Tera® MT40 in the inner layers. This hybrid construction is ideal for demanding RF/microwave applications requiring controlled impedance, blind vias, and reliable thermal management in a medium-to-large form factor.

 

By integrating RO4350B as the high-frequency material and MT40 as the bonding/core layers, this design achieves an optimal balance of electrical performance, mechanical robustness, and manufacturability for mass production of RF modules.

 

 

Rogers RO4350B™ High-Frequency Laminate

RO4350B™ is a hydrocarbon ceramic laminate from Rogers Corporation's RO4000® series, designed to offer superior high-frequency performance and low-cost circuit fabrication. It is a low-loss material that can be fabricated using standard epoxy/glass (FR-4) processes at competitive prices.

 

 

Key Specifications (RO4350B):

RO4350B Typical Value
Property	RO4350B	Direction	Units	Condition	Test Method
Dielectric Constant,εProcess	3.48±0.05	Z		10 GHz/23℃	IPC-TM-650 2.5.5.5 Clamped Stripline
Dielectric Constant,εDesign	3.66	Z		8 to 40 GHz	Differential Phase Length Method
Dissipation Factortan,δ	0.0037 0.0031	Z		10 GHz/23℃ 2.5 GHz/23℃	IPC-TM-650 2.5.5.5
Thermal Coefficient of ε	+50	Z	ppm/℃	-50℃to 150℃	IPC-TM-650 2.5.5.5
Volume Resistivity	1.2 x 1010		MΩ.cm	COND A	IPC-TM-650 2.5.17.1
Surface Resistivity	5.7 x109		MΩ	COND A	IPC-TM-650 2.5.17.1
Electrical Strength	31.2(780)	Z	Kv/mm(v/mil)	0.51mm(0.020")	IPC-TM-650 2.5.6.2
Tensile Modulus	16,767(2,432) 14,153(2,053)	X Y	MPa(ksi)	RT	ASTM D 638
Tensile Strength	203(29.5) 130(18.9)	X Y	MPa(ksi)	RT	ASTM D 638
Flexural Strength	255 (37)		MPa (kpsi)		IPC-TM-650 2.4.4
Dimensional Stability	<0.5	X,Y	mm/m (mil/inch)	after etch+E2/150℃	IPC-TM-650 2.4.39A
Coefficient of Thermal Expansion	10 12 32	X Y Z	ppm/℃	-55℃to288℃	IPC-TM-650 2.4.41
Tg	>280		℃ TMA	A	IPC-TM-650 2.4.24.3
Td	390		℃ TGA		ASTM D 3850
Thermal Conductivity	0.69		W/M/oK	80℃	ASTM C518
Moisture Absorption	0.06		%	48hrs immersion 0.060" sample Temperature 50℃	ASTM D 570
Density	1.86		gm/cm3	23℃	ASTM D 792
Copper Peel Stength	0.88 (5.0)		N/mm (pli)	after solder float 1 oz. EDC Foil	IPC-TM-650 2.4.8
Flammability	(3)V-0				UL 94
Lead-free Process Compatible	Yes

 

 

Isola I-Tera® MT40 Very Low-Loss Laminate and Prepreg

I-Tera® MT40 is a very low-loss laminate and prepreg system from Isola, designed for high-frequency and high-speed applications. It features a high Tg and excellent thermal reliability, making it ideal for multilayer boards requiring sequential lamination.

 

Key Specifications (I-Tera MT40):

I-Tera® MT40
Property		Typical Value	Units	Test Method
			Metric (English)	IPC-TM-650 (or as noted)
Glass Transition Temperature (Tg) by DSC		215	°C	2.4.25C
Glass Transition Temperature (Tg) by DMA		230	°C	2.4.24.4
Glass Transition Temperature (Tg) by TMA		210	°C	2.4.24C
Decomposition Temperature (Td) by TGA @ 5% weight loss		360	°C	2.4.24.6
Time to Delaminate by TMA (Copper removed)	A. T260	>60	Minutes	2.4.24.1
	B. T288
Z-Axis CTE	A. Pre-Tg	55	ppm/°C	2.4.24C
	B. Post-Tg	290	ppm/°C
	C. 50 to 260°C, (Total Expansion)	2.8	%
X/Y-Axis CTE	Pre-Tg	12	ppm/°C	2.4.24C
Thermal Conductivity		0.61	W/m·K	ASTM E1952
Thermal Stress 10 sec @ 288ºC (550.4ºF)	A. Unetched	Pass	Pass Visual	2.4.13.1
	B. Etched
Dk, Permittivity	A. @ 2 GHz	3.45	—	2.5.5.5
	B. @ 5 GHz
	C. @ 10 GHz
Df, Loss Tangent	A. @ 2 GHz	0.0031	—	Bereskin Stripline
	B. @ 5 GHz
	C. @ 10 GHz
Volume Resistivity	C-96/35/90	1.33 x 10^7	MΩ-cm	2.5.17.1
Surface Resistivity	C-96/35/90	1.33 x 10^5	MΩ	2.5.17.1
Dielectric Breakdown		45.4	kV	2.5.6B
Arc Resistance		139	Seconds	2.5.1B
Electric Strength (Laminate & laminated prepreg)		45 (1133)	kV/mm (V/mil)	2.5.6.2A
Comparative Tracking Index (CTI)		3	Class (Volts)	UL 746A
				ASTM D3638
Peel Strength	1 oz. EDC foil	1.0 (5.7)	N/mm (lb/inch)	2.4.8C
Flexural Strength	A. Length direction	490 (71.0)	MPa (kpsi)	2.4.4B
	B. Cross direction	400 (58.0)
Tensile Strength	A. Length direction	269 (39.0)	MPa (kpsi)	ASTM D3039
	B. Cross direction	241 (35.0)
Young's Modulus	A. Length direction	3060	ksi	ASTM D790-15e2
	B. Cross direction	2784
Poisson's Ratio	A. Length direction	0.234	—	ASTM D3039
	B. Cross direction	0.222
Moisture Absorption		0.1	%	2.6.2.1A
Flammability (Laminate & laminated prepreg)		V-0	Rating	UL 94
Relative Thermal Index (RTI)		130	°C	UL 796

 

 

Design Logic for High-Frequency Performance

The top and bottom layers utilize 0.168mm RO4350B high-frequency laminate, which offers a low dissipation factor (Df 0.0037 at 10 GHz) and minimal dielectric loss. This meets the requirements for 50Ω single-ended RF transmission lines on the outer layers, effectively reducing microwave signal attenuation.

 

The center layer employs a 0.381mm MT40 high-Tg laminate. This provides structural support and thermal stability for the multilayer board while significantly reducing production costs compared to an all-ceramic high-frequency board.

 

By incorporating 0.1245mm MT40 prepreg, the inter-layer thickness can be precisely adjusted. This ensures stable control over the total laminated thickness of 1.636mm, with a well-maintained thickness tolerance.

 

Hybrid Construction Logic:

 

RF Signal Layers (L1, L8): Use RO4350B for critical high-frequency signal layers to maintain low loss and stable Dk (3.48).

 

Bonding/Inner Layers (L2–L7): Use MT40 prepreg and core for bonding, power/ground planes, and digital signal routing, leveraging its cost-effectiveness and high thermal reliability.

 

 

CTE Mismatch Management

Differences in the Coefficient of Thermal Expansion (CTE) between the RO4350B ceramic-based material and the MT40 FR4-based material can lead to delamination or board warpage during lamination. This requires a multi-stage, gradient heating process and a slow cooling cycle.

 

 

CTE Comparison:

1. RO4350B: X/Y = 14/16 ppm/°C, Z = 32 ppm/°C
2. MT40: X/Y = 12 ppm/°C, Z = 55 ppm/°C (Pre-Tg)

 

The Z-axis CTE mismatch (RO4350B: 32 ppm/°C vs. MT40: 55 ppm/°C) requires careful lamination process control.

 

Recommended strategies include:

1. Slow ramp rates in the critical Tg region (150–200°C)
2. Extended dwell time at peak temperature to allow stress relaxation
3. Controlled cooling to prevent warpage

 

 

Layer Misalignment Prevention

Layer misalignment is a risk when laminating thick, multi-layer boards (1.636 mm). This necessitates:

 

1. Addition of registration targets on each layer
2. Optimization of pressure uniformity across the lamination press plates
3. Precision tooling pins to ensure accurate layer-to-layer registration

 

Impedance Sensitivity Control

In a stack-up alternating between thick and thin dielectrics, impedance is highly sensitive to fluctuations in dielectric thickness. Therefore, strict control over the thickness tolerances of incoming base materials is essential. The 0.168mm RO4350B and 0.1245mm MT40 prepreg must be specified with tight thickness tolerances to maintain consistent impedance.

 

Cost-Performance Advantages

Replacing inner layers with lower-cost MT40 material significantly reduces overall material costs, making it well-suited for the mass production of medium-to-large-sized RF modules.

 

MT40 offers high thermal resistance and can withstand multiple SMT reflow soldering cycles, effectively compensating for the thermal reliability limitations of pure ceramic-based boards.

 

By utilizing RO4350B only for the outer signal layers—where high-frequency performance is critical—and employing standard FR4 for the internal power and ground layers, the design achieves more efficient resource utilization.

 

Cost Savings Estimate: Hybrid stackups place Rogers on the RF signal layers and FR-4 on digital/power layers, reducing cost by 40–60% versus all-Rogers construction while maintaining RF performance.

 

 

Blind Vias (L1–L2 and L7–L8)

Blind vias (L1–L2 and L7–L8) reduce via stub effects and improve high-frequency performance by eliminating unnecessary via lengths. This is critical for maintaining signal integrity in high-layer-count designs.

 

Implementation: Sequential lamination is required. The first lamination step bonds L1–L2 (RO4350B + MT40 prepreg), followed by the second lamination step to bond the remaining layers.

 

50Ω Single-Ended Impedance (6-mil Traces)

The top layer features 6-mil traces designed for 50Ω single-ended impedance. This is achieved through precise control of the RO4350B dielectric thickness (0.168mm) and copper trace width, ensuring optimal RF signal transmission.

 

 

PCB Construction Details

Parameter	Specification
Base Material (High-Frequency Layers)	Rogers RO4350B (0.168 mm)
Base Material (Core)	Isola I-Tera MT40 (0.381 mm)
Bonding Material (Prepreg)	Isola I-Tera MT40 (0.1245 mm)
Number of Layers	8
Finished Board Size	179 mm × 121 mm (1 piece)
Finished Board Thickness	1.636 mm
Outer Layer Copper Weight	1 oz (35 μm)
Inner Layer Copper Weight	0.5 oz (18 μm)
Surface Finish	Electroless Palladium (EP)
Top Solder Mask	Green
Bottom Solder Mask	Green
Top Silkscreen	White
Bottom Silkscreen	White
Blind Vias	L1–L2, L7–L8
Impedance Control	Top layer: 6-mil traces, 50Ω single-ended
Electrical Testing	100% prior to shipment
Accepted Quality Standard	IPC-Class-2
Artwork Format	Gerber RS-274-X
Global Availability	Worldwide shipping

 

 

PCB Stackup

 

 

Features & Benefits Summary

Feature	Benefit
Hybrid RO4350B + MT40 Construction	Optimizes high-frequency performance and cost-effectiveness
8-Layer Design	Supports complex RF/microwave circuits with power/ground planes
0.168mm RO4350B Outer Layers	Low loss (Df 0.0037) for RF signal transmission
MT40 Core and Prepreg	High Tg (215°C), excellent thermal reliability, cost-effective
Blind Vias (L1–L2, L7–L8)	Reduces stub effects, improves signal integrity
50Ω Single-Ended Impedance	6-mil traces for precise RF signal control
Electroless Palladium (EP) Finish	Flat, solderable, wire-bondable, and corrosion-resistant
Green Solder Mask / White Silkscreen	Comprehensive protection with clear component identification
1.636mm Finished Thickness	Optimized for medium-to-large RF module designs
100% Electrical Testing	Guaranteed quality and performance
IPC-Class-2	Meets industry-standard quality requirements

 

 

Typical Applications

1. RF/Microwave Modules: Phased array antennas, beamforming networks, and transceiver modules.
2. Cellular Infrastructure: Base station antennas and remote radio heads.
3. Automotive Radar: 24 GHz and 77 GHz radar modules for ADAS.
4. Satellite Communications: Spaceborne and ground-based satellite equipment.
5. Test & Measurement Equipment: Vector network analyzers and high-frequency test fixtures.
6. Medium-to-Large RF Modules: Cost-sensitive applications requiring high-frequency performance.

 

 

Conclusion

As a specialized supplier of custom high-frequency circuit boards, we combine the proven performance of Rogers RO4350B (low loss, stable Dk, excellent dimensional stability) and Isola I-Tera MT40 (high Tg, excellent thermal reliability, cost-effective) with our precision manufacturing capabilities to deliver world-class hybrid PCB solutions.

 

The 8-layer hybrid offering fully leverages RO4350B's low-loss high-frequency performance and MT40's thermal reliability and processing advantages, while incorporating advanced features such as blind vias (L1–L2, L7–L8) and 50Ω single-ended impedance control to ensure optimal signal integrity and reliability in a 1.636mm form factor.

 

Whether you are developing phased array antennas, cellular infrastructure equipment, automotive radar modules, or satellite communication payloads, our engineering team is ready to support your project. For technical consultations, samples, or volume orders, please contact us today. We look forward to being your trusted partner in high-frequency innovation.