Introduction

Printed circuit boards (PCBs) are essential components in nearly all modern electronics. The laminate and prepreg materials that make up the PCB substrate play a critical role in determining the performance and reliability of the final PCB assembly. As electronic devices become smaller, faster, and more complex, the demands on PCB materials continue to increase.

One of the leading high performance PCB substrate materials on the market today is Isola’s R408HR laminate and prepreg system. R408HR is an epoxy resin system reinforced with electrical grade woven glass fabric that is designed to meet the needs of high frequency RF/microwave circuits as well as other advanced applications.

In this article, we will take an in-depth look at R408HR laminate and prepreg, including:

• Key properties and performance characteristics
• Available product formats
• Typical applications
• Fabrication considerations
• Quality and reliability data
• Frequently asked questions

Understanding the capabilities and proper utilization of these materials is critical for PCB designers, fabricators, and assemblers to achieve the best results.

Key Properties and Characteristics

R408HR exhibits an excellent combination of electrical, mechanical, thermal, and processing properties that make it well-suited for a wide range of demanding applications. Some of the key highlights include:

• High Frequency Performance – Low dielectric constant (Dk) of 3.66 and loss tangent (Df) of 0.0037 (IPC-TM-650 2.5.5.5 Clamped Stripline at 10 GHz) support excellent high frequency signal integrity. Stable electrical properties vs. frequency.
• Low Thermal Expansion – X-Y coefficient of thermal expansion (CTE) of 10-15 ppm/$^{\circ}$C minimizes copper stress and reliably plated through holes (PTHs).
• Dimensional Stability – Low Z-axis CTE of 60 ppm/$^{\circ}$C and composite construction minimize laminate expansion and contraction under temperature changes. Excellent registration stability for fine features.
• High Glass Transition Temperature – Tg > 170$^{\circ}$C allows robust lead-free assembly processes.
• Excellent Layer-to-Layer Adhesion – No special treatment needed for high layer count designs.
• Good Fabrication Yields – Tight material controls support high drilling yields and plating quality. Good hole wall integrity.
• UL 94V-0 Flame Resistance – Meets strict flammability requirements for many end products.
• RoHS Compliant – Conforms to RoHS 3 standards for lead-free electronics.
• Excellent Quality and Reliability – Stringent IPC 4101 slash sheet testing. Long-term field performance data.

R408HR’s balanced set of electrical, mechanical, thermal, and processing attributes make it an excellent choice for a wide range of networking, telecommunications, military/aerospace, industrial, and harsh environment applications.

Available Product Formats

R408HR laminates and prepregs are available in a variety of formats to support different fabrication needs:

Laminate Types

• R408HR – Core laminate sheets, available in thicknesses from 0.002″ (0.05 mm) to 0.062″ (1.6mm). Common copper cladding options.
• R408HR-M – Core laminate with MATTE on one side and standard foil on the other side. Used for MLBs.
• R408HR-ST – Core laminate with copper foil on one side only. Other side has no foil. Allows direct heat sink bonding.

Prepreg Types

• R408HR-2 – B-stage prepreg for conventional multilayer PCBs. Standard 106 glass weave. Resin content adjusted for laminate and prepreg flow matching.
• R408HR-2-1500ST – Higher resin content prepreg for structurally reliable stagger-vias. 1500 glass style.
• R408HR-2-106-NH – Low Dk, low Df prepreg. Uses non-halogenated resin system.
• R408HR-2-1080 – Low loss prepreg for high frequency applications. 1080 glass weave.

This range of available laminates and prepregs enables optimization across many different design scenarios.

Typical Applications

With its well-balanced set of properties, R408HR materials are used across a wide variety of end products and applications, including:

• Networking/Telecom – Routers, switches, servers, base stations, fiber optic transceivers.
• Military/Avionics – Radar systems, navigation, communications, control circuits.
• Industrial – Process control equipment, robotics, power conversion, motor drives.
• Transportation – Automotive, aviation, marine, mass transit.
• Medical – Imaging equipment, patient monitoring, analyzers, diagnostics.
• Consumer – Set top boxes, networking gear, appliances.
• Aerospace – Guidance systems, engine controls, satellite payloads.
• Harsh Environments – Oil and gas, mining, sustainable energy, heavy machinery.

R408HR supports the combination of high frequency performance, ruggedness, and reliability required by advanced electronic systems.

PCB Fabrication Considerations

While R408HR offers excellent fabrication and assembly yields, there are some key guidelines that PCB manufacturers should follow:

Imaging and Etching

• Standard lithography processes are recommended. R408HR is compatible with both alkaline and acid cupric chloride etching.
• For high density designs, a bump plate may be required to achieve the desired line width, space, and wall angle.
• Fine features down to 2 mil line/space and 3/3 mil line/space routing have been demonstrated.

Drilling

• Standard entry/exit parameters typically used for epoxy FR-4 should be employed.
• Hole walls are smooth and clean with good resin curing.
• Hits > 2 million are readily achieved when drilling under standard conditions.

Via Formation

• R408HR has excellent hole wall integrity for both filled and unfilled vias.
• For filled vias, a desmear process may be used but is often not required. Direct metallization is preferred.
• Both trenched and filled caps are reliable for blind/buried vias.

Outer Layers

• For MLBs, a MATTE side prep is recommended prior to bonding.
• Good urethane and acrylic soldermasks have proven successful.
• Compatible with most common final finishes (OSP, immersion Ag, Au, ENIG, HASL).

MLB/HLB Bonding

• Bake presses just below the Tg prior to bonding.
• Standard bonding cycles with 200-300 psi (1.4-2.1 MPa) pressure are used.
• Total pressed cycles for 48 layer boards ~3 hours at 185°C.
• Robust layer-to-layer adhesion is achieved without any special chemical processes.

Final Surfaces

• For direct heat sink attach, nickel barrier + immersion silver is recommended.
• Low loss surfaces can be achieved for high frequency designs.

Following these general guidelines will help maximize yields and optimize electrical performance. As always, work closely with your material supplier to dial-in ideal parameters.

Quality and Reliability Testing

Isola employs an extensive battery of quality and reliability tests to ensure consistent performance of R408HR laminate and prepreg materials:

• IPC 4101 Rev D /slash sheets – Full evaluation of properties and requirements per IPC specification.
• UL 94V-0 flammability – Materials meet UL 94 vertical flame test criteria for V-0 rating.
• Raw material qualification – Resin, glass fabric, and copper foil suppliers are qualified through rigorous approval process.
• Spectrographic analysis – Verifies chemistry of resins and glass reinforcements.
• Lot controls – Incoming materials sampling, testing, and SPC monitoring ensures repeatable performance.
• CAF resistance – 100V testing per IPC-TM-650 verifies resistance to conductive anodic filaments.
• Reliability testing – Simulated aging, temperature/humidity cycling, TMA, DMA.
• Correlation testing – Compares PCB-level performance to baseline established during qualification.
• First article inspection – Verifies PCB quality requirements on initial production runs.
• Conductor spacing analysis – Ongoing testing of line width, spacing and via annular rings.

This combination of rigorous qualification, inspection, process control, and field validation ensures long-term reliability and performance of designs manufactured using R408HR materials.

Frequently Asked Questions

Here are answers to some of the most common questions about R408HR high performance laminate and prepreg:

What are the key benefits of R408HR versus standard FR-4 materials?

R408HR provides much higher frequency performance and insulation properties than standard FR-4 while maintaining excellent thermal and mechanical stability. This enables improved signal integrity and miniaturization for advanced RF and high speed digital designs.

Is R408HR compatible with halogen-free (green) initiatives?

Yes, R408HR is halogen-free and compatible with green requirements. An additional non-halogenated prepreg (R408HR-2-106-NH) is available for designs requiring completely halogen-free materials.

What is R408HR’s thermal performance? Can it withstand lead-free assembly?

With a Tg over 170°C, R408HR easily withstands lead-free assembly processes, including higher temperature SAC305 soldering. The high Tg also provides a wide process window for multilayer bonding.

Does R408HR require special chemical processes for high layer count boards?

No, tight resin control allows R408HR to achieve very good layer-to-layer adhesion without special chemical processes like reduced oxides or plasma treatments.

What design guidelines should be followed to optimize fabrication with R408HR?

General guidelines are to use ≥ 8 mil hole sizes for designs >6 layers, ≥ 10 mil annular rings, and ≥ 5 mil isolation borders on fine features. Your PCB shop can advise on specific stackups and geometries.

Conclusion

R408HR laminate and prepreg materials provide an enabling technology solution for advanced PCBs used in today’s high performance electronic systems. The balanced electrical, mechanical, thermal, and processing properties allow reliable fabrication and assembly of complex, fine featured designs. Rigorous product qualification and process control provides long-term reliability.

As demands continue to increase for higher frequencies, greater miniaturization, and challenging operating environments, R408HR stands ready as a proven material system to fulfill future needs. Ongoing developments by materials engineers ensure these products continue their evolution to support next generation requirements.