FR408HR PCB: Properties, Stackup & Design Guide

What Is an FR408HR PCB?

An FR408HR PCB is a printed circuit board built on Isola’s FR408HR laminate—a high-performance, glass-reinforced epoxy with a dielectric constant (Dk) of 3.68 and a dissipation factor (Df) of 0.0092 at 10 GHz. It fills the gap between commodity FR-4 (Df ≈ 0.020) and specialty low-loss laminates like Megtron 6 or Rogers hydrocarbon ceramics. With a glass-transition temperature (Tg) of 200 °C (DSC), lead-free reflow compatibility, and halogen-free UL 94 V-0 flame rating, the FR408HR PCB is widely specified as the core and prepreg in hybrid RF/digital stackups for wireless infrastructure, radar IF boards, and high-speed backplanes up to roughly 10 GHz.

Key Electrical and Thermal Properties

The table below summarizes the values most relevant to RF and high-speed FR408HR PCB designs. All electrical data are from Isola’s FR408HR datasheet.

A Dk of 3.68 sits close enough to standard FR-4 (~4.2–4.4 at 1 GHz) that trace geometries remain familiar, yet the Df of 0.0092 at 10 GHz cuts insertion loss roughly in half compared to standard-loss FR-4. That makes an FR408HR PCB a practical choice when a design needs better signal integrity without the cost and fabrication complexity of PTFE or ceramic-filled laminates.

Where FR408HR PCB Fits in the Loss Spectrum

Understanding where FR408HR sits relative to other laminates helps you decide whether it is good enough or whether you need to step up—or can step down.

FR408HR’s mid-loss category makes it the default digital-side material in many hybrid boards: the RF front-end uses a PTFE or ceramic-filled layer, while FR408HR handles the power, control and lower-frequency signal layers. This keeps the bill of materials reasonable and the fabrication process within reach of most high-reliability shops.

Designing a Hybrid FR408HR PCB Stackup

A hybrid stackup pairs FR408HR cores and prepregs with one or two layers of specialty RF laminate. The classic architecture for a six-layer radar IF board or small-cell radio looks like this:

• Layer 1 (Top): RF signal on a low-loss core (e.g., RO4350B, 10 mil / 0.254 mm)
• Layer 2: Ground plane (copper on the underside of the RF core)
• Layer 3: High-speed digital routed on FR408HR prepreg
• Layer 4: Power / ground split on FR408HR core
• Layer 5: Low-speed control on FR408HR prepreg
• Layer 6 (Bottom): Ground plane

The RF-to-FR408HR bond is the critical interface. Isola’s FR408HR prepreg (available in 106, 1080, 2116 and 7628 glass styles) bonds well to most hydrocarbon ceramic and thermoset laminates during a standard 185 °C lamination cycle. However, bonding to pure PTFE substrates—such as glass-reinforced PTFE—typically requires a sodium-etch or plasma surface treatment to promote adhesion, plus a bonding film like Rogers CuClad 6700 or Taconic FastRise FR-27.

Use the builder below to experiment with different layer counts and mixed-dielectric arrangements. Enter your target impedance, frequency and layer materials to see how FR408HR core thicknesses interact with an RF laminate on top.

After running the builder, pay attention to the total board thickness and the symmetry of the stackup. For example, a typical six-layer FR408HR PCB hybrid might report a total thickness of 58 mil (1.47 mm) with a 10 mil RO4350B top core and two 12 mil FR408HR inner cores—well within the 62 mil limit for standard SMA edge-launch connectors. An asymmetric hybrid (RF laminate only on one side) will bow during reflow unless balanced by a matching CTE layer on the opposite side. A common fix is mirroring the RF core on the bottom even if it carries no RF traces, or using a thicker FR408HR core in the center to dominate the mechanical neutral axis.

Prepreg Selection Within the FR408HR System

FR408HR prepreg comes in multiple resin-content grades. For RF-adjacent layers where Dk consistency matters, choose a high-resin-content (HR) style such as 1080HR or 2116HR. Higher resin content reduces the glass-weave effect—periodic Dk variation caused by the knuckles of woven fiberglass—which can create skew on differential pairs and phase ripple on RF traces. The RF laminate thickness chart lists common pressed thicknesses for these glass styles.

Fabrication Considerations

Drill and Via Reliability

FR408HR’s Td of 360 °C and low Z-axis CTE (40 ppm/°C below Tg) make it well-suited for HDI builds with microvias and multiple lamination cycles. It passes IPC-6012 Class 3 reliability testing, including IST (Interconnect Stress Test) at 150 °C with 500+ cycles to failure on 0.3 mm vias. For laser-drilled microvias, the epoxy resin ablates cleanly under UV (355 nm) and CO₂ lasers without excessive smear.

Hybrid Lamination Process

Standard press cycles for FR408HR use 185 °C peak temperature, 200–300 psi, with a 60–90 minute dwell. When co-laminating with Rogers RO4350B or similar thermoset RF laminates, the cure profiles are compatible—both materials flow and cure in the same temperature window. PTFE-based laminates, by contrast, require higher pressures (400–500 psi) and careful ramp rates to prevent delamination at the hybrid interface.

Surface Finish Compatibility

FR408HR is compatible with all common surface finishes: ENIG, ENEPIG, immersion silver, immersion tin, OSP, and hard gold. For RF layers that will carry wire-bonded components (e.g., bare-die LNAs or PAs), ENEPIG is preferred for both the RF laminate and the FR408HR portions to maintain a uniform bonding surface across the hybrid board.

FR408HR PCB vs. Competing Mid-Loss Laminates

Choose FR408HR when your fabricator already qualifies it and you need the broadest global supply chain. Choose Megtron 4 when the extra thermal headroom of a higher Td matters (e.g., sequential lamination with three or more press cycles). Choose S7136H when cost is the dominant driver and your supply chain can source from Asian mills. All three are close enough electrically that the practical differences often come down to fab qualification and lead time.

Test Your FR408HR PCB Stackup Knowledge

Hybrid stackups involve balancing electrical performance, mechanical symmetry and fabrication constraints. The interactive challenge below tests whether a proposed stackup meets impedance, loss and reliability targets—try plugging in an FR408HR-based design to see how it scores.

If the challenge scores your design below 70 %, the most common culprit is a CTE mismatch between dissimilar dielectric layers. For instance, pairing a single 10 mil RO4350B core on Layer 1 with five FR408HR layers underneath often triggers a bow-and-twist warning because the Z-axis expansion rates differ by roughly 15 ppm/°C. Swapping in a 20 mil (0.508 mm) FR408HR core opposite the RF side, or mirroring a thin RO4350B layer on the bottom, typically pushes the score above 85 % and keeps total thickness under 62 mil (1.57 mm) for standard connectors. Re-run the challenge after each adjustment to confirm the improvement.

Common Applications

• 5G small-cell radios: FR408HR digital layers with an RF front-end on RO4350B or Wangling TP-2 as a cost-effective RO4350B equivalent.
• Automotive radar IF and baseband: 77 GHz antenna layers use PTFE or LCP, while FR408HR handles the MMIC supply, SPI and CAN buses.
• Phased-array beamformer boards: Distribution networks and digital beamforming ASICs on FR408HR, radiating elements on low-loss RF laminate.
• High-speed backplanes (25–56 Gbps NRZ): FR408HR provides adequate loss for channels under 20 inches at 28 Gbps; above that, step up to Megtron 6 or better.
• Test and measurement instruments: IF processing boards where cost matters but FR-4 loss is too high.

Design Tips and Pitfalls

Dk Variation with Resin Content

Published Dk values assume a 50 % resin-content laminate. Actual Dk shifts with glass style: a 7628 construction (lower resin content, more glass) pushes Dk toward 4.0, while a 106HR sheet (higher resin) drops Dk closer to 3.5. Always use the Dk for the specific construction your fabricator will press, not the headline number. Request the Isola Design Dk tables for the exact glass style and copper weight.

Copper Roughness Impact

FR408HR is commonly supplied with standard HTE copper (Rz ≈ 6–8 µm). At 10 GHz, this roughness can add 0.3–0.5 dB/inch of conductor loss beyond what a smooth-copper model predicts. For loss-sensitive traces on an FR408HR PCB, specify RTF (reverse-treat foil, Rz ≈ 2–3 µm) or HVLP (Rz ≈ 1.5 µm) copper. The improvement is measurable on any trace longer than about 3 inches at 5 GHz and above.

Moisture Sensitivity

FR408HR’s moisture absorption of 0.15 % is low for an epoxy system but not negligible. Absorbed moisture raises Df and can cause delamination during reflow. Pre-bake panels at 105 °C for 4–6 hours if they have been stored unsealed for more than 72 hours, per IPC-1601 handling guidelines.

Frequently Asked Questions

Is FR408HR the same as FR-4?

No. Both are glass-reinforced epoxy systems, but FR408HR uses a proprietary high-performance resin that delivers roughly half the dielectric loss of standard FR-4 at 10 GHz, plus a significantly higher Tg (200 °C vs. 130–170 °C). It is sometimes called “enhanced FR-4” or “high-performance FR-4,” but its electrical behavior is a clear step above commodity grades.

Can FR408HR be used for millimeter-wave frequencies?

FR408HR is generally not recommended above 10–12 GHz as the primary signal-carrying dielectric. At 28 GHz, its Df climbs above 0.012, producing insertion losses that are unacceptable for most antenna feeds and RF front-ends. Use it for digital and power layers in a hybrid stackup, pairing it with a very-low-loss or PTFE material for the millimeter-wave signal layers.

What is the maximum layer count practical with FR408HR?

FR408HR has been qualified in production stackups exceeding 30 layers for server backplanes. In hybrid RF builds, 8–16 layers is more typical, limited by the number of sequential lamination cycles needed to bond dissimilar materials rather than by the laminate itself.

Does FR408HR require special drilling or plating?

No. It drills, desmears and plates using the same processes as high-Tg FR-4. Permanganate desmear at standard concentrations (55–65 g/L) is effective. Plasma desmear can also be used and is preferred when hybrid panels include PTFE layers that need plasma treatment anyway.

How does FR408HR PCB pricing compare to standard FR-4?

FR408HR typically costs 1.5–2.5× more per panel than standard-Tg FR-4, depending on thickness and copper weight. The premium is modest compared to low-loss or PTFE laminates (which can be 5–15× FR-4 cost). Check the RF laminate price guide for current relative pricing across material families.