Isola PCB Materials Comparison: 370HR vs. FR408HR vs. I-Speed vs. Tachyon 100G

In the modern PCB design landscape, the “standard FR-4” label is no longer a sufficient specification. As an engineer, you’re faced with a tiered hierarchy of laminates where the choice between a mid-loss and an ultra-low-loss material can determine whether your 25Gbps SerDes link actually closes or if your 100-watt power module delaminates during the third reflow cycle.

Isola Group has dominated the high-reliability and high-speed digital (HSD) markets by offering a clear migration path. Whether you are building a ruggedized industrial controller or a 100G networking switch, understanding the nuances of the Isola PCB materials comparison is critical to balancing performance with BOM cost.

In this deep-dive guide, we will contrast four of Isola’s most significant laminates: the “industry-standard” 370HR, the high-performance FR408HR, the signal-integrity-focused I-Speed, and the ultra-low-loss Tachyon 100G. We’ll look at these through the lens of a stack-up engineer—focusing on Dk/Df stability, thermal robustness, and glass weave inhomogeneities.

The Spectrum of Performance: Why We Compare These Four

Before diving into the data, we must establish the “why.” These four materials represent the evolution of the epoxy-resin system.

370HR is the baseline for high-reliability thermal performance.

FR408HR is the bridge material, offering better electricals for 1-5 GHz designs.

I-Speed is engineered specifically for high-speed digital transitions (10-25 Gbps).

Tachyon 100G is the peak of thermoset technology, competing directly with PTFE for 100G+ applications.

Isola 370HR: The High-Reliability Benchmark

If you are designing a board that needs to “just work” in a harsh environment, 370HR is usually the first name on the fab note. It is a high-performance, 180°C Tg FR-4 system.

Thermal Robustness and CAF Resistance

The “HR” stands for High Reliability, and 370HR earns it through its Z-axis CTE of 2.8% (50°C to 260°C). This low expansion rate is non-negotiable for thick multilayer boards (20+ layers) where via-barrel cracking is a primary failure mode.

Furthermore, 370HR is the gold standard for CAF (Conductive Anodic Filament) resistance. Its resin-to-glass bond is exceptionally tight, preventing the electrochemical migration that kills dense HDI designs in humid environments.

The Trade-off: Signal Loss

The downside? 370HR is a “standard loss” material. With a Dissipation Factor (Df) of 0.0210 at 1 GHz, it is not meant for long-reach high-speed traces. If you try to route 10Gbps Ethernet over 10 inches of 370HR, the insertion loss will likely exceed your budget.

Isola FR408HR: The “Electrical” Upgrade to 370HR

FR408HR is often confused with 370HR because they share the same 180°C Tg and thermal specs. However, FR408HR is a distinct resin system designed for better electrical performance.

Dk and Df Improvements

FR408HR offers a lower Dielectric Constant (Dk of 3.65 vs. 4.04) and a significantly lower Df (0.0092 vs. 0.0210). This makes it the ideal “bridge” material. If your design is pushing into the 5-8 GHz range—typical of many RF and mid-speed digital applications—FR408HR allows you to maintain the exact same fabrication “recipe” as 370HR while significantly extending your signal reach.

Why Use FR408HR over 370HR?

Choose FR408HR when your impedance targets require thinner dielectrics or wider traces than 370HR can comfortably provide, or when you need to reduce the “attenuation wall” without jumping to a full HSD laminate.

Isola I-Speed: Engineered for High-Speed Digital (HSD)

As we move toward 10Gbps and 25Gbps (NRZ and PAM4), the parasitic losses of standard FR-4 become insurmountable. Isola I-Speed is the first tier of “Low-Loss” HSD materials in this comparison.

Reducing the “Glass Weave Effect”

A major feature of I-Speed is its compatibility with “spread glass” weaves. Standard glass weaves have large resin-filled windows that cause phase skew in differential pairs. I-Speed is often paired with mechanically spread glass (like 1067 or 1086 styles) to ensure a homogeneous Dk across the signal path.

Thermal Integrity in HDI

I-Speed maintains a high Tg (180°C) and is designed for sequential lamination. If you are building an HDI board with multiple microvia layers (2+N+2), I-Speed provides the thermal headroom to survive the repeated lamination cycles that standard FR-4 cannot.

Isola Tachyon 100G: The PTFE-Killer

Tachyon 100G represents the absolute “state of the art” for Isola’s thermoset portfolio. It was designed specifically for 100G networking (28Gbps per lane and higher).

Ultra-Low Loss Performance

With a Df of 0.0021 at 10 GHz, Tachyon 100G is in a different league. It competes directly with PTFE (Teflon) materials but, crucially, it processes like FR-4. You don’t need specialized plasma desmear or exotic lamination cycles.

Phase Stability and mmWave Potential

Tachyon 100G exhibits remarkably flat Dk and Df across a massive frequency range (up to 40 GHz and beyond). This makes it a favorite for hybrid boards where you might have 77GHz radar or 5G mmWave signals on the outer layers and Tachyon 100G handling the high-speed backplane routing.

Technical Comparison Table: Data-Driven Selection

As an engineer, the datasheet is your source of truth. Here is how these four materials stack up across the critical metrics.

Property	Isola 370HR	Isola FR408HR	Isola I-Speed	Tachyon 100G
Glass Transition (Tg)	180°C	180°C	180°C	200°C
Decomposition (Td)	340°C	360°C	360°C	360°C
Dk @ 10 GHz	3.92	3.65	3.45	3.02
Df @ 10 GHz	0.0250	0.0092	0.0060	0.0021
Z-Axis CTE (Pre-Tg)	45 ppm/°C	40 ppm/°C	40 ppm/°C	32 ppm/°C
Z-Axis Expansion (Total %)	2.8%	2.5%	2.5%	1.8%
Relative Cost	$ (Baseline)	$$	$$$
Material Category	Standard Loss	Mid Loss	Low Loss	Ultra Low Loss

The “Glass Weave Effect” and Signal Integrity

When performing an Isola PCB materials comparison, you cannot ignore the reinforcement. The resin is only half the story.

At speeds above 10Gbps, the physical weave of the fiberglass (E-glass) creates a “speed bump” effect. Glass has a Dk of ~6.0, while the resin has a Dk of ~3.0. A trace running over a glass bundle travels slower than a trace running over a resin-rich gap.

370HR: Usually uses standard weaves. Fine for power and low-speed signals.

I-Speed & Tachyon 100G: Specifically designed to be used with Square Weave or Mechanically Spread Glass. This flattens the bundles, closing the resin windows and eliminating the Dk variations that cause jitter and skew.

Fabrication Considerations: The Hidden Costs

A common mistake in material selection is only looking at the laminate price. You must also consider the “Fabrication Yield” and “Process Compatibility.”

370HR and FR408HR: The “Easy” Materials

These are extremely forgiving. Almost any board house in the world can process these with high yields. They use standard permanganate desmear and standard lamination cycles.

I-Speed and Tachyon 100G: The “Advanced” Materials

While they are “FR-4 like,” they are harder and more brittle. Fabricators must use sharper drill bits (reduced hit counts) to prevent resin chipping. Because they are ultra-high-Tg (up to 200°C for Tachyon), the lamination press must be able to reach higher temperatures to ensure a full cure. If the resin isn’t fully cross-linked, the board will be susceptible to delamination during the assembly reflow.

When sourcing these advanced materials, it is vital to work with a shop that carries them in stock to avoid 6-week lead times. You can find vetted manufacturers and material stock info at ISOLA PCB.

High-Voltage and Automotive Applications

As EVs migrate to 800V architectures, the Isola PCB materials comparison shifts toward dielectric breakdown and CAF resistance.

Isola 370HR is the traditional choice for automotive BMS (Battery Management Systems).

However, for high-frequency motor controllers and SiC (Silicon Carbide) inverters, Tachyon 100G is being increasingly used not for its “speed,” but for its extremely low moisture absorption (0.10%) and superior dielectric stability under high-voltage stress.

Hybrid Stack-ups: The Engineer’s Secret Weapon

You don’t always have to choose just one. In a 12-layer board, you might only have two layers of 25Gbps signals.

Pro Tip: Build a hybrid stack-up. Use Tachyon 100G for the critical high-speed signal layers and 370HR for the inner power/ground cores. This allows you to meet your signal integrity targets while keeping the total board cost much lower than an all-Tachyon build. Isola has specifically engineered these resin systems to be “chemically compatible” for hybrid bonding.

Useful Resources and Tools

To finalize your design, don’t guess—model it. Use these resources to pull exact data for your specific stack-up:

Isola Technical Library: Access the full data sheets for Dk/Df across varying resin contents (the “Dk tables”).

IPC-4101 Standards: These materials are governed by specific slash sheets (e.g., 370HR follows /98 and /101).

Impedance Calculators: Use tools like Polar Speedstack or Simberian to import the Isola Dk tables for accurate 2D/3D field solving.

Sourcing Database: Check current material availability and fabrication specs at the ISOLA PCB portal.

5 Frequently Asked Questions (FAQs) About Isola Materials

1. Is FR408HR a direct drop-in replacement for 370HR?

Thermally, yes. Electrically, no. If you swap 370HR for FR408HR, your trace impedance will change because the Dk drops from 4.04 to 3.65. You must recalculate your trace widths to maintain your impedance targets.

2. Can I use Tachyon 100G for 77GHz automotive radar?

Yes. While Astra MT77 is the “dedicated” radar material, Tachyon 100G has the electrical stability and low-loss profile to handle 77GHz signals, especially in hybrid designs where HSD and RF coexist.

3. What is the main cause of delamination in high-Tg Isola boards?

Moisture. High-Tg materials are hygroscopic. If the prepreg or cores are not stored in a climate-controlled environment, they will absorb moisture. During the 260°C reflow, that moisture turns to steam, causing “popcorning” or delamination.

4. Why is I-Speed called “I-Speed”?

It was specifically branded to highlight its optimization for High-Speed Digital transitions. It focuses on reducing the Df and managing the glass weave to minimize jitter in multi-gigabit data streams.

5. How does Isola 370HR compare to Rogers 4350B?

They are fundamentally different. 370HR is a thermoset epoxy FR-4; Rogers 4350B is a hydrocarbon ceramic. Rogers 4350B is better for RF (higher Dk stability), but 370HR is much cheaper and easier to fabricate in high-layer-count multilayer boards.