Isola I-Speed laminate delivers Dk 3.63 and Df 0.006–0.007 at 10 GHz — 25% more bandwidth than competitive FR-4 at near-identical processing. Full technical guide: actual Dk/Df at 1–10 GHz, construction options, spread glass selection, vs FR408HR and I-Tera MT40 comparison, 10–25 Gbps applications, and stack-up design tips.

Primary keyword: Isola I-Speed laminate | ~2,800 words

Every PCB materials conversation eventually reaches a fork: keep running standard FR-4 and accept the signal loss, or jump to premium ultra-low-loss materials and deal with higher cost and tighter process constraints. For a substantial share of high-speed digital designs — those running at 5 to 25 Gbps per lane, with moderate to long trace lengths — that binary framing misrepresents the actual choice. Isola I-Speed laminate occupies exactly the middle ground that serves those designs: a low-loss epoxy system that delivers 25% more electrical bandwidth than competitive FR-4-class products, processes almost identically to standard FR-4, carries Tg 180°C and Td 360°C thermal credentials that support demanding lead-free assembly, and does all of this at a cost tier well below premium low-loss grades.

This guide covers the full technical picture — precise Dk/Df values across frequency, thermal properties, construction options, processing discipline, competitive positioning across the Isola family and against common alternatives, and the application segments where I-Speed laminate is the correct specification rather than a compromise.

What Is Isola I-Speed Laminate?

I-Speed is a proprietary high-performance 180°C Tg FR-4 system for multilayer Printed Wiring Board applications where maximum thermal performance and reliability are required. I-Speed laminate and prepreg products are manufactured with Isola’s patentable high-performance multifunctional resin system, reinforced with electrical grade (E-glass) glass fabric.

The core positioning statement from Isola: this system delivers a 15% improvement in Z-axis expansion and offers 25% more electrical bandwidth (lower loss) than competitive products in this space. These properties, coupled with superior moisture resistance at reflow, result in a product that bridges the gap from both a thermal and electrical perspective.

That “bridges the gap” phrase is doing real work in that description. Standard high-Tg FR-4 materials in the 370HR class carry Df around 0.018–0.021 at 10 GHz. I-Speed at Df 0.006–0.007 at 10 GHz is roughly a 65–70% reduction in loss tangent. The signal integrity improvement moving from 370HR to I-Speed at 10 Gbps per lane over a 20-inch trace is the difference between an eye diagram with adequate margin and one that requires active equalization. That’s not incremental — it’s an enabling step.

Isola I-Speed Laminate Full Electrical and Thermal Specifications

The table below consolidates properties from the official Isola datasheet. Dk and Df values shown at nominal 56% resin content (Bereskin Stripline method for GHz measurements). Actual Dk/Df for specific constructions vary with resin content — refer to the full Dk/Df tables for layer-specific stack-up modeling.

Electrical Properties Across Frequency

Frequency	Dk (Permittivity)	Df (Loss Tangent)	Test Method
1 GHz	3.65	0.0058	HP4291A / IPC-TM-650 2.5.5.9
2 GHz	3.64	0.0060	Bereskin Stripline
5 GHz	3.63	0.0067	Bereskin Stripline
10 GHz	3.63	0.0071	Bereskin Stripline

The Dk across this frequency range — 3.65 at 1 GHz dropping to 3.63 at 10 GHz — is remarkably flat. That stability matters for broadband signal channels: a Dk that shifts significantly with frequency causes dispersion, meaning different frequency components of a digital signal propagate at different velocities, degrading eye opening at high data rates. I-Speed’s flat Dk profile through 10 GHz means the material itself is not a significant source of dispersion on channels below that frequency range.

The Df trend from 0.0058 at 1 GHz to 0.0071 at 10 GHz is expected behavior for any organic resin — loss tangent rises with frequency as dielectric relaxation mechanisms become more active. The practical implication: use the 10 GHz Df value (0.0071) for worst-case insertion loss calculations at data rates where the Nyquist frequency approaches 5–7 GHz, and verify stack-up-specific Dk/Df from the full tables rather than nominal values when modeling controlled impedance at high frequencies.

Full Thermal and Mechanical Properties

Property	Typical Value	Test Method
Glass Transition Temp (Tg) — DSC	180°C	IPC-TM-650 2.4.25C
Glass Transition Temp (Tg) — DMA	Documented	Rev H update (March 2024)
Decomposition Temp (Td)	360°C	IPC-TM-650 2.4.24.6
T260	>60 minutes	IPC-TM-650 2.4.24.1
T288	>60 minutes	IPC-TM-650 2.4.24.1
Z-Axis CTE (pre-Tg)	60 ppm/°C	IPC-TM-650 2.4.24
Z-Axis CTE (post-Tg)	230 ppm/°C	IPC-TM-650 2.4.24
Z-Axis CTE (50–260°C, total)	2.7% (2.5% per Rev I, 6×2116)	IPC-TM-650 2.4.24
X/Y-Axis CTE (pre-Tg)	16 ppm/°C	IPC-TM-650 2.4.24
Thermal Conductivity	0.4 W/m·K	ASTM E1952
Moisture Absorption	~0.1%	IPC-TM-650
UL Flammability	V-0	UL 94
RTI (Relative Thermal Index)	Documented	UL 746
RoHS	Compliant

Understanding the T260 and T288 Ratings

T260 and T288 greater than 60 minutes each means the material can withstand isothermal exposure at those temperatures for an extended period before delamination occurs. These figures directly support lead-free assembly compatibility — SAC305 peak reflow temperatures of 250–260°C are well within I-Speed’s thermal margin. For programs involving multiple assembly passes, high-temperature rework, or dense BGA packages that create local thermal concentration, the >60-minute T288 rating provides meaningful process buffer.

The “25% More Electrical Bandwidth” Claim: What It Actually Means

Isola’s positioning statement comparing I-Speed to “competitive products in this space” requires some translation into practical terms. The comparison is against standard FR-4-class Tg 180°C materials like Isola’s own 370HR or competitive mid-range laminates.

At 10 GHz, I-Speed’s Df is approximately 0.007. Isola 370HR’s Df at 10 GHz is approximately 0.021. The ratio is roughly 3:1 in favor of I-Speed — a 65–70% reduction in loss tangent. The “25% more electrical bandwidth” figure is Isola’s characterization comparing to the nearest competitive FR-4-class products, not to 370HR specifically. The point is not the exact percentage — it’s the category: I-Speed is meaningfully less lossy than standard high-Tg FR-4 at signal frequencies where loss tangent dominates insertion loss.

For a PCB engineer, the translation is: at 10 Gbps NRZ (Nyquist at 5 GHz), on a 20-inch trace, the difference between 0.021 Df and 0.007 Df is roughly 2–3 dB of insertion loss. That 2–3 dB is often the margin between an eye diagram that closes without equalization and one that requires a DSP equalizer in the SerDes — which adds cost, complexity, and power. I-Speed earns its specification at 5–25 Gbps lane rates on moderate-to-long trace lengths.

Product Availability and Construction Options

Standard Material Offering

Parameter	Available Options
Laminate thickness	2 to 28 mil (0.05 to 0.71 mm) — 28 mil added Rev H
Standard copper foil	HVLP (VLP2) ≤2.5 µm Rz JIS
Alternate copper foil	RTF (Reverse Treat Foil)
Standard HTE foil	HTE Grade 3
Copper weight	½, 1, 2 oz (18, 35, 70 µm); heavier and thinner available
Prepreg	Roll or panel form; tooling of panels available
Glass fabric	Standard E-glass; Low Dk glass; square weave glass; spread glass

The 28 mil laminate thickness added in Revision H (March 2024) extends I-Speed’s coverage into thicker backplane-style cores that previously required material substitutions. For designs where a single thick core layer simplifies the stack-up compared to multiple thinner cores, this construction option is useful.

The glass fabric options deserve specific attention. While standard E-glass is the common offering, I-Speed is also available with spread glass and square weave glass. For designs operating above 5 GHz where fiber weave effect causes differential skew, specifying spread glass on I-Speed is the same mitigation technique used with premium low-loss materials — and it’s available at I-Speed’s price tier.

The VLP2 (HVLP, ≤2.5 µm Rz JIS) copper foil is the standard offering, providing meaningful conductor loss reduction over standard HTE copper at multi-GHz frequencies. The skin depth effect that makes copper roughness a loss contributor starts becoming significant above 1–2 GHz. VLP2 at 2.5 µm roughness is not as smooth as the HVLP3 used in Tachyon 100G, but it’s appropriate for I-Speed’s target data rate range.

I-Speed Performance and Processing Feature Summary

Category	Attribute
Performance	25% more electrical bandwidth vs competitive FR-4
	15% Z-axis CTE improvement
	T260 and T288 >60 min
	Lead-free assembly compatible
	Low moisture absorption
	Superior moisture resistance at reflow
Processing	FR-4 process compatible
	Closest to conventional FR-4 of all HSD materials
	UV blocking
	Laser fluorescing (AOI compatible)
	Multiple lamination cycles
	Lead-free compatible
Compliance	UL 94 V-0
	UL File E41625
	RoHS compliant
	IPC-TM-650 qualified

The AOI and UV Blocking Features: Practical Shop Floor Value

The I-Speed resin system is laser fluorescing and UV blocking for maximum compatibility with Automated Optical Inspection (AOI) systems, optical positioning systems, and photoimageable solder mask imaging. This is worth a brief explanation because it affects manufacturing yield in ways that aren’t obvious from the electrical specs.

UV blocking in the laminate prevents UV light from the AOI or optical positioning system from passing through the dielectric and creating false readings from copper structures on inner layers. In standard FR-4 without UV blocking, inspection systems can generate spurious signals that trigger false defect alerts. UV blocking material eliminates that false-positive source.

Laser fluorescing means the material emits visible light when excited by a UV or laser source, providing a bright, high-contrast background for automated inspection. This improves defect detection sensitivity for opens, shorts, and solder bridges during optical inspection, and supports the fine-pitch fiducial targeting that high-density PCB assembly lines use for accurate component placement.

Both of these features reduce false-reject rates and improve first-pass yield on the assembly line — factors that matter economically on high-volume production programs.

I-Speed Laminate Processing: Discipline Points That Matter

I-Speed is closest to conventional FR-4 processing of all Isola’s high-speed digital material grades. That’s Isola’s own characterization, and it’s accurate — any shop running 370HR or FR408HR can process I-Speed with minimal parameter adjustment. Several specific practices deserve attention.

Lamination

I-Speed prepreg and laminate cure at parameters consistent with standard high-Tg FR-4. The material supports multiple lamination cycles, making it suitable for sequential lamination HDI designs. Full lamination cycles should be used for both subassembly and final lamination. The superior moisture resistance at reflow that Isola highlights is a function of the multifunctional resin system’s lower moisture uptake versus competing FR-4 materials — this translates directly to lower risk of measling and blistering during high-temperature assembly.

For prepreg storage, keep in original moisture barrier packaging, handle with clean gloves, and use FIFO inventory management. If I-Speed prepreg absorbs moisture before lamination, it will show depressed Tg and alter flow properties during the press cycle.

Drilling

Use lower chiploads and cutting speeds than standard FR-4 as the starting point. I-Speed’s multifunctional resin system is somewhat harder than standard epoxy, which can accelerate drill wear if FR-4 parameters are applied directly. Undercut drill geometries and high-helix tools are recommended. For thick boards (above 2.5 mm) with high layer counts, peck drilling may be required.

Desmear

Standard permanganate chemical desmear at standard FR-4 parameters. No plasma treatment or special through-hole treatments are required — this is FR-4 process territory throughout the fabrication sequence.

Stack-Up and Impedance Design

When using spread glass or low Dk glass fabric on I-Speed, note that these constructions produce different Dk values than standard E-glass. The spread glass constructions with high resin content produce lower Dk (toward the 3.44–3.46 range in some constructions from the Dk/Df tables) versus standard E-glass constructions (toward 3.55–3.65). Use the Dk/Df tables specific to your construction and resin content when calculating controlled impedance — do not use a single nominal Dk value for all layers.

Where Isola I-Speed Laminate Sits in the Full Material Selection Decision

I-Speed in the Isola PCB Material Hierarchy

Understanding where I-Speed fits relative to the rest of the Isola portfolio is the most useful context for making the right selection.

Material	Dk (10 GHz)	Df (10 GHz)	Tg	Halogen-Free	Position
370HR	~4.04	~0.021	180°C	No	Standard high-Tg FR-4
FR408HR	~3.68	~0.0092	190°C	No	High-perf FR-4, better loss
I-Speed	3.63	~0.007	180°C	No	Low-loss entry point
IS580G	3.80	0.006	205°C	Yes	Halogen-free, higher Tg
I-Tera MT40	3.45	0.0031	215°C	No	Very low loss, HSD+RF
Tachyon 100G	3.02	0.0021	215°C	No	Ultra-low loss, 100G+
TerraGreen 400GE	3.29	0.0026	200°C	Yes	Halogen-free low loss

The two comparisons that define I-Speed’s position in practice are its relationship to FR408HR below it, and to I-Tera MT40 above it.

I-Speed vs FR408HR: Which Low-Loss Step Up Do You Need?

FR408HR delivers Df approximately 0.0092 at 10 GHz — roughly 25–30% higher loss than I-Speed at that frequency. FR408HR has a higher Tg (190°C vs 180°C DSC) and is specifically engineered for the highest thermal reliability in the FR-4 system class, with a 30% improvement in Z-axis expansion compared to standard FR-4.

For a design that is primarily thermal-reliability driven — high-layer-count boards with dense BGA components, programs requiring multiple reflow passes, complex sequential lamination stack-ups — FR408HR’s combination of high-Tg reliability and very well-characterized fabrication history is compelling. When the design is more signal-integrity driven and the channel data rate is starting to strain FR408HR’s Df, I-Speed is the step up.

In rough practical terms: if your channels run at 5–10 Gbps and trace lengths are below 15 inches, FR408HR usually closes. Above 10 Gbps per lane or on longer traces, I-Speed provides enough Df improvement to matter.

I-Speed vs I-Tera MT40: When to Move Up

I-Tera MT40 at Df 0.0031 at 10 GHz is more than twice as good on loss tangent as I-Speed’s 0.007. For channels running at 25–50 Gbps per lane, or at data rates where 100 Gbps aggregate per port is being targeted, I-Tera MT40’s loss performance becomes the enabling specification. I-Speed at those data rates leaves channel loss budgets too tight, particularly on backplane designs with traces exceeding 20 inches.

The decision point: if your channel loss simulation with I-Speed’s Dk/Df shows insufficient noise margin at your SerDes data rate and trace geometry, the next step is I-Tera MT40 — not a small incremental adjustment. The Df jump from 0.007 to 0.003 is the kind of step that changes whether a 400G Ethernet switch achieves extended reach on the backplane.

I-Speed works well for designs in the 10–25 Gbps range. Tachyon 100G is optimized for 100 Gbps and beyond. I-Tera MT40 covers the middle ground and extends into RF/microwave applications.

Target Applications for Isola I-Speed Laminate

Networking and Communications Infrastructure: Campus and enterprise switches, mid-range routers, and SDH/SONET transport equipment running at 1–25 Gbps per lane where standard FR-4 creates eye diagram margin problems. I-Speed has been widely deployed in this segment. Data center access layer switches, aggregation switches, and enterprise core routers where 10GbE and 25GbE interfaces are the dominant channel type all fall within I-Speed’s performance envelope.

Industrial and Commercial Computing: Server motherboards and workstation PCBs using DDR4/DDR5 memory at standard data rates, PCIe Gen 3/4 interface boards, and NVMe storage controller cards. The modest data rate requirements of these interfaces combined with the demanding thermal environment of dense server chassis make I-Speed’s combination of Df 0.006 and T288 >60 min a strong value proposition.

Aerospace and Defense Ground Equipment: Mission planning systems, ground station electronics, data link equipment, and command-and-control hardware where high thermal reliability and verified lead-free compatibility are required. The T260/>60 min and T288/>60 min ratings along with I-Speed’s established qualification history at aerospace-grade PCB shops make it appropriate for these applications at the data rates involved.

Medical Electronics: Monitoring systems, imaging acquisition boards, and telemetry processing electronics where strict thermal reliability, RoHS compliance, and UV blocking for AOI-compatible manufacturing are required. Medical programs often run relatively modest data rates (5–10 Gbps) through complex, high-layer-count multilayer assemblies — exactly the regime where I-Speed adds value.

Telecom Access and CPE: DSL/VDSL customer premises equipment, optical network termination units, and cable modem termination system (CMTS) line cards where channel data rates are in the 1–10 Gbps range and halogen-free compliance is not a hard requirement.

For design support and fabrication using ISOLA PCB materials including I-Speed laminate, working with a fabricator who regularly processes the full Isola laminate family ensures correct parameter calibration between I-Speed and the adjacent FR-4 grades it sits between in process terms.

Design Tips for I-Speed Laminate Stack-Ups

Always use construction-specific Dk/Df: The nominal Dk 3.63–3.64 in the headline datasheet is for a 56% resin content construction. Constructions with 67% or 72% resin content (e.g., 1035 or 1067 glass styles) have different Dk values — ranging from 3.44 to 3.59 depending on the specific construction. Download the full I-Speed Dk/Df table PDF from Isola and model your specific prepreg and core constructions individually. Stack-up tools that use a single Dk for all layers of I-Speed will give you incorrect controlled impedance.

Spread glass for critical differential pairs: If any layers in your design carry differential pairs above 5 GHz where fiber weave effect causes skew, specify spread glass on those layers. Spread glass is available for I-Speed and provides the same skew reduction as spread glass on premium grades, at I-Speed’s cost point.

Copper foil choice at data rates above 10 Gbps: At 10+ GHz, HVLP (VLP2) copper’s 2.5 µm Rz roughness is appropriate. If you’re using I-Speed in a design that’s at the upper edge of its data rate range (20–25 Gbps), the RTF option can provide marginally better conductor loss performance for the most loss-sensitive channels.

Z-axis CTE for via reliability on thick boards: I-Speed’s total Z-axis expansion of 2.7% (50–260°C) is good but not the lowest in its performance tier. For boards above 24 layers or total thickness above 4 mm, the via barrel stress model should be verified using I-Speed’s CTE values. FR408HR with its explicitly lower Z-axis CTE may be the better choice for the most thermally demanding thick-board constructions.

Useful Resources and Data Downloads

Resource	Type	Link
I-Speed Official Product Page	Isola product page	isola-group.com/i-speed
I-Speed Datasheet PDF	Official datasheet	isola-group.com (PDF)
I-Speed Dk/Df Tables PDF	Construction-specific data	isola-group.com (Dk/Df)
FR408HR Product Page	Comparison — lower tier	isola-group.com/fr408hr
370HR Product Page	Standard high-Tg FR-4	isola-group.com/370hr
I-Tera MT40 Product Page	Comparison — upper tier	isola-group.com/i-tera-mt40
IS580G Product Page	Halogen-free equivalent	isola-group.com/is580g
IsoDesign Impedance Calculator	Online stack-up tool	isola-group.com/design-tools
Insulectro I-Speed Distributor Page	US Distributor	insulectro.com/i-speed
IPC-TM-650 Test Methods	IPC Standards	ipc.org
UL Product iQ (File E41625)	UL Certification DB	iq.ul.com

Frequently Asked Questions About Isola I-Speed Laminate

1. What data rate range is Isola I-Speed laminate designed for?

I-Speed is well matched to designs operating at 5–25 Gbps per SerDes lane. At 5 Gbps, the improvement over FR408HR is modest, and either material can close typical channel budgets. As data rates approach 10–25 Gbps and trace lengths exceed 15 inches, I-Speed’s Df advantage over FR408HR (0.007 vs 0.009 at 10 GHz) becomes meaningful — typically 1–2 dB of additional insertion loss headroom on long traces. Above 25 Gbps per lane, I-Tera MT40 (Df 0.003) is the more appropriate specification, as I-Speed’s loss tangent begins to close off channel margin at those signaling speeds. Run your actual channel loss simulation with I-Speed’s Dk/Df values at your specific trace length and SerDes operating frequency before deciding between I-Speed and I-Tera MT40.

2. How close is I-Speed laminate processing to standard FR-4?

Isola describes I-Speed as the closest to conventional FR-4 processing of all its high-speed digital material grades. Lamination press cycles, drilling parameters (with modest chipload reduction), permanganate desmear, imaging chemistry, and plating processes are all compatible with standard FR-4 processing. The main disciplines are: use lower chiploads and cutting speeds when drilling, keep prepreg in moisture barrier packaging until use, and model Dk/Df using construction-specific values rather than a single nominal Dk. Any shop running Isola 370HR or FR408HR can qualify I-Speed with a straightforward process characterization cycle — no new chemistry, equipment, or press cycle profiles required.

3. Is Isola I-Speed a halogen-free material?

No. Standard Isola I-Speed laminate is not halogen-free. It uses a brominated flame retardant system to achieve its UL 94 V-0 flammability rating. If your program requires halogen-free compliance per IEC 61249-2-21 and a similar electrical performance tier (Df around 0.006), Isola IS580G is the appropriate material — it delivers a comparable Df with halogen-free compliance and a higher Tg (205°C DMA). For halogen-free compliance with lower Df values (0.0026 range), Isola TerraGreen 400GE is the option to evaluate.

4. What is the difference between I-Speed’s Dk values in the datasheet and in the Dk/Df tables?

The headline Dk value on the I-Speed product page (3.64) and datasheet is measured at approximately 56% resin content — the nominal condition used for headline comparison. The full Dk/Df tables show construction-specific values at different glass styles and resin contents. A 1×1035 construction at 67% resin content produces Dk around 3.55–3.57 across the frequency range, while a 1×1067 construction at 72% resin content produces Dk around 3.44–3.47. These differences are significant for controlled impedance calculations: a stack-up mixing multiple prepreg styles will have different Dk values on different layers, and using the 3.64 headline Dk for all layers will produce an impedance calculation error. Always use the Dk/Df table PDF and apply the construction-specific Dk/Df for each layer in your stack-up model.

5. Can I-Speed laminate be used in sequential lamination HDI designs?

Yes. I-Speed supports multiple lamination cycles and is rated for HDI technology applications. The multifunctional resin system retains adhesion and dielectric performance through repeated press cycles, which is the fundamental requirement for sequential lamination. The material’s UV blocking and laser fluorescing features are directly useful in HDI production, where AOI inspection happens after each lamination cycle and optical alignment accuracy is critical for blind/buried via registration. For HDI programs that need lower loss than standard FR-4 but aren’t in the I-Tera MT40 performance range, I-Speed is a practical and proven material choice with well-established fabrication process parameters.

The Case for Isola I-Speed Laminate

Isola I-Speed laminate earns its specification through a combination that is harder to find than it looks: a 65–70% reduction in loss tangent versus standard high-Tg FR-4, a Tg and Td sufficient for aggressive lead-free assembly, a process compatibility that doesn’t require special equipment or chemistry, and a glass fabric option set that includes spread glass for fiber weave effect control. That package, at I-Speed’s cost tier, covers a large share of production PCBs that have clearly outgrown 370HR but don’t need the full capability — and cost — of I-Tera MT40.

The selection logic is straightforward. When your channel loss analysis shows that standard FR-4 won’t close the budget at your data rate and trace geometry, and when I-Tera MT40 or Tachyon 100G would be over-specified, I-Speed is the right tool. It bridges the gap efficiently, processes like FR-4, and has a fabrication track record across thousands of deployed designs in networking, computing, industrial, and aerospace applications.

For fabrication services and engineering support using Isola PCB materials including I-Speed laminate, visit RayPCB’s ISOLA PCB resource page.