Isola FR408HR delivers 190°C DSC / 230°C DMA Tg, 30% better Z-axis CTE, and Df ~0.009 at 10 GHz — the benchmark mid-loss high-reliability FR-4 laminate. Full guide: actual Dk/Df at 100 MHz–10 GHz, via reliability physics, processing parameters, spread weave glass standard, and comparison with 370HR, I-Speed, and I-Tera MT40.

Primary keyword: Isola FR408HR | ~3,200 words

Ask any experienced PCB fabrication engineer which laminate occupies the position of go-to high-reliability mid-loss material for demanding multilayer designs, and the answer you’ll hear most often is Isola FR408HR. It’s been deployed in everything from telecommunications line cards and enterprise switches to aerospace electronics and medical imaging systems — boards where standard FR-4 thermal reliability doesn’t make the grade but where paying for ultra-low-loss premium laminates isn’t justified by the signal frequencies involved. The combination of 190°C DSC Tg, 230°C DMA Tg, 30% better Z-axis CTE than competitive products, and Df in the 0.009–0.010 range at 10 GHz has made FR408HR a reference specification in high-performance PCB design for the better part of two decades.

This guide provides the full technical picture: verified electrical and thermal specifications, construction availability, processing discipline, how FR408HR compares with both the materials below and above it in the Isola portfolio, and the precise application scenarios where its specific combination of properties makes it the right choice.

What Is Isola FR408HR?

FR408HR is a proprietary high-performance 230°C DMA glass transition temperature FR-4 system for multilayer Printed Wiring Board applications where maximum thermal performance and reliability are required. FR408HR laminate and prepreg products are manufactured with Isola’s patented high-performance multifunctional resin system, reinforced with electrical grade (E-glass) glass fabric.

The “HR” designation stands for High Reliability — a classification that reflects the specific engineering tradeoffs built into the multifunctional resin system. The key quantified claims: this system delivers a 30% 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.

Two aspects of that positioning deserve unpacking. “30% improvement in Z-axis expansion” means FR408HR’s Z-axis CTE is 30% lower than standard high-Tg FR-4-class materials — reducing the thermal expansion stress on plated through-holes during lead-free assembly and thermal cycling. “25% more electrical bandwidth” means the Df is approximately 25% lower than competing mid-loss materials, reducing insertion loss on high-frequency signal channels. Both improvements address real failure modes in high-layer-count, high-frequency multilayer boards.

The FR408HR system is also laser fluorescing and UV blocking for maximum compatibility with Automated Optical Inspection (AOI) systems, optical positioning systems, and photoimageable solder mask imaging — which translates to better fabrication yield on complex assemblies.

The Distinction Between DSC Tg and DMA Tg for FR408HR

One of the most frequently confusing aspects of FR408HR’s datasheet is the two Tg values: 190°C (DSC) and 230°C (DMA). Engineers encountering FR408HR for the first time sometimes question which value to use.

DSC (Differential Scanning Calorimetry) measures the heat flow changes in the resin as it’s heated, identifying the glass transition as the temperature at which the specific heat changes. It tends to produce a conservative Tg value — practical and representative of the transition in dimensional behavior. TMA (Thermomechanical Analysis) measures dimensional change and also produces a 190°C range value for FR408HR.

DMA (Dynamic Mechanical Analysis) measures the viscoelastic properties of the cured resin and typically yields a higher Tg, reflecting the thermally active frequency-dependent stiffness of the polymer network. For FR408HR, this produces the 230°C DMA value.

For practical PCB engineering purposes: use the DSC/TMA value of 190°C for assembly process planning (it’s the conservative reference for lead-free reflow margin calculations), thermal management analysis, and sequential lamination cycle planning. The DMA value of 230°C tells you that the resin system has exceptional thermoset network crosslinking, which is why FR408HR achieves its outstanding thermal reliability — the network is highly crosslinked, explaining both the high thermal stability and the low Z-axis CTE.

Isola FR408HR Full Electrical and Thermal Specifications

The table below consolidates properties from the official Isola datasheet (Revision G, January 5, 2026), the most current version. Electrical properties shown at nominal 55% resin content (2×2116 construction).

Electrical Properties by Frequency

Frequency	Dk (Permittivity)	Df (Loss Tangent)	Test Method
100 MHz	3.72	0.0072	IPC-TM-650 2.5.5.3
1 GHz	3.69	0.0091	HP4291A
2 GHz	3.68	0.0092	Bereskin Stripline
5 GHz	3.64	0.0098	Bereskin Stripline
10 GHz	3.65	0.0095	Bereskin Stripline

The nominal Dk of 3.68 at 2 GHz is the headline value, dropping slightly to 3.65 at 5–10 GHz — typical of the slight decrease with frequency seen in all organic resin systems. The Df rising from 0.007 at 100 MHz to approximately 0.009–0.010 at 5–10 GHz reflects the increasing dielectric relaxation contribution at higher frequencies. For controlled impedance calculations, use the Dk from the Dk/Df table at your specific operating frequency and construction — the variation across different glass styles and resin content percentages is more significant than the frequency variation.

The Dk range across different FR408HR constructions spans approximately 3.30 to 3.82 at 100 MHz, and narrows toward 10 GHz. High-resin-content constructions (72% RC with 1086 glass style, for example) produce lower Dk in the 3.3 range; low-resin-content constructions (42% RC with 1652 glass) push toward 4.0. This spread matters enormously for stack-up design — a single nominal Dk is not adequate for controlled impedance modeling on boards with multiple prepreg types.

Full Thermal and Mechanical Properties

Property	Typical Value	Test Method
Glass Transition Temp (Tg) — DSC	190°C	IPC-TM-650 2.4.25C
Glass Transition Temp (Tg) — DMA	230°C	IPC-TM-650 2.4.24.4
Decomposition Temp (Td)	360°C	IPC-TM-650 2.4.24.6
T260	60 minutes	IPC-TM-650 2.4.24.1
T288	>30 minutes	IPC-TM-650 2.4.24.1
Z-Axis CTE (pre-Tg)	55 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)	2.8%	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
Thermal Stress 10 sec @ 288°C	Pass	IPC-TM-650 2.4.13.1
Moisture Absorption	Low	IPC-TM-650
Flexural Strength (length/cross)	72.5 / 58.0 ksi	IPC-TM-650 2.4.4B
Tensile Modulus (length/cross)	3,695 / 3,315 ksi	ASTM D3039
Peel Strength (after thermal stress)	~5.5 lb/in	IPC-TM-650 2.4.8.2A
Comparative Tracking Index (CTI)	Class 2 (250–499 V)	UL 746A
UL Flammability	V-0	UL 94
UL File Number	E41625
IPC Classifications	IPC-4101/98, /99, /101, /126
RoHS	Compliant

Interpreting T260 and T288 for FR408HR

FR408HR’s T260 = 60 minutes and T288 >30 minutes positions it well for standard and moderately aggressive lead-free assembly processes. The T260/T288 test measures the time-to-delamination when held at constant temperature — it’s a stress indicator for assembly processes approaching those temperatures.

SAC305 lead-free assembly profiles peak at 250–260°C. FR408HR’s 60-minute T260 means the material can withstand extended exposure at the assembly peak temperature without delaminating — necessary when multiple reflow passes, long-zone ovens, or thick boards with slow thermal equilibration are involved.

The T288 of >30 minutes is somewhat lower than materials like I-Speed (>60 min T288) or IS580G (>60 min T288). For the most aggressive assembly profiles, or programs requiring extensive rework cycles at the hottest solder float temperatures, this distinction matters. For standard lead-free assembly in typical production environments, FR408HR’s T288 is fully adequate.

Product Availability and Construction Options

Standard Material Offering

Parameter	Available Options
Laminate thickness	2 to 59 mil (0.05 to 1.5 mm)
Standard copper foil	RTF (Reverse Treat Foil)
Alternate copper foil	HVLP (VLP2) ≤2.5 µm Rz JIS
Standard HTE foil	HTE Grade 3
Copper weight	½ to 2 oz (18 to 70 µm); heavier and thinner available
Prepreg	Roll or panel form; tooling of panels available
Glass fabric	Standard E-glass; square weave glass; spread glass
All FR408HR glass	Spread weave in both directions (all constructions)

The designation “All FR408HR glass is Spread Weave in both directions” on the product page is a significant update. Spread weave glass in all constructions means every FR408HR design benefits from reduced fiber weave effect — the periodic Dk variation caused by alternating glass bundle and resin-rich zones that creates differential skew in fast differential pairs. For boards operating above 3–5 GHz on any layer, spread weave glass is a signal integrity improvement that FR408HR delivers as standard rather than as a special-request option.

The FR408HRIS variant mentioned in the official documentation provides access to low-Dk glass styles for designs where a lower composite Dk is needed for impedance matching without moving to a premium low-loss material. This is worth noting when designing controlled impedance stack-ups with specific Dk requirements that the standard E-glass constructions don’t satisfy.

Performance and Processing Summary

Category	Attribute
Performance	30% Z-axis CTE improvement vs competitive FR-4
	25% more electrical bandwidth (lower loss)
	T260: 60 min; T288: >30 min
	6x 260°C reflow capable
	6x 288°C solder float capable
	CAF resistant
	Lead-free assembly compatible
	Low moisture absorption
Processing	FR-4 process compatible
	Via filling capability
	Multiple lamination cycles
	Laser fluorescing (AOI compatible)
	UV blocking
Compliance	UL 94 V-0 (File E41625)
	IPC-4101/98, /99, /101, /126
	RoHS compliant

The 30% Z-Axis CTE Improvement: Why It Defines FR408HR’s Reliability Position

The 30% improvement in Z-axis CTE versus competitive products is the technical specification that most directly determines FR408HR’s suitability for high-reliability multilayer boards with plated through-holes. Understanding why requires a brief explanation of the via barrel stress problem.

During lead-free assembly reflow, a PCB heats from room temperature to approximately 250–260°C peak. In the Z-axis (thickness direction), the dielectric material tries to expand. Copper — the via barrel and annular ring — has a much lower Z-axis CTE (~17 ppm/°C). The dielectric expanding more than the copper creates tensile stress in the copper barrel. Under repeated thermal cycles, this fatigue accumulates. Eventually, the barrel cracks — a via-open failure that is intermittent at first and becomes permanent as the crack propagates.

A lower Z-axis CTE means less differential expansion between the dielectric and the via copper, which means lower barrel stress per thermal cycle, which means more thermal cycles before fatigue failure. FR408HR’s pre-Tg Z-axis CTE of 55 ppm/°C, producing 2.8% total expansion from 50–260°C, is meaningfully lower than standard FR-4 materials, which typically run 3.5–4% total Z-axis expansion over that range. That difference is why FR408HR is specified for high-layer-count boards with through-holes on 0.8 mm pitch BGAs and for programs requiring extended thermal cycling qualification.

The CAF resistance that accompanies the multifunctional resin system reinforces the reliability positioning. CAF (Conductive Anodic Filament) formation under voltage bias and humidity is the other dominant failure mode for high-density multilayer boards. FR408HR’s resin system provides enhanced interlaminar adhesion that resists the glass-resin interface degradation where CAF growth initiates.

FR408HR vs Neighboring Isola Materials: The Correct Selection Framework

Isola Portfolio Comparison Table

Material	Dk (10 GHz)	Df (10 GHz)	Tg DSC	Td	Halogen-Free	Z-CTE (total)
370HR	~4.04	~0.021	180°C	340°C	No	~3.5%
FR408HR	~3.65	~0.009–0.010	190°C	360°C	No	2.8%
I-Speed	~3.63	~0.007	180°C	360°C	No	2.7%
IS580G	3.80	0.006	205°C	385°C	Yes	1.8%
I-Tera MT40	3.45	0.0031	215°C	360°C	No	~2.8%
Tachyon 100G	3.02	0.0021	215°C	360°C	No	~2.5%

FR408HR vs Isola 370HR: When to Step Up

Standard FR-4 materials like 370HR carry Df around 0.018–0.021 at 10 GHz and Tg of 180°C with Td 340°C. The decision to move from 370HR to FR408HR is driven by two independent triggers that can appear separately or together.

The first trigger is thermal: when the board requires a higher Tg or lower Z-axis CTE than 370HR can provide. For boards with multiple 2 oz copper inner layers, thick constructions above 3.5 mm, 0.8 mm pitch BGAs with high ball count, or programs requiring formal thermal cycling qualification at aggressive parameters, FR408HR’s 190°C DSC Tg and 30% better Z-axis CTE deliver the margin that 370HR cannot.

The second trigger is electrical: when signal frequencies or data rates have reached the point where 370HR’s Df (~0.021) is causing unacceptable insertion loss. At 5 Gbps over a 15-inch trace, the difference between Df 0.021 and Df 0.009 is approximately 2–3 dB — enough to close an eye diagram that 370HR would leave marginal. FR408HR at Df ~0.009 gets the channel back into spec without requiring the price premium of genuinely low-loss materials.

FR408HR vs Isola I-Speed: The Mid-Loss Tier Decision

Both FR408HR and I-Speed occupy the same general price tier in the Isola mid-performance lineup. The comparison is frequently asked because both are FR-4 compatible, both target the same fabrication community, and both are positioned between 370HR and I-Tera MT40.

The key differences: FR408HR has a higher Tg (190°C DSC vs 180°C DSC) and a better-documented T260/T288 profile for demanding thermal applications. I-Speed has a slightly lower Df (~0.007 vs ~0.009 at 10 GHz). FR408HR explicitly carries the 6x 260°C and 6x 288°C solder float ratings on its product page; I-Speed’s T288 rating of >60 min actually exceeds FR408HR’s >30 min on paper.

For programs where thermal reliability is the primary driver — thick multilayer boards, heavy copper layers, 0.8 mm BGA pitch, or aerospace-grade thermal cycling qualification — FR408HR’s Tg and Z-axis CTE profile is the stronger specification. For programs where signal loss at 10–25 Gbps is the primary constraint and thermal requirements are standard, I-Speed’s lower Df provides a tangible advantage.

In practice, FR408HR has more field qualification history in aerospace, defense, and high-reliability computing applications; I-Speed has stronger positioning in networking and communications where signal integrity is the primary driver.

FR408HR vs I-Tera MT40: When Do You Need the Jump?

I-Tera MT40 at Df 0.0031 at 10 GHz is roughly three times better on loss tangent than FR408HR’s 0.009–0.010. For SerDes channels running at 25–50 Gbps per lane, that Df difference is usually what separates channels that close with FR408HR from channels that don’t. Below approximately 10–15 Gbps per lane on moderate trace lengths, FR408HR typically closes the link budget without leaving insufficient margin. Above 15–25 Gbps, I-Tera MT40 becomes the practical step up.

The cost difference between FR408HR and I-Tera MT40 is substantial — I-Tera MT40 is premium pricing relative to FR408HR’s high-performance FR-4 cost tier. Running your channel loss simulation with actual FR408HR Dk/Df values from the construction-specific tables is the technically correct way to make this decision, not the headline Df comparison alone.

FR408HR Processing Guidance

FR408HR processes as a conventional high-performance FR-4 material. It is the closest to conventional FR-4 processing of all Isola’s high-speed material grades — a distinction that separates it from materials like I-Tera MT40 or Tachyon 100G that require process parameter adjustments.

Lamination

Lamination follows standard high-Tg FR-4 press cycles. The 190°C DSC Tg means press cure temperatures and times are calibrated to that Tg, using standard multilayer press equipment. FR408HR supports multiple lamination cycles — the via filling capability listed in the product features supports sequential lamination HDI designs where resin needs to fill complex via structures during intermediate lamination cycles.

Key prepreg handling discipline: store in moisture barrier packaging, handle with clean gloves, use FIFO inventory management. FR408HR prepreg will absorb moisture if improperly stored, leading to depressed Tg, bubbling during lamination, and measling in the finished board.

Drilling

FR408HR drilling follows standard high-performance FR-4 parameters. The multifunctional resin is slightly harder than commodity FR-4, so chiploads and cutting speeds should be reduced relative to standard 180°C Tg FR-4 as a starting point. Undercut drill geometries and high-helix tools support clean hole wall quality.

For high-layer-count boards above 2.5 mm total thickness: peck drilling is often required. The IPC-recommended approach of removing 7628 glass from the construction near drilled areas reduces the energy required to cut through the coarse weave, improving hole wall quality and reducing smear generation. Ensure drill parameters are properly adjusted on cured FR408HR laminates to minimize smear production and improve desmear performance.

Desmear

Plasma desmear is effective for FR408HR. Standard plasma gas mixtures used for conventional FR-4 epoxy are the appropriate starting point. A plasma desmear with a permanganate pass combination provides enhanced hole quality for high-aspect-ratio vias in thick boards. Care should be taken with the permanganate chemistry to avoid excessive resin removal (etchback) — Isola’s guidance is to use plasma parameters comparable to standard FR-4 epoxy, not the more aggressive parameters sometimes used for specialty resin systems.

Standard permanganate-only desmear is also compatible for standard-aspect-ratio via designs. The choice between permanganate and plasma/permanganate combination depends primarily on board thickness, via aspect ratio, and your fabricator’s process capability.

Assembly Compatibility

FR408HR is 6x 260°C reflow capable and 6x 288°C solder float capable — sufficient for standard lead-free SAC305 assembly including substantial rework and multiple assembly passes. The superior moisture resistance at reflow means FR408HR boards have low delamination risk during assembly even under rapid thermal ramp conditions.

Target Applications for Isola FR408HR

Telecom and Datacom Switching Infrastructure: Enterprise switches, campus routers, data center access switches, and DSL aggregation equipment operating at 1–10 Gbps per lane. This is the heart of FR408HR’s commercial application history. The combination of Tg 190°C, Df ~0.009, and well-established fabrication process at every major PCB shop in the world makes it the default specification for this segment.

Aerospace and Defense Ground Electronics: Radar signal processors, satellite ground station receivers, military command-and-control electronics, and airborne electronic warfare support equipment where lead-free qualification, aggressive thermal cycling specifications, and long service life requirements all converge. FR408HR’s Tg margin above standard FR-4, its low Z-axis CTE for via reliability under thermal cycling, and its RoHS compliance make it the practical choice for aerospace and defense programs that don’t require ultra-low-loss RF performance.

High-Reliability Computing: Server boards, storage controller PCBs, workstation motherboards, and data center line cards where board complexity — high layer counts, multiple BGA devices, 0.8 mm pitch packages, heavy copper power layers — demands the Z-axis CTE and Tg margin that FR408HR provides beyond what 370HR can deliver.

Medical Electronics: Imaging system acquisition boards, telemetry processing PCBs, and medical monitoring devices where strict reliability requirements, RoHS compliance, and the signal frequency range of 1–5 GHz align with FR408HR’s performance profile. The UV blocking and laser fluorescing features support medical electronics manufacturing environments where automated optical inspection accuracy is a quality system requirement.

Industrial Automation and Control: Industrial Ethernet switches, motor drive controllers, PLC processing boards, and industrial IoT gateways where thermal reliability in high-ambient-temperature industrial environments requires Tg well above the 150–160°C operating range that standard FR-4 supports with margin. FR408HR’s 190°C DSC Tg provides the buffer needed for boards mounted in industrial panel enclosures.

For fabrication and stack-up engineering support on ISOLA PCB designs using FR408HR across any of these application segments, working with a shop experienced with high-performance FR-4 processing is the path to realizing the material’s full reliability credentials.

Stack-Up Design Guidance for FR408HR

Always use construction-specific Dk from the Dk/Df tables. FR408HR’s Dk varies from approximately 3.30 to 3.82 at 100 MHz across different glass styles and resin content percentages. A stack-up mixing 1086 prepreg at 71% RC on signal layers and 2116 core at 54% RC as the dielectric will have substantially different Dk values layer by layer. Using the single headline Dk of 3.68 for all layers will produce controlled impedance calculations that miss the target by several ohms. Download the FR408HR Dk/Df table PDF from Isola’s website and apply construction-specific values in your stack-up modeling tool.

Spread weave glass on all constructions. The “All FR408HR glass is Spread Weave in both directions” note on the current product page means fiber weave effect is mitigated in every FR408HR design without special requests. For designs with differential pairs above 3 GHz, this is already accounted for. No need to specifically request spread weave — it’s the default for all current FR408HR constructions.

Z-axis CTE modeling for via reliability. For boards above 20 layers or total thickness above 3.5 mm, run via barrel stress calculations using FR408HR’s pre-Tg Z-axis CTE of 55 ppm/°C. The pre-Tg value governs during normal operating temperature cycling; the post-Tg value of 230 ppm/°C governs during assembly reflow. In designs where via pitch is 0.8 mm or below with large BGA devices, include thermal cycling via reliability analysis in the design qualification plan.

Moisture barrier packaging for long shelf life. Finished FR408HR boards that will wait extended periods before lead-free assembly should be packaged in Moisture Barrier Bags with Humidity Indicator Cards and adequate desiccant. Even with FR408HR’s superior moisture resistance at reflow, moisture uptake during extended storage can affect assembly performance.

Useful Resources and Data Downloads

Resource	Type	Link
FR408HR Official Product Page	Isola product page	isola-group.com/fr408hr
FR408HR Datasheet PDF	Official datasheet	isola-group.com (PDF)
FR408HR Dk/Df Tables PDF	Construction-specific data	isola-group.com (Dk/Df)
370HR Product Page	Standard FR-4 (below)	isola-group.com/370hr
I-Speed Product Page	Low-loss (beside)	isola-group.com/i-speed
I-Tera MT40 Product Page	Very low loss (above)	isola-group.com/i-tera-mt40
IsoDesign Impedance Calculator	Stack-up modeling tool	isola-group.com/design-tools
FR408HR Dk/Df Table (external PDF)	Older table version	isola-group.com Dk/Df revision G
Northwest Engineering FR408HR Tables	Third-party reference	nwengineeringllc.com
IPC-4101/98, /99, /101, /126	IPC Standards	ipc.org
UL Product iQ (File E41625)	UL Certification DB	iq.ul.com

Frequently Asked Questions About Isola FR408HR

1. Why does the FR408HR datasheet show both 190°C and 230°C as Tg values?

FR408HR has a DSC Tg of 190°C and a DMA Tg of 230°C. These are two different measurement methods that quantify different properties of the cured resin. DSC measures the heat flow transition and yields 190°C — the conservative, practical reference for lead-free assembly process planning and PCB operational temperature limits. DMA measures the viscoelastic stiffness transition of the polymer network, which yields the higher 230°C value and reflects the exceptionally high crosslink density of FR408HR’s multifunctional resin. For assembly compatibility decisions, use 190°C. For understanding why FR408HR has such excellent thermal reliability versus competing 180°C Tg materials, the DMA Tg of 230°C explains the story: the network is so highly crosslinked that it retains mechanical stiffness to a much higher temperature than the DSC measurement suggests.

2. What is the practical significance of FR408HR’s 30% Z-axis CTE improvement?

The 30% improvement means FR408HR’s pre-Tg Z-axis CTE is approximately 55 ppm/°C versus roughly 70–80 ppm/°C for standard high-Tg FR-4 materials. During lead-free assembly reflow from 25°C to 260°C, FR408HR expands approximately 2.8% in the Z-direction (50–260°C total) versus 3.5%+ for standard FR-4. For a 3 mm thick board, that’s approximately 0.21 mm of absolute expansion versus 0.30+ mm — roughly 30% less strain on via barrel copper per thermal cycle. Accumulated over hundreds to thousands of thermal cycles in field service, that lower strain per cycle directly extends via lifetime. For programs specifying IPC-6012 class 3 via reliability or military thermal cycling qualification, this improvement is not theoretical — it’s the difference between meeting the specification and failing it.

3. Is FR408HR process-compatible with standard FR-4 equipment and chemistry?

FR408HR is described as the closest to conventional FR-4 processing of all high-speed Isola materials. Lamination press cycles, desmear chemistry (permanganate or plasma), imaging chemistry, plating processes, and solder mask processing are all compatible with standard FR-4 equipment. The main adjustments are: modest reduction in drill chipload and cutting speed versus commodity FR-4, and attention to desmear dwell time to avoid excessive etchback. Any shop running standard high-Tg FR-4 production can qualify FR408HR without capital equipment investment or process line redesign. This is the primary manufacturing advantage of FR408HR over materials like I-Tera MT40 or Tachyon 100G, which require more careful drilling parameter optimization.

4. What is the difference between Isola FR408 and Isola FR408HR?

FR408 is an earlier Isola mid-loss material with Tg 180°C (DSC) and Df approximately 0.012 at 10 GHz — slightly higher loss than FR408HR’s 0.009–0.010 and lower Tg. FR408HR (“High Reliability”) was developed as an upgrade with the multifunctional resin system that delivers the higher 190°C DSC Tg (230°C DMA), improved Z-axis CTE, and lower loss. Most modern PCB programs specifying this performance tier use FR408HR rather than FR408 — the “HR” designation reflects real improvements in thermal and reliability performance, not just marketing. FR408 remains in the portfolio for legacy designs and programs that have qualified it, but FR408HR is the recommended new-design material in this tier.

5. When should I specify FR408HR versus I-Tera MT40?

The decision depends on which constraint is binding in your design. FR408HR is the right choice when: thermal reliability is the primary specification driver (high Tg, low Z-axis CTE, aggressive thermal cycling qualification), the data rate per lane is at or below approximately 10–15 Gbps, and cost is a significant factor — FR408HR costs substantially less than I-Tera MT40. I-Tera MT40 is the right choice when: the data rate per lane exceeds 15 Gbps and channel loss simulation with FR408HR’s Df (~0.009–0.010) shows insufficient noise margin; or when the design includes RF or microwave content that benefits from I-Tera MT40’s Dk 3.45 and Df 0.003. The cleanest diagnostic tool: run your channel loss simulation with FR408HR’s actual Dk/Df from the construction-specific tables. If the eye diagram closes with adequate margin, FR408HR is the correct specification. If not, I-Tera MT40 is the step up.

Why Isola FR408HR Remains the Industry Reference Laminate

Isola FR408HR has earned its position as the reference mid-loss, high-reliability laminate through two decades of deployment across the most demanding multilayer PCB application segments. The combination of 190°C DSC Tg, 230°C DMA Tg, 30% Z-axis CTE improvement versus standard FR-4, Df ~0.009–0.010 at 10 GHz, multiple IPC-4101 slash sheet certifications, spread weave glass as standard, and closest-to-FR-4 processability is a package that no single alternative fully replicates at FR408HR’s cost tier.

The selection logic is straightforward. When standard 370HR either can’t meet the thermal reliability specification or can’t close the channel loss budget at the design’s signal frequencies — and when the full low-loss premium of I-Speed, I-Tera MT40, or Tachyon 100G isn’t required — FR408HR is the answer. It’s been that answer for a long time, and the current Revision G datasheet with spread weave glass as the standard offering across all constructions makes it even more relevant for modern high-frequency multilayer designs.

For PCB fabrication support, stack-up design, and material sourcing across the full Isola laminate portfolio including FR408HR, visit RayPCB’s ISOLA PCB resource page.