ย Isola 370HR is the industry-leading 180ยฐC Tg FR-4 laminate. Learn about its thermal stability, CAF resistance, and electrical specs in our complete PCB engineerโs guide to high-reliability design.
At its core, Isola 370HR is a high-performance 180โC glass transition temperature (Tgโ) FR-4 system. But the Tgโ is only half the story. What makes 370HR unique is its multifunctional epoxy resin system, which is reinforced with electrical-grade (E-glass) glass fabric.
Unlike cheaper FR-4 variants that use bi-functional resins, the multifunctional chemistry of 370HR creates a more robust molecular cross-linking. This translates to superior thermal performance, lower expansion rates, and mechanical properties that donโt just meet but often exceed traditional FR-4 materials.
Why High Tgโ is Critical for 2026 Designs
As of 2026, lead-free soldering is no longer a “new” requirement, but the thermal stresses of modern reflow profiles are still the primary cause of PCB delamination. With a Tgโ of 180โC, 370HR remains in its “glassy” state during most of the assembly process, significantly reducing the risk of via-barrel cracking and pad cratering.
Technical Specifications: The Data Behind the Reliability
When I evaluate a material for a new stackup, the datasheet is my first stop. For Isola 370HR, the numbers tell a story of consistent performance across thermal and electrical domains.
Table 1: Thermal and Physical Properties of Isola 370HR
| Property | Value | Test Method |
| Glass Transition Temp (Tg) | 180โC | DSC |
| Decomposition Temp (Td) | 340โC | TGA @ 5% loss |
| T260 (Time to Delaminate) | 60 minutes | IPC-TM-650 2.4.24.1 |
| T288 (Time to Delaminate) | 30 minutes | IPC-TM-650 2.4.24.1 |
| Z-Axis CTE (Pre-Tg) | 45 ppm/โC | IPC-TM-650 2.4.24C |
| Z-Axis Expansion (50-260โC) | 2.8% | IPC-TM-650 2.4.24 |
| Thermal Conductivity | 0.4 W/mยทK | ASTM E1952 |
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Table 2: Electrical Performance for Signal Integrity
| Frequency | Dielectric Constant (Dk) | Dissipation Factor (Df) |
| 100 MHz | 4.24 | 0.0150 |
| 1 GHz | 4.17 | 0.0161 |
| 2 GHz | 4.04 | 0.0210 |
| 5 GHz | 3.92 | 0.0245 |
| 10 GHz | 3.92 | 0.0250 |
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Note: These values are typical for a 50% resin content. Your specific Dk will vary based on the glass style (e.g., 1080 vs. 7628) and resin percentage in your stackup.
Superior CAF Resistance: Preventing the “Hidden” Failure
One of the most insidious failure modes in high-density PCBs is Conductive Anodic Filament (CAF) growth. This is essentially an electrochemical migration of copper along the glass-to-resin interface, leading to internal shorts.
Isola has patented the way they treat the glass fabric in 370HR to ensure a superior bond with the resin. This makes Isola 370HR one of the most CAF-resistant materials on the market. In my designs for automotive radar or medical imaging, where a short-circuit can have catastrophic results, this is a non-negotiable feature.
Dimensional Stability in HDI Designs
In High-Density Interconnect (HDI) designs, the registration of microvias and small pads is critical. 370HR offers excellent dimensional stability. Its low Z-axis CTE (2.8% total expansion) means that your via structures are under significantly less stress during thermal cycling than they would be with a “standard” 140โC Tgโ material.
Processing Advantages: A Fabricatorโs Perspective
A material is only as good as its manufacturability. One reason I often default to Isola 370HR is that it is incredibly “fabricator-friendly.” It uses standard FR-4 processing equipment and chemistries, which keeps yields high and costs predictable.
1. Drilling and Desmear
Because of its multifunctional resin, 370HR is more brittle and “crisp” than lower-grade epoxies. This actually helps during drilling; the debris consists of fine chips rather than a gummy smear. This results in cleaner hole walls and better plating adhesion.
Pro Tip: While it doesn’t require plasma desmear, a light plasma cycle before permanganate is a “best practice” for high-reliability Class 3 boards to ensure total reliability of the via-to-inner-layer connection.
2. Sequential Lamination
If you are designing complex boards with blind and buried vias, you need a material that can survive multiple “press cycles.” 370HR has proven performance in sequential lamination. It doesn’t lose its dimensional integrity after the second or third lamination, which is where many mid-range materials fail.
3. UV Blocking and AOI Compatibility
The resin system is designed to be UV-blocking and offers AOI (Automated Optical Inspection) fluorescence. For the shop floor, this means high throughput and fewer false errors during the inspection of internal layers.
Typical Market Applications
Where do we actually see 370HR? Itโs rarely in your basic TV remote, but itโs everywhere in “infrastructure” electronics.
Telecommunications: High-speed routers, switches, and base station controllers where 24/7 uptime is required.
Automotive: Engine control units (ECU), ADAS systems, and power electronics for EVs.
Medical: Portable imaging equipment and patient monitoring systems that require high durability.
Industrial: Power distribution systems and robotics that operate in high-temperature factory environments.
Aerospace/Defense: Flight control computers and radar subsystems where IPC Class 3 compliance is a must.
Designing the Stackup: 370HR vs. The World
When Iโm designing a board, I often have to justify the material cost to the procurement team. Here is how I position 370HR against other common ISOLA PCB materials:
vs. FR406: 370HR is the direct upgrade. It offers better Tgโ, better Tdโ, and significantly better CAF resistance. If your board has a lead-free finish (ENIG, HASL, OSP), go with 370HR.
vs. IS400: While similar, 370HR has a slightly higher Tgโ and better sequential lamination performance.
vs. FR408HR: If you need lower loss (Df < 0.010) for high-speed digital above 10 Gbps, FR408HR is the better choice. However, for most “general” high-reliability needs, 370HR is the more cost-effective workhorse.
Useful Resources for PCB Designers
To really master this material, you need access to the raw engineering data:
Isola 370HR Official Data Sheet: Download PDF
Processing Guide for Fabricators: Full Guidelines
IPC-4101 Standards: 370HR is qualified under slash sheets /98, /99, /101, and /126.
RayMing Isola Materials Center: Comprehensive look at ISOLA PCB manufacturing capabilities.
Frequently Asked Questions (FAQs)
1. Is Isola 370HR halogen-free?
Standard 370HR is not halogen-free. If your project has strict “green” requirements, you should look into Isolaโs TerraGreen or IS550H families. However, 370HR is fully RoHS compliant.
2. Can I use 370HR for 28 GHz 5G applications?
For 28 GHz, the Df of 0.0210 might be too high for long signal paths. You would experience significant insertion loss. For those frequencies, I recommend Astra MT77 or I-Tera MT40. 370HR is better suited for applications up to 5-10 GHz.
3. What is the shelf life of 370HR prepreg?
Isola recommends storing prepreg at <23โC and below 50% humidity. Under these conditions, it generally has a shelf life of 3 to 6 months. Always keep it in the original moisture-barrier bag (MBB) until use.
4. Why is it called “multifunctional” epoxy?
Standard FR-4 uses “di-functional” epoxy (two reactive groups per molecule). 370HR uses “multifunctional” epoxy (three or more reactive groups), which creates a denser, more thermally stable molecular network once cured.
5. Is 370HR compatible with laser drilling?
Yes. Its stable resin-to-glass ratio and consistent Dk make it an excellent candidate for laser-ablated microvias in HDI stackups.
Conclusion: The Engineerโs Safe Bet
In my decade of designing PCBs, Iโve learned that the “cheapest” material is often the most expensive once you account for field failures and assembly reworks. Isola 370HR provides the thermal “headroom” you need for modern lead-free assembly while offering the electrical stability required for todayโs telecommunications and automotive standards.
If you arenโt sure where to start with a high-reliability design, 370HR is almost always the safest starting point. It’s predictable, reliable, and every major fabricator in the world knows how to build with it.
Meta Description: Isola 370HR is the industry-leading 180ยฐC Tg FR-4 laminate. Learn about its thermal stability, CAF resistance, and electrical specs in our complete PCB engineerโs guide to high-reliability design.
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