Direct Answer
Use Isola 370HR when the board is still fundamentally an FR-4-class design, but the thermal and reliability requirements are too important for a vague "high-Tg FR-4" note. It is a specific laminate/prepreg system, so it gives purchasing, CAM, and reliability teams a clearer target than generic Tg170 language.
Specification Snapshot
| Property | 370HR value | Engineering note |
|---|---|---|
| Material family | FR-4-class epoxy laminate and prepreg | High-reliability, lead-free compatible system |
| Tg | 180 C | Higher than generic Tg170 class |
| Td | 340 C | Useful margin for lead-free processing and rework exposure |
| Dk | About 4.04 in Isola summary data | Use construction-specific data for impedance |
| Df | About 0.021 in Isola summary data | Standard-loss material, not low-loss RF laminate |
| Thermal conductivity | 0.4 W/m-K | Board heat spreading still depends mostly on copper |
| Moisture absorption | About 0.15 percent | Helps reliability assessment but is not the only humidity factor |
| IPC/UL references | IPC-4101 slash sheets and UL file E41625 listed by Isola | Confirm project-specific requirements with supplier documentation |
Why Engineers Choose 370HR
370HR solves a reliability problem, not an RF problem. When a multilayer PCB sees lead-free reflow, rework, thick copper, buried structures, or field thermal cycling, the laminate has to survive mechanical and chemical stress without delamination, barrel cracking, or excessive expansion.
The material gives engineers a named system with published thermal performance. That matters because "high-Tg FR-4" can hide major differences in Td, CTE, resin content, prepreg availability, and actual electrical behavior.
Where 370HR Fits Best
Use 370HR for:
- Industrial controls with long service life
- Telecom and networking boards that need stable multilayer reliability
- Medical or instrumentation electronics where traceability matters
- Multilayer lead-free boards with rework exposure
- Designs where customer qualification requires a named laminate family
Do not choose 370HR solely for microwave loss reduction. Its Df is in standard-loss FR-4 territory. If insertion loss, phase stability, or antenna tuning is the main problem, compare Rogers RO4003C or RO4350B instead.
Stackup and Fabrication Notes
For impedance-controlled designs, request a fabricator stackup based on actual 370HR cores and prepregs. The final Dk depends on construction, resin percentage, glass style, frequency, and pressed thickness. Use test coupons when tolerance matters.
For reliability, review aspect ratio, copper distribution, lamination cycles, soldering profile, and any planned rework. A better material does not compensate for excessive drill stress, resin starvation, poor copper balance, or unclear bake/storage controls.
Worked Example
A 10-layer industrial controller uses BGAs, two lead-free assembly passes, and functional test rework on some early builds. The board is not RF-critical, but field reliability matters. FR4 Tg170 may work, but 370HR provides a named 180 C Tg / 340 C Td material system that is easier to control across prototype and repeat production.
In the fabrication drawing, specify Isola 370HR or approved equivalent by written approval, not simply "high-Tg FR-4." That gives purchasing and CAM a clear substitution boundary.
Comparison
| Material | Best use | Main tradeoff |
|---|---|---|
| Isola 370HR | High-reliability FR-4-class multilayer builds | Standard-loss electrical behavior |
| FR4 Tg170 | Cost-controlled high-Tg boards | Broad category unless a material grade is named |
| RO4350B | RF boards needing UL 94 V-0 and stable Dk | Higher material cost |
| RO4003C | Cost-sensitive microwave boards | Not UL 94 V-0 rated |
| MEGTRON 6 | High-speed digital channels with low insertion loss | Specialty high-speed material |
Datasheet References
Summary
Isola 370HR is a reliability-driven material choice. It makes sense when the board needs a named FR-4-class system with stronger lead-free thermal margin than commodity or generic high-Tg FR-4. It does not replace low-loss RF laminates, and it still requires construction-specific stackup validation.