# FR-4 glass-epoxy laminate

Specifying FR-4 industrial sheet and PCB materials by named grade, construction, test conditions and processing requirements.

Reviewed: 2026-11-01

Tags: materials, composites, epoxy, electrical-insulation

Isola’s **FR406** construction data at **1 GHz** give **Dk/Df 3.77/0.0180** for a **1×1080, 58% resin, 0.0025-inch core** and **4.09/0.0150** for a **1×2113, 44% resin, 0.0030-inch core**. Use the selected construction’s data and the fabricator’s validated stackup for impedance design. [Isola FR406 construction data](https://www.isola-group.com/pcb-laminates-prepreg/fr406-laminate-and-prepreg/)

## Identify the material being purchased

Norplex-Micarta describes **NP510A** as woven glass fabric combined with an epoxy resin system, engineered to provide NEMA FR-4 properties. Its resin backbone contains bromine. The product page identifies industrial electrical and structural applications and an approximate glass-transition temperature of 130 °C. These are named-product statements; they do not establish every FR-4 formulation's composition or temperature capability. [Norplex-Micarta, NP510A](https://www.norplex-micarta.com/grades/np510a/)

Commercial naming can conceal important distinctions. Accurate Plastics separately lists **Epoxyglas G10/FR4** as FR-4, MIL-I-24768/27 Type GEE-F, and **G10 non FR** as G10, MIL-I-24768/2 Type GEE. A seller's combined G10/FR4 label is therefore not permission to erase a drawing's flame-retardancy or material-specification requirement. Preserve the manufacturer's product designation, required standard and edition, and delivered certification. [Accurate Plastics, Thermoset Product List](https://acculam.com/thermoset-product-list)

For a machined insulator, specify thickness and tolerance, sheet orientation, surface condition, hole and edge requirements, and whether copper cladding is prohibited. For a PCB, also specify glass construction, resin content, dielectric thickness, copper type/weight and stackup. A generic FR-4 entry cannot resolve all those procurement choices.

## Cured laminate and prepreg differ

Isola's **FR406 Processing Guide**, printed **Last Updated May 28, 2026**, distinguishes fully cured laminates from prepreg: glass fabric impregnated with partially polymerized resin. Prepreg is a bonding/process input whose resin flow, cure and moisture exposure affect lamination results. It should not be substituted for a cured sheet in a mechanical assembly.

The guide advises controlled prepreg storage and equilibration in its packaging before opening to avoid condensation. It also says fabrication suggestions are starting points requiring adjustment for the actual board and equipment. Qualify the press cycle and resulting board rather than treating a published profile as a universal FR-4 cure recipe. [Isola, FR406 Processing Guide](https://www.isola-group.com/wp-content/uploads/data-sheets/fr406-laminate-and-prepreg_Processing_Guide_new.pdf)

## Read electrical values with their test basis

NP510A's **June 14, 2023** IEC technical bulletin explicitly labels its data **typical**, not specification values:

| Property | Typical result | Specimen and test basis |
| --- | ---: | --- |
| Breakdown voltage | 65 kV | 3.18 mm; Condition A; IEC 60893-2/6.1 |
| Electric strength | 25 kV/mm | 1.59 mm; Condition A; IEC 60893-2/6.1 |
| Vertical flammability | V-0 | 1.59 mm; Condition A; IEC 60893-2/7.2 |

Breakdown voltage in kV and electric strength in kV/mm are different reported quantities. Do not extrapolate one specimen's result linearly to arbitrary thicknesses or assume it establishes a working voltage. Preserve electrode arrangement, conditioning and applicable test requirements when comparing data. The same bulletin lists longitudinal/crosswise results separately for mechanical tests, reinforcing that sheet direction belongs in a property comparison. [Norplex-Micarta, NP510A IEC Technical Data Bulletin](https://www.norplex-micarta.com/wp-content/uploads/2017/07/TDB_NP510A_IEC.pdf)

Compare permittivity and loss at the operating frequency and confirm the construction with the fabricator.

## Thermal quantities answer different questions

Isola's FR406 datasheet, **revision C, last updated April 15, 2021**, labels its table typical values:

| Quantity | FR406 typical value and method |
| --- | --- |
| Glass-transition temperature, Tg | 170 °C by DSC; IPC-TM-650 2.4.25C |
| Decomposition temperature, Td | 300 °C at 5% mass loss by TGA; 2.4.24.6 |
| Relative thermal index, RTI | 130 °C; UL 796 |
| Z-axis CTE | 60 ppm/°C before Tg; 250 ppm/°C after Tg; 2.4.24C |
| X/Y-axis CTE | 13 ppm/°C before Tg; 2.4.24C |

Tg, decomposition and RTI are not interchangeable maximum-use temperatures. Their different values and test definitions must remain visible. A short soldering exposure likewise cannot establish continuous loaded service. The larger post-Tg Z expansion is relevant to a PCB stack's thermal assessment, but does not by itself predict plated-hole reliability in a particular design. [Isola, FR406 datasheet](https://www.isola-group.com/wp-content/uploads/data-sheets/fr406.pdf)

For dimensional assessment, identify direction, temperature interval, constraints and construction. Do not apply an in-plane coefficient to through-thickness growth or extend a pre-Tg coefficient across the transition without an appropriate material model.

## Flame classification is bounded

UL explains that UL 94 evaluates small specimens under controlled flame or radiant-heat conditions. A V classification addresses specified burning behavior; it is not a declaration that a material cannot burn or a substitute for required building-product fire tests. Check the actual named material, applicable thickness and end-product requirements rather than treating FR-4 as an assembly approval. UL's explanatory page is dated **July 15, 2019** and identifies its underlying newsletter material as **2007**. [UL Solutions, UL 94 certifications and limitations](https://code-authorities.ul.com/about/blog/understanding-ul-94-rating-certifications-and-limitations/)

## Fabrication, exposure and handling

Mechanical-property claims need the same discipline as electrical claims. Accurate Plastics' Epoxyglas sheet, marked **Rev 08.01.24**, lists typical flexural values separately for lengthwise and crosswise **0.125-inch, Condition A** specimens. This is not an allowable for a differently oriented, notched or hot part. Establish the load direction and qualify fastener-bearing regions, thin sections and machined edges against the actual application. [Accurate Plastics, Epoxyglas G10/FR4 datasheet](https://acculam.com/Portals/0/DS/Acculam-Epoxyglas-G10-FR4-Data-Sheet.pdf)

Norplex SDS **ILN0209**, covering NP510A among other epoxy-glass products, has a **last revision of June 24, 2025**; its September 23 print date is different. It distinguishes the solid product as shipped from dust generated by machining/punching and fumes from decomposition. It calls for local exhaust to control dust, protection against eye/skin contact and appropriate respiratory protection when exposure guidelines are exceeded. The SDS also identifies combustible airborne dust and hydrogen bromide/CO/CO₂ decomposition products. A flame-retardant grade does not remove the need for machining-dust controls. [Norplex-Micarta, SDS ILN0209](https://www.norplex-micarta.com/wp-content/uploads/2017/07/SDS_ILN0209.pdf)

For release, retain named grade, batch identity, full specification, dimensions, orientation and required test documentation. Verify moisture/chemical exposure, working voltage, creepage/clearance, mechanical loading and temperature at assembly level. Treat substitutions as comparisons of complete material and process requirements, rather than acceptance based on the letters FR-4 alone.

## Related reference

[Material selection through production](../grades-tempers-and-conditions.md)