FR-4
Flame Retardant grade 4
FR-4 is a flame-retardant woven glass-reinforced epoxy laminate. It is the default base material for most rigid PCBs, balancing mechanical strength, electrical insulation, thermal performance, and cost. Variants exist for high-Tg, halogen-free, and high-speed applications.
What it is
FR-4 is a composite of woven fibreglass cloth impregnated with epoxy resin. The "FR" stands for flame retardant — FR-4 self-extinguishes when ignited, meeting the UL94 V-0 flammability rating required for most electronic equipment. The "4" refers to grade 4 in the NEMA classification system.
Key properties of a standard FR-4 laminate include a glass transition temperature (Tg) of around 130-140°C, a dielectric constant near 4.5, and good mechanical rigidity. The material is supplied as cores (fully cured laminate with copper foil on one or both sides) and prepregs (B-stage uncured resin used to bond layers in multilayer construction).
Standard FR-4 is sufficient for most commercial electronics, but specialised variants address specific needs: high-Tg FR-4 (Tg 170°C and above) for lead-free assembly and thermally demanding applications; halogen-free FR-4 to meet environmental specifications; low-loss FR-4 variants for higher-frequency designs where signal integrity matters. Specific Tg, dielectric constant, and loss tangent values vary between manufacturers — consult the datasheet for any critical application.
When it matters
FR-4 selection affects assembly compatibility, signal integrity, and long-term reliability. Standard Tg-140 FR-4 can struggle with lead-free reflow temperatures (peak around 245-260°C) over multiple cycles, leading to delamination — high-Tg FR-4 is the safer choice for any lead-free assembly. For high-speed digital (above roughly 1 GHz) or RF designs, standard FR-4 introduces noticeable losses and a low-loss laminate or alternative material such as Rogers is usually required. For typical low-frequency designs, standard FR-4 remains the most cost-effective choice.
At Nordic PCB
Standard FR-4 is included in our default quote. If your design needs high-Tg, halogen-free, or a specific low-loss laminate, note it in the RFQ — our certified suppliers stock the most common variants and can quote alternatives such as Rogers or Isola on request. Material choice is also reviewed during DFM to flag combinations that could affect manufacturability.
Related terms
- Stack-up
A PCB stack-up is the cross-sectional arrangement of copper layers, dielectric materials, and bonding films that make up a multilayer board. It defines layer thickness, copper weight, dielectric properties, and is critical for impedance control, signal integrity, and manufacturability.
- Multilayer PCB
A multilayer PCB has more than two copper layers separated by dielectric material and bonded together under heat and pressure. Common configurations are 4, 6, 8, 10, and 12 layers. Multilayer designs enable higher routing density, dedicated power and ground planes, and better signal integrity for complex circuits.
- DFM
DFM (Design for Manufacturability) is a structured review of your PCB design against a fabricator's process limits — trace widths, drill sizes, annular rings, solder mask clearances, and stack-up choices — to catch issues before tooling starts. A good DFM review prevents rework, scrap, and missed delivery dates.
- Impedance control
Impedance control is the practice of specifying and manufacturing PCB traces so their characteristic impedance meets a precise target — typically 50 Ω single-ended or 100 Ω differential. Required for high-speed digital, RF, and signal integrity-critical designs, it depends on trace width, dielectric thickness, copper weight, and material properties.