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FR-2 (Flame Resistant 2) is a NEMA designation for synthetic resin bonded paper, a composite material made of paper impregnated with a plasticized phenol formaldehyde resin, used in the manufacture of printed circuit boards. Its main properties are similar to NEMA grade XXXP (MIL-P-3115) material, and can be substituted for the latter in many applications.

Applications

[edit]

FR-2 sheet with copper foil lamination on one or both sides is widely used to build low-end consumer electronics equipment. While its electrical and mechanical properties are inferior to those of epoxy-bonded fiberglass, FR-4, it is significantly cheaper. It is not suitable for devices installed in vehicles, as continuous vibration can make cracks propagate, causing hairline fractures in copper circuit traces. Without copper foil lamination, FR-2 is sometimes used for simple structural shapes and electrical insulation.

Properties

[edit]
Property Value
Dielectric constant, or relative permittivity 4.5 @ 1 MHz
Dissipation factor 0.024–0.26 @ 1 MHz
Dielectric strength 29 kV/mm (740 V/thou)

Fabrication

[edit]

FR-2 can be machined by drilling, sawing, milling and hot punching. Cold punching and shearing are not recommended, as they leave a ragged edge and tend to cause cracking. Tools made of high-speed steel can be used, although tungsten carbide tooling is preferred for high volume production.

Adequate ventilation or respiration protection are mandatory during high-speed machining, as it gives off toxic vapors.

Trade names and synonyms

[edit]
  • Carta[1]
  • Haefelyt
  • Lamitex
  • Paxolin, Paxoline
  • Pertinax,[1] taken over by Lamitec[2] and Dr. Dietrich Müller GmbH in 2014[3]
  • Getinax (in the Ex-USSR)
  • Phenolic paper
  • Preßzell[1]
  • Repelit[1]
  • Synthetic resin bonded paper (SRBP)
  • Turbonit[1]
  • Veroboard
  • Wahnerit[1]

See also

[edit]

References

[edit]

Further reading

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

FR-2

View on Grokipedia
from Grokipedia
FR-2 is a NEMA-designated grade of flame-retardant bonded , a composed of impregnated with a plasticized phenolic to achieve fire-retardant properties compliant with UL94 HB standards. This electrical-grade laminate is characterized by its cold-punchability, which allows for easy fabrication of intricate shapes at , and offers a temperature (Tg) of approximately 105°C, a dielectric constant of around 4.5, and good electrical insulation with a of 0.024–0.26. Primarily utilized as a low-cost substrate for single- and double-layer printed circuit boards (PCBs) in , such as toys, calculators, and basic appliances, FR-2 provides adequate mechanical strength and fair water resistance but is limited in chemical resistance and not suitable for multilayer, high-frequency, or high-temperature applications exceeding its thermal limits. Compared to other NEMA flame-retardant grades, FR-2 differs from FR-1 by including additional , enhancing punchability and slightly improving water resistance while having a lower Tg of 105°C compared to FR-1's 130°C, making it preferable for electrical rather than mechanical uses. In contrast to , which uses woven glass fabric reinforced with resin for superior mechanical and thermal performance (Tg 140–170°C), FR-2's paper base results in lower rigidity, reduced (typically 400–600 V/mil versus FR-4's 800–900 V/mil), and greater susceptibility to moisture absorption, positioning it as an economical choice for non-critical, volume-production PCBs rather than demanding environments like or . FR-3, another paper-based variant, incorporates resin for better heat resistance but is less common than FR-2 in modern low-end applications. The material's development aligns with NEMA standards for industrial laminates (LI 1), emphasizing its role in cost-sensitive manufacturing since the mid-20th century, though its usage has declined with the rise of more robust alternatives like due to evolving performance requirements in compact, high-density devices. Despite these shifts, FR-2 remains relevant in prototyping, educational projects, and disposable where affordability outweighs longevity.

Overview

Composition

FR-2 is a NEMA-standard grade of bonded , defined as a consisting of fibers impregnated with plasticized . The base is typically derived from cotton linters, alpha , or wood-pulp kraft variants, such as bleached or unbleached saturation kraft, selected based on the intended thickness and application. This material features a layered structure formed by stacking multiple plies of the impregnated paper and consolidating them under heat and high pressure to create rigid sheets, usually ranging from 0.5 to 3.2 mm in thickness. The phenol formaldehyde (phenolic) resin serves as the primary binder, incorporating plasticizers to enhance flexibility and machinability, particularly for punching operations. Flame-retardant additives are integrated into the formulation to achieve a V-1 rating, distinguishing FR-2 from non-retardant phenolic grades. The provides the primary binding for the to ensure structural integrity while maintaining electrical insulation properties.

Historical Development

FR-2, a flame-retardant grade of phenolic laminate, emerged in the as an economical alternative to earlier bakelite-based materials for electrical insulation in wartime applications. Phenolic resins, first commercialized in 1910, saw widespread adoption during for producing durable, low-cost components in radios, proximity fuzes, and other military electronics, where provided sufficient mechanical strength and electrical properties at a fraction of the cost of more advanced composites. By the 1950s, post-war demand for consumer and industrial electronics drove refinements in these materials, leading to the development of flame-retardant variants to address safety concerns in enclosed devices. formalized the FR-2 designation in the early 1960s as part of its industrial laminating standards (LI 1), distinguishing it from non-flame-retardant grades like XXXP and alongside FR-1 for similar paper-phenolic constructions with varying punchability. This standardization facilitated consistent production for emerging (PCB) applications, where FR-2's cold-punchable properties suited single-sided boards. FR-2 reached peak usage in the 1970s and 1980s, powering the boom in affordable such as televisions, radios, and early personal computers, thanks to its low material cost—often under $1 per —and ease of fabrication in high-volume manufacturing. Key milestones included Underwriters Laboratories (UL) recognition under the flammability standard in the mid-1970s, enabling broader commercial certification, and integration into IPC Association standards in the late , such as IPC-4101 for base materials, which specified FR-2 for general-purpose PCBs. However, its limitations in moisture resistance and thermal stability contributed to a decline starting in the , as glass-epoxy laminates gained dominance for higher-performance requirements in and . As of 2025, FR-2 persists in niche roles within developing markets and prototyping, where cost constraints outweigh demands for advanced durability, supporting low-end consumer goods and educational electronics in regions like and .

Properties

Mechanical Properties

FR-2, a flame-retardant grade of paper-based phenolic laminate, demonstrates anisotropic mechanical properties due to its layered construction, with higher values typically observed along the grain direction. The material's tensile strength ranges from 70-110 MPa along the grain and 40-60 MPa across the grain, as determined by standard testing procedures outlined in ASTM D638. These values indicate moderate load-bearing capacity suitable for low-stress applications, though the lower cross-grain strength highlights the importance of orientation in . Flexural strength for FR-2 is reported at 100-210 MPa, accompanied by a modulus of elasticity of 5-7 GPa, reflecting its ability to resist forces without permanent deformation under typical operating conditions. This combination provides adequate rigidity for structural support in , but the material's overall is less than that of glass-reinforced alternatives. Impact resistance is low, with impact values ranging from 40-110 J/m, making FR-2 prone to under sudden shock loads or drops. The of FR-2 falls between 1.3-1.4 g/cm³, contributing to its lightweight nature while maintaining sufficient mass for stability. Water absorption after 24-hour immersion, measured per ASTM D570, is 2-5%, which can affect dimensional stability in humid environments if not properly sealed. In terms of , FR-2 is easily drilled and punched, facilitating cost-effective fabrication for prototyping, though unsealed edges are susceptible to during handling or exposure to . These properties collectively position FR-2 as a durable yet economical choice for non-critical mechanical demands, with brief consideration for thermal stability limits in prolonged use.

Electrical and Thermal Properties

FR-2, a flame-retardant phenolic paper laminate, demonstrates reliable electrical insulation properties suitable for low-voltage, single-sided printed circuit boards. Its dielectric strength, measured perpendicular to the laminate per ASTM D149, ranges from 15-25 kV/mm (400-650 V/mil, short time), enabling it to withstand moderate electrical stresses without breakdown. Volume resistivity exceeds 10^8 ohm-cm at 50% relative humidity, while surface resistivity surpasses 10^6 ohms, ensuring effective prevention of current leakage under typical operating conditions. The dielectric constant of FR-2 is 4.5-5.5 at 1 MHz, paired with a dissipation factor of 0.024-0.26, which supports signal integrity in low-frequency applications while minimizing energy loss. Thermally, FR-2 supports continuous use up to 105-130°C as defined by NEMA FR-2 grade specifications, making it appropriate for environments with limited heat exposure. The temperature (Tg) falls around 120-140°C, marking the point where the material shifts from a rigid to a more compliant state, influencing its stability during or operational heating. The coefficient of measures 15-20 ppm/°C in the in-plane (x-y) direction, with higher values perpendicularly (z-axis), which helps mitigate warpage in simple board designs but requires consideration for multilayer compatibility. These properties collectively position FR-2 as a cost-effective option where extreme thermal cycling or high-speed signaling is not required.

Fabrication

Production Methods

The production of FR-2, a flame-retardant phenolic laminate, begins with the preparation of base paper sheets, typically high-quality , which serves as the reinforcing substrate due to its strength and absorbency. These sheets may also incorporate in some formulations for enhanced properties, and they are pre-treated if necessary to optimize uptake. The paper is then treated with a phenolic formulated for flame retardancy, often through dipping in a resin bath or coating to ensure even application. Following initial treatment, the impregnation process saturates the with the , sometimes under to remove air pockets and achieve uniform penetration. The -saturated undergoes partial curing, known as B-staging, in controlled ovens to advance the to a tack-free, semi-cured state while evaporating solvents; this typically occurs at temperatures around 130°C for several minutes, resulting in a resin content of 50-60% by weight. This B-stage material is cut to size and stored under controlled conditions to prevent full curing until . Lamination forms the core of FR-2 production, where 5-20 layers of the B-staged impregnated sheets are stacked to achieve the desired thickness, with copper foil added to one or both sides for clad variants used in PCB bases. The stack is placed between steel plates in a and subjected to heat and pressure—typically 150°C and 5-10 MPa (approximately 0.5-0.75 tons per square inch)—for 30-60 minutes to fully the and bond the layers into a rigid sheet. Unclad FR-2 omits the copper foil, producing a plain laminate sheet for non-conductive applications. After pressing, the laminate undergoes post-curing through controlled heating and gradual cooling to complete cross-linking and minimize internal stresses, ensuring final thickness uniformity within ±0.1 mm. The resulting sheets are trimmed, inspected for defects, and conditioned to meet mechanical specifications, such as punchability for electrical uses. Variations in production account for single- or double-sided cladding, which integrates the foil directly during lamination for direct PCB substrate use, while unclad processes focus on bulk insulating material.

Standards Compliance

FR-2 laminates are defined under the ANSI/NEMA IM 60000-2021 standard (superseding NEMA LI 1-1998) as a grade of industrial thermoset laminate composed of cellulose paper reinforcement impregnated with a flame-retardant phenolic resin, suitable for electrical applications with a maximum continuous operating temperature of 105°C (for thicknesses >0.062 in) or 75°C (for <0.062 in). This standard outlines requirements for manufacturing, physical properties, and performance testing of such materials in sheet form, ensuring uniformity and reliability for use in electrical insulation. FR-2 achieves the V-1 flammability rating in the vertical burn test, where specimens must self-extinguish within 30 seconds after two 10-second flame applications, with no flaming drips allowed to ignite below. This certification verifies the material's ability to limit fire propagation in electronic devices, distinguishing it from non-retardant grades. The IPC-4101 specification designates FR-2 under subclass /03 for flame-retardant paper-based laminates, requiring minimum peel strength exceeding 9 lb/in (1.6 N/cm) after to ensure robust adhesion in circuit boards. Additionally, it mandates passing solder float tests at 260°C for 20 seconds without or blistering, confirming thermal stability during assembly processes. Since the implementation of the RoHS Directive in 2006, FR-2 formulations have been adapted to comply with restrictions on hazardous substances, limiting lead and other to below 0.1% by weight in the resin system to minimize environmental impact. Internationally, FR-2 corresponds to grade PF CP 308 in the IEC 60893 series for industrial rigid laminated sheets based on thermosetting resins, providing equivalent specifications for phenolic paper composites in electrical insulation applications.

Applications

In Printed Circuit Boards

FR-2 serves as a low-cost substrate material primarily for single-sided and double-sided printed circuit boards (PCBs) in , where simplicity and affordability are prioritized over advanced performance. It is commonly employed in devices such as remote controls, calculators, clocks, toy , LED lamps, low-end , and disposable gadgets, enabling basic circuit functionality without the need for high-density interconnects. This material exhibits strong compatibility with through-hole components, facilitating straightforward and assembly for low-complexity designs. Typical FR-2 boards are produced in sizes ranging from small prototypes (e.g., 70 × 90 mm) to larger panels up to approximately 300 × 400 mm, suitable for batch manufacturing of consumer products. In PCB fabrication, FR-2 undergoes standard processing steps, including to define traces via photolithographic methods and mechanical for component holes. Drilling requires controlled feed rates to prevent cracking in the brittle phenolic-paper composite, ensuring reliable hole quality for through-hole mounting. Its electrical properties provide adequate basic insulation for low-voltage applications, as detailed in the electrical and thermal properties section. However, FR-2 has notable limitations in PCB design, including unsuitability for multilayer or high-density boards due to its structural constraints and higher dielectric losses, which restrict its use to low to moderate frequencies, typically below 30 MHz, due to its moderate dielectric constant and elevated moisture absorption, making it inappropriate for high-speed requirements. FR-2 remains in use in low-cost segments within emerging economies where budget constraints dominate over performance demands, though its overall market share is small.

In Prototyping and Low-Cost Electronics

FR-2 phenolic laminate finds significant application in prototyping environments, particularly through its use in perfboards and stripboards, which enable hobbyists and beginners to assemble circuits with basic skills rather than advanced fabrication techniques. These pre-perforated boards, featuring a grid of holes spaced at 0.1 inches (2.54 mm), allow components to be directly soldered into place, facilitating the transition from conceptual sketches to functional prototypes without the need for chemical or professional equipment. Key advantages of FR-2 in this context include its ease of cutting with hand tools, such as utility knives for severing tracks on stripboards, and its very low cost for raw sheets. This affordability, combined with widespread availability in standard pre-perforated formats, makes it ideal for budget-conscious DIY projects and small-scale experimentation. Its mechanical ease of machining further supports manual modifications, allowing users to trim boards or create custom shapes without specialized machinery. Examples of FR-2 applications include simple sensor boards for environmental monitoring and custom prototyping for Arduino-compatible shields in hobbyist electronics, as well as repairs on vintage devices from the 1980s that originally utilized phenolic laminates. In repair scenarios, FR-2 sheets can replace damaged sections or extend original boards, preserving the low-cost, single-sided nature of era-specific designs. Prototyping workflows with FR-2 often involve manual routing of traces using knives to isolate conductive strips or, for small runs, CNC milling to define custom copper patterns on copper-clad sheets. These methods suit low-power circuits, with FR-2 typically supporting voltages up to 12 V and currents around 1 A per trace, depending on copper thickness and layout. In educational settings, FR-2 perfboards have played a role in STEM kits since the 2000s, providing accessible platforms for students to learn circuitry fundamentals through exercises and basic project assembly. This hands-on approach fosters understanding of electrical connections and component integration without the complexities of multilayer boards.

Comparisons

With FR-1

FR-1 and FR-2 are closely related grades of paper-based phenolic laminates standardized under NEMA LI 1, both incorporating flame-retardant additives for electrical insulation and early PCB applications, but they differ in processing, performance, and suitability. FR-1 serves as a mechanical-grade material optimized for hot punching at elevated temperatures, while FR-2 is an electrical-grade variant designed for cold punching at ambient conditions, enabling higher production efficiency for intricate components. Both utilize a base of linter or alpha paper impregnated with phenolic , but FR-2 includes an additional to enhance formability without heat, whereas FR-1's formulation prioritizes rigidity for mechanical applications. In terms of flame retardancy, both FR-1 and FR-2 achieve a UL 94 V-1 rating, providing self-extinguishing properties suitable for enclosed electronics. This stems from retardant additives enhancing char formation and reducing flame propagation. Thermal performance is similar, with both rated for continuous use up to approximately 130°C (Tg), though FR-2 may have a practical limit of 105°C for thinner sheets (<0.062 in). FR-1 remains cheaper due to its simpler processing and lower material demands, but it is less common today, though still used in low-cost single-layer boards where fire risks are managed. FR-2 persists in viable applications for low-cost, single-layer boards in prototyping and non-critical consumer goods. Property comparisons reveal similar (typically 500-750 V/mil perpendicular to laminate for both). The shift from non-flame-retardant phenolics like NEMA XPC to FR grades such as FR-1 and FR-2 was accelerated by 1970s safety mandates. FR-3, a related paper-based variant using , offers better heat resistance (Tg 140-170°C) than FR-2 but is less common in modern low-end applications.

With FR-4

FR-2, composed of phenolic reinforced with , contrasts fundamentally with , which uses reinforced with woven cloth, resulting in FR-4's greater mechanical rigidity due to its higher tensile strength of 300-400 MPa compared to FR-2's 50-80 MPa. This structural difference makes FR-4 more suitable for demanding structural integrity in PCBs, while FR-2 offers adequate strength for basic designs but is prone to deformation under stress. In terms of applications, FR-4 supports high-frequency and multilayer PCBs operating up to GHz ranges with a of 130-180°C, enabling reliable performance in complex , whereas FR-2 is restricted to low-end, single-layer boards due to its Tg around 130°C and limited thermal stability. FR-2 also exhibits poorer heat resistance, often leading to during assembly processes that exceed 250°C, unlike FR-4's robustness in lead-free . FR-2's cost is approximately 20-30% that of , making it attractive for budget-constrained projects, though holds about 56.5% of the PCB laminate as of 2024 due to its versatility and performance advantages. Despite FR-2's higher absorption of 1-4% versus FR-4's 0.1-0.5%, which can degrade electrical properties in humid environments, FR-2 remains viable for high-volume production of simple, non-critical circuits in cost-sensitive regions like consumer toys or basic appliances.

Environmental Aspects

Flame Retardancy Mechanisms

FR-2 achieves flame retardancy primarily through the inherent properties of its phenolic resin matrix, which decomposes upon heating to form a carbon-rich char residue. This char serves as a and oxidative barrier, insulating the underlying material and limiting the supply of combustible volatiles to the . The char-forming process involves the breakdown of rings in the resin, accompanied by the release of non-flammable gases such as , CO, and CO₂, resulting in high char yields of up to 33 wt% under pyrolytic conditions. In UL 94 vertical burn testing, FR-2 materials self-extinguish rapidly due to char formation, achieving the V-1 classification. This involves intumescent swelling of the char layer into a foam-like barrier that further shields the material from oxygen and heat, with afterglow persisting for less than 30 seconds after flame removal. The peak heat release rate during combustion remains low, typically in the range of 100-200 kW/m² under standard cone calorimeter conditions (50 kW/m² incident flux), significantly lower than non-flame-retardant paper-phenolic composites which exceed 1000 kW/m². FR-2's flame retardancy effectiveness diminishes above its temperature of approximately 130°C. Unlike modern systems based on ammonium polyphosphate, FR-2 does not exhibit significant expansion of the char layer, relying instead on dense residue formation without the volumetric foaming that enhances barrier properties in advanced alternatives.

and Disposal

The production of FR-2 laminates involves significant environmental considerations, particularly in the upstream processes of paper manufacturing and resin synthesis. , the primary reinforcement material in FR-2, requires substantial inputs during pulping and bleaching, with consumption ranging from 32.7 to 36.8 cubic meters of freshwater per ton of pulp depending on the bleaching method used. Additionally, the curing of phenolic resins emits volatile organic compounds (VOCs) such as , , and phenol, with U.S. regulations mandating a 51% reduction in organic hazardous air pollutant emissions from existing facilities, equivalent to 361 tons per year across major sources. At end-of-life, FR-2's cross-linked phenolic polymers render it non-biodegradable, contributing to persistent waste in landfills or incinerators where secondary pollution from residual additives can occur. is constrained by the material's heterogeneity, with mechanical methods like grinding limited to recovering low-value fillers; overall recovery rates for such composite laminates remain below 10% globally due to economic and technical barriers. offers a more disposal alternative, decomposing non-metallic fractions into recoverable phases while minimizing toxic emissions compared to landfilling. Regulatory frameworks address these challenges by promoting responsible handling and phase-out of harmful additives. The EU's Waste Electrical and Electronic Equipment (WEEE) Directive mandates separation and proper treatment of -containing circuit boards from general e-waste to prevent environmental release of materials. Industry trends favor transitioning from to alternatives like (epoxy-glass) or CEM-1 (paper-epoxy) for enhanced recyclability, as these materials benefit from more established recovery infrastructures despite similar thermoset challenges. 's typically halogen-free composition reduces risks from persistent organic pollutants compared to halogenated alternatives. Ongoing research mitigates these impacts through bio-based phenolic resins derived from or , which as of 2025 show potential for 30% reductions in environmental load by lowering content and fossil-derived inputs in formulations suitable for FR-2-like laminates.

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