What Is FR2 PCB? A Complete Guide to Paper-Based Circuit Boards

If you’ve been in the PCB industry for any length of time, you’ve probably come across FR2 material at some point. While FR4 dominates most conversations about circuit board substrates, FR2 holds its own in specific applications where cost efficiency is the primary concern. After working with various PCB materials over the years, I’ve found that understanding when to use FR2 versus other substrates can make a significant difference in both project budgets and product performance.

The history of FR2 dates back to the 1950s and 1960s when it emerged as an enhanced PCB substrate, utilizing cotton paper reinforced with phenolic resin. During that era, FR2 expanded PCB capabilities to meet the needs of industrial controls and instrumentation, including early aerospace applications. While technology has evolved significantly since then, FR2 still serves valuable purposes in modern electronics manufacturing.

In this comprehensive guide, I’ll walk you through everything you need to know about FR2 PCBs—from material composition and electrical properties to practical design considerations and real-world applications. Whether you’re designing low-cost consumer electronics or evaluating substrate options for a new project, this article will give you the technical foundation to make informed decisions.

What is FR2 PCB?

FR2 stands for “Flame Retardant 2,” a NEMA designation for synthetic resin bonded paper used in printed circuit board manufacturing. The material consists of paper substrate (typically cotton paper or cellulose fibers) impregnated with plasticized phenol formaldehyde resin. This combination creates a rigid, flat laminate that provides adequate electrical insulation for many low-frequency applications.

The phenolic resin composition falls into two categories: resoles and novolacs. Both work effectively for PCB applications, offering moisture resistance and good adhesion to copper foil. When you see an FR2 PCB, you’re looking at a brownish board that’s noticeably different from the green glass-epoxy appearance of FR4.

One key point about FR2: it’s compliant with the UL94V-0 standard, which is the benchmark for determining the flammability of plastic materials in electronics. This flame-retardant property is essential for meeting safety regulations in consumer products.

FR2 Material Composition and Structure

Understanding what goes into an FR2 board helps explain its performance characteristics. The base structure is relatively straightforward compared to more advanced substrates.

Core Components

The substrate uses paper reinforcement—specifically cellulose or cotton paper fibers—soaked in phenol-formaldehyde resin. This phenolic compound provides rigidity and some degree of moisture resistance. The paper base makes FR2 significantly lighter than glass-reinforced alternatives like FR4, coming in at approximately 15% lighter at equivalent thicknesses.

Copper foil is laminated onto one or both sides of the substrate, typically at 35 µm (1 oz/ft²) standard weight. For applications requiring higher current capacity, 50 µm copper is available. The thin copper layer can be etched or milled away to create the circuit traces needed for your design.

Flame Retardant Properties

The “FR” designation indicates that brominated flame retardants are incorporated into the resin system. Standard FR2 grades typically meet UL-94 HB (horizontal burn) requirements, though V-0 rated phenolic grades are available for applications requiring enhanced fire resistance. This is particularly important for power supplies and any equipment used in environments with fire risk.

Read more about FR materils:

Key Technical Properties of FR2 PCB

Before specifying FR2 for any project, you need to understand its electrical, thermal, and mechanical characteristics. Here’s a breakdown of the critical specifications:

Property	FR2 Value	Notes
Dielectric Constant (Dk)	4.5 (typical)	Higher than FR4
Dissipation Factor	0.024 – 0.026	Signal loss factor
Dielectric Strength	~29 kV/mm	Good insulation
Glass Transition Temp (Tg)	105°C	Lower than FR4
Thermal Conductivity	~0.25 W/m·K	Limited heat spreading
Standard Thickness	1.2mm or 1.6mm	Thinner may warp
Copper Weight	35 µm (standard)	50 µm for higher current
Flammability Rating	UL94 HB / V-0	V-0 on request

A few things stand out from these specifications. The glass transition temperature of 105°C is notably lower than FR1’s 130°C and significantly below FR4’s typical 130-180°C range. This limits FR2’s suitability for lead-free soldering processes that typically require temperatures above 245°C. If you’re planning to use lead-free assembly, you’ll need to work carefully with your thermal profile.

FR2 vs FR4: A Direct Comparison

This is probably the comparison most engineers want to see. Both materials serve as PCB substrates, but they’re designed for very different use cases. Here’s how they stack up:

Characteristic	FR2	FR4
Base Material	Paper/Cotton + Phenolic	Woven Fiberglass + Epoxy
Cost	~60% of FR4 cost	Standard pricing
Layer Support	Single-sided primarily	Up to 32+ layers
Through-Hole	Not recommended (PTH)	Excellent
Mechanical Strength	Lower rigidity	High strength
Thermal Performance	Tg: 105°C	Tg: 130-180°C
Frequency Range	Low frequency (<30 MHz)	Up to GHz range
Moisture Resistance	Fair (higher absorption)	Excellent (~0.10%)
Manufacturing	Punch-friendly, easy to cut	Requires routing/drilling

The cost difference is significant—FR2 runs about 60% of FR4 pricing, making it attractive for high-volume, cost-sensitive products. However, that savings comes with trade-offs in performance and design flexibility. FR4 dominates the industry (over 90% of rigid PCBs) for good reason: it handles everything from single-layer to complex multilayer designs with excellent electrical properties.

Common Applications for FR2 PCB

FR2 PCBs work best in applications where cost is the primary driver and performance requirements are modest. Here’s where you’ll typically find them:

Consumer Electronics

• Remote controls and TV controllers
• Basic calculators and digital clocks
• Low-cost toys and electronic games
• Simple household appliances
• LED lighting fixtures
• Basic audio equipment

Industrial and Power Applications

• Simple power supplies and chargers
• Electrical connectors and switches
• Basic control panels
• Solar cell controllers (non-critical)
• Simple instrumentation

Prototyping and Education

FR2 is excellent for prototype work and educational purposes. It’s easy to solder, affordable enough for experiments, and works well for learning PCB assembly techniques. Many DIY electronics projects use FR2 boards because the material is forgiving and inexpensive to replace if something goes wrong.

Design Considerations for FR2 PCB

If you’re designing with FR2, there are several important constraints to keep in mind. Ignoring these can lead to manufacturing problems or field failures.

Layout Guidelines

1. Stick to single-layer designs: FR2 isn’t suitable for plated through-holes, so multilayer designs are essentially out. Plan your routing accordingly.
2. Use adequate pad sizes: Minimum 0.30mm for pads and annular rings. Square pads can help resist peel-off during thermal stress.
3. Design for punch manufacturing: FR2’s paper base makes it excellent for die-cutting, so take advantage of this for cost-effective outline and slot creation.
4. Allow for thermal relief: Use 4-spoke thermal relief patterns with spoke widths ≥0.30mm to prevent cold solder joints.
5. Maintain proper clearances: For 250V basic insulation, maintain 2.5mm creepage/clearance per IEC 60335-1.

Thermal Management

FR2’s thermal conductivity (0.25 W/m·K) limits heat spreading significantly compared to FR4 or metal-core alternatives. For components like TO-220 packages that generate heat, consider adding thick copper jumpers or external aluminum heat slugs. The board itself won’t dissipate heat effectively, so external thermal management is essential for any power application.

When designing power circuits on FR2, pay special attention to trace widths for current-carrying conductors. The general rule is to add 2mm of width per amp for traces carrying more than 2A. Wider traces help distribute heat more effectively across the limited thermal capacity of the paper substrate. Additionally, consider placing high-power components near the board edges where heat can dissipate more easily into the surrounding air.

Soldering and Component Mounting

This is critical: FR2 cannot withstand lead-free reflow temperatures (≥245°C). If you must use lead-free solder, stick to Sn-Pb wave or selective-solder profiles under 245°C. Heavy BGAs should be avoided entirely. The paper substrate will delaminate or blister under excessive thermal stress.

You have two primary mounting technology options for FR2 PCBs: Surface Mount Technology (SMT) and Through-Hole Mounting (THM). SMT works well for compact designs with surface mount devices, though you need to manage thermal profiles carefully. Through-hole components can be used, but remember that plated through-holes aren’t reliable on FR2. For through-hole work, you’re essentially soldering to pads on one side only, which affects mechanical strength of the solder joint.

Advantages and Limitations of FR2 PCB

Advantages

Advantage	Benefit
Low Cost	~40% savings vs FR4 for high-volume production
Easy Machining	Punch-friendly, burr-free edges, longer tool life
Lightweight	15% lighter than FR4 at same thickness
Good Insulation	Adequate dielectric strength for low-voltage applications
RoHS Compliant	Meets environmental regulations

Limitations

Limitation	Impact
Single-Layer Only	Cannot create complex multilayer designs
No PTH Support	Use wire jumpers, rivets, or solder bridges for crossovers
Lower Thermal Rating	Cannot withstand lead-free reflow (>245°C)
Higher Moisture Absorption	Not suitable for high-humidity environments
Frequency Limitations	Not recommended above ~30 MHz
Mechanical Weakness	Can crack under vibration; avoid automotive use

FR2 PCB Manufacturing Process

Understanding how FR2 boards are manufactured helps explain some of their characteristics and limitations.

Production Steps

1. Substrate Preparation: Paper substrate is impregnated with phenolic resin and cured to form the base laminate.
2. Copper Lamination: Copper foil is bonded to one or both sides under heat and pressure.
3. Circuit Imaging: Photoresist is applied and exposed to transfer the circuit pattern.
4. Etching: Unwanted copper is removed chemically to leave the circuit traces.
5. Surface Finish: HASL (Hot Air Solder Leveling) is commonly applied for solderable surfaces.
6. Punching/Cutting: Steel-rule punch dies cut board outlines, slots, and mounting holes.
7. Testing: Electrical testing verifies continuity and isolation.

The punching capability is a significant advantage for FR2. Unlike FR4, which requires routing or drilling for complex outlines, FR2 can be punched with steel-rule dies, dramatically reducing per-piece costs in high-volume production. This is one reason why FR2 remains popular for disposable consumer electronics despite its technical limitations.

Industry Standards and Compliance

When specifying FR2 for production, you should reference appropriate industry standards:

• NEMA LI 1-1998: Defines FR-2 material properties and designations
• IPC-4101 (/21 & /22): Laminate specifications for paper-reinforced phenolic materials
• UL 94: Flammability testing (HB or V-0 ratings)
• IPC-A-600: Acceptability criteria for printed boards
• RoHS/REACH: Environmental compliance for hazardous substances

Useful Resources and References

For further reading and technical specifications, these resources are valuable:

• IPC Standards: www.ipc.org – Official IPC standards documentation
• NEMA Standards: www.nema.org – NEMA LI 1 material specifications
• UL Standards: www.ul.com – UL 94 flammability certification
• Wikipedia FR-2 Entry: en.wikipedia.org/wiki/FR-2 – General overview
• Altium Designer Resources: resources.altium.com – PCB design software documentation
• PCB Material Databases: Manufacturer datasheets for specific FR2 laminate grades

Frequently Asked Questions About FR2 PCB

Can FR2 PCB be used for multilayer boards?

No, FR2 is not suitable for multilayer PCB construction. The material doesn’t support plated through-holes (PTH) reliably, which are essential for connecting layers in multilayer designs. FR2 works best for single-sided boards. For double-sided applications, you’ll need to use wire jumpers, rivets, or solder bridges for crossovers instead of traditional vias. If your design requires multiple layers, FR4 or similar glass-reinforced materials are the appropriate choice.

Is FR2 PCB compatible with lead-free soldering?

FR2 has limited compatibility with lead-free soldering processes. The material’s glass transition temperature of 105°C and its paper-based construction cannot withstand the typical lead-free reflow temperatures of 245°C and above. If you must use lead-free solder, wave soldering or selective soldering with profiles under 245°C can work, but you need to be careful with thermal management. Heavy components like BGAs should be avoided. For full lead-free compatibility, consider FR4 with higher Tg ratings.

What is the maximum operating frequency for FR2 PCB?

FR2 is recommended for low-frequency applications, typically below 30 MHz. The material’s higher dielectric constant (around 4.5) and dissipation factor cause greater signal loss at higher frequencies compared to FR4. For RF applications or high-speed digital circuits, the signal integrity issues become significant. If your application involves frequencies above 30 MHz, you should use FR4 or specialized high-frequency laminates like Rogers materials.

How much cheaper is FR2 compared to FR4?

FR2 typically costs about 60% of FR4 pricing, representing roughly 40% savings on material costs. However, the total cost picture depends on your production volume and manufacturing processes. FR2’s punch-friendly nature can add significant savings in high-volume production where die-cutting is faster than routing. But the limitation to single-layer designs and potential compatibility issues with modern assembly processes may offset some savings. For simple, high-volume products like remote controls or basic toys, FR2 offers meaningful cost advantages.

When should I choose FR2 over FR4 for my project?

Choose FR2 when your project meets these criteria: cost is the primary concern, the circuit is simple enough for single-layer design, operating frequency is below 30 MHz, power requirements are low to moderate, the product won’t be exposed to high humidity or extreme temperatures, and Sn-Pb soldering processes are acceptable. FR2 is ideal for disposable consumer electronics, educational projects, prototypes, and high-volume products where the savings justify the technical constraints. If any of these conditions aren’t met, FR4 is likely the better choice.

Conclusion

FR2 PCB occupies a specific niche in the electronics industry—one where cost efficiency outweighs performance requirements. For the right applications, it’s an excellent choice that can significantly reduce production costs while still delivering reliable functionality. The key is understanding its limitations and designing within them.

As a PCB engineer, I’ve used FR2 successfully in numerous cost-sensitive projects where the technical constraints weren’t limiting factors. Remote controls, simple LED controllers, basic power supplies—these are all applications where FR2 makes sense. But I’ve also seen projects fail when designers tried to push FR2 beyond its capabilities, particularly when attempting multilayer designs or high-frequency applications.

The material’s historical significance shouldn’t be overlooked either. FR2 helped enable the consumer electronics revolution of the latter 20th century, making affordable calculators, radios, and toys possible for mass markets. Even today, billions of FR2 boards are manufactured annually for products where cost-effectiveness is paramount.

The bottom line: know your requirements, understand the material’s properties, and make informed decisions. FR2 won’t replace FR4 as the industry standard, but it has its place. When the application fits, it’s a cost-effective solution that gets the job done reliably. Consider FR2 when designing high-volume, cost-sensitive products with simple circuit requirements, and you may find it’s exactly the right tool for the job.