FR4 PCB: Properties, Types & How to Choose
FR4 PCB is one of the most widely used PCB types as it offers an excellent balance of performance, cost-effectiveness, and reliability. However, FR-4 is not a single material with one definitive set of properties; it is a class of materials whose properties can differ among grades and manufacturers. This guide discusses the properties of FR-4, the various types of FR-4, the limitations of FR-4, and how to choose the right FR-4 material for a design.
What Is FR4 PCB?
FR-4 PCB is a printed circuit board (PCB) made using FR-4 laminate as the base material. The laminate is formed using woven glass fiber reinforcement in an epoxy resin system: the glass fiber provides dimensional stability and mechanical strength, and the epoxy resin provides electrical insulation and holds the structure together.
To manufacture an FR4 PCB, a copper foil is laminated to the FR-4 dielectric material to make copper-clad laminate (CCL). Then the copper layers are patterned to form the conductive traces, pads, and other circuit features. FR4 PCBs are used in a wide range of electronic applications due to their good combination of electrical, mechanical, thermal, and manufacturing characteristics.
What Does FR-4 Mean?
FR4 is an abbreviation that stands for “Flame Retardant 4.” FR-4 is a generic term for a family of flame-retardant glass-reinforced epoxy laminates (not necessarily the same formulation), and different FR-4 laminates may have different Tg, Dk, Df, thermal performance, and reliability properties. In design-critical applications, always refer to the specific laminate manufacturer’s datasheet, and do not assume that FR-4 has equal characteristics.
Properties and Specifications of FR4 PCB
Flame Retardancy
Most FR4 PCBs are qualified to UL 94 V-0 and are flame retardant, which means that they can self-extinguish to reduce the fire risk in end products. Depending on the grade of laminate (FR-4.0, FR-4.1 etc), the exact rating may vary and should be checked with the manufacturer.
Electrical Insulation
FR4 PCBs are feature with good electrical insulation, making them suitable for multilayer construction and a wide range of digital and analog circuits. The dielectric material helps limit leakage current and prevents unintended electrical connections between conductors.
Mechanical Strength
Thanks to the woven glass fiber reinforcement in the FR4 laminate, FR4 PCBs provide good strength and dimensional stability. This provides the boards with resistance to mechanical stresses during manufacture, assembly, installation, and normal operation.
Dielectric Properties
FR4 boards have moderate dielectric properties — Dk typically 4.0–4.8, Df around 0.02, depending on resin and glass content. These suit most general-purpose and mid-speed designs, though high-speed/RF applications often need lower, tighter-controlled Dk/Df.
Thermal Performance
FR4 PCBs can handle the temperatures of most conventional applications; however, there are several different temperature specifications that should be checked individually: Tg, Td, CTE, thermal conductivity, and operating temperature. If the thermal stress is high or if the boards are required to be assembled and disassembled multiple times with lead-free solder, a higher- performance FR4 grade should be considered.
The following is a chart that lists key parameters of FR4 printed circuit boards:
Further reading: A Comprehensive Guide to FR4 Thermal Conductivity
5 Common Types of FR-4 PCB Materials
Based on various thermal, electrical, mechanical, environmental, and reliability criteria, FR4 PCB materials can be divided into 5 types:
Standard-Tg FR-4 — used for general-purpose electronics, moderate operating temperature and thermal reliability. Chosen for its balance of performance, reliability, and affordability.
Mid-Tg FR-4 — higher thermal performance than standard-Tg; used in designs when improved thermal reliability is needed without special laminate.
High-Tg FR-4 — able to resist higher temperature and thermal stress. Usually used for automotive electronics, industrial equipment, power electronics, high-layer-count PCBs and products that have strict thermal requirements.
Halogen-Free FR-4 — keeps FR-4’s basic properties and meets environmental and material standards. Performance is dependent on the standard used; refer to the manufacturer’s certification.
High-CTI FR-4 — built for resistance to electrical tracking; considered for higher working voltages, tighter creepage requirements, or demanding electrical safety needs.
Limitations of FR4 Circuit Board
• Insulating Stability Constraints
• Impedance Instability at High Frequencies
• Signal Loss Considerations
FR4 PCBs have certain limitations that should be considered, especially in specific applications or environments:
• Insulating Stability Constraints
One of the drawbacks of FR4 is its limited operating range when exposed to excessive power, voltage, or heat. Although FR4 acts as an electrical insulator between copper layers, its dielectric properties can degrade if the operating limits are exceeded. In scenarios where high temperatures are involved, such as aerospace applications, FR4 PCBs may not be the ideal choice as the insulation can deteriorate, leading to potential electrical conduction.
• Impedance Instability at High Frequencies
Another limitation of FR4 is its inability to maintain stable impedance in high-frequency designs. The dielectric constant (DK) of FR4 can vary across the board’s length and width, and it can also be affected by temperature changes. These variations can impact signal integrity, making FR4 less suitable for high-frequency applications.
• Signal Loss Considerations
FR4 PCBs have a relatively high dissipation factor (Df), which increases as the frequency increases. A higher Df translates to greater overall signal loss. While a certain degree of signal loss may be acceptable in non-high-frequency situations, it can become a significant issue in high-frequency designs. In applications where signal loss is critical, alternative high-frequency laminates may be more appropriate than FR4.
How to Choose the Right FR4 PCB Material
When selecting a specific FR4 PCB material, consider the following factors:
Operating Temperature
Determine the expected continuous and peak operating temperatures. Higher operating temperatures or repeated thermal cycling may favor a higher-Tg grade.
Manufacturing Temperature
The material should also be able to resist the thermal profile applied during PCB assembly. In most designs, the temperature that PCBs need to handle during the lead-free soldering process is likely to be elevated relative to that of Sn-Pb soldering, which increases the importance of Tg, Td, and CTE in that case.
Signal Frequency and Data Rate
It is normally fine to use standard FR-4 for traditional analog and digital circuits. However, the dielectric loss, Dk variation, glass-weave effects, and impedance control become more significant as frequency and data rate increase. A low-loss laminate would be more suitable in designs requiring high speeds or high frequencies.
Mechanical Requirements
When choosing the FR4 PCB material, board size, layer count, thickness, vibration, shock, and installation requirements should be considered as well. These factors can influence the laminate construction and PCB thickness.
Electrical Requirements
If your application needs to withstand high voltage or demanding insulation and electrical requirements, factors such as dielectric strength, CTI, creepage and clearance, insulation resistance, and controlled impedance must be considered.
Environmental Requirements
Humidity, temperature cycling, chemical exposure, vibration – all need to be taken into consideration when choosing the right FR-4 grade.
Cost
Specialized laminates can offer enhanced thermal, high-frequency, or reliability performance, but are more expensive than standard laminates. In general-purpose applications, a conventional FR-4 provides a good compromise between performance and cost.
Is FR4 PCB Right for Your Application?
Although FR-4 is a widely used and cost-effective substrate for printed circuit boards, it may not be the ideal choice in certain situations where specific properties or performance requirements are crucial.
When to Use FR4 PCBs:
General-purpose applications: FR4 PCBs are well-suited for a wide range of general-purpose electronic devices, such as consumer electronics, computers, telecommunications equipment, and industrial control systems, where the operating conditions are not extreme.
Cost-sensitive projects: FR4 PCBs offer an economical solution for cost-sensitive projects, making them an attractive choice for applications that do not require specialized or high-performance materials. Moderate temperature environments: FR4 PCBs can withstand moderate temperatures, typically up to 130°C (266°F), making them suitable for applications where the operating temperature is within this range.
Low to moderate frequency applications: FR4 PCBs perform well in low to moderate frequency applications, where signal integrity requirements are not stringent, and controlled impedance is not a critical factor.
Prototyping and development: Due to their widespread availability and cost-effectiveness, FR4 PCBs are often used for prototyping and development purposes before transitioning to more specialized materials for final production.
When FR4 Isn't the Right Choice:
High-temperature environments: FR4 PCBs may not be suitable for applications that involve prolonged exposure to high temperatures, such as aerospace or automotive applications, where the operating temperatures can exceed the material’s limits.
High-frequency or high-speed designs: For high-frequency or high-speed applications that require precise signal integrity and controlled impedance, alternative materials like Rogers or Polyimide may be more appropriate than FR4 PCBs.
Harsh chemical environments: While FR4 PCBs offer decent chemical resistance, they may not be the best choice for applications involving exposure to aggressive chemicals or solvents, where more chemically resistant materials would be preferable.
Extreme mechanical stress: In applications that involve severe mechanical stress, such as vibrations or impacts, more robust materials like metal-core PCBs or specialized laminates may be better suited than FR4 PCBs.
Frequently Asked Questions about FR4 PCBs
1. What is FR4 in PCB?
FR4 is one of the most widely used laminate materials for making PCBs. “FR” indicates its flame-retardant property, and the “4” denotes its grade as a glass-reinforced epoxy laminate.
2. Is FR4 suitable for high-frequency applications?
Some FR4 materials with lower Dk/Df values and tighter manufacturing tolerances can be used in high-frequency and high-speed applications, while standard FR4 cannot be used in such demanding applications.
3. Is FR4 flammable?
No, FR4 is not a flammable material in the conventional sense, it’s flame-retardant.
4. What is the standard FR4 PCB thickness?
The most common FR4 thickness is 1.6 mm, but 0.8 mm, 1.0 mm, 1.2 mm, and 2.0 mm are also widely used. The right thickness is decided by the number of layers, mechanical needs, and type of connector or enclosure that the board will fit into.
5. What Tg options are available for FR4?
FR4 comes in a variety of Tg grades, Standard-Tg (around 130°C), Mid-Tg (around 150°C), and High-Tg (170°C or higher).
FR4 PCB Manufacturing at MOKO Technology
At MOKO Technology, we specialize in providing high-quality PCB solutions tailored to your unique requirements. Whether you need FR4 printed circuit boards or boards made from other advanced materials, our knowledgeable team is here to guide you. With years of industry experience, we pride ourselves on maintaining stringent quality standards, on-time delivery, and competitive pricing.
Our FR-4 PCB Manufacturing Capabilities
| Capability | Spec |
| Minimum PCB Size | 10 × 10 mm |
| Maximum PCB Size | Up to 570 × 1200 mm, depending on layer count |
| Standard FR4 PCB Thickness | 0.8, 1.0, 1.2, 1.6, 2.0 mm |
| Other Available Thicknesses | 0.3, 0.4, 0.6, 2.4, 3.0, 3.2, 5.0 mm |
| Minimum BGA Pad Size | 7 mil |
| Outer Layer Copper Thickness | 0.5–5 oz |
| Inner Layer Copper Thickness | 0.5–2 oz |
| Minimum Line Width | 4 mil* |
| Minimum Line Spacing | 4 mil* |
| Minimum Through-Hole Diameter | 0.15 mm |
| Through-Hole Via Copper Thickness | 25 μm average |
| Buried/Blind Via Copper Thickness | 20 μm average |
| Available Surface Finishes | HASL, ENIG, OSP, ENEPIG, Immersion Silver, Immersion Tin |
