The FR-4 Material System: Classification, Characteristics, and Application Analysis

The detailed classification system of FR-4 materials reveals the depth and breadth of this fundamental material. From the most traditional standard type to variants that address specific needs—such as high-Tg, halogen-free, high-CTI, and low-loss—FR-4 has evolved into a family of materials tailored to meet diverse performance requirements. This refined classification directly corresponds to its wide range of applications, from ubiquitous consumer electronics to demanding sectors like automotive electronics and industrial control. Through precise adjustments in material formulation, FR-4 plays an indispensable role in modern electronic industries.

Detailed Classification of FR-4 Materials

FR-4 is not a single specific material but rather a designation for a class of materials (Flame Retardant 4, i.e., fourth-level flame-retardant material). It refers to a substrate composed of a composite of glass fiber cloth and epoxy resin.

Therefore, when discussing “what types of PCB FR-4 materials exist,” it typically refers to classifications based on their reinforcing materials and resin systems.

By Reinforcement Material Type (Glass Fiber Cloth)

Standard fiberglass cloth (E-Glass) is the most common and fundamental type. It typically utilizes woven fabrics in various specifications such as 7628, 2116, and 1080. This material strikes a good balance between mechanical strength, electrical insulation properties, and cost, making it the default choice for the vast majority of consumer electronics and standard circuit boards. Its comprehensive performance is reliable, and its manufacturing process is mature, capable of meeting the reliability and cost control requirements of general electronic products.

Thin glass cloth (e.g., 106, 1080) primarily serves applications demanding high precision and lightweight construction. Featuring finer yarns and denser weaves, these fabrics achieve exceptionally thin sheet thicknesses, delivering outstanding conformability and flexibility. They are commonly used as inner core materials in high-layer-count boards or in microvia (HDI) designs requiring precision circuitry. Their uniform thickness ensures etching accuracy for fine traces and laminating consistency. Employing thin cloth effectively controls overall board thickness and enhances the miniaturization of electronic devices.

Thick glass cloths (e.g., 7628, 2116) emphasize stronger mechanical support and superior dimensional stability. Their inherently sturdier structure yields substrates with higher rigidity, better resisting bending and deformation. These are commonly applied in PCBs supporting heavy components or where exceptional board flatness is critical. Particularly when manufacturing high-current power boards using thick copper foil (e.g., 2oz and above), the robust glass fiber cloth skeleton effectively counters stresses generated by thick copper foil during lamination and high-temperature processing, preventing board warping.

Classified by Resin System and Technology

First is the most fundamental and common standard FR-4. This material utilizes traditional polyfunctional or difunctional epoxy resin systems, with a glass transition temperature typically ranging between 130°C and 140°C. Its mature manufacturing processes, exceptional reliability, and lowest cost make it the preferred choice for the vast majority of ordinary consumer electronics. Whether for computer peripherals, home appliances, or hardware with modest performance requirements, standard FR-4 offers a solid balance, meeting routine production and operational needs.

However, with the widespread adoption of lead-free soldering and increasingly demanding operating environments for electronic devices, high-Tg FR-4 emerged. High-Tg refers to a high glass transition temperature. These materials enhance thermal resistance by using modified epoxy resins (such as phenolic epoxy resins), typically achieving Tg values of 180°C or higher. Under the high temperatures required for lead-free soldering, high-Tg materials significantly better maintain their mechanical strength and dimensional stability. This effectively prevents excessive board expansion, delamination, or the “exploding board” phenomenon where copper foil separates from the substrate at elevated temperatures. Consequently, they are widely adopted in highly reliability-critical applications like automotive electronics, industrial control equipment, and power modules.

Amid increasingly stringent environmental regulations, halogen-free FR-4 has emerged as a critical variant. Traditional FR-4 relies on halogen compounds like bromine for flame retardancy, whereas halogen-free materials utilize eco-friendly alternatives such as phosphorus-based or nitrogen-based flame retardants. This approach prevents the generation of toxic halogenated gases like dioxins during combustion, meeting stringent environmental directives such as RoHS. It should be noted that halogen-free materials typically impose specific requirements on lamination and drilling processes, with a relatively narrow processing window.

For electronics operating in high-voltage environments, high-CTI FR-4 provides superior reliability assurance. CTI, or the Contact Tracking Index, measures a material's surface resistance to electrical tracking (the formation of conductive pathways). High-CTI materials achieve CTI values of 600V or higher through specialized resin formulations. This enables superior resistance to arcing and carbonized conductive pathways under humid, contaminated conditions with surface potential differences, thereby preventing short circuits and failures. Consequently, they are indispensable in high-voltage applications like power supplies, new energy vehicle charging stations, and automotive control systems.

Finally, amid the ongoing pursuit of higher signal speeds, a compromise solution has emerged: high-frequency/low-loss FR-4. Strictly speaking, it is not a traditional epoxy resin system but rather epoxy resin filled with ceramic or other low-loss fillers (such as modified hydrocarbons) to enhance its dielectric properties. This material exhibits a lower dielectric constant and dielectric loss factor than standard FR-4, reducing signal loss and distortion during transmission. While its performance cannot rival specialized high-frequency materials like Rogers, its significantly lower cost makes it widely adopted in medium-to-high-speed digital circuits and certain automotive radar modules where signal integrity is required but cost constraints are significant.

Application Scenarios and Industries

1. Consumer Electronics

2. Industrial Control and Automation

3. Automotive Electronics

Note: For high-reliability components like engine control and braking systems, materials with superior high-temperature resistance (e.g., high-Tg variants of FR-4 or ceramic substrates) may be employed.

4. Communication Equipment

Note: For high-frequency signal processing components (e.g., 5G antennas), FR-4 exhibits significant loss and is typically unsuitable.

5. LED Lighting

6. Medical Electronics

Note: For implantable or high-precision medical devices, extremely stringent stability and reliability requirements may necessitate advanced materials.

Application Limitations

In high-frequency or microwave applications, FR-4 proves inadequate. Such circuits are commonly found in 5G communication base stations, millimeter-wave radar, and satellite receiving equipment, where signal integrity demands are extremely high. The dielectric constant of FR-4 material becomes unstable with increasing frequency, and its inherent loss factor is relatively high. This leads to severe attenuation and phase distortion of high-frequency signals during transmission, causing a significant degradation in circuit performance. In such cases, engineers typically opt for specialized high-frequency laminates produced by companies like Rogers or Taconic as replacements for FR-4.

FR-4 also faces challenges when circuit design enters the high-speed digital domain. In high-speed servers, premium switches, or communication backplanes operating at rates exceeding 10Gbps, digital signal edges become extremely steep and contain abundant higher-order harmonics. FR-4's significant attenuation of high-frequency components degrades signal integrity, causing waveform distortion and eye diagram closure, thereby increasing bit error risk. To address this, mid-loss/low-loss FR-4 variants—offering performance between standard FR-4 and pure high-frequency materials—have emerged. Alternatively, high-speed materials like the MEGTRON series provide a better cost-performance balance.

In high-temperature environments or those demanding exceptional long-term reliability, standard FR-4's durability faces significant challenges. For instance, in automotive engine compartments, aerospace electronics, or deep-well drilling equipment, operating temperatures may remain persistently elevated or undergo extreme fluctuations. Standard FR-4's limited glass transition temperature (Tg) means prolonged exposure to high temperatures can cause delamination, aging, and degradation of both electrical properties and mechanical strength. For such applications, higher-Tg FR-4 with elevated glass transition temperatures is required, or materials with superior heat resistance like polyimide or ceramic substrates should be directly employed.

FR-4 also reveals limitations in high-power-density applications demanding excellent thermal dissipation. In high-power LED lighting, IGBT power modules, and high-current power converters, chips generate substantial heat. FR-4 itself is a poor thermal conductor with low thermal conductivity. If heat cannot be dissipated promptly, it leads to elevated device junction temperatures, compromising operational efficiency, reliability, and lifespan. In such cases, employing materials with superior thermal conductivity, such as metal or ceramic substrates, offers a more ideal solution.

In structural designs requiring circuit boards to bend or dynamically fold, the rigid FR-4 material is clearly inadequate. Hinge connections in wearable devices and smartphones, or compact camera modules, all demand circuit boards capable of repeated bending or adapting to irregular spaces. FR-4 material is brittle and rigid, unable to withstand bending stresses. In such cases, flexible circuit boards become the inevitable choice, typically using flexible polyimide or PET film as the substrate material.

In summary, the classification and application of FR-4 materials form a closely interconnected ecosystem. The more refined the classification, the more accurately it can meet the needs of specific application scenarios. Although limitations exist in extreme environments—such as high-frequency, high-temperature, or specialized physical form requirements—the continuous innovation and classification optimization of the FR-4 family continue to expand its capabilities. Understanding this classification system not only helps engineers make informed material choices in design but also demonstrates how foundational materials evolve to respond to and drive the advancement of the electronics industry.

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• Micriavia filling with copper: laser via size 0.1-0.125mm, priority 0.1mm
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• Max aspect ratio for via-in-pad filling with resin PCB - 12: 1
• Min resin plugged PCB thickness: 0.2mm
• Max via-filling ith resin PCB thickness: 3.2mm
• Making different hole sizes with via filling in one board: Yes
• Via filling with copper/silver: Yes

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Conclusion

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