FR 4 Substrate Classification Performance and Application Research

What is FR 4? FR 4 is the common abbreviation for glass fiber-reinforced epoxy resin materials widely adopted in the printed circuit board (PCB) industry. Strictly speaking, FR 4 is not an exclusive material name but a flame retardancy grade designation. This specification standard is formulated by the National Electrical Manufacturers Association (NEMA) of the United States. FR stands for Flame-Retardant; FR 4 specifically refers to copper clad laminate (CCL) substrates with epoxy resin as the resin matrix, woven glass fiber cloth as the reinforcement material, and a flame retardancy rating of UL94 V-0.

Currently, a wide range of FR 4 grade composite materials are available for circuit board manufacturing. Most mainstream formulations are fabricated by compounding tetrafunctional epoxy resin with fillers and electronic-grade glass fiber.

Different PCB substrates correspond to distinct flame retardancy grades. The classification, raw material composition, and flame retardant performance of various substrates are listed in the table below:

Introduction to Each Substrate Type

1.Paper Substrates
Paper substrates use phenolic resin as the binder and wood pulp cellulose paper as the surface reinforcement material. XPC and XXPC deliver no flame retardant capability, while FR-1, FR-2 and FR-3 feature flame retardant properties. With low overall cost, they are primarily used in low-end electronic products such as toys, radios, landline telephones and calculators.

2.Woven Glass Cloth Substrates
Also known as epoxy sheets, fiberglass sheets or fiber boards, FR 4 falls under this category. These substrates adopt epoxy resin as the binder and electronic-grade woven glass cloth as the reinforcing base material; both FR 4 and FR-5 are flame retardant grades. Epoxy fiberglass CCLs boast high mechanical strength, superior heat resistance and outstanding dielectric properties. As the most widely consumed substrate with broad application coverage, they are extensively utilized in mobile communication, digital televisions, consumer electronics and other fields.

3.Composite Substrates
The mainstream models are CEM-1 and CEM-3. Their mechanical properties and production costs sit between woven glass cloth substrates and phenolic paper substrates.
CEM-3 exhibits electrical performance comparable to standard FR-4 materials, with superior drilling machinability. Certain CEM-3 products outperform ordinary FR 4 in Comparative Tracking Index (CTI), dimensional accuracy and dimensional stability.

Performance Indicators of FR 4

Core performance parameters of FR 4 include glass transition temperature (Tg), thermal decomposition temperature (Td), dissipation factor (Df), dielectric constant (Dk), Comparative Tracking Index (CTI), coefficient of thermal expansion (CTE), water absorption, and copper foil peel strength.

Parameter comparison of mainstream FR 4 materials from three major manufacturers: Isola, Nelco and Ventec

Glass Transition Temperature (Tg) & Thermal Decomposition Temperature (Td)
Tg denotes the critical temperature at which a material transitions from rigid glassy state to deformable rubbery state. Thermal transition is reversible when the temperature stays below Td; the material regains rigidity after cooling down below Tg. Once temperatures exceed Td, FR 4 undergoes irreversible thermal decomposition and permanent functional failure.

The industry categorizes FR 4materials into three tiers by Tg value:

• Low Tg FR-4: Tg ≈ 135℃
• Mid Tg FR-4: Tg ≈ 150℃
• High Tg FR-4: Tg ≈ 170℃

Selection Recommendation: High Tg materials are mandatory for multilayer PCB lamination, high-layer-count boards, soldering temperatures ≥230℃, long-term operating temperatures over 100℃, wave soldering and other working conditions with severe thermal stress.

2.Dissipation Factor (Df) & Dielectric Constant (Dk)
These two fundamental dielectric properties vary with signal frequency. Higher Df leads to severe signal transmission attenuation, while Dk mainly governs trace impedance stability.

FR 4 grading by Df value:

1. Standard loss materials: Df ≥ 0.02
2. Mid-loss materials: 0.01 ≤ Df ＜ 0.02
3. Low-loss materials: 0.005 ≤ Df ＜ 0.01
4. Ultra-low-loss materials: Df ＜ 0.005
5. Comparative Tracking Index (CTI)
   CTI measures an insulating material’s resistance to electrical tracking breakdown. Heat-induced carbonization on insulation surfaces forms conductive paths, eventually triggering short circuits between electrodes. CTI value positively correlates with insulation performance; higher CTI allows reduced creepage distance between conductors.

Performance Level Category (PLC) corresponding to CTI voltage ranges:

Selection Recommendation: Prioritize FR 4 with high CTI grades for aerospace, marine, high-voltage high-current equipment and other scenarios requiring strict insulation performance.

Limitations of FR 4 Materials

FR 4 delivers advantages including low cost, excellent machinability, stable electrical performance, high mechanical strength and decent heat resistance. However, it has prominent drawbacks for high-speed high-frequency and high-power products.

1.Severe Signal Transmission Loss
Signal loss becomes prominent for FR 4 at elevated transmission rates and longer trace lengths. Conventional FR 4carries a Df of approximately 0.020, while high-speed dedicated substrates achieve Df as low as 0.004, with only one quarter of FR-4’s loss magnitude. Moreover, FR-4’s dielectric loss rises sharply and linearly with increasing frequency, whereas high-frequency substrates exhibit mild, converging loss growth. High-speed PCB design requires matching low-loss substrates corresponding to the operating frequency.

2.Insufficient Impedance Control Accuracy
Dk directly determines trace impedance; discontinuous impedance causes signal integrity issues such as overshoot, reflection and ringing. FR 4 features a maximum Dk deviation of up to 10%, while high-frequency high-speed substrates control deviation within 2%. FR-4’s upper limit of impedance precision fails to meet requirements for high-precision impedance design.

3.Poor Thermal Conductivity
FR 4 boasts a thermal conductivity of merely 0.3~0.4 W/m·K, resulting in weak heat dissipation capacity. Solutions for high-power products include high thermal conductivity substrates, embedded copper posts/copper blocks, and metal core PCBs to boost heat dissipation efficiency.

4.Inadequate High-Temperature Thermal Stability
FR 4 is prone to warpage and deformation under prolonged high-temperature exposure. The peak temperature of lead-free reflow soldering reaches 250℃, exceeding the Tg value of most FR 4 grades. Thermal expansion and contraction of the material generate internal stress, easily causing component cold solder joints and solder joint cracking. For PCBA with component dimensions larger than 3.2×1.6mm, substrates with low coefficient of thermal expansion (CTE) are recommended.