What Are the Differences Between Various PCB Materials

August 22, 2025

The flammability of materials, also referred to as flame retardancy, self-extinguishing properties, combustion resistance, fire resistance, or combustibility, is a measure of a material's ability to withstand burning.

To evaluate flammability, a material sample is ignited with a flame that meets specific standards. After a defined period, the flame is removed, and the extent of burning is assessed to determine the flammability rating. The ratings are divided into three levels. When the sample is placed horizontally, it is tested using the horizontal method and classified into FH1, FH2, or FH3. When placed vertically, it is tested using the vertical method and classified into FV0, FV1, or FV2.

Therefore, PCB substrates are classified into HB-grade and V0-grade materials.

HB-grade substrates have low flame retardancy and are commonly used in single-layer boards. V0-grade substrates have high flame retardancy and are typically used in double-layer and multilayer boards. PCB substrates that meet the V-1 fire resistance rating are referred to as FR-4 substrates. The fire resistance ratings are classified as V-0, V-1, and V-2.

PCBs must be flame-resistant, meaning they should not burn but only soften at certain temperatures. The temperature at this transition point is called the glass transition temperature (Tg), which is related to the dimensional stability of the PCB.If standard PCB substrate materials are exposed to high temperatures, they may soften, deform, melt, and experience a sharp decline in mechanical and electrical properties.

What is High-Tg PCB and What Are Its Advantages?

When the temperature of a high-Tg PCB rises to a certain range, the substrate transitions from a "glassy state" to a "rubbery state." The temperature at which this occurs is called the glass transition temperature (Tg). In other words, Tg is the highest temperature at which the substrate maintains rigidity.

Generally, substrates with a Tg above 130°C are considered standard, those with a Tg above 150°C are medium-Tg, and those with a Tg above 170°C are classified as high-Tg PCBs. When the Tg of the substrate is increased, the PCB's heat resistance, moisture resistance, chemical resistance, and stability are enhanced. Higher Tg values indicate better temperature resistance, which is particularly important in lead-free manufacturing processes.

High Tg refers to high heat resistance. With the rapid development of the electronics industry, especially in high-functionality and multilayer products such as computers, higher heat resistance of PCB substrates is essential. The emergence and advancement of high-density mounting technologies like SMT and CMT require PCBs with smaller apertures, finer circuits, and thinner profiles, all of which depend on high heat resistance.

The key differences between standard FR-4 and high-Tg FR-4 lie in their mechanical strength, dimensional stability, adhesion, water absorption, thermal decomposition, and thermal expansion under heat, particularly after moisture absorption. High-Tg materials perform significantly better than standard PCB substrates. In recent years, the demand for high-Tg PCBs has been increasing.

What are the specific types of PCB substrates?

They are classified from lowest to highest grade as follows:
94HB → 94V0 → 22F → CEM-1 → CEM-3 → FR-4

Detailed descriptions:
94HB: Standard paper substrate, non-flame retardant (lowest-grade material, suitable for punch drilling, not used for power boards).
94V0: Flame-retardant paper substrate (suitable for punch drilling).
22F: Single-sided semi-glass fiber substrate (suitable for punch drilling).
CEM-1: Single-sided glass fiber substrate (requires CNC drilling, not suitable for punch drilling).
CEM-3: Double-sided semi-glass fiber substrate (the lowest-end material for double-sided boards except paper-based substrates; suitable for simple double-sided boards; costs 5–10 RMB/square meter less than FR-4).
FR-4: Double-sided glass fiber substrate.

Classification of PCB Substrates by Reinforcement Material

1.Phenolic PCB Paper-Based Substrate
Often referred to as paper board, V0 board, flame-retardant board, or 94HB, this substrate is made from wood pulp fiber paper reinforced with phenolic resin. It is non-flame-retardant, suitable for punch drilling, low-cost, and lightweight. Common examples include XPC, FR-1, FR-2, and FE-3. 94V0 is a flame-retardant paper board.

2.Composite PCB Substrate
Also known as composite board, it uses wood pulp or cotton pulp fiber paper as the base reinforcement, with glass fiber cloth as the surface reinforcement, bonded with flame-retardant epoxy resin. Common types include single-sided semi-glass fiber 22F, CEM-1, and double-sided semi-glass fiber CEM-3. CEM-1 and CEM-3 are the most widely used composite copper-clad laminates.

3.Glass Fiber PCB Substrate
Also called epoxy board, fiberglass board, FR4, or fiberboard, this substrate uses epoxy resin as the adhesive and glass fiber cloth as the reinforcement. It operates well at high temperatures, is less affected by environmental conditions, and is commonly used in double-sided PCBs. However, it is more expensive than composite substrates. The typical thickness is 1.6mm. This substrate is suitable for power boards and high-layer PCBs, widely used in computers, peripherals, and communication devices.

Additional Technical Specifications

File Acceptance: Protel, AutoCAD, PowerPCB, OrCAD, Gerber, or physical board cloning.
Substrate Types: CEM-1, CEM-3, FR-4, high-Tg materials.
Maximum Board Size: 600mm × 700mm (24,000 mil × 27,500 mil).
Board Thickness: 0.4mm–4.0mm (15.75 mil–157.5 mil).
Maximum Layers: 16 layers.
Copper Thickness: 0.5–4.0 oz.
Finished Board Thickness Tolerance: ±0.1mm (4 mil).
Dimension Tolerance: CNC milling: 0.15mm (6 mil); Die punching: 0.10mm (4 mil).
Minimum Line Width/Spacing: 0.1mm (4 mil); Line width control capability: < ±20%.
Minimum Finished Drill Hole Size: 0.25mm (10 mil).
Minimum Punched Hole Size: 0.9mm (35 mil).
Hole Size Tolerance: PTH: ±0.075mm (3 mil); NPTH: ±0.05mm (2 mil).
Finished Hole Wall Copper Thickness: 18–25 μm (0.71–0.99 mil).
Minimum SMT Spacing: 0.15mm (6 mil).
Surface Finishes: ENIG, HASL, overall nickel/gold plating (hard/soft gold), solder mask printing, etc.
Solder Mask Thickness: 10–30 μm (0.4–1.2 mil).
Peel Strength: 1.5N/mm (59N/mil).
Solder Mask Hardness: >5H.
Solder Mask Plugging Capability: 0.3–0.8mm (12–30 mil).
Dielectric Constant: ε = 2.1–10.0.
Insulation Resistance: 10KΩ–20MΩ.
Characteristic Impedance: 60 Ω ±10%.
Thermal Shock Resistance: 288°C, 10 seconds.
Board Warpage: <0.7%.
Applications: Communication devices, automotive electronics, instruments, GPS, computers, MP4 players, power supplies, home appliances, etc.

Standards for PCB Substrates

With advancements in electronics, PCB substrate materials continue to evolve, leading to ongoing updates in standards. Major standards include:

National Standards: China’s GB/T 4721–4722 (1992) and GB 4723–4725 (1992); Taiwan’s CNS standards (based on Japanese JIS standards, published in 1983).
Other International Standards: Japanese JIS standards
U.S. ASTM, NEMA, MIL, IPC, ANSI, UL standards
British BS standards
German DIN and VDE standards
French NFC and UTE standards
Canadian CSA standards
Australian AS standards
Russian GOST standards
International IEC standards