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Flame Retardant 4, or FR4, is a material grade used in PCB manufacturing. It is made of fibreglass and "4" represents the UL94 V-0 flammability standard (it must self-extinguish after being ignited). Fibreglass reinforced woven glass fibre and epoxy resin makes it excellent for electrically insulated applications. FR4 is a widely used material due to its low cost and high reliability.
FR4 represents flame-retardant grade 4 glass-reinforced epoxy laminate. The material's dielectric constant and dissipation factor are moderate. FR4 can be used in low-speed and high-power applications, making it the first choice for many hobbyists.
Dielectric constant: around 4.5 at 1MHz. Loss tangent: around 0.02 at 1MHz. The glass transition temperature (Tg) determines how the material behaves under heat.
FR4 comes in three different temperature ranges:
FR-4 is affordable compared to advanced laminates like Rogers. Its low cost makes it popular and the industry standard for most applications.
The woven fiberglass provides higher tensile strength, making it more resistant to warping, bending, and mechanical stress.
Due to a moderate dielectric constant, it is excellent for low-frequency and higher-power applications with higher signal isolation.
Thicker FR-4 increases dielectric spacing, raising impedance. Thickness can be adjusted for impedance control.
Thinner FR-4 has smaller spacing, lowering impedance. However, higher dielectric loss limits performance above a few GHz.
Thicker FR-4 offers improved heat spreading and higher thermal capacity.
Thinner FR-4 has limited heat-spreading capability, more susceptible to hot spots.
Thicker boards are more rigid and resistant to bending.
Thinner boards are more flexible and lightweight, ideal for compact devices but more prone to warping.
FR4 PCBs balance cost and strength for everyday electronic devices. From smartphones to wearables, FR-4 provides durable and reliable PCB solutions.
Control systems, power supplies, and automation boards often use thicker FR-4 laminates for mechanical rigidity and thermal cycling.
Instrument clusters, infotainment units, and sensor modules frequently use FR-4 PCBs for stable performance under vibration and heat.
| Features | Capabilities |
|---|---|
| Layer count | 1, 2, 4, 6, ...32 layers |
| Controlled Impedance | 4/6/8/.../32 layers |
| Impedance Tolerance | ±10% (±5Ω if ≤ 50Ω) |
| UL Number | E479892 |
| Maximum Dimensions | 670 × 600mm (≥0.8mm); 500×600mm (<0.8mm); 2-layer: 1020×600mm |
| Minimum Dimensions | 3 × 3mm; 70×70 for V-cut panel |
| Outer Copper Thickness | 1oz, 2oz (2.5oz, 3.5oz for 2-layer) |
| Inner Copper Thickness | 0.5 oz, 1 oz, 2 oz |
| Surface finish | HASL (leaded/lead-free), ENIG(1u"/2u") |
| Minimum Via Hole/Diameter | 0.15mm / 0.25mm |
| Minimum Track Width | 0.10/0.10mm (1oz); 0.16/0.16mm (2oz) |
| Gold Fingers Bevelling | 30° or 45° |
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FR4 typically has a dielectric constant of around 4.2–4.8 at 1 MHz. This affects signal transmission speed and impedance control.
Standard FR4 has a Tg of about 130–140°C, while high-Tg FR4 options can reach 170–180°C or higher.
FR4 works well for most low- and mid-frequency applications. For RF above ~1 GHz, specialized materials like Rogers are recommended.
Yes. Most modern FR4 laminates are RoHS- and REACH-compliant; halogen-free options are also available.