For printed circuit boards (PCBs), the base material shapes performance, reliability, and cost. FR4 PCB is the industry standard—strong, affordable, and suitable for most electronics. But what is FR-4, and when would you use a different material?
What Is FR-4?
FR-4 is a fibreglass-reinforced epoxy laminate used as the core substrate in most rigid PCBs. The name “FR” means flame retardant, while “4” identifies the specific NEMA grade of woven glass cloth combined with epoxy resin. In simple terms, FR-4 provides insulation between copper layers and gives the PCB its mechanical strength.
It’s the most common PCB base because it offers a reliable balance between cost, performance, and manufacturability. From consumer electronics to industrial controllers, FR-4 fits nearly every standard application where extreme heat or ultra-high frequency is not involved.
What Are PCB Boards Made Of?
A PCB is made up of several layers:
- Copper foil – The conductive layer that carries electrical signals.
- Substrate (FR-4 or other materials) – The insulating core that supports copper layers.
- Solder mask – The green (or other colour) coating that protects copper from oxidation and bridges.
- Silkscreen and surface finish – Used for component labelling and solder protection.
FR-4 acts as the core structural and dielectric layer in this stack. Its properties influence everything from impedance control to thermal expansion and solder reliability.
Key FR-4 Material Specifications
While all FR-4 materials share a similar composition, their performance varies depending on formulation and glass weave type. The most important parameters include:
- Glass Transition Temperature (Tg) – This is the point where the resin changes from rigid to rubbery. Standard FR-4 has a Tg around 130–140 °C, while high-Tg versions reach 170–200 °C. Choose high-Tg FR-4 for lead-free assembly or multiple reflow cycles.
- Dielectric Constant (Dk) and Dissipation Factor (Df) – Dk (typically 3.8–4.8) affects impedance; Df determines signal loss. For high-speed designs, use lower-loss FR-4 variants to minimise attenuation.
- Board Thickness and Copper Weight – Standard thickness is 1.6 mm, with copper layers of 0.5, 1, or 2 oz per square foot. Thicker copper increases current capacity and heat dissipation but reduces fine-line capability.
- Thermal Conductivity – FR-4 has poor thermal conductivity (~0.3 W/m·K), so it’s not ideal for high-power or LED applications without additional thermal design measures.
These parameters help designers balance performance, cost, and manufacturability for different PCB applications.
When FR4 PCB Is the Right Choice
FR-4 remains the go-to option for:
- General digital and analogue circuits
- Industrial controllers, power supplies, and consumer electronics
- Multilayer boards (up to 8–10 layers in most cases)
- Cost-sensitive projects that don’t require extreme thermal or frequency performance
Its versatility, availability, and strong mechanical stability make it ideal for both mass production and quick prototyping.
When FR4 PCB Isn’t Enough
Even though FR-4 covers about 80% of all PCB needs, there are cases where other materials perform better:
- High-Speed or RF Designs – At gigahertz frequencies, standard FR-4 introduces too much signal loss. In these cases, low-loss materials like Rogers or PTFE-based laminates are preferred.
- High-Power or Thermal-Intensive Boards – FR-4’s low thermal conductivity limits heat spreading. Aluminium or copper-based substrates work better for LED lighting, power converters, and motor controllers.
- Extreme Reliability Conditions – Under repeated thermal cycling or high mechanical stress, FR-4 can crack or delaminate. High-Tg or ceramic materials handle these stresses more effectively.
FR-4 vs. Aluminium and High-Frequency Materials
| Thermal Conductivity | Low (~0.3 W/m·K) | Very High (~1–2 W/m·K) | Low to Medium |
| Dk / Df Stability | Moderate | Moderate | Excellent |
| Layer Count | Up to 10+ | Usually 1–2 | 2–6 typical |
| Cost | Low | Medium | High |
| Best For | General Electronics | LED / Power / Thermal | RF / Microwave / High-Speed |
This quick comparison shows why FR-4 remains dominant — it’s the most flexible choice for common multilayer designs. But when thermal or high-frequency performance matters most, switching to aluminium or PTFE can be worth the higher cost.
Layer Stack-Up and Thickness Selection
FR-4 is available in many core and prepreg combinations for 2-, 4-, and 6-layer boards.
- 4-Layer Example: Signal – Ground – Power – Signal
- 6-Layer Example: Signal – Ground – Signal – Power – Ground – Signal
When choosing board thickness:
- Start from the mechanical or connector requirement.
- Then confirm impedance targets and available prepreg thicknesses with your PCB manufacturer.
- Stick to standard thicknesses (1.6 mm or 1.0 mm) for cost and availability.
For copper weight:
- 1 oz is the industry standard for general designs.
- Use 2 oz for higher current or better heat dissipation.
Frequently Asked Questions
What is FR-4 in PCB?
It’s a flame-retardant fibreglass and epoxy laminate used as the base of most rigid PCBs.
What are PCB boards made of?
Copper foil, FR-4 substrate, solder mask, and silkscreen layers.
Is FR-4 good for high-frequency circuits?
It works for low- to mid-frequency designs, but for high-speed or RF circuits, use a low-loss material.
What is the typical thickness of an FR-4 board?
1.6 mm is standard, though thinner (1.0 mm or 0.8 mm) boards are common for compact electronics.
When should I choose an aluminium PCB instead of FR-4?
For LED lighting, power electronics, or designs that generate significant heat.
Conclusion
FR4 PCB remains the foundation of modern PCB manufacturing — cost-effective, durable, and suitable for the majority of electronic products. While it isn’t perfect for high-frequency or high-heat environments, its balance of electrical, mechanical, and economic advantages makes it the best all-around material for general PCB design.
If your design requires improved signal integrity or better heat management, talk to your PCB supplier about high-Tg FR-4, aluminium-based boards, or low-loss laminates. Selecting the right substrate early ensures your circuit stays stable, reliable, and cost-efficient from prototype to production.
