FR-4 is the most common material used for printed circuit boards, composed of glass fiber and epoxy resin. It is strong, lightweight, and provides good insulation, making it best suited for many electronics. This article explains the structure, properties, grades, limitations, and design factors of FR-4, providing detailed information on when and how it should be used.
FR-4 Overview
FR-4 is the most common material used to make printed circuit boards (PCBs). It is made from glass fiber and epoxy resin, which makes it both strong and good at insulating electricity. The FR means flame retardant, meaning it can resist burning, but this doesn’t always mean it meets the strict UL 94 V-0 fire safety standard.
This material is popular because it is lightweight, durable, and affordable. It also does a good job resisting moisture and heat, which helps electronic circuits stay stable. Another reason FR-4 is used is that it can be easily shaped into single-layer or multilayer boards without adding much cost.
FR-4 Laminate Structure
This image shows the layered structure of an FR-4 laminate; the most common material used in printed circuit boards (PCBs). At the top and bottom, copper foil sheets form the conductive layers that will later be etched into circuit patterns. Between these copper sheets lies the core: woven glass fabric impregnated with epoxy resin. The glass weave provides mechanical strength and dimensional stability, while the epoxy binds the fibers and adds rigidity. Together, they create an insulating yet durable base. The combination of copper foil, glass fiber, and epoxy makes FR-4 strong, flame-resistant, and ideal for supporting and protecting PCB traces.
Electrical Properties of FR-4
| Parameter | FR-4 Range |
|---|---|
| Dielectric Constant (Dk) | 3.8 – 4.8 |
| Dissipation Factor (Df) | \~0.018 – 0.022 |
| Dielectric Strength | >50 kV/mm |
| Stability | Varies with frequency and glass weave |
Thermal Properties of FR-4
| Property | Standard FR-4 | High-Grade FR-4 |
|---|---|---|
| Glass Transition Temperature (Tg) | 130–150 °C | ≥180 °C |
| Decomposition Temperature (Td) | >300 °C | >300 °C |
| Time to Delamination (T260 / T288) | Lower resistance | Higher resistance |
FR-4 Thickness and Stackup Options
| Thickness / Type | Advantages | Limitations |
|---|---|---|
| Thin (<0.5 mm) | Lightweight, compact, and can be made flexible | Fragile, more difficult to handle during assembly |
| Standard (1.6 mm) | Industry default, widely available, cost-effective | Can limit ultra-compact or high-density designs |
| Thick (>2 mm) | Provides stiffness and better resistance to vibration | Increases overall weight and cost |
| Custom Multilayer Stackups | Enables impedance control, supports high-speed signals, and improves EMI shielding | Requires precise fabrication processes, more expensive |
Using FR-4 for PCB Design
• Consumer Electronics - It provides a stable base material that can handle everyday use and basic power needs.
• Industrial Controls and Automation - FR-4 offers steady performance in systems that need durability and consistent function over time.
• Power Supplies and Converters - For circuits that work below very high frequencies, FR-4 delivers insulation and performance that meet the requirements.
• Cost-Sensitive Designs - When budgets matter, FR-4 allows you to keep production costs lower without giving up reliability.
Limits of FR-4 and Better Alternatives
When FR-4 Is Not Suitable
• High-Frequency Circuits - Above about 6–10 GHz, FR-4 causes higher signal loss, which makes it unsuitable for advanced RF or microwave designs.
• Ultra-High Data Rates - For speeds like PCIe Gen 5 and above (25+ Gbps), FR-4 adds too much delay and insertion loss, reducing signal integrity.
• High-Temperature Conditions - Standard FR-4 begins to break down faster when exposed to temperatures higher than about 150 °C, making it unreliable for long-term use in such environments.
Alternatives to FR-4
| Material | Use Case |
|---|---|
| Rogers laminates | RF and microwave designs needing low signal loss |
| PTFE composites | Ultra-low dielectric loss for precision, high-frequency circuits |
| Polyimide | High-temperature endurance in harsh environments |
| Ceramics | Extreme performance and durability under stress |
FR-4 Grades and Uses
Standard FR-4
Standard FR-4 has a glass transition temperature (Tg) of about 130–150 °C. It is the most common grade, used in electronics, office equipment, and standard industrial control systems.
High-Tg FR-4
High-Tg FR-4 offers a Tg of 170–180 °C or higher. This grade is required for lead-free soldering processes and is used in automotive electronics, aerospace boards, and other designs that need higher thermal stability.
High-CTI FR-4
High-CTI FR-4 provides a comparative tracking index (CTI) of 600 or greater. It is chosen for power supplies, converters, and high-voltage circuits where safe creepage and clearance distances are required.
Halogen-Free FR-4
Halogen-free FR-4 has properties similar to standard or high-Tg types, but it avoids halogen-based flame retardants. It is used in eco-friendly designs that must comply with RoHS and REACH environmental standards.
Signal Integrity Issues in FR-4
Problem
FR-4 uses a woven glass fabric for strength, but this weave is not perfectly uniform. When routing differential pairs, one trace may pass mainly over the glass bundles, which have a higher dielectric constant, while the other trace passes over the resin, which has a lower dielectric constant. This uneven exposure causes the signals to travel at slightly different speeds, creating what is called fiber-weave skew.
Impact
The difference in speed between the two signals leads to timing mismatches. At high data rates, this mismatch appears as differential skew, added jitter, and even eye-diagram closure. These effects can reduce signal integrity and limit the performance of high-speed communication channels.
Solutions
Routing differential pairs at a 10–15° angle to the weave helps prevent traces from aligning directly with the glass bundles. Choosing spread glass fabrics, such as 3313 styles, makes the dielectric properties more uniform across the board. Staggering differential pairs ensures both traces encounter a similar material mix. Budgeting skew in timing simulations allows you to predict and account for these effects before fabrication.
Moisture and Reliability Risks in FR-4
Effects of Moisture
• Tg Reduction During Reflow - Absorbed moisture lowers the glass transition temperature, which makes the material less stable during soldering and can lead to delamination.
• Dielectric Degradation - At high frequencies, moisture increases dielectric loss, which reduces signal quality in GHz-speed designs.
• Conductive Anodic Filamentation (CAF) - One of the most serious risks, CAF occurs when copper ions migrate through the epoxy under electrical bias, forming hidden conductive paths that may cause shorts between traces or vias.
Reducing Moisture Problems
• Store boards dry and sealed to keep out moisture.
• Bake boards before use if they’ve been exposed to humidity.
• Choose CAF-resistant FR-4 for high-density or high-voltage designs.
• Follow spacing rules from IPC to reduce the risk of shorts.
Factors to Check Before Buying FR-4
• Specify the laminate grade and IPC-4101 slash sheet to avoid confusion.
• Include frequency-specific dielectric constant (Dk) and dissipation factor (Df) values for the intended operating band.
• Confirm thermal requirements with Tg ≥ 170 °C and Td > 300 °C for lead-free soldering and long-term heat stability.
• Call out copper foil roughness for high-speed layers to minimize insertion loss.
• Note the comparative tracking index (CTI) rating when designing for high-voltage paths.
• Select CAF-resistant laminate for dense via fields or high-voltage applications.
• Add handling or storage instructions to control moisture and prevent delamination.
• Request spread glass fabric for differential pairs to reduce fiber-weave skew.
Conclusion
FR-4 offers strength, insulation, and cost efficiency, which is why it remains the standard PCB material. Still, it has limits in high-frequency, high-speed, or high-temperature conditions. By knowing its electrical, thermal, and reliability factors, and choosing the right grade, you can ensure stable performance or switch to better alternatives when designs demand it.
Frequently Asked Questions [FAQ]
What is IPC-4101 in FR-4?
It’s a standard that defines FR-4 laminate properties like Tg, Dk, and moisture absorption.
How is FR-4 different from metal-core PCBs?
FR-4 is for general PCBs, while metal-core PCBs use aluminum or copper for better heat dissipation.
Can FR-4 be used in flexible PCBs?
No, FR-4 is rigid. It can only be part of rigid-flex designs with polyimide layers.
What is the moisture absorption of FR-4?
Around 0.10–0.20%, which can lower stability if not baked or stored properly.
Is FR-4 good for high-voltage circuits?
Yes, high-CTI grades (CTI ≥ 600) are used in power supplies and converters.
Why does copper foil roughness matter in FR-4?
Rough foils increase signal loss; smooth foils improve high-speed performance.