An Overview of FR4 Material in PCB Fabrication

FR4 is the most commonly used dielectric material in PCB fabrication. Renowned for its unwavering reliability, widespread availability, and cost-effectiveness, it's no surprise that it's the go-to choice in the electronic industry.

Highlights:

• FR4 has relatively low D_k ranging from 8 to 4.8, ensuring minimal signal loss.
• A high T_g, typically ranging from 150°C to 170°C ensures that the material doesn't deform under high-temperature conditions.
• FR4 exhibits a low moisture absorption of 10%.
• The higher CTI value of 600V indicates better electrical tracking resistance.

What is FR4?

FR4 stands for flame retardant 4. It is a type of composite material that consists of layers of woven glass fabric impregnated with epoxy resin. The "4" in FR4 refers to the grade or classification of the material, denoting its flame-retardant properties.

FR4 laminate

It is extensively used in the electronics industry, particularly in the production of printed circuit boards.

Properties of FR4 material

Flame retardant

Flame retardants are substances that are added to materials to stop or slow the spread of fire. As the name suggests, FR4 is inherently flame-retardant, meaning it does not readily catch fire or support combustion, making it a safer choice for electronic devices. Manufacturers can produce flame retardant laminates and prepregs using a variety of methods and can rely on them to deliver predictable outcomes due to their incredible versatility.

Excellent electrical performance

Signal integrity and impedance issues depend heavily on the electrical qualities of a PCB material. They define the speed at which an electrical signal travels through the material and the amount of electrical charge it can hold in a specific volume.

• Electrical insulation: FR4 provides excellent electrical insulation. It prevents electrical current from leaking or arcing between conductive traces on a PCB, reducing the risk of short circuits.
• Dielectric constant (D_k): FR4 has a stable and relatively low dielectric constant as a result of a lower resin content of 20%. This property ensures consistent signal propagation and minimal signal loss in electronic circuits.

The table below includes a few significant electrical properties, along with their typical values. 

Depending on the manufacturer, the indicated values may change. For instance, the thickness, resin content, and glass weaving method all affect the dielectric constant of FR4, which ranges from 3.8 to 4.8.

Mechanical strength

FR4 gives the circuit board a high load-bearing capacity and mechanical robustness by combining fiberglass and epoxy resin. FR4 is well-known for its mechanical robustness and durability. It can withstand physical stresses, such as bending or mechanical impacts, without breaking or deforming easily. However, the thickness of the material has the most significant impact on strength and load bearing.

Moisture resistant

A PCB material's capacity to withstand water absorption when immersed in water is known as moisture absorption. It is indicated by a percentage increase in a circuit board material's weight as a result of water absorption under regulated circumstances. When submerged in water for 24 hours, FR4 materials exhibit a low moisture absorption of 0.10%.

Higher glass transition temperature (T_g)

The glass transition temperature is the temperature at which the PCB begins to soften and lose its shape. Higher T_g values guarantee improved thermal resistance. In addition to this, moisture and chemical resistance are also impacted by the T_g value of PCB.

FR4 has a high T_g, typically ranging from 150°C to 170°C. This ensures that the material remains stable and doesn't soften or deform under high-temperature conditions.

Types of FR4 PCB material

FR4 PCB material comes in various types, each designed to meet specific requirements and applications. Here are some common types of FR4 circuit board materials.

Standard FR4

This is the most widely used type of FR4 material. It offers good electrical insulation, thermal stability, and mechanical strength. Standard FR4 is suitable for a wide range of general-purpose applications with heat resistance of about 140℃ to 150℃.

High T_g FR4

High-T_g FR4 materials are engineered to have a higher T_g compared to standard FR4. Printed circuit boards constructed using high T_g FR4 materials exhibit will be capable to retain their structural integrity even when exposed to temperatures as high as 170°C. This makes them more suitable for lead-free soldering processes, where higher temperatures are involved. High T_g FR4 is ideal for applications requiring improved thermal resistance.

High CTI FR4

High comparative tracking index FR4 refers to a type of FR4 PCB material that has a high CTI value. The CTI value is a measure of a material's electrical breakdown resistance under humid conditions. A higher CTI value of 600V indicates better electrical tracking resistance, which means the material is less prone to forming conductive paths or tracking when exposed to moisture or contaminants.

FR4 materials with high CTI values are designed to meet specific safety and reliability requirements in applications where there is a risk of exposure to humidity or contaminants. These materials are often used in critical applications where safety and reliability are paramount, such as automotive, aerospace, and industrial electronics.

FR4 with no laminated copper

FR4 without laminated copper is essentially a substrate made from FR4 material that doesn't have a copper layer bonded to it. This is typically not used for traditional PCB fabrication but may find applications in non-electrical components or insulating layers within electronic assemblies.

Halogen-free FR4

Halogen-free FR4 materials do not contain halogenated flame retardants, making them environmentally friendly and compliant with regulations like RoHS (restriction of hazardous substances). They are commonly used in eco-conscious applications.

The difference between FR4 material and high-frequency laminate

FR4 material and high-frequency laminate are two distinct types of substrates used in circuit board fabrication, and they serve different purposes and have contrasting characteristics. Here are the key differences between FR4 material and high-frequency laminate:

 The main difference between FR4 material and high-frequency laminate lies in their dielectric properties, operating frequency range, and suitability for high-frequency applications.

While FR4 material is suitable for standard PCBs and lower-frequency applications, high-frequency laminates are engineered to excel in high-frequency, RF, and microwave applications where signal integrity and low losses are critical. The choice between the two depends on the specific requirements of the PCB design and the intended operating frequency range.

Benefits of using FR4 substrate for PCB fabrication

FR4 offers several advantages that make it a preferred choice for efficient PCB fabrication:

1. Dimensional stability

Stable board dimensions during fabrication and operation are ensured by a low Z-axis coefficient of thermal expansion CTE and robust construction.FR4 maintains its dimensions and shape even under varying environmental conditions, contributing to the precision and reliability of PCBs.

2. Easily etchable

FR4 substrate offers excellent etchability, allowing for precise and intricate patterns to be etched onto the board. The epoxy resin offers good etchability characteristics for creating fine-featured copper traces using subtractive processes.

3. Clean drilling capability

When drilling holes, the reinforcing weave allows for clean punch-through. As a result, plated-through holes for layer interconnections become possible.

4. Easy to bind

For multilayer boards, the surface of FR4 easily binds with popular laminates like copper foils and offers high peel-off strength. Complex PCBs may be built up at an inexpensive cost because of this.

5. Easily reworkable

If there are damaged components on the board, technicians can remove and replace damaged surface mount components without significantly damaging the PCB pad surfaces.

6. Widely available

FR4 material is readily available from various manufacturers in standardized forms, ensuring a consistent supply chain, which is essential for large-scale production.

7. Mass-production compatible

FR4 is ideal for mass-production processes. It is a practical option for mass-produced electronics because PCB manufacturers readily accept it. PCB manufacturers worldwide have perfected the art of working with FR4, resulting in streamlined and efficient production methodologies.

8. Cost-effective

FR4 is a cost-effective choice for PCB fabrication. It strikes a balance between performance and affordability, making it suitable for a wide range of electronic applications.

Material properties of FR4 from different manufacturers 

 Isola 370HR datasheet

Isola 370HR datasheet

Limitations of using FR4 circuit board materials

FR4 circuit board materials do have certain limitations that engineers and designers should be aware of when considering it for specific applications. Here are the key limitations of FR4 circuit board material:

Signal losses

Signal loss stands as a critical consideration in PCB design, especially when dealing with high-frequency applications. FR4 may not be the most suitable PCB material for such scenarios due to its higher dissipation factor (D_f) compared to materials specifically engineered for high-frequency use.

FR4 exhibits a D_f of approximately 0.020, whereas most high-frequency laminates boast a significantly lower D_f, typically around 0.004, merely a quarter of FR4's D_f . A smaller D_f corresponds to reduced signal loss.

FR4's D_f tends to escalate as signal frequencies increase. Consequently, PCBs constructed with this material experience more pronounced signal losses compared to their counterparts utilizing high-frequency laminates.

Insulating stability

While FR4 is well-known for its excellent insulating properties, it does have its limits when confronted with high power, voltage, or heat. When these limits are exceeded, the insulating characteristics of the material can degrade, leading it to transition from insulating to conducting electricity. This transition becomes the root cause of circuit board failures.

Controlled impedance

Unlike high-speed board materials, FR4 does not provide a consistent dielectric constant. The D_k fluctuates as frequency increases. For high-speed materials, dielectric constant tolerances are smaller than 2%, whereas for FR4, it can reach 10%. This makes the material an undesirable option for controlled impedance boards.

Temperature stability

Do to use FR4 materials for PCBs in electronics that are subjected to very high temperatures (>200 °C). Additionally, due to the fact that in no-lead PCB assembly, the reflow temperatures can reach up to 250 °C, these materials do not support lead-free soldering. This is considerably greater than the T_g of many FR4 variants.

3 tips for selecting the right FR4 PCB material

1. Choose thicker FR4 materials for circuit boards with grooves.The thickness of the FR4 materials range from 0.2 mm to 3.2 mm.

PCB with V-groove

2. Opt high Tg materials if the operating temperature exceeds 150 °C. These materials feature lower expansion rates and superior thermal performance while maintaining the same manufacturability. An example of a high-performance FR4 material is Isola 370HR.

3. Select a dielectric material that provides a consistent dielectric constant (D_k) throughout a broad frequency range. Variations in the dielectric constant will have an impact on the parasitic capacitance between a trace and its reference conductor, as well as between a trace and any other neighboring conductors.

IPC-A-600 standard for FR4 materials

To make FR4 PCBs, manufacturers intertwine glass strands in a square grid. They then transform this woven glass sheet into a prepreg ply by impregnating it with epoxy resin and semi-curing it. In the section below, we will go over the IPC-A-600 standard with respect to base materials.

Standards related to the base material surface

Weave exposure

Weave exposure on FR4 materials

 In this state, as you can see in the image above, resin partially covers unbroken woven cloth fibers on the surface. In all classes (class 1, 2, and 3 boards), weave exposure is permissible as long as the remaining distance between the conductors satisfies the minimal spacing requirement.

Weave texture

Weave texture on FR4 materials

The weave pattern is plainly apparent on the surface, yet the woven cloth’s fibers are entirely covered in resin, as can be seen in the image above. Although weave texture is permissible in all classes, weave exposure is sometimes mistaken for it because of their similar appearances.

Standards related to base material subsurface

This section provides a description of the subsurface conditions of laminated materials that are externally visible. Some of the most common subsurface base material conditions include measling, crazing, delamination/blistering, and foreign materials.

Measling

Measling in FR4 materials

Measling is the term for a state where discrete white dots appear in a PCB's base material. When the resin is not applied to the board optimally during the lamination process, measling develops. All applications aside from those involving high-voltage products allow measles. However, excessive measling could harm the board's overall and electrical performance.

Crazing

Crazing in FR4 laminates

 The emergence of a sequence of interconnected white spots or crosses within the substrate material characterizes this phenomenon. It signifies the separation of glass cloth fibers and the connecting weave junctions.

Class 1 acceptability:

• Ensure that the gap between conductive patterns meets or exceeds the minimum conductor spacing requirement.

Class 2 and 3 acceptability:

• Same as class 1 acceptability.
• The distance between adjacent crazing patterns should not exceed 50% of the distance between adjacent conductive patterns.
• Crazing occurring at the edge of the board does not impact the minimum distance between the board's edge and the conductive pattern.

Delamination/Blister

Delamination/blistering on PCB materials

Any planar division between layers of the base material or between the material and the conductive foil, is referred to as delamination. When delamination is accompanied by localized swelling, it is specifically labeled as a blister.

Class 1 acceptability:

• The area of the board affected by delamination does not exceed 1% of the total area.
• The blister or delamination should be less than 25% of the distance between conductors.
• Delamination should not be closer to the edge of the board.

Class 2 and 3 acceptability:

• Same as class 1 acceptability.
• The space between conductive patterns should not be less than the minimum conductor spacing.

Foreign materials

It is possible to identify foreign substances in the raw (B-stage) laminate, and they may be either conductive or non-conductive. For all classes, the presence of translucent particles confined within the PCB is acceptable as long as it does not impact the electrical and thermal characteristics of the board.

Key takeaways:

• Different types of FR4 material include, standard FR4, high-T_g, High CTI, FR4, and Halogen-Free FR4.
• FR4 is suitable for standard PCB applications only and should be avoid in high-frequency and RF designs.
• FR4 offers dimensional stability, excellent etchability, drillability, bondability, solderability, reworkability, wide availability, mass production compatibility, and cost-effectiveness.
• Limitations include signal losses, insulating stability, controlled impedance challenges, temperature stability, and lead-free soldering issues.
• The IPC-A-600 standard outlines specific acceptability criteria for FR4 materials, including aspects such as weave exposure, weave texture, measling, crazing, delamination/blistering, and foreign materials.
• Specific values of FR4 material properties may vary based on the manufacturer. 

FR4 stands as the prevailing material choice in PCB fabrication. These boards exhibit durability, resistance to moisture, and exceptional insulation properties between copper layers, thereby minimizing interference and preserving signal integrity. FR4 may not be an optimal material for use in high-frequency or challenging operating conditions.