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 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.
It is extensively used in the electronics industry, particularly in the production of printed circuit boards.
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.
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.
The table below includes a few significant electrical properties, along with their typical values.
|
Electrical property |
Typical value |
|
Dielectric constant (Dk) |
5 at 1 MHz |
|
Electrical strength |
800 to 1800 volts/mil |
|
Dissipation factor |
0.009 at 1 MHz |
|
Surface resistivity (Ohm/square unit) |
105 to 1010 |
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.
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.
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%.
The glass transition temperature is the temperature at which the PCB begins to soften and lose its shape. Higher Tg values guarantee improved thermal resistance. In addition to this, moisture and chemical resistance are also impacted by the Tg value of PCB.
FR4 has a high Tg, 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.
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.
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-Tg FR4 materials are engineered to have a higher Tg compared to standard FR4. Printed circuit boards constructed using high Tg 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 Tg FR4 is ideal for applications requiring improved thermal resistance.
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 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 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.
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:
|
|
FR4 |
High-frequency laminate |
|
Dielectric constant (Dk) |
Relatively higher Dk in the range of 3.8 to 4.8. This value can vary slightly depending on the specific FR4 grade. |
Lower Dk values, typically ranging from 2.2 to 3.5 or even lower. Lower Dk values are desirable for high-frequency applications to minimize signal loss and maintain signal integrity. |
|
Dissipation factor (Df) |
Relatively higher dissipation factor, typically around 0.020, which can lead to higher signal losses at high frequencies. |
Lower Df values, often less than 0.004, result in reduced signal loss and improved performance at high frequencies.
|
|
Operating frequency |
It is suitable for standard PCB applications and is not optimized for high-frequency operation. |
Specifically engineered for high-frequency applications, including microwave and RF designs. |
|
Thermal properties |
Typically, 200 ppm/°C change in temperature, which can result in significant variations over a wide temperature range.
Its moderate thermal properties are suitable for standard soldering processes. They may not be ideal for applications with extreme thermal demands. |
About 40 ppm/°C change in temperature results in much lesser variations in the circuit over a wide temperature range.
It showcases enhanced thermal properties to withstand high-temperature processes and the heat generated by high-power components. |
|
Cost |
Cost-effective, making it the preferred choice for standard PCBs. |
These are specialized materials and tend to be more expensive due to their advanced properties. |
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.
FR4 offers several advantages that make it a preferred choice for efficient PCB fabrication:
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.
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.
When drilling holes, the reinforcing weave allows for clean punch-through. As a result, plated-through holes for layer interconnections become possible.
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.
If there are damaged components on the board, technicians can remove and replace damaged surface mount components without significantly damaging the PCB pad surfaces.
FR4 material is readily available from various manufacturers in standardized forms, ensuring a consistent supply chain, which is essential for large-scale production.
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.
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.
|
Manufacturer and Name |
Glass transition temperature (Tg) (°C) |
Decomposition temperature (Td) (°C) |
Dielectric constant (Dk) @ 10GHz |
Dissipation factor (Df) @ 10 GHz |
Moisture absorption (%) |
Application area
|
|
Isola - 370 HR |
180 |
340 |
3.92 |
0.025 |
0.15 |
Normal speed, normal loss |
|
Isola - FR408HR |
200 |
360 |
3.65 |
0.0095 |
0.15 |
High Speed, low loss |
|
Nelco - N4000-13 |
210 |
350 |
3.6 |
0.009 |
0.1 |
Medium speed, low loss |
|
Nelco - N4000-13SI |
210 |
350 |
3.2 |
0.008 |
0.1 |
Medium speed, low loss |
|
Ventec - VT-47 |
170 |
340 |
4.27 |
0.016 |
0.12 |
Normal speed, normal loss |
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 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 (Df) compared to materials specifically engineered for high-frequency use.
FR4 exhibits a Df of approximately 0.020, whereas most high-frequency laminates boast a significantly lower Df, typically around 0.004, merely a quarter of FR4's Df . A smaller Df corresponds to reduced signal loss.
FR4's Df 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.
Unlike high-speed board materials, FR4 does not provide a consistent dielectric constant. The Dk 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.
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 Tg of many FR4 variants.
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 (Dk) 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.
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.
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.
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.
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 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.
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:
Class 2 and 3 acceptability:
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:
Class 2 and 3 acceptability:
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:
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.