The FR-4 is the most common dielectric material used in Printed Circuit Boards. This base material separates copper layers in the PCBs and provides mechanical support to the PCBs. It is a National Electrical Manufacturers Association (NEMA) standard for glass-reinforced epoxy laminate material. The substrate is a composite material in nature, achieved by combining fiberglass and epoxy resin.
FR is an abbreviation for "Flame Retardant"and indicates that the material meets the standard of the UL94V-0 on plastic material inflammability. While the number 4 indicates its grade in the family of glass epoxy laminate materials. This material ensures stopping the expansion of fire and its immediate extinguishing when the material burns.
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FR-4 is the most common printed circuit board material that uses Bromine, a halogen that has the properties to prevent or slow down the spread of fire. It is also a standard material used for PCB by National Electrical Manufacturers Association (NEMA).
The FR-4 is used as the base material which makes its electrical properties important to be noticed. As the electrical properties of a PCB material are crucial for signal integrity, impedance control, and the quality of insulation.
The following chart emphasizes the fundamental electrical properties of FR-4 Material.
The raw material of FR-4 (Fiberglass and epoxy resin) makes it lightweight, cheap, and easily accessible. Fiberglass provides good strength while epoxy resin holds the structure. Together they provide an insulator with exceptional properties.
FR-4 printed circuit board material is widely available and low in cost. Still, they provide exceptional quality functioning. However, the price for a high glass transition temperature and high comparative tracking index material is a bit high.
FR-4 material typically provides a dielectric constant (Dk) somewhere between 4.25-4.55 depending on different factors such as glass weave style, thickness, resin content, and copper foil roughness. The dielectric constant describes how the substrate stores electric energy and is one of the key parameters for insulation and impedance control. So, choosing a material with stable and well-characterized Dk helps ensure consistent electrical performance.
As we know that FR-4 is a combination of fiberglass and epoxy resin, and it provides high load-bearing capacity and mechanical strength to the overall PCB. But mainly, the strength and load bearing depend on the thickness of the material. The thickness of a standard FR-4 material varies between 0.2 to 3.2 mm
FR-4 provides moisture resistance. Humidity cannot affect FR-4 printed circuit boards such as contracting and expanding them. However, moisture absorptions affect the electrical and thermal of the material as well as the power of the material to resist conductive anode filament (CAF) formation when the circuit is powered on. Therefore, providing high moisture resistance gives an edge to FR-4 to use as the base PC material and makes it compatible to use under highly humid areas and other marine PCB applications.
FR-4 is highly temperature resistant providing a Tg value of 150Tg or 170Tg. The Tg denotes the glass transition temperature of the PCB. Glass transition temperature defines at which point or temperature the PCB will start getting softened and losing its shape. That is why the Tg value of PCB matters a lot. Because a higher number of Tg values ensures better performance of PCB. The Tg value of PCB not only affects temperature resistance but also affects moisture and chemical resistance.
The decomposition temperature (Td) defines the temperature where a PCB's almost 5% of the laminate's mass is lost due to decomposition. FR-4 PCB material provides a higher decomposition temperature of >345 Celsius. A high decomposition temperature offers better protection and long life for the printed circuit board.
The FR-4 material is available with different values regarding its properties. This variety of materials with individually improved properties helps PCB engineers to choose the equitable material that matches the requirements of their circuits.
There are four different types of FR-4 material available in the market based on individual properties, they are listed:
Standard FR-4 refers to the normal material, with heat resistance ranging from 150-160 Celsius. The properties and advantages of the standard FR-4 are listed above.
High TG FR-4 stands for high glass transition temperature. PCBs made with high glass transition temperature FR-4 material have the ability to resist and maintain their shape at a temperature of 170 ℃. In this type of base material, Higher TG is achieved through extra-efficient lamination. PCBs made with high TG FR-4 material have the ability to resist and maintain their shape at a temperature of 170 ℃. The standard Fr-4 material is a great insulator and flame retardant, but still, it has some limits when exposed to high power, voltage, or heat. If a certain limit exceeds, the materials insulating properties can get weakened. It can start conducting electricity instead of insulating it. Thereby high Tg FR-4 is used to avoid these unwanted circumstances.
Applications of High TG FR-4 Material:
The Comparative Tracking Index is the limit to which the FR-4 or any other insulating material can resist the unwanted current flowing between the tracks on the printed circuit board. The CTI value indicates how resistive PCB base material is against unpleasant environmental and electrical conditions such as moisture and leakages on board. The high CTI material with a maximum value of 600 V ensures that the base is more resistant and can withstand harsh environmental and electrical conditions.
Applications of High CTI FR-4 Material:
This type of FR-4 is a little different with distinct functions. As the name itself explains that this type of material has no copper lamination on it and, therefore, is used to insulate and support other boards, etc.
Though FR-4 PCBs are widely famous for their quality and low prices, providing good electrical and mechanical properties. The application of FR-4 PCB material is vast. From small circuit boards to large and complex systems, they have been in use for years. But could this material be the answer for every type of circuit, especially for the circuits with high-frequency requirements?
Related reading: If you’re choosing materials for RF / high-frequency designs, see Rogers vs FR4: How to Choose PCB Materials for High-Frequency Designs.
When it comes to high-frequency laminate, there are multiple factors to consider, such as:
In many passive circuits, the signal loss increases with the gain in frequency. As we know that FR-4 PCB material has a higher dissipation factor (Df ) than laminates designed for high-frequency use, and due to this, the circuits fabricated on FR-4 suffer more loss in signal than similar circuits constructed on a PCB with high-frequency laminate material. The typical value for signal loss is about 0.020 for FR-4 and about 0.004 for a high-frequency laminate or any other base material with a dissipation factor that is about one-fourth of FR-4. The difference between the signal loss of both materials due to the dissipation factor is huge and can affect the performance of the whole system. Therefore, the capacity of designs to tolerate signal loss is different and majorly depends on the loss budget of individual projects.
Bottom Line:Due to the high dissipation factor (Df), signal loss is higher in standard FR-4 PCBs as compared to high-frequency laminates.
Stable impedance is crucial for many designs, such as high-speed digital circuits. Stable impedance is achieved by maintaining a stable dielectric constant (DK) across the substrate. The dielectric constant in FR-4 varies with changes in frequency. In FR-4 PCB material, the dielectric constant (Dk) may change as frequency increases, and it can also be influenced by resin system, glass weave style, and manufacturing tolerances. Though, the dielectric constant tolerances for high-speed materials are less than 2%, whereas FR-4 can fluctuate up to 10%. Therefore, FR-4 makes it almost impossible to achieve stable impedance. On the other hand, high-frequency laminates provide a stable dielectric constant across the whole board. The stable dielectric constant helps in maintaining stable impedance.
Bottom Line: Due to the unstable dielectric constant, FR-4 is unable to provide a stable impedance. Hence, it cannot be used for circuits that require controlled impedance. In this case, high-frequency laminates are great options to avail.
Just like the dielectric constant, FR-4 also has a higher thermal coefficient of the dielectric constant (TCDK) too. TCDK is a way to conclude how much the Dk changes over a given temperature. For FR-4, it is typically 200 parts per million (ppm) per Celsius. This number seems too little to be worried about, but a wide temperature range can result in huge variations. On the other hand, high-frequency lamination provides an 80% smaller value of 40 parts per million (ppm) per Celsius. This property of FR-4 made it incapable of being used with designs that require little variations over a wide temperature range. In this case, high-frequency lamination is best to use.
Bottom Line: FR-4 has a higher TCDK compared to high-frequency lamination, which makes it incapable of using in circuits that require little variation over a wide range of temperatures.
In comparison to high-frequency laminate, FR-4 has so many downsides. But there are still some properties that make FR-4 stand out. Such as, moisture absorption of FR-4 is higher than high-frequency lamination. The moisture absorption property of the substrate is crucial to note while working with marine and outdoor applications and moisture-sensitive devices or circuits.
Bottom Line: FR-4 has a higher ability to absorb water. Hence, it is better to use FR-4 for circuits where moisture could be a problem.
On a wavelength-dependent circuit, the results of using FR-4 or high-frequency lamination could be different. On a wavelength-dependent circuit such as a high-frequency circuit, the dielectric constant will affect the size of the whole circuit. Therefore, circuit boards with higher Dk can produce smaller circuits and vice versa. As we know, the dielectric constant value of the FR-4 is about 4.5, which is still higher than most PTEF high-frequency materials but lower than several high-frequency laminates, which can provide a dielectric constant (Dk) of about 6.15-11.0. So, high-frequency laminate or any other material with such high Dk can save up to 25-30% or much more of the size of a circuit board compared to FR-4.
Bottom Line: For wavelength-dependent devices such as radio, filters, power amplifiers, etc. High-frequency laminate is better to use due to the high dielectric constant, which will help to reduce the size of the circuit.
In a nutshell, the difference between FR-4 PCB material and high-frequency laminate is mainly of dielectric constant (Dk) and dissipation factor (Df). High-frequency laminate provides high dielectric constant Dk and low dissipation factor Df, which makes it perfect to use mainly for Radio PCBs, antennas, filter circuits, power amplifiers, and high-speed digital PCBs.
We all know that FR-4 is a standard set by National Electrical Manufacturers Association (NEMA) for glass-reinforced epoxy laminate material. But it is not the only substrate to use as a PCB base.
As far as it is concerned about simple and basic circuit designs that run at low frequencies and are not subject to extreme environmental conditions, FR-4 might be a great choice. But for modern and high-frequency circuits with more accurate material requirements, there are other materials available to consider.
Why do we need FR-4 alternative materials? Not every circuit is the same, and neither their specifications are. Some boards might have high-temperature requirements, and some could have high frequency. The desired requirements of substrate change from circuit to circuit, and a single material cannot fulfill all of them.
To understand and compare FR-4 with other substrate materials, first we need to understand the limitations of FR-4.
