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Multilayer Fr4 PCB Substrate

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Multilayer Fr4 PCB Substrate

Choosing the right PCB (Printed Circuit Board) substrate is crucial in electronic design and manufacturing. The substrate serves as the foundation for the PCB, providing mechanical support and electrical insulation for the components and traces on the board. The choice of substrate material can significantly impact the performance, reliability, and manufacturability of the PCB.

This Article we will discuss how to choose multilayer FR4 PCB Substrate, and why it’s so important in the PCB Manufacturing.

Now, Let’s go!

What Is FR4?

FR4 is a widely used type of substrate material in the manufacturing of printed circuit boards (PCBs). It is a composite material composed of a woven glass fabric reinforcement impregnated with an epoxy resin binder. The term “FR4” stands for Flame Retardant 4, referring to its fire-resistant properties.

The glass fabric reinforcement provides mechanical strength and stability to the PCB, while the epoxy resin serves as a dielectric material that isolates the conductive traces on the board. FR4 is known for its excellent electrical insulation properties, dimensional stability, and cost-effectiveness, making it a popular choice for a wide range of applications.

FR4 is available in various grades and thicknesses to meet different performance requirements. It is compatible with standard PCB manufacturing processes and can be easily fabricated into multilayer boards, making it suitable for complex designs. FR4 substrates are commonly used in consumer electronics, telecommunications equipment, automotive electronics, industrial control systems, and many other electronic devices and systems.

What’s Mean for Multilayer FR4 PCB Substrates?

Multilayer FR4 PCB substrates refer to printed circuit boards that consist of multiple layers of FR4 material stacked together. In a multilayer PCB, the FR4 layers are separated by thin layers of copper foil, which serve as the conductive pathways for electrical signals.

The construction of a multilayer FR4 PCB involves laminating multiple FR4 layers together using heat and pressure. Each layer is typically etched with copper traces and vias that connect the different layers, allowing for complex interconnections and increased circuit density.

The number of layers in a multilayer FR4 PCB can vary depending on the complexity of the circuit and the design requirements. Commonly used layer counts include 4-layer, 6-layer, 8-layer, and higher. The use of multiple layers allows for better signal integrity, reduced electromagnetic interference (EMI), and more efficient use of space compared to single or double-sided PCBs.

Factors to Consider When Choosing Multilayer FR4 PCB Substrate

When choosing a multilayer FR4 PCB substrate, several factors should be considered to ensure the optimal performance and reliability of the PCB. Here are some key factors to consider:

1.Layer count:

Determine the required number of layers based on the complexity of the circuit and the desired density of components. Higher layer counts allow for more complex designs and increased routing flexibility.


Select the appropriate thickness of the FR4 substrate based on the mechanical and electrical requirements of the PCB. Thicker substrates provide better rigidity and can handle higher current levels, while thinner substrates offer more compactness and flexibility.

3.Dielectric constant (Dk):

Consider the dielectric constant of the FR4 material, as it affects the impedance and signal propagation characteristics of the PCB. Choose a substrate with a consistent and controlled Dk value to maintain signal integrity.

4.Thermal properties:

Evaluate the thermal conductivity and coefficient of thermal expansion (CTE) of the FR4 substrate. These properties impact the PCB’s ability to dissipate heat and withstand thermal stresses. Higher thermal conductivity and a compatible CTE with other components are desirable for efficient thermal management.

5.Flame retardancy:

Ensure that the chosen FR4 substrate meets the required flame retardancy standards for the intended application. FR4 materials are typically flame retardant, but different grades may have varying levels of flame resistance.

6.Surface finish:

Consider the surface finish options available for the multilayer FR4 PCB. Common surface finishes include HASL (Hot Air Solder Leveling), ENIG (Electroless Nickel Immersion Gold), and OSP (Organic Solderability Preservative). The surface finish affects solderability, corrosion resistance, and overall reliability.

7.Manufacturing capabilities:

Verify that the chosen multilayer FR4 PCB substrate is compatible with the manufacturing processes and equipment available. Ensure that the PCB manufacturer has experience and expertise in handling multilayer designs.


Evaluate the cost-effectiveness of the multilayer FR4 PCB substrate, considering factors such as material cost, manufacturing complexity, and any additional features or requirements.

By considering these factors, you can select a multilayer FR4 PCB substrate that meets the specific needs of your application in terms of performance, reliability, and cost.

How to Select Multilayer Fr4 Substrate Thickness?

Selecting the appropriate multilayer FR4 PCB substrate thickness involves considering several factors related to the specific application and design requirements. Here are some considerations to help you make an informed decision:

1. Mechanical requirements: Evaluate the mechanical constraints of your application. Consider factors such as the available space for the PCB, mounting constraints, and any weight limitations. Thicker substrates may provide more mechanical strength and rigidity, which can be beneficial in applications with higher mechanical stress or when components need to be mounted securely.

2. Electrical performance: The substrate thickness can affect the electrical performance of the PCB. Thinner substrates generally offer improved high-frequency performance due to reduced signal propagation delays and lower parasitic capacitance. Thicker substrates, on the other hand, may provide better impedance control and power handling capabilities.

3. Heat dissipation: Consider the thermal management requirements of your PCB. Thicker substrates can have better heat dissipation capabilities due to increased thermal mass. This can be advantageous in applications where heat generation is a concern, as it helps in spreading and dissipating heat more effectively.

4. Layer count and complexity: The layer count and complexity of your PCB design can influence the substrate thickness selection. Higher layer counts may require thicker substrates to accommodate the additional layers and provide sufficient insulation between them. Complex designs with fine-pitch components or high-density interconnects may benefit from thinner substrates to facilitate precise routing and reduce parasitic capacitance.

5. Manufacturing capabilities: Evaluate the manufacturing capabilities of your PCB manufacturer or supplier. Some manufacturers may have limitations on the minimum or maximum substrate thickness they can handle. Ensure that the selected thickness falls within their manufacturing capabilities to avoid any production issues.

6. Cost considerations: Thicker substrates generally cost more due to the increased material usage. Consider your budget and cost constraints when selecting the substrate thickness. Find a balance between the desired performance and the cost-effectiveness of the chosen thickness.

It’s important to note that there isn’t a one-size-fits-all approach to selecting the substrate thickness. The optimal thickness will depend on the specific requirements of your application. It is recommended to consult with PCB manufacturers, engineers, or industry experts who can provide guidance based on their experience and knowledge of similar applications.

Additionally, performing simulations or prototypes can help evaluate the impact of different substrate thicknesses on electrical performance, thermal management, and mechanical stability, allowing you to make an informed decision.

Evaluating Different Multilayer FR4 PCB Substrate Options

When evaluating different multilayer FR4 PCB substrate options, it’s important to consider the following aspects:

1. Material quality: Assess the quality and reliability of the FR4 material used in the substrate. Look for reputable suppliers and ensure that the material meets industry standards and certifications.

2. Performance requirements: Determine the specific performance requirements of your PCB, such as signal integrity, impedance control, power handling, and thermal management. Choose a substrate that can meet these requirements effectively.

3. Layer count and stackup design: Consider the required layer count based on the complexity of your circuit design. Evaluate the stackup design options offered by different substrate options to ensure they can accommodate your specific needs.

4. Dielectric properties: Analyze the dielectric properties of the substrate, including the dielectric constant (Dk), dissipation factor (Df), and insulation resistance. These properties affect signal propagation, impedance control, and overall circuit performance.

5. Thermal properties: Evaluate the thermal conductivity, CTE, and glass transition temperature (Tg) of the substrate. These properties determine the substrate’s ability to dissipate heat and withstand thermal stresses, which is crucial for reliable operation.

6. Flame retardancy: Verify that the substrate meets the required flame retardancy standards for your application. Consider the specific flame retardant rating and any additional certifications or compliance requirements.

7. Surface finish compatibility: Ensure that the substrate is compatible with the desired surface finish options for your PCB. Different surface finishes offer various advantages in terms of solderability, corrosion resistance, and wire bonding compatibility.

8. Manufacturing capabilities: Assess the manufacturing capabilities of the PCB suppliers or manufacturers offering the substrate options. Consider their experience, expertise, and equipment capabilities in handling multilayer FR4 PCB production.

9. Cost-effectiveness: Compare the costs of different substrate options, considering factors such as material cost, manufacturing complexity, and any additional features or requirements. Balance the cost with the desired performance and reliability to find the most cost-effective option.

By carefully evaluating these factors, you can make an informed decision when selecting the most suitable multilayer FR4 PCB substrate option for your specific application.

How Testing and Validation of Multilayer FR4 PCB Substrate?

Testing and validation of multilayer FR4 PCB substrates are essential steps in ensuring the quality, reliability, and performance of the PCB. Here are some common testing and validation methods:

1. Electrical testing: Electrical testing involves checking the functionality and performance of the PCB by verifying the continuity of the circuitry, measuring impedance, checking signal integrity, and performing functional tests. This can be done using automated test equipment (ATE) or specialized testing tools. Various tests, such as continuity tests, resistance checks, and signal integrity measurements, are performed to ensure proper electrical connectivity and performance.

2. Thermal testing: Thermal testing is conducted to evaluate the thermal characteristics of the multilayer FR4 PCB substrate. This includes assessing its ability to dissipate heat, thermal conductivity, and thermal expansion properties. Thermal imaging, thermal cycling, and thermal resistance measurements are some of the techniques used to assess the thermal performance of the PCB.

3. Mechanical testing: Mechanical testing is performed to assess the mechanical strength, durability, and reliability of the multilayer FR4 PCB substrate. This includes tests such as bend tests, flex tests, vibration tests, and shock tests. These tests help ensure that the PCB can withstand mechanical stress, environmental conditions, and handling during its lifecycle.

4. Environmental testing: Environmental testing involves subjecting the multilayer FR4 PCB substrate to various environmental conditions to evaluate its performance and reliability. This includes tests such as temperature cycling, humidity testing, salt spray testing, and thermal shock testing. These tests assess the PCB’s ability to withstand temperature variations, moisture, corrosion, and other environmental factors.

5. Reliability testing: Reliability testing is conducted to assess the long-term performance and reliability of the multilayer FR4 PCB substrate. This includes accelerated life testing (ALT), which involves subjecting the PCB to accelerated aging conditions to simulate the effects of prolonged use. ALT helps identify potential failure modes and estimate the lifespan of the PCB.

6. Signal integrity testing: Signal integrity testing is performed to evaluate the quality and integrity of the signals transmitted through the multilayer FR4 PCB substrate. This includes tests such as eye diagram analysis, jitter measurements, and impedance measurements. These tests ensure that the PCB can maintain signal integrity and minimize signal degradation or distortion.

7. Design rule checks (DRC): DRC involves verifying the PCB design against specific design rules and guidelines to ensure compliance and manufacturability. DRC checks for issues such as trace spacing, pad sizes, solder mask clearances, and other design-related parameters. This helps identify potential design flaws or manufacturing issues that could affect the performance or reliability of the PCB.

It’s important to note that the specific testing and validation methods may vary depending on the application, industry standards, and specific requirements of the multilayer FR4 PCB substrate.

Important of Multilayer FR4 Substrate in Circuit Boards Manufacturung Process

Multilayer FR4 PCB substrates play a crucial role in the manufacturing process of circuit boards. Here are some important aspects of their significance:

1.Complex interconnections:

Multilayer FR4 PCB substrates allow for the creation of complex interconnections between different components and layers of a circuit board. The multiple layers provide ample routing space, enabling the design of intricate circuitry and high-density interconnects.

2.Increased circuit density:

Multilayer FR4 PCB substrates enable the placement of more components and circuitry within a limited board space. By stacking multiple layers, the overall circuit density can be significantly increased, allowing for more functionality in a compact form factor.

3.Better signal integrity:

The use of multilayer FR4 PCB substrates helps in achieving better signal integrity by providing controlled impedance routing. The dielectric properties of FR4 material, such as its consistent dielectric constant (Dk), allow for precise impedance control, reducing signal reflections and maintaining signal integrity.

4.Reduced electromagnetic interference (EMI):

The multiple layers in multilayer FR4 PCB substrates act as shielding, reducing the chances of electromagnetic interference. The separation between layers helps contain electromagnetic emissions and provides better isolation between different circuit sections, minimizing EMI-related issues.

5.Improved reliability:

Multilayer FR4 PCB substrates offer enhanced reliability due to their robust construction. The multiple layers provide mechanical strength and stability, reducing the chances of warping or flexing. This ensures the longevity and durability of the circuit board, especially in demanding environments.

6.Design flexibility:

Multilayer FR4 PCB substrates provide design flexibility, allowing for the integration of various circuit elements, such as power planes, ground planes, and signal layers. This flexibility enables the implementation of complex circuit designs, including high-speed digital circuits, RF circuits, and mixed-signal circuits.

7.Manufacturing efficiency:

Multilayer FR4 PCB substrates streamline the manufacturing process by consolidating the circuitry into fewer layers. This simplifies the assembly and testing processes, reduces the number of interconnections required, and improves overall manufacturing efficiency.


While multilayer FR4 PCB substrates may have a higher initial cost compared to single-layer or double-layer boards, they offer cost advantages in terms of space utilization, reduced wiring complexity, and improved manufacturing efficiency. The overall cost per unit functionality can be lower when using multilayer FR4 PCB substrates.

In Conclusion

Selecting the appropriate multilayer FR4 PCB substrate thickness requires considering various factors such as mechanical requirements, electrical performance, heat dissipation, layer count and complexity, manufacturing capabilities, and cost considerations. It’s important to find a balance between the desired performance and the practical constraints of the application.

By carefully considering the requirements of your specific application, seeking expert advice, and conducting thorough testing and validation, you can ensure the selection of an appropriate multilayer FR4 PCB substrate thickness and achieve a reliable and high-performance PCB solution.

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