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PCB Substrate: A Comprehensive Guide to PCB Base Materials

PCB design and manufacturing are essential for modern electronics, and the choice of PCB substrate and base material can have a significant impact on the resulting product's performance and quality.

Printed circuit boards (PCBs) form the backbone of modern-day electronic devices. They are vital for ensuring the proper functioning of electronic devices, ranging from smartphones and laptops to medical equipment and automotive systems. A PCB serves as a platform for components to be mounted and connected through conductive traces, allowing electrical signals to flow between them. One of the key considerations in PCB design and manufacture is the choice of substrate material. The substrate, also known as the base material or the PCB laminate, plays a crucial role in determining the board's overall performance and reliability. In this article, we will explore the different types of substrates available for PCBs, their properties, and factors to consider when selecting a substrate.

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Introduction to PCB basics

A PCB is a board that connects various electronic components in a device. It provides a mechanical foundation for the device and allows for efficient communication between components.

There are various types of PCBs that are designed to meet different requirements. Some common types include single-sided PCBs, double-sided PCBs, and multi-layer PCBs. Each type is designed to accommodate a specific set of components and requires different manufacturing techniques.

The choice of base material (also known as substrate) is an important factor in the design of a PCB. The base material provides mechanical support and electrical insulation to the copper traces on the circuit board. Different types of base materials have different properties, such as thermal conductivity and dielectric constant, which can impact the performance of the finished PCB.

Common Types of Base Materials

We are known that PCBs are essential components in electronic devices, and selecting the right base material for a PCB is critical to ensure its performance and durability. There are a variety of base materials available, each with its own unique set of properties and applications. In this discussion, we will explore some of the common types of base materials used for PCBs, their physical and mechanical characteristics, as well as the factors to consider when selecting a base material for a specific application.

1 - Introduction to Base Materials

Base materials are the substrates on which printed circuits are created. They serve as the foundation for the conductive circuitry and provide the mechanical support for the components and solder connections. The choice of base material is a critical factor in the overall performance and reliability of the finished PCB. Base materials are available in a broad range of compositions, each with its unique properties and characteristics. Therefore, selecting the right base material is an essential element of successful PCB manufacturing.

4 Properties of PCB substrate dielectric materials

1.1 - Thermal Properties

Thermal properties are important factors to consider when selecting materials for PCB manufacturing. These properties include the glass transition temperature, decomposition temperature, coefficient of thermal expansion, and thermal conductivity.

The glass transition temperature (Tg) is the temperature at which a material changes from a rigid, glassy state to a more flexible, rubbery state. For PCBs, it is important to select a material with a high Tg to ensure the board maintains its shape and properties under high temperatures during operation.

The decomposition temperature (Td) is the temperature at which a material begins to break down and lose its structural integrity. In PCB manufacturing, it is important to select a material with a higher Td to ensure the board can withstand high temperatures during assembly and operation.

The coefficient of thermal expansion (CTE) is the rate at which a material expands or contracts based on changes in temperature. For PCBs, it is important to select a material with a low CTE to reduce the risk of stress and damage from thermal cycling during operation.

Thermal conductivity is the ability of a material to transfer heat. For PCBs, it is important to select a material with high thermal conductivity to dissipate heat generated by components during operation, reducing the risk of overheating and component failure.

1.2 - Electrical properties

Relative Permittivity or Dielectric Constant (Dk or Er) is the measure of a material’s ability to store electric charge, also known as capacitance. It is an important electrical property in PCB manufacturing as it affects the signal propagation speed and attenuation. The higher the dielectric constant, the slower the signal speed and the higher the signal loss. Typical values for Dk range from 2-12 for PCB materials.

Dissipation Factor or Dielectric Loss Tangent (Df Tan δ) is a measure of the energy lost in the material as a result of dielectric heating and is also an important electrical property in PCB manufacturing. It is the ratio of the energy dissipated to the energy stored in a dielectric material and is an indication of the loss of signal energy in dielectric materials. Typical values for tan δ range from 0.002-0.05 for PCB materials.

Volume Resistivity is the measure of a material’s ability to resist electrical current flow through the bulk or volume of the material. It is an important electrical property in PCB manufacturing as it helps ensure that current flows only in the intended circuit and not through unintended paths. The higher the volume resistivity of a material, the better it is able to prevent current leakage. Typical values for volume resistivity range from 1e4 to 1e15 ohm-cm for PCB materials.

Surface Resistivity (ρS) is the measure of a material’s ability to resist electrical current flow across the surface of the material. It is an important electrical property in PCB manufacturing as it helps ensure that current flows only through the intended circuit and not through the surface of the material. The higher the surface resistivity, the better it is able to prevent current leakage. Typical values for surface resistivity range from 10e4 to 10e13 ohm/sq for PCB materials.

Electrical Strength is the maximum voltage that a material can withstand under a given set of conditions without breaking down or suffering permanent damage. It is an important electrical property in PCB manufacturing as it helps ensure the reliability of the PCB under high voltage conditions. Typical values for electrical strength range from 10-100 kV/mm for PCB materials.

1.3 - Chemical Properties

Flammability Specs – UL94: UL94 is a standard to evaluate the flammability of plastic materials. It is based on test methods to assess the ignition and burning behavior of the material. This standard classifies plastic materials into different categories according to their burning behavior, with Class V0 being the most flame-retardant and Class HB being the least. The higher the classification, the more resistant the material is to flame propagation.

Moisture Absorption: Moisture absorption is the ability of a material to absorb moisture from the environment. It can lead to changes in the material’s physical and electrical properties, including dimensional changes and deterioration of dielectric properties. The amount of moisture absorption varies depending on the material type and environmental conditions. For example, the moisture absorption rate of FR-4 is typically less than 0.2%.

Methylene Chloride Resistance: Methylene chloride is a common solvent used in the manufacturing of PCBs. However, it can cause damage to some materials and affect their properties. Methylene chloride resistance refers to the ability of a material to resist this solvent without undergoing significant changes or damage. The methylene chloride resistance of different materials can vary widely. For example, CEM-3 is more resistant to methylene chloride than FR-4.

1.4 - Mechanical Properties

Peel Strength: This refers to the strength required to separate two bonded layers. Commonly used units are N/m or lbs/in. For FR4 PCBs, the typical peel strength is around 1.5 N/mm or 8.5 lbs/in, which is sufficient for most applications.

Flexural Strength: This is the stress required to bend or deform a material until it breaks. It is usually measured in MPa or psi. For FR4 PCBs, the flexural strength is around 340 MPa or 49,000 psi, which is high enough to withstand typical assembly and use.

Young’s Modulus: This is a measure of the stiffness of a material and is measured in GPa or psi. For FR4 PCBs, the typical Young’s Modulus is around 17-20 GPa or 2.5-3.0 million psi, which is also high enough to provide good support and stability.

Density: This is the mass per unit volume of a material, usually measured in g/cm^3 or lbs/in^3. For FR4 PCBs, the density ranges from 1.80 to 1.90 g/cm^3 or 0.065 to 0.069 lbs/in^3, which is typical for most circuit board applications.

Time to Delamination: This refers to the time it takes for a PCB to lose its bond strength and separate into layers due to mechanical or thermal stress. For FR4 PCBs, the time to delamination is typically longer than 200 hours at 260℃, which is considered good performance for most applications.

The following is a comparison of PCB substrate materials:

PCB Material	Typical Usage	DK	Tg (^oC)	Recommended Board Type
FR-4	Substrate, Laminate	4.2 to 4.8	135	Standard
CEM-1	Substrate, Laminate	4.5 to 5.4	150 – 210	High Density
RF-35	Substrate	3.5	130	High Density
Teflon	Laminate	2.5 to 2.8	160	Microwave, High Power, High Frequency
Polyimide	Substrate	3.8	>= 250	High Power, Microwave, High Frequency
PTFE	Substrate	2.1	240 to 280	Microwave, High Power, High Frequency

2 - FR-4 Materials

2.1 - What is FR-4 Materials?

FR-4 materials are a type of fiberglass-reinforced epoxy laminate that is commonly used for electronic PCBs. The name “FR-4” refers to the grade of the material, which meets specific fire-retardancy requirements. It is a widely used PCB material due to its low cost, high mechanical strength, and excellent electrical insulation properties.

2.2 - Physical and mechanical properties of FR-4 materials

FR-4 is a composite material made of woven fiberglass cloth impregnated with a thermosetting resin. The material is rigid and provides excellent dimensional stability, even at high temperatures. It has excellent electrical insulation properties and a dielectric constant of about 4.5. The material is also resistant to moisture, chemicals, UV radiation, and high-energy radiation. However, FR-4 materials are brittle and prone to cracking under stress.

2.3 - Advantages and disadvantages of FR-4 materials

Advantages of FR-4 materials include their low cost, high mechanical strength, excellent electrical insulation properties, and resistance to environmental damage. They are also easy to fabricate and can be used in a wide range of applications. The disadvantages of FR-4 materials include their brittleness, which can make them prone to cracking and fracturing, as well as their limited thermal conductivity.

2.4 - Common applications of FR-4 materials

FR-4 materials are commonly used for electronic printed circuit boards, particularly in consumer electronic devices such as smartphones, tablets, and laptops. They are also used in automotive and aerospace industries due to their high mechanical strength, resistance to environmental damage, and electrical insulation properties. Other applications of FR-4 materials include insulation panels, lighting fixtures, and industrial machinery.

3 - CEM Series Materials

3.1 - Definition and overview of CEM series materials

CEM series materials are a type of PCB substrate made from woven glass and a resin binder. This substrate is used as the base material for all PCBs and is responsible for providing mechanical support and insulation for the copper traces and components that will be placed onto it.

The resin binder used in CEM series materials can be of different types, leading to different grades of CEM materials with different properties. CEM series materials have excellent physical and mechanical properties, and they are popularly used in high-performance electronic applications.

3.2 - Types of CEM series materials based on resin type

The different grades of CEM series materials are classified based on the resin binder used in their manufacture. There are four main types of CEM series materials, namely CEM-1, CEM-2, CEM-3, and CEM-4.

CEM-1 is made with a phenolic resin binder that gives it good mechanical strength and rigidity. CEM-2 also uses phenolic resin, but it is mixed with epoxy resins to improve its electrical and mechanical properties. CEM-3 is made entirely with epoxy resin, and it typically offers better thermal management and improved dimensional stability. CEM-4 uses polyimide resin that is known for its high-temperature resistance and excellent chemical resistance.

3.3 - Physical and mechanical properties of CEM series materials

CEM series materials typically have excellent physical and mechanical properties, including high-temperature resistance, chemical resistance, and excellent electrical insulation. The specific properties of CEM series materials depend on their resin type and thickness.

CEM series materials are highly durable and have excellent dimensional stability, making them ideal for high-performance PCB applications. They also have good mechanical strength and rigidity, and can withstand harsh environmental conditions, making them suitable for a wide range of applications.

3.4 - Advantages and disadvantages of CEM series materials

The main advantages of CEM series materials are their high strength and durability, excellent thermal and chemical resistance, and good electrical properties. They are also highly resistant to physical impact and can withstand harsh environmental conditions.

However, the manufacturing process for CEM series materials is more complex than for other types of substrate materials, resulting in a higher cost. Additionally, they are not as flexible as other substrate materials, which can reduce their suitability for some applications.

3.5 - Common applications of CEM series materials

CEM series materials are commonly used in a wide range of electronic applications that demand high-performance PCBs. They are particularly well-suited for telecommunications, computer manufacturing, medical devices, and automotive electronics.

CEM series materials are ideal for high-temperature applications, such as soldering or reflow, and offer excellent chemical resistance to withstand exposure to solvents, acids, and other chemicals. They are also suitable for applications where high-frequency circuitry is required.

4 - Metal Substrate Materials

4.1 - Definition and overview of metal substrate materials

Metal substrate materials are a type of PCB base material that uses a metallic layer as the foundation, instead of traditional organic polymers. This metallic layer is typically made of aluminum, copper, or iron-nickel alloys. Metal substrate materials provide superior thermal dissipation and mechanical stability, making them ideal for high-power applications.

4.2 - Types of metal substrate materials based on metal type

The most common metal substrate materials are aluminum-based (aluminum nitride, aluminum oxide, and aluminum-clad copper), copper-based (copper-clad boards), and iron-nickel alloy-based (Kovar). Each type has unique properties and is chosen based on specific application requirements.

4.3 - Physical and mechanical properties of metal substrate materials

Metal substrate materials have excellent thermal conductivity, high melting points, and superior mechanical stability. They can withstand high temperatures and are resistant to warping, cracking, and thermal expansion. Additionally, they have low thermal resistance, allowing for efficient heat dissipation.

4.4 - Advantages and disadvantages of metal substrate materials

The main advantage of metal substrate materials is their thermal dissipation capabilities, making them ideal for high-power applications. They also offer superior mechanical stability, reducing the likelihood of failure due to physical stresses. However, metal substrate materials are more expensive than traditional organic polymers and are more difficult to manufacture. Additionally, they have a limited range of flexibility, making them unsuitable for applications that require complex shapes or high levels of bending.

4.5 - Common applications of metal substrate materials

Metal substrate materials are commonly used in high-power LED lighting, power converters, and power supplies, where heat dissipation is critical. They are also used in military and aerospace applications due to their high reliability and durability. Other applications include communication devices, automotive electronics, and renewable energy systems.

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5 - High-Frequency Board Materials

5.1 - What is high-frequency board materials?

High-frequency board materials refer to a class of materials specifically designed to operate in high-frequency electronic applications, typically above 1 GHz. These materials have a low dielectric constant and low loss tangent, which enables them to transmit signals with minimal attenuation and distortion.

5.2 - Types of high-frequency board materials based on dielectric constant and loss tangent

There are two main types of high-frequency board materials based on dielectric constant: low-loss materials with a dielectric constant of 3 or less, and ultra-low-loss materials with a dielectric constant of 2 or less. Materials with a low loss tangent, typically less than 0.002, are also highly desired for high-frequency applications.

5.3 - Physical and mechanical properties of high-frequency board materials

High-frequency board materials typically have an excellent combination of physical and mechanical properties, such as high-temperature stability, low coefficient of thermal expansion, and good dimensional stability. Other key properties include high mechanical strength, good chemical resistance, and low moisture absorption.

5.4 - Advantages and disadvantages of high-frequency board materials

The advantages of high-frequency board materials include low signal attenuation, low distortion, and excellent thermal stability. They are also able to provide a high degree of reliability and durability in demanding applications. However, high-frequency board materials are often more expensive than other types of materials, and can be more difficult to manufacture.

5.5 - Common applications of high-frequency board materials

High-frequency board materials are widely used in various high-frequency applications, including wireless communication, microwave, and millimeter-wave devices, radar systems, and satellite communications. Other common applications include automotive electronics, aerospace electronics, and medical electronics.

Rigid PCB substrate vs. Flexible PCB substrate

A rigid PCB substrate is a type of printed circuit board that has a solid and inflexible base material. This base material, also known as the substrate, is typically made of fiberglass-reinforced epoxy resin, which is a type of composite material that is very strong and durable. Because of this, rigid PCBs can be used in applications that require high reliability and stability, such as in aerospace, military, and medical equipment. The rigidity of the substrate also allows for a more precise placement of components on the board.

On the other hand, a flexible PCB substrate is made of thin, flexible material, such as polyimide or polyester. This allows the board to bend and flex without breaking, making it ideal for applications that require some degree of flexibility, such as in wearable devices or medical implants. The flexibility of the substrate also makes it easier to design and manufacture more complex PCBs that cannot be achieved with a rigid substrate.

Factors to consider when selecting base materials

When selecting base materials for PCB fabrication, there are several factors that need to be taken into consideration. These include:

1. Performance requirements: The choice of base material will depend on the specific performance requirements of the PCB, such as the desired electrical, mechanical, thermal, and chemical properties.

2. Signal frequency: For high frequency applications, a material with low dielectric loss and high dielectric constant is preferred.

3. Thermal management: The base material should be able to dissipate heat efficiently to prevent overheating of the PCB. This becomes particularly important for high power applications.

4. Cost: The cost of the base material is an important consideration, as it can significantly impact the overall cost of the PCB.

5. Fabrication complexity: Some base materials may require more complex fabrication processes, which can increase the cost and turnaround time of the PCB production.

6. Environmental considerations: The application environment can also impact the choice of base material. For example, the PCB may need to be resistant to moisture, chemicals, or other environmental factors.

Overall, the selection of base materials for PCB fabrication requires careful consideration of a variety of factors, with the aim of achieving the necessary electrical, mechanical, thermal, and chemical properties, while also balancing cost and manufacturing complexity.

To ensure a manufacturable PCB design, it is important to choose a material brand that complies with and holds certification for IPC-4101. This certified material can be utilized for rigid or multilayer printed boards. By selecting a compliant material system, you can have confidence that, when paired with a reliable manufacturing process, the material will offer a certified and verifiable level of performance.

Below is a table indicating some of the key characteristics of IPC-4101 material systems:

IPC Spec Number	Reinforcement	Resin System	ID Reference	Tg ⁰C min.	Z-Axis CTE max. (50 to 260⁰C)	Td ⁰C min.	Dk @ 1MHz
IPC-4101/99	Woven E-Glass	Epoxy (1), Multifunctional Epoxy (2), Modified Epoxy or Non-Epoxy (max wt. 5%) (3), Flame Resistant	UL/ANSI FR-4/99	150⁰C	3.5%	325⁰C	5.4
IPC-4101/101	Woven E-Glass	Difunctional Epoxy (1), Multifunctional Epoxy (2), Modified Epoxy or Non-Epoxy (max wt. 5%) (3), Flame Resistant	UL/ANSI FR-4/101	110⁰C	4.0%	310⁰C	5.4
IPC-4101/121	Woven E-Glass	Difunctional Epoxy (1), Multifunctional Epoxy (2), Modified Epoxy or Non-Epoxy (max wt. 5%) (3), Flame Resistant	UL/ANSI FR-4/121	110⁰C	4.0%	310⁰C	5.4
IPC-4101/124	Woven E-Glass	Epoxy (1), Multifunctional Epoxy (2), Modified Epoxy or Non-Epoxy (max wt. 5%) (3), Flame Resistant	UL/ANSI FR-4/124	150⁰C	3.5%	325⁰C	5.4
IPC-4101/126	Woven E-Glass	Epoxy (1), Multifunctional Epoxy (2), Modified Epoxy or Non-Epoxy (max wt. 5%) (3), Flame Resistant	UL/ANSI FR-4/126	170⁰C	3.0%	340⁰C	5.4
IPC-4101/129	Woven E-Glass	Epoxy (1), Multifunctional Epoxy (2), Modified Epoxy or Non-Epoxy (max wt. 5%) (3), Flame Resistant	UL/ANSI FR-4/129	170⁰C	3.0%	340⁰C	5.4

In summary, selecting the appropriate material for your design involves considering various factors. If you are uncertain which materials are most suitable for your PCB design, do not hesitate to reach out to us at sales@pcbjhy.com. We are well-equipped to guide you in selecting the materials that will best serve your needs while also streamlining the production process and ensuring prompt delivery of your printed circuit boards.

FAQs about PCB Substrates

What materials are commonly used as a PCB substrate?

The most commonly used materials are FR-4, polyimide, and Rogers.

What is FR-4?

FR-4 is a fiberglass-reinforced epoxy laminate material used as a substrate for PCBs.

What is polyimide?

Polyimide is a high-temperature plastic material used for its thermal and dimensional stability in PCBs.

What is Rogers?

Rogers is a high-frequency laminate material used for radio frequency (RF) applications.

What is the thickness tolerance for FR-4 substrates?

The standard tolerance for the thickness of an FR-4 substrate is +/- 10%.

What is the dielectric constant of polyimide?

The dielectric constant of polyimide ranges from 3.0 to 3.5.

Why is Rogers material used in RF applications?

Rogers has a low dielectric constant, which makes it an ideal material for high-frequency applications.

What is the maximum temperature that FR-4 can withstand?

The maximum operating temperature for FR-4 is typically between 130°C and 140°C.

What is the thermal conductivity of polyimide?

The thermal conductivity of polyimide is low, ranging from 0.1 to 0.5 W/m-K.

What is the Tg of Rogers material?

The glass transition temperature (Tg) of Rogers material is typically in the range of 180°C to 200°C.

What is the difference between FR-4 and CEM-1?

CEM-1 is a lower cost alternative to FR-4 and has a lower dielectric constant.

What is the difference between polyimide and FR-4?

Polyimide has a higher temperature resistance and lower dielectric constant than FR-4.

What is the dielectric constant of Rogers material?

The dielectric constant of Rogers material ranges from 2.2 to 10.2, depending on the specific material.

What is the thermal expansion coefficient of polyimide?

The thermal expansion coefficient of polyimide is typically in the range of 20-40 ppm/K.

What is the difference between Rogers 4003 and Rogers 4350?

Rogers 4350 has a lower dielectric constant and a higher loss tangent than Rogers 4003, making it more suitable for high-frequency applications.

What is the dielectric breakdown voltage of polyimide?

The dielectric breakdown voltage of polyimide ranges from 100 to 200 volts/mil.

What is the difference between Rogers 4003 and Rogers 5880?

Rogers 5880 has a lower dielectric constant and lower loss tangent than Rogers 4003, making it better suited for high-frequency applications.

What is the thermal conductivity of Rogers material?

The thermal conductivity of Rogers material varies depending on the specific material, but typically ranges from 0.2 to 1.0 W/m-K.

What is the maximum operating frequency of a PCB made with Rogers 4350?

PCBs made with Rogers 4350 can typically operate at frequencies up to 40 GHz.