PCB Material Usage Guidelines
Printed circuit boards play a critical role in various electronic applications by serving two main functions: providing reliable electrical connections to components and acting as a support for these components. To fulfill their role effectively, choosing the right PCB manufacturing material is crucial. The right material can enhance electrical performance while ensuring the PCB’s resilience in extreme environments. In this article, FS Technology will explore three key aspects: circuit structure, substrate material and material selection.
Materials Used in PCB Manufacturing
Multiple parts together form a completed PCBA board, and then the components are connected through the pads, lines and traces on the laminated sheet of copper. This board comprises copper circuit layers embedded inside or mounted on a flat laminate composite substrate, which includes materials like fiberglass and composite epoxy. To achieve higher circuit densities, multilayer structures are designed, enabling the construction of multilayer PCB where the layers are fused together using heat to create a compact single object. Let’s delve into the materials utilized in building the various layers of the PCB:
The substrate serves as the foundational material for all printed circuit boards, providing insulation for the conductive layers. In rigid circuits, the most prevalent build substrate is FR4 material, renowned for its excellent mechanical strength and electrical insulation. However, some projects opt for more cost-effective alternatives like phenolic resins or epoxies. Flex circuits, on the other hand, utilize polyimide or polyester materials to imbue the board with flexibility. In high-power applications, metal core PCB substrate materials such as copper or aluminum are preferred. Engineers must carefully consider project requirements and material characteristics during the design phase to ensure optimal functionality and durability of the final PCB product.
The conductive layer is the crucial copper layer responsible for forming electrical paths or traces, enabling component connections on the PCB. This thin copper foil is applied to a non-conductive base, typically a glass fiber reinforced epoxy, with multiple insulating layers sandwiched in between. During the PCB manufacturing process, heat and adhesives are utilized to laminate the copper foil to the board.
In the case of a double-sided PCB, both sides of the board are laminated with copper to accommodate the necessary electrical connections. For multi-layer boards, each layer is equipped with its own conductive traces. The thickness of the conductive copper material directly influences crucial electrical parameters such as inductance, capacitance, and resistance.
The thickness of the copper layer can be specified in ounces per square foot or microns per square inch, with 35 microns being a common thickness. The copper thickness can vary based on the number of layers in the board. Therefore, it is essential to tailor the design of the conductive layer to the complexity and functionality of the desired device.
For designers, it is highly recommended to review PCB copper thickness guidelines before commencing the design process. Adhering to these guidelines ensures that the conductive layer meets the electrical requirements and performs optimally in the final PCB product.
The solder mask, also known as solder resist, is a protective layer applied above the copper layer on the circuit board. Its primary function is to insulate and shield the copper traces and conductive layers from direct contact, preventing unintended conduction and short circuits. By serving as a barrier, the solder mask safeguards all circuits on the outer layer of the PCB, whether during the assembly soldering process or in challenging environmental conditions during application.
While green is the common color associated with PCBs, manufacturers offer a variety of options for solder mask colors. For instance, FS Technology provides red, yellow, white, and other material choices, allowing designers to customize the appearance of their PCBs according to their preferences or branding requirements.
The silkscreen layer is positioned above the solder mask layer and serves as a valuable visual aid for users. It allows for the addition of numbers, symbols, and other markings that aid in understanding the PCB board’s layout and functionality. By providing clear indications of the purpose and role of each component and pin on the board, the silkscreen layer facilitates easier assembly, debugging, and maintenance processes.
Typically, the silkscreen is presented in white color, ensuring good visibility against the contrasting background of the solder mask. However, various other color options, such as grey, red, yellow, and black, are also available depending on the designer’s preferences or specific project requirements.
Common PCB Materials and Their Features
We have already discussed which materials the different parts of the PCB are made of. Next, we will analyze the PCB substrate materials in detail and introduce their properties.
FR4 is the most widely used and fundamental circuit board material, with its full name being Flame Retardant 4, which denotes its flame retardant properties. Comprised of a composite of glass fiber and epoxy resin, FR4 materials are enhanced with specific additives during the manufacturing process to improve their thermal, electrical, and flame-retardant characteristics. This makes FR4 an ideal choice for constructing high thermal stress and high-speed circuit designs at a relatively lower cost.
Here are the technical characteristics of FR4 materials provided:
- Dielectric Constant: This material has a 4.4 dielectric constant value, is stable across frequency ranges and is suitable for a variety of general-purpose projects.
- Dielectric Loss: FR4 exhibits low dielectric losses, typically ranging from 0.015 to 0.035, depending on the manufacturer and specific grades. This low dielectric loss ensures minimal signal attenuation and helps maintain signal integrity and quality.
- Thermal Conductivity: FR4 PCB board materials typically have lower thermal conductivity, which limits their heat dissipation capabilities in high-power projects. However, for standard applications, their thermal properties are effective and sufficient.
- Operating Temperature: The glass transition temperature (Tg) of FR4 is approximately 130 to 140°C. This temperature represents the point where the material changes its state. The Tg value of FR4 makes it suitable for applications that require moderate operating temperatures.
- Mechanical Strength: FR4 boasts good mechanical strength and rigidity, which prevents warping and bending. This robustness enables the board to withstand mechanical stress in various design applications, including industrial instruments, automotive projects, and consumer electronics.
PTFE, commonly known as Teflon, is a special material extensively used in the production of PCBs, Teflon laminates, and adhesive materials. It boasts exceptional electrical and thermoelectric properties, making it ideal for applications in microwave, high-speed, and high-frequency circuits. One of the key advantages of this plastic material is its negligible electrical resistance, ensuring minimal signal loss and superior performance in high-frequency environments.
Furthermore, PTFE is highly flexible, making it suitable for applications with tight tolerances. It is also flame retardant and offers remarkable temperature stability, allowing it to withstand demanding operating conditions.
PTFE can be combined with glass fibers to enhance its mechanical properties. By incorporating additives and special fillers, the mechanical strength of the glass fibers can be further improved, providing enhanced reinforcement for the overall material structure.
Here are the technical characteristics of PTFE materials provided:
- Dielectric Constant: The value of the dielectric constant for PTFE is very low about 2.2 to 2.65.
- Dielectric Loss: It comes with less dielectric losses about 0.001 to 0.002.
- Thermal Conductivity: This material has less thermal conductivity, than polyimide and FR4
- Operating Temperature: PTFE provides a high glass transition temperature of almost 200°C or high, making it best for high-power and high-temperature circuits
- High-Frequency Performance: its good electrical parameter and less value of dielectric constant make it best for microwave and high frequency circuits.
- Chemical Resistance: This material is resistant to solvents and chemicals and offered protection from different environmental conditions
As electronic devices become increasingly complex, the demand for flexible laminates has grown, allowing for the accommodation of folding requirements. Unlike traditional rigid PCB that utilize fiberglass for support, flexible PCB employs plastic films as their base material. These flexible materials are highly versatile and can be equally effective in both one-off projects, such as flex installations, and continuous applications like dynamic flex.
Various flexible materials are available in the market, including polyimide, LCP (liquid crystal polymer), polyester, and PEN. Among these options, polyimide PCB stands out due to its excellent thermal stability, making it well-suited for use in high-temperature conditions.
Here are the technical characteristics of Polyimide materials provided:
- Dielectric Constant: The value of the dielectric constant for polyimide is about 3.3.
- Dielectric Loss: Polyimide also comes with fewer dielectric losses, that value is about 0.003 to 0.006.
- Thermal Conductivity: it also comes with less thermal conductivity usually between 0.12 and 0.15 W/(m K).
- Operating Temperature: Polyimide materials come with a high value of glass transition temperatures (Tg) that is more than 250°C.
- Flexibility: Used in projects where boards require bending or transformation to different shapes like wearable devices
- Chemical Resistance: This material provides resistance to different chemicals and solvents, so the best option for corrosive environments.
Rogers Corporation is a famous manufacturer of high-frequency laminates specially made for microwave circuits and projects where high frequency is involved.
Here are the technical characteristics of Rogers PCB materials provided:
- Dielectric Constant: Rogers materials provide different values of dielectric constant, from a minimum of 2.2 to high more than 10, which is helpful to handle high-frequency design. Engineers can select the best Dk values according to their required features.
- Dielectric Loss: These materials have less dielectric loss, about 0.001 to 0.005.
- Thermal Conductivity: Some types of Rogers materials have a high thermal conductivity than standard board materials.
- Operating Temperature: Their Tg value is high, which is more than 250°C, making sure of their reliability in high-temperature conditions.
CEM-3 or Composite Epoxy Material 3 is a less costly option than FR4 material providing the same electrical and mechanical features. It is created with woven fabric and epoxy resin and adding cellulose paper as a filler material.
Here are the technical characteristics of CEM-3 materials provided:
- Dielectric Constant: The value of the dielectric constant for CEM-3 is about 4.6.
- Dielectric Loss: This material comes with high value of dielectric losses than FR-4, about 0.02 to 0.035.
- Thermal Conductivity: Usually between 0.25 and 0.35 W/(m K), slightly lower than FR4 materials.
- Operating Temperature: Its Tg value is about 135 to 140 °C, like the standard FR-4. This temperature value is best for electronic circuits that have moderate thermal needs.
- Cost-Effectiveness: It is mostly used due to cost-effectiveness parameters than other materials like polyimide and specialized high-frequency laminates.
PCB Fabrication Material Selection Guide
When selecting materials for PCB fabrication, the primary consideration revolves around the electrical performance, as it directly impacts the transmission of electrical signals and the overall functionality of the circuit board.
- Dielectric Constant: Also known as the relative dielectric constant, determines how a material influences the speed at which electrical signals travel through it. In high-frequency engineering, materials with lower dielectric constant values are preferred to reduce signal distortion and ensure efficient signal transmission.
- Dielectric Loss: Also referred to as the Dissipation Factor, this parameter quantifies the amount of energy lost as heat during signal transmission. Materials with low dielectric loss values are highly beneficial for high-speed and high-frequency circuits, as they minimize signal attenuation and help maintain signal quality.
- Signal Integrity: For certain projects, maintaining signal quality is of utmost importance to avoid issues such as signal reflection, crosstalk, and impedance mismatch. Board materials with controlled impedance parameters play a crucial role in preserving signal quality and reducing signal attenuation.
Thermal parameters play a crucial role in projects where high heat is generated or when operating in high-temperature conditions. Effective thermal management ensures reliable operation and prolongs the longevity of components.
- Thermal Conductivity: Materials with high thermal conductivity efficiently dissipate heat from components, minimizing the risk of overheating and ensuring reliable operation.
- Thermal Expansion Coefficient (CTE): The CTE measures how much a material contracts or expands with changes in temperature. Choosing materials with compatible thermal expansion coefficients for the board and components reduces mechanical stress during temperature variations and helps prevent damage to solder joints.
- Tg (Glass Transition Temperature): The Tg denotes the temperature at which the board material transitions from a rigid to a flexible state. High Tg materials are used in projects involving temperature fluctuations during operation or manufacturing processes.
- Thermal Dissipation Capability: Materials with good thermal dissipation properties are essential to prevent unnecessary temperature increases in different components.
The mechanical strength of PCB materials structural quality and the board’s ability to withstand mechanical stress during production, transportation, and application.
- Flexibility vs. Rigidity: Depending on the application, PCBs can be either rigid or flexible. Flexible materials like polyimide or polyester are ideal for projects requiring bending or for space-constrained designs.
- PCB Thickness: The thickness of the board directly influences its mechanical strength. Thick boards provide a highly rigid structure and enhanced durability, while thin boards are suitable for projects with space constraints.
- Mechanical Shock and Vibration Resistance: Materials with good damping properties are essential for applications exposed to mechanical shocks or vibrations. These damping features help minimize stress-induced failures and ensure the board’s reliability during challenging conditions.
Especially for industries operating under harsh conditions, designers need to look at the ability of PCB materials to withstand the influence of chemicals and different environmental parameters
- Environmental Exposure: Chemical resistance protects the board from degradation and corrosion, ensuring its longevity and reliability in challenging environments.
- Moisture Resistance: Materials with good moisture resistance prevent moisture absorption, reducing issues such as delamination and electrical malfunctions caused by water ingress.
- Environmental Regulations: Selecting materials that are RoHS-compliant ensures that the PCBs are environmentally friendly and do not pose any hazards to human health or the environment.
- Cost and Availability: When creating any project, cost parameters are crucial as the choice of PCB material directly impacts the overall production cost. Additionally, the availability of materials varies based on suppliers and regions. Striking a balance between the required features and budget constraints is essential.
- Manufacturing Process: The selected PCB material must be compatible with the required manufacturing process. Some materials may necessitate specialized manufacturing methods, such as controlled environment handling and high-temperature lamination. Evaluating the viability and financial efficiency of the manufacturing process using the chosen board materials is important.
- Specific Requirements: Each electronic device comes with unique requirements, and the selection of PCB materials should align with these specific needs. Some applications may prioritize specific characteristics, such as high-frequency performance, while others may emphasize thermal management or cost-effectiveness.
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