A Comprehensive Analysis of Printed Circuit Board (PCB) Technology: Types, Materials, Applications, and Future Trends
As a crucial component of electronic devices, the Printed Circuit Board (PCB) serves as the nerve center of an electronic system, undertaking the important tasks of providing mechanical support and electrical connection for electronic components. With the rapid development of electronic technology, ranging from the thin and lightweight portability of smartphones to the high-performance computing in data centers, from the high-speed transmission in 5G communication to the complex control in automotive autonomous driving, different application scenarios pose diverse requirements for the performance, structure, and process of PCBs. This has led to the emergence of a wide variety of PCB types. A thorough understanding of the various characteristics of PCBs is essential for electronic engineers, researchers and developers, as well as practitioners in related industries, to optimize electronic designs and enhance product performance.
一、Technical Analysis of PCBs Classified by Substrate Materials
(一) Rigid PCBs
Rigid PCBs, with their outstanding mechanical stability and strength, have become the fundamental choice for numerous electronic devices. Glass fiber materials, such as E-glass and S-glass, are widely used in the manufacturing of rigid PCBs due to their excellent insulation properties and mechanical strength. Among them, E-glass fiber offers high cost-effectiveness and is commonly found in general electronic products; S-glass fiber, on the other hand, has higher strength and modulus, making it suitable for fields with stringent requirements for mechanical properties.
Rigid PCBs made of polyimide (PI) material possess characteristics such as high temperature resistance (capable of operating continuously above 200°C), high insulation (with a dielectric constant as low as approximately 3), and excellent chemical stability. They are often used in high-demand scenarios such as aerospace and military electronics. FR-4, as the most commonly used substrate material for rigid PCBs, is laminated from epoxy resin-impregnated glass fiber cloth. It has good electrical properties (with a volume resistivity of over 10^14Ω·cm) and flame retardancy (meeting the UL94V-0 flame retardant grade), and is widely applied in civil electronic products such as computers and communication devices.
As composite base copper-clad laminates, CEM-1 and CEM-3 have their own characteristics. CEM-1, composed of a combination of glass mat and paper base, has a relatively low cost and certain electrical properties, making it suitable for cost-sensitive consumer electronic products. CEM-3, based on a glass fiber core, excels in thinning and mechanical processing performance, and is often used in miniaturized electronic devices.
（二）、Flexible PCBs (FPCs)
Flexible PCBs (FPCs), with their unique bendability and thinness, hold an important position in electronic devices with limited space. Polyimide (PI) is one of the main materials for FPCs. It has excellent flexibility (able to withstand millions of bending cycles), high temperature resistance (with a long-term operating temperature of over 150°C), and good electrical properties (with a dielectric loss tangent as low as 0.002).
Polyester film materials such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) are also commonly used in FPCs. They have the advantages of low cost and good processability, but are slightly inferior to PI in terms of high temperature resistance and electrical properties. Key parameters for measuring the performance of FPCs, such as the bend radius and the number of bending cycles it can endure, directly affect the reliability and service life of FPCs in practical applications.
(三) Rigid-Flex PCBs
Rigid-flex PCBs ingeniously combine the advantages of rigid PCBs and flexible PCBs. In the rigid areas, the same substrate materials as those used in rigid PCBs, such as FR-4 or PI rigid boards, are usually employed to provide the necessary mechanical support and stability for the circuit board, ensuring the reliable installation of electronic components and electrical connections. In the flexible areas, flexible substrate materials, such as PI flexible films, are used to achieve the bendability of the circuit board.
The key technology of rigid-flex PCBs lies in the connection process between the rigid and flexible areas. Common connection methods include laser welding and adhesive bonding. The reliability of the connection parts and the design of stress relief are important factors for ensuring the performance of rigid-flex PCBs. A reasonable design can effectively prevent problems such as connection failure or abnormal signal transmission during the bending process.
二、Technical Characteristics of PCBs Classified by the Number of Conductive Layers
(一) Single-Sided PCBs
As a basic type of PCB, a single-sided PCB has copper foil on only one side and realizes circuit functions through single-sided wiring. Its circuit structure is relatively simple, making it suitable for some electronic devices with low functional requirements, such as simple home appliance control boards and toy circuit boards. The production process of single-sided PCBs is relatively straightforward, mainly including steps such as copper foil etching, solder mask printing, and character printing, and the cost is relatively low. However, due to the limited wiring space, the circuit complexity and functional expandability of single-sided PCBs are somewhat restricted.
(二) Double-Sided PCBs
A double-sided PCB has copper foil on both sides of the circuit board and achieves electrical connection between the two sides through vias (metallized vias). The manufacturing process of vias usually includes steps such as drilling, electroless copper plating, and electroplating to ensure good electrical conductivity of the inner wall of the vias. The circuit complexity and functional implementation ability of double-sided PCBs have significantly improved compared to single-sided PCBs, and they can be used in computer motherboards, some modules of communication devices, etc.
In the design of double-sided PCBs, wiring planning and via layout are key aspects. A reasonable design can effectively reduce signal interference and improve the integrity and stability of signal transmission.
(三) Multilayer PCBs
Multilayer PCBs have multiple conductive layers (usually 4 layers or more), which are separated by insulating layers (such as prepreg sheets, PP sheets). In addition to common through holes, blind via and buried via technologies are also widely applied in multilayer PCBs. A blind via is a conducting hole that extends from the surface of the circuit board to a certain inner layer and does not penetrate the entire circuit board; a buried via is a connecting conducting hole between inner layers and is not exposed on the surface of the circuit board.
The application of blind via and buried via technologies effectively reduces the number of vias on the surface of the circuit board and improves the wiring density and signal transmission performance. Multilayer PCBs can achieve high-density wiring and high-performance signal transmission, and are widely used in high-end electronic devices, such as server motherboards, smartphone motherboards, and high-performance graphics cards. In the manufacturing process of multilayer PCBs, factors such as the lamination process, drilling accuracy, and electroplating quality have a significant impact on the performance and reliability of the circuit board.
三、Technical Key Points of PCBs Classified by Special Processes
(一) High-Frequency PCBs
High-frequency PCBs are mainly applied in electronic devices that handle high-frequency signals, such as wireless communication, radar, and other fields. During the transmission of high-frequency signals, problems such as signal loss and phase distortion are relatively prominent. Therefore, high-frequency PCBs need to use special materials to reduce signal loss and improve the quality of signal transmission.
Rogers material is one of the commonly used substrate materials for high-frequency PCBs. It has an extremely low dielectric constant (Dk can be as low as about 2.2) and loss tangent (Df as low as 0.0009), which can effectively reduce signal loss and distortion during the transmission process. Polytetrafluoroethylene (PTFE) and its filled materials are also often used in high-frequency PCBs, as they have good high-frequency electrical properties and chemical stability.
In the design and manufacturing process of high-frequency PCBs, in addition to material selection, the design of aspects such as circuit layout, impedance matching, and electromagnetic shielding is also crucial. A reasonable circuit layout can reduce interference between signals, precise impedance matching can ensure efficient signal transmission, and effective electromagnetic shielding can prevent high-frequency signals from interfering with surrounding circuits.
(二) Metal-Based PCBs
Metal-based PCBs, with their excellent heat dissipation performance, have become an ideal choice for high-power electronic devices. Common metal substrate materials include aluminum-based and copper-based. The aluminum-based substrate has good heat dissipation performance and a relatively low cost, and is widely used in fields such as LED lighting and power modules. The copper-based substrate has a higher thermal conductivity (about 1.6 times that of aluminum) and is suitable for occasions with extremely high heat dissipation requirements, such as high-power motor drive circuits.
The heat dissipation principle of metal-based PCBs is to quickly conduct the heat generated by electronic components through the metal substrate. To improve the heat dissipation efficiency, a thermally conductive insulating layer, such as a thermally conductive silicone pad or thermally conductive insulating adhesive, is usually added between the metal substrate and the electronic components. In the manufacturing process of metal-based PCBs, the thickness control of the insulating layer and the bonding strength with the metal substrate are key factors for ensuring their performance.
(三) HDI PCBs (High-Density Interconnect PCBs)
HDI PCBs (High-Density Interconnect PCBs), with their characteristics of high-density wiring and miniaturization, meet the strict requirements of modern electronic devices for space and performance. The key technologies of HDI PCBs lie in the fabrication of micro-vias (Micro-Vias) and the etching of fine lines. The diameter of micro-vias is usually less than 0.1mm and is fabricated using advanced technologies such as laser drilling or plasma etching.
The etching of fine lines requires high-precision etching equipment and process control to achieve fine wiring with a line width/line pitch of less than 30μm/30μm. HDI PCBs usually adopt the build-up technology, achieving high-density interconnection by stacking insulating layers and conductive layers layer by layer. HDI PCBs are widely used in miniaturized electronic devices such as smartphones, tablets, and wearable devices, and are one of the key technologies for achieving high performance and miniaturization of these devices.
四、IC Substrates (IC Carrier Boards)
IC substrates (IC carrier boards) are mainly used for chip packaging, such as BGA (Ball Grid Array) packaging, QFN (Quad Flat No-Lead) packaging, and FC (Flip Chip) packaging, etc. IC substrates need to have the characteristics of high-density interconnection and high precision to achieve a reliable connection between the chip and the external circuit.
IC substrates usually adopt a multilayer board design, and there are strict requirements for line precision, via size, and position precision during the manufacturing process. At the same time, IC substrates also need to consider heat dissipation management and electrical performance to ensure the stability and reliability of the chip during operation. With the continuous development of chip technology, the requirements for the performance and precision of IC substrates are also constantly increasing.
五、Technical Characteristics of PCBs Classified by the Structure of Conductive Vias
(一) Through-Hole Boards
The through-hole board is one of the most common types of PCBs. Its conductive vias penetrate from the top layer to the bottom layer of the circuit board, and the electrical connection between layers is achieved through the via electroplating process. The via diameter and the copper thickness of the via wall of the through-hole board are important parameters affecting its electrical performance and mechanical strength.
A larger via diameter is beneficial for the installation of plug-in components, but it will occupy more wiring space; insufficient copper thickness of the via wall may lead to unreliable electrical connections. During the manufacturing process of the through-hole board, the drilling precision of the vias and the quality of electroplating have an important impact on the performance of the circuit board. In addition, the through-hole board can also adopt a via filling process, such as resin filling, to improve its reliability and moisture resistance.
(二) Micro-Via Boards
Micro-via boards use conductive vias with small diameters (usually less than 0.15mm), such as blind micro-vias and buried micro-vias. The formation of micro-vias usually adopts laser drilling or plasma etching technology, and these technologies can achieve the fabrication of micro-vias with high precision to meet the requirements of high-density wiring.
The micro-vias of micro-via boards have small diameters and a large number, which can achieve more electrical connections within a limited space and improve the integration level and performance of the circuit board. During the design and manufacturing process of micro-via boards, the filling and surface treatment processes of micro-vias need to be considered to ensure the electrical performance and reliability of micro-vias.
(三) Blind Via Boards
The conductive vias of blind via boards only extend from the surface of the circuit board to a certain inner layer and do not penetrate the entire circuit board. The existence of blind vias can reduce the number of vias on the surface of the circuit board, increase the wiring density, and also reduce the interference of signals during the transmission process.
The formation processes of blind vias include laser drilling, electroplating after mechanical drilling, etc. Different process methods have different impacts on the quality and cost of blind vias. In the design of blind via boards, the position and quantity of blind vias need to be reasonably planned to give full play to their advantages and improve the performance of the circuit board.
(四) High-Density Interconnect (HDI) Boards
High-Density Interconnect (HDI) boards are characterized by high-density interconnection, small via diameters, and fine lines, and are one of the key technologies for achieving the miniaturization and high performance of electronic devices. In addition to using tiny conductive vias, HDI boards also achieve fine wiring with a line width/line pitch of less than 30μm/30μm through fine line etching technology.
HDI boards usually adopt the build-up technology, achieving high-density interconnection by stacking insulating layers and conductive layers layer by layer. During the manufacturing process of HDI boards, the requirements for materials, equipment, and processes are very high, and the quality of each link needs to be strictly controlled to ensure the performance and reliability of the circuit board
Conclusion
As an important foundation of electronic technology, the diversity of types and the complexity of technologies of printed circuit boards provide solid support for the innovation and development of electronic devices. From the selection of substrate materials to the design of conductive layers, from the application of special processes to the consideration of surface treatment methods, and then to the technical requirements of different application fields and the characteristics of the structure of conductive vias, each link contains rich technical connotations.
With the continuous progress of electronic technology, such as the rapid development of emerging technologies like artificial intelligence, the Internet of Things, and big data, higher requirements will be put forward for the performance and functions of PCBs. In the future, PCB technology will develop towards the direction of higher density, higher performance, lower cost, and greater environmental protection, continuously promoting the miniaturization, intelligence, and high-performance development of electronic devices. Electronic engineers and practitioners in related industries need to pay close attention to the development trends of PCB technology, continuously innovate and optimize designs to meet the growing market demands and technical challenges.
FAQS：
Q1.What are the common substrate materials for rigid PCBs?
Common substrate materials for rigid PCBs include glass fibers (such as E-glass, S-glass, etc.), polyimide (PI), FR-4 (a flame-retardant copper-clad laminate composed of epoxy resin and glass fiber cloth), CEM-1 (a composite base copper-clad laminate containing glass mat and paper base), and CEM-3 (a composite base copper-clad laminate with a glass fiber core).
Q2.Why are flexible PCBs suitable for wearable devices?
Flexible PCBs are suitable for wearable devices because their main substrate materials such as polyimide (PI), polyethylene terephthalate (PET), etc. endow them with good flexibility, enabling them to bend, fold, and curl within a certain range, which can adapt to the complex shapes of wearable devices that conform to the human body. At the same time, they also have the characteristics of being thin and light, and will not impose too much burden on the wearer, meeting the requirements of wearable devices for space and comfort.
Q3.What are the respective functions of the rigid area and the flexible area in a rigid-flex PCB?
In the rigid area of a rigid-flex PCB, rigid substrate materials such as FR-4 or PI rigid boards are mainly used to provide mechanical support and stability, ensuring the structural strength of the circuit board during installation and use. The flexible area, on the other hand, uses flexible substrate materials such as PI flexible films to achieve the bending function, so as to adapt to the shape changes of the device in different usage scenarios and meet the requirements of complex mechanical and electrical performance.
Q4.What are the main differences between single-sided PCBs and double-sided PCBs?
A single-sided PCB has copper foil on only one side and is used for single-sided wiring. Its circuit complexity is relatively low, and it is suitable for simple electronic devices. The production process is simple and the cost is low, but its functions and wiring space are limited. A double-sided PCB has copper foil on both sides, and the electrical connection between the two sides is achieved through vias (usually metallized vias). The circuit complexity is moderate, and it can achieve more functions with a wider application range, such as computer motherboards, communication devices, etc.
Q5.What are the functions of blind vias and buried vias in multilayer PCBs?
Blind vias in multilayer PCBs are conductive holes that extend from the surface to a certain inner layer and do not penetrate the entire circuit board. They can be used for electrical connections between specific layers, reducing signal interference and improving signal integrity. Buried vias are connections between inner layers and are not exposed on the surface. They can achieve interlayer connections without occupying surface space, which helps to realize high-density wiring and improve the performance and integration level of the circuit board.
Q6.Why do high-frequency PCBs need to use special materials?
High-frequency PCBs are mainly used to process high-frequency signals, such as microwave frequency bands and millimeter-wave frequency bands, and signals are prone to loss during the transmission process. Special materials such as Rogers materials, polytetrafluoroethylene (PTFE), and ceramic-filled materials are used because these materials have the characteristics of low dielectric constant (Dk) and low loss tangent (Df), which can effectively reduce signal loss, ensure signal integrity, and meet the requirements of high-frequency signal transmission.
Q7.Which high-power electronic devices are metal-based PCBs suitable for?
Metal-based PCBs are suitable for a variety of high-power electronic devices. For example, in power modules, a large amount of heat is generated during operation, and metal-based PCBs can effectively dissipate heat to ensure their normal operation. For LED lighting devices, they can quickly dissipate the heat generated by LEDs, improving the lighting efficiency and the lifespan of the lamps. For motor drive circuits, they can help dissipate the heat generated during motor operation, ensuring the stable operation of the circuit.
Q8.What are the key technical characteristics of HDI PCBs?
The key technical characteristics of HDI PCBs include the use of tiny via diameters (Micro-Via, usually less than 0.1mm), which are formed through advanced technologies such as laser drilling and plasma etching; having fine lines (Fine Line), which can achieve fine wiring with a line width/line pitch of less than 30μm/30μm; adopting the build-up technology to increase the number of layers and achieve high-density interconnection; and having good compatibility with the surface mount technology (SMT) assembly process, which is suitable for the needs of miniaturized electronic devices.
Q9.What are the types of surface tin layers for tin-sprayed PCBs?
The types of surface tin layers for tin-sprayed PCBs include pure tin (Pure Tin), tin-lead alloy (Tin-Lead Alloy), and lead-free tin spray, such as Sn-Cu (tin-copper alloy), Sn-Ag-Cu (tin-silver-copper alloy), etc. Lead-free tin spray is a type that has been gradually popularized to meet environmental protection requirements.
Q10.Why are gold-plated PCBs often used in high-end electronic devices?
Gold-plated PCBs are often used in high-end electronic devices because the metallic gold covering their surface (divided into hard gold and soft gold) can provide good electrical conductivity, reduce contact resistance, and ensure the reliability of electrical connections. At the same time, gold has excellent anti-oxidation performance, which can effectively resist environmental corrosion and chemical corrosion, extend the service life of the circuit board, and meet the strict requirements of high-end electronic devices such as aerospace, medical equipment, and communication base stations for reliability and stability.
Q11.What should be paid attention to when storing OSP PCBs?
OSP PCBs are surface-treated with an organic solderability preservative film. Their storage life is relatively short, and attention should be paid to moisture prevention. They should be stored in a dry and low-humidity environment to avoid the failure of the organic solderability preservative film due to moisture, which may affect the solderability of the circuit board. At the same time, attention should also be paid to surface cleaning to prevent dust and impurities from contaminating the surface of the circuit board, which may affect subsequent welding and usage performance.
Q12.What performance requirements do computer boards have for PCBs?
Computer boards such as motherboards, graphics cards, and server boards have requirements for the high-speed data processing and transmission capabilities of PCBs. They need to support high-speed signal transmission protocols such as the PCI-E bus, USB interfaces, and SATA interfaces. They should have good electrical performance to ensure signal integrity and stability. The motherboard needs to integrate a variety of key components, such as the chipset, BIOS chip, and power supply circuit, etc., requiring the PCB to have a reasonable layout and a high integration level. Server boards also need to support multiple CPUs and large-capacity memory, imposing higher requirements on the carrying capacity and reliability of the PCB.
Q13.Why do automotive boards need to have high reliability?Automotive boards are applied to automotive electronic systems. During the driving process, vehicles will face various complex environmental conditions, such as vibration, impact, and changes in high and low temperatures (usually required to work normally within the temperature range of -40°C to 125°C). If the automotive board has insufficient reliability, it may lead to failures in the automotive electronic system, affecting the normal operation of the vehicle and driving safety. Therefore, automotive boards need to have high reliability to ensure stable operation in harsh environments.
Q14.What role does an IC substrate (IC carrier board) play in chip packaging?
An IC substrate (IC carrier board) mainly plays the role of connecting the chip and the external circuit in chip packaging. For example, packaging methods such as BGA (Ball Grid Array) packaging, QFN (Quad Flat No-Lead) packaging, and FC (Flip Chip) packaging all need to achieve reliable connections through the IC substrate. It needs to have the characteristics of high-density interconnection and high precision to meet the requirements of a large number of signal transmissions between the chip and the external circuit. At the same time, heat dissipation management and electrical performance also need to be considered to ensure that the heat generated during the operation of the chip can be effectively dissipated and the signal can be stably transmitted, ensuring the normal operation of the chip.
Q15.How are the micro-vias of micro-via boards formed?
The micro-vias of micro-via boards are usually formed by laser drilling or plasma etching technology. Laser drilling uses a high-energy laser beam to irradiate the circuit board, causing the material to vaporize instantly to form micro-vias. Plasma etching, on the other hand, uses a plasma to chemically react with the surface material of the circuit board to remove the unnecessary parts, thereby forming tiny via diameters, such as blind micro-vias and buried micro-vias, etc., to achieve high-density wiring.