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Copper-clad laminates (CCL) are the core base material used in the manufacture of printed circuit boards (PCBs). The material composition, formulation and performance parameters of the laminate directly determine the PCB’s applications, stability, flame retardancy and signal transmission quality. Within the industry, copper-clad laminates are standardised and classified according to four key dimensions: reinforcing substrate, resin adhesive, flame retardancy rating and performance characteristics. Furthermore, laminates of different specifications exhibit significant variations in key parameters such as thickness, copper foil thickness, dielectric properties and heat resistance, catering to the production requirements of various electronic products. The following provides a comprehensive and detailed analysis of these laminates.
Classification of Copper-Clad Laminates (CCL)
Classification by Reinforcement Material
1.Paper-based CCL: Utilising specialised insulating wood pulp paper as the core reinforcement material, this type is low-cost and easy to process, making it a common substrate for low-end general-purpose circuit boards. It generally offers average rigidity and has relatively poor heat resistance.
2. Glass Fibre Cloth-Based Copper-Clad Laminates: Utilising alkali-free glass fibre cloth as the reinforcing material, these offer high strength, excellent heat resistance, superior insulation and good dimensional stability. With the best overall performance, they are currently the most widely used mainstream laminates on the market.
3. Composite-based copper-clad laminates (CEM series): These utilise a hybrid reinforcement structure combining paper and glass fibre cloth, balancing cost-effectiveness with fundamental performance. They address the shortcomings of pure paper-based laminates (insufficient performance) and all-glass-fibre laminates (high cost), and are frequently used in mid-range general-purpose electronic products.
4. Laminated Multilayer Copper-Clad Laminates: A substrate specifically developed for multilayer precision PCBs, designed to accommodate multilayer circuit lamination processes. It offers excellent flatness, interlayer adhesion and insulation consistency, and is widely used in multilayer circuit board applications such as precision electronics and telecommunications equipment.
5. Special Material-Based Copper-Clad Laminates: These are high-end specialised laminates using special materials such as ceramics or metal cores as the substrate. They possess characteristics such as ultra-high heat resistance, efficient heat dissipation and low thermal expansion, and are primarily used in specialised applications such as defence, aerospace and high-end precision chip packaging.
Classification by Resin Adhesive Type
1.Resins for paper-based copper-clad laminates: The mainstream option is phenolic resin, with corresponding grades including XPc, XxxPC, FR-1, FR-2, etc. These serve as the base adhesives for paper-based laminates, offering good value for money but with average heat resistance and moisture resistance; the second option is epoxy resin (FE-3 grade), which outperforms phenolic resin and offers greater stability; Additionally, there are polyester resin systems, which feature good toughness and lightweight properties, making them suitable for certain lightweight consumer electronics.
2.Resins specifically for glass fibre cloth substrates: The most fundamental and widely used are epoxy resin systems, with representative grades being FR-4 and FR-5. Thanks to their balanced mechanical, insulating and processing properties, they have become the standard resins for glass fibre laminates. In addition, there are various high-end specialty resin systems, combined with reinforcing materials such as glass fibre cloth, polyamide fibres and non-woven fabrics, to meet the demands of high-end electronics.
These include bis-maleimide-modified triazine resin (BT), polyimide resin (PI), diphenyl ether resin (PPO), maleic anhydride imide-styrene resins (MS), polycyanurate resins and polyolefin resins. These resin-based sheets offer advantages such as high heat resistance, low dielectric constant and ageing resistance, and are widely used in high-end telecommunications, automotive electronics and precision instrumentation sectors.
Classification by flame-retardant properties
1.Non-flame-retardant sheets: Corresponding to UL94-HB rating, these have no fire-retardant properties and are highly flammable when exposed to open flames. They are suitable only for simple electronic devices operating at low voltage and low power consumption, where safety requirements are minimal.
2.Flame-retardant boards: Corresponding to UL94-V0 and UL94-V1 flame-retardant ratings, these possess self-extinguishing properties and do not sustain combustion when exposed to open flames; they are the mainstream choice for industrial and consumer electronics. With the continuous upgrading of global environmental standards, a new category of flame-retardant laminates has emerged: green halogen-free flame-retardant CCL. This laminate eliminates traditional brominated flame retardants, contains no toxic or harmful substances, and produces low smoke and low toxicity when burning. It meets environmental certification requirements and is currently the material of choice for high-end green electronic products.
Classification by Overall Performance
1. General-purpose CCL: Provides basic insulation and meets mechanical performance standards, with no specific requirements for high-temperature resistance or low loss. Suitable for low-end applications such as standard small household appliances, toys and simple digital products.
2. Low-dielectric-constant CCL: Its core advantages lie in low signal transmission loss and high speed, making it specifically suited to high-frequency, high-speed signal transmission applications, such as 5G communication equipment, routers and high-speed server circuit boards.
3. High-temperature-resistant CCL: The board has a high glass transition temperature and can operate stably at temperatures above 150°C for extended periods. It offers excellent high-temperature resistance and anti-ageing properties, making it suitable for high-temperature operating environments such as power supply equipment, automotive electronics and industrial control equipment.
4. Low thermal expansion coefficient CCL: The board exhibits minimal deformation under temperature fluctuations and offers exceptional dimensional stability. It is primarily used in applications with stringent dimensional accuracy requirements, such as chip packaging substrates and precision micro-circuit boards.
PCB Board Brand Quality Grades and Core Applications
Within the industry, a standardised quality grading system has been established based on board material, performance and process compatibility. From low-end to high-end, the grades are: 94HB → 94VO → 22F → CEM-1 → CEM-3 → FR-4. The material structure, processing methods and suitable applications of boards in different grades differ significantly. Specific parameters and applications are as follows:
1. 94HB: The lowest-grade standard paper-based board. It offers no flame-retardant or fire-resistant properties and has average mechanical performance. It supports only die-cutting processing and exhibits poor heat and pressure resistance.Its use in power supply circuit boards is strictly prohibited; it is suitable only for low-voltage, simple, small-scale electronic products.
2. 94VO: A flame-retardant paper-based board. This grade adds flame-retardant properties to the 94HB base, meeting basic fire safety standards. It continues to utilise die-cutting processing and offers good value for money, making it widely used in standard low-voltage consumer electronic circuit boards.
3. 22F: Single-sided semi-glass fibre board, featuring a paper-based substrate with a localised glass fibre composite structure. It offers superior rigidity and heat resistance compared to pure paper boards and supports die-cutting. It is commonly used for single-sided, simple functional circuit boards.
4. CEM-1: Single-sided full-glass-fibre composite board. The stability of the substrate is significantly improved. Processing is limited to computer-controlled precision drilling only; die-cutting is not supported. Suitable for single-sided circuit boards with basic precision requirements.
5. CEM-3: Double-sided semi-glass fibre composite board. As an entry-level product among double-sided boards, it outperforms standard double-sided paper-based boards. Priced 5–10 RMB per square metre lower than mainstream FR-4 boards, it offers outstanding value for money. Primarily used for standard double-sided circuit boards with simple structures and no high-frequency or high-temperature requirements.
6. FR-4: A double-sided, fully glass-fibre laminate, this is currently the most versatile and best-performing standard board on the market. It combines multiple advantages, including high strength, high heat resistance, high insulation, dimensional stability, flame retardancy and moisture resistance. It is suitable for the vast majority of consumer and industrial double-sided and multilayer PCBs, and is the mainstream general-purpose board in the electronics industry.
Classification of Board Flame Retardancy Ratings
In accordance with the UL 94 flame retardancy standard, the flame retardancy protection of PCB boards is classified into four levels from weakest to strongest: 94HB, V2, V1 and 94V0. The higher the rating, the faster the self-extinguishing speed and the greater the fire safety. Among these, 94V0 is the standard flame retardancy rating commonly used in industry and meets the safety certification requirements for the vast majority of electronic products.
Specifications and Parameters of PCB Pre-pregs
Pre-pregs serve as the core bonding material in the lamination of multilayer PCBs. Different grades correspond to fixed standard thicknesses and are key parameters in the design of multilayer board stacks. The mainstream standard grades and thicknesses in the industry are as follows: Grade 1080 has a thickness of 0.0712 mm, Grade 2116 has a thickness of 0.1143 mm, and Grade 7628 has a thickness of 0.1778 mm. In the production of multilayer PCBs, different types of prepreg can be selected according to the required board thickness to ensure stable interlayer bonding and the overall thickness accuracy of the board.
Dielectric Constant of PCB Board Materials
The Significance of the Dielectric Constant
The dielectric constant (abbreviated as Dk, ε or Er) is a key parameter for assessing the electrical performance of PCB base materials, directly determining the transmission speed and integrity of electrical signals on the circuit board. During high-speed signal transmission in electronic products, the value of the board’s dielectric constant directly affects issues such as signal delay, signal attenuation and transmission distortion, making it a core reference indicator for high-frequency, high-speed PCB design. Most hardware designers pay little attention to this parameter, primarily because once the PCB board has been manufactured and formed, the dielectric constant is fixed by the substrate formulation. There are inherent differences in parameters between boards from different brands; for example, the dielectric constant of Shengyi PCB boards is approximately 3.7, whilst that of Ultrasonic PCB boards is approximately 4.2.
Working Principle and Characteristics of Dielectric Constant
From a technical definition, the relative permittivity refers to the ratio of the applied electric field strength in a vacuum to the final electric field strength within the dielectric material, where the dielectric generates induced charges that weaken the original electric field under the influence of an external electric field. This parameter is not a fixed value but is closely related to the signal frequency. The absolute dielectric constant is the product of the relative dielectric constant and the absolute dielectric constant of a vacuum. Simply put, the higher the dielectric constant, the stronger the medium’s attenuation of the electric field and the greater the loss of the electric field within the medium; the relative dielectric constant of an ideal conductor approaches infinity.
Furthermore, the dielectric constant can be used to determine the polarity of polymeric laminates: a relative dielectric constant >3.6 indicates a polar material, 2.8–3.6 indicates a weakly polar material, and <2.8 indicates a non-polar material. The polarity of the material directly influences the laminate’s secondary properties, such as insulation, temperature resistance and moisture resistance.
The relationship between dielectric constant and signal transmission
The speed at which electrical signals propagate through a PCB dielectric is inversely proportional to the square root of the board’s dielectric constant; the lower the dielectric constant, the lower the signal transmission resistance, the faster the speed, and the better the signal integrity. This can be understood through a simple analogy: signal transmission is like running on a beach, where the dielectric constant corresponds to the viscosity of the seawater. The higher the viscosity (the greater the dielectric constant), the greater the resistance to running (the slower the signal transmission and the higher the loss). The industry-standard conclusion is that the signal transmission speed for conventional boards is approximately 6 mils per picosecond (6 mil/ps) or 6 inches per nanosecond (6 in/ns).
Factors Affecting the Dielectric Constant
The dielectric constant is not a constant parameter; it is influenced by multiple environmental and testing conditions: Firstly, it is related to the material formulation of the board itself, which is the fundamental determining factor; secondly, it is affected by the test frequency, test methods and the processing state of the board; simultaneously, the effects of temperature and humidity are most significant. As water has a dielectric constant as high as 70, even slight moisture absorption by the board can cause significant fluctuations in the dielectric constant, affecting signal transmission stability. Therefore, during the production and use of high-end PCB boards, the temperature and humidity environment must be strictly controlled.
Detailed Core Parameters of FR4 Board
Dielectric Loss Parameters
Dielectric loss, abbreviated as DL, refers to the internal energy loss generated in insulating boards under the action of an alternating electric field due to dielectric conductivity and polarisation hysteresis. It is a key indicator for measuring signal transmission loss in the board. The standard dielectric loss value for FR4 laminates is 0.02. Its loss characteristics are directly proportional to signal frequency; as transmission frequency increases, internal energy loss within the laminate gradually increases. Consequently, low-dielectric-loss modified FR4 laminates must be selected for high-frequency, high-speed applications. The dielectric loss angle δ is the key parameter for determining the magnitude of dielectric loss; it is the complementary angle of the power factor angle. The smaller the angle, the lower the dielectric loss.
TG (Glass Transition Temperature)
The TG value, or glass transition temperature, refers to the critical temperature at which the board transitions from a rigid glassy state to a flexible, highly elastic state. It directly determines the board’s high-temperature resistance and long-term operational stability. Conventional FR4 laminates are categorised into four TG gradients: 130°C, 140°C, 150°C and 170°C. The higher the TG value, the greater the laminate’s resistance to high temperatures, deformation and ageing. High-TG FR4 laminates with a TG of 170°C are frequently used in high-temperature operating environments such as industrial control, automotive and power supply applications.
Standard Board Thicknesses
FR4 boards are available in standardised thickness specifications to suit various PCB design requirements. Common industry thicknesses include: 0.3 mm, 0.4 mm, 0.5 mm, 0.6 mm, 0.8 mm, 1.0 mm, 1.2 mm, 1.5 mm, 1.6 mm, 1.8 mm and 2.0 mm. There is a slight tolerance in the actual thickness of the laminate; the margin of error is determined by the manufacturer’s equipment precision and manufacturing standards. For high-precision PCB production, premium laminates with tighter tolerances should be selected.
Standard Copper Foil Thickness
The thickness of the copper foil on the surface of FR4 copper-clad laminates（CCL） determines the current-carrying capacity of the circuit board. The industry-standard copper thicknesses are 0.5 oz, 1 oz and 2 oz, which meet the current-carrying requirements of the vast majority of circuits. For applications involving high current, high power or specialised precision circuits, other copper thickness specifications can be customised; specific parameters must be confirmed in conjunction with the PCB manufacturer’s production process.