PCB Circuit Board Parameters
1. Dielectric Constant (DK Value): This usually indicates the ability of a material to store electrical energy. The smaller the ∑ value, the lower the energy storage capacity, and the faster the transmission speed.
2. Glass Transition Temperature (TG): When the temperature rises to a certain range, the substrate transitions from a “glass state” to a “rubber state”. This temperature point is referred to as the glass transition temperature (Tg) of the board. Tg is the highest temperature (°C) at which the substrate maintains its “rigidity”.
3. Comparative Tracking Index (CTI): This indicates the quality of insulation. The higher the CTI value, the better the insulation.
4. Thermal Decomposition Temperature (TD): This is an important indicator of the heat resistance of the board material.
5. Coefficient of Thermal Expansion (CTE) – Z-axis: This reflects a performance indicator of the board’s thermal expansion decomposition. The smaller the CTE value, the better the performance of the board.
PCB Material Knowledge and Standards
Currently, there are various classification methods for copper-clad laminates widely used in China. Generally, they can be classified based on the reinforcing material of the board into five categories: paper-based, glass fiber cloth-based, composite-based (CEM series), multilayer laminated boards, and special material bases (ceramic, metal core, etc.). If classified according to the type of resin adhesive used, common paper-based CCLs include: phenolic resin (XPc, XxxPC, FR-1, FR-2, etc.), epoxy resin (FE-3), polyester resin, and various types. Common glass fiber cloth-based CCLs include epoxy resin (FR-4, FR-5), which is currently the most widely used type of glass fiber cloth-based material.
Additionally, there are other special resins (using glass fiber cloth, polyamide fiber, non-woven fabric, etc. as reinforcing materials): bismaleimide-modified triazine resin (BT), polyimide resin (PI), diphenyl ether resin (PPO), maleic anhydride-imide-styrene resin (MS), polycyanate resin, polyolefin resin, etc. Based on the flame-retardant performance of CCLs, they can be divided into flame-retardant types (UL94-V0, UL94-V1) and non-flame-retardant types (UL94-HB). In recent years, with increasing attention to environmental issues worldwide, a new type of bromine-free CCL has emerged within the flame-retardant CCL category, commonly referred to as “green flame-retardant CCL”. With the rapid development of electronic product technology, the requirements for CCLs have also become higher.
Therefore, based on the performance classification of CCLs, they can be divided into general performance CCLs, low dielectric constant CCLs, high heat-resistant CCLs (generally with L above 150°C), and low thermal expansion coefficient CCLs (commonly used in packaging substrates). As electronic technology continues to develop and progress, new requirements for PCB substrate materials are constantly being proposed, thus promoting the continuous development of copper-clad laminate standards. Currently, the main standards for substrate materials are as follows:
1. National Standards: The national standards related to substrate materials in China include GB/T4721—47221992 and GB4723—4725—1992. The standards for copper-clad foil boards in Taiwan are based on the CNS standard, which was formulated based on the Japanese JIS standard and released in 1983.
2. International Standards: Japan’s JIS standard, the United States’ ASTM, NEMA, MIL, IPC, ANSI, UL standards, the UK’s BS standard, Germany’s DIN, VDE standards, France’s NFC, UTE standards, Canada’s CSA standard, Australia’s AS standard, the former Soviet Union’s FOCT standard, and international IEC standards; common and frequently used PCB design material suppliers include Shengyi, Jiantao, and others.
PCB materials are classified by brand quality level from low to high as follows: 94HB-94VO-CEM-1-CEM-3-FR-4
The specific parameters and uses are as follows:
94HB: Ordinary paperboard, not flame-retardant (lowest grade material, can be punched, cannot be used for power boards)
94V0: Flame-retardant paperboard (can be punched)
22F: Single-sided semi-glass fiber board (can be punched)
CEM-1: Single-sided glass fiber board (must be drilled with a computer, cannot be punched)
CEM-3: Double-sided semi-glass fiber board (the lowest grade material for double-sided boards, simple double-sided boards can use this material, which is 5-10 yuan/m² cheaper than FR-4)
FR-4: Double-sided glass fiber board
1. The classification of flame-retardant properties can be divided into four types: 94VO-V-1-V-2-94HB.
2. Prepreg: 1080=0.0712mm, 2116=0.1143mm, 7628=0.1778mm.
3. FR4 and CEM-3 both refer to board materials; FR4 is glass fiber board, and CEM3 is composite board.
4. Halogen-free refers to substrates that do not contain halogens (fluorine, bromine, iodine, etc.) because bromine produces toxic gases when burned.
5. Tg is the glass transition temperature, i.e., the melting point.
6. Circuit boards must be flame-resistant, meaning they cannot burn at a certain temperature but can only soften. This temperature point is called the glass transition temperature (Tg), which relates to the dimensional durability of the PCB.
What is High Tg? Advantages of Using High Tg PCBs
High Tg refers to high heat resistance. When the temperature of a high Tg PCB reaches a certain threshold, the substrate transitions from a “glass state” to a “rubber state”. This temperature is referred to as the glass transition temperature (Tg) of the board. Essentially, Tg is the highest temperature (°C) at which the substrate maintains its rigidity. Ordinary PCB substrate materials tend to soften, deform, and melt under high temperatures, which also leads to a sharp decline in mechanical and electrical properties, thus affecting the product’s lifespan. Typically, Tg materials are above 130°C, high Tg is usually above 170°C, and medium Tg is around 150°C; PCBs with Tg ≥ 170°C are referred to as high Tg PCBs. As the Tg of the substrate increases, the heat resistance, moisture resistance, chemical resistance, and stability of the circuit board improve. The higher the Tg value, the better the temperature resistance performance of the material, especially in lead-free processes where high Tg applications are more common.
With the rapid development of the electronics industry, especially in electronic products represented by computers, there is a trend towards higher functionality and multilayering, necessitating higher heat resistance in PCB substrate materials. The emergence and development of high-density mounting technologies represented by SMT and CMT have made it increasingly reliant on the support of high heat resistance in PCB substrates for small apertures, fine line processing, and thinness.
Therefore, the differences between ordinary FR-4 and high Tg are as follows: under high temperatures, especially when heated after moisture absorption, there are significant differences in mechanical strength, dimensional stability, adhesion, water absorption, thermal decomposition, and thermal expansion between the materials. High Tg products are significantly better than ordinary PCB substrate materials.
What Are the Important Parameters of High-Frequency PCBs?
The dielectric constant (Dk) of high-frequency circuit board substrates must be low and stable. Generally, the smaller the better, as the signal transmission rate is inversely proportional to the square root of the material’s dielectric constant. A high dielectric constant can lead to signal transmission delays.
The dielectric loss (Df) of high-frequency circuit board substrate materials must be low, as this primarily affects the quality of signal transmission. The lower the dielectric loss, the smaller the signal loss.
The impedance of high-frequency circuit boards refers to the parameters of resistance and reactance. Since PCB lines must consider the installation of electronic components, the conductivity and signal transmission performance after installation must be taken into account, thus requiring the impedance to be as low as possible.
The water absorption of high-frequency circuit board substrates must be low. High water absorption can cause changes in dielectric constant and dielectric loss when exposed to moisture.
To meet the signal integrity requirements of different applications, PCBs must not only test S-parameters and TDR impedance but also analyze the physical properties of the materials themselves, such as dielectric constant and dielectric loss. Accurate dielectric constants not only enable effective design but also make simulation and actual test results more consistent, improving the efficiency of design and development, which is significant for PCB material suppliers and PCB production R&D manufacturers.
Screenshots of Some Electronic Books
Screenshots of Some Course PPTs
【Complete Set of Hardware Learning Materials Collection】
