Common PCB Board Materials and Dielectric Constants
Introduction of PCB Materials
They are generally divided into five categories according to the different reinforcement materials used for the boards: paper-based, glass fiber cloth-based, composite-based (CEM series), laminated multi-layer board-based, and special material-based (ceramic, metal core-based, etc.).
If categorized by the resin adhesive used for the boards, for common paper-based CCI, there are various types such as phenolic resin (XPC, XXXPC, FR-1, FR-2, etc.), epoxy resin (FE-3), polyester resin, etc. For common glass fiber cloth-based CCL, there is epoxy resin (FR-4, FR-5), which is the most commonly used type. There are also other special resins (using glass fiber cloth, polyimide fiber, non-woven fabrics, etc., as reinforcing materials) such as bismaleimide-triazine modified resin (BT), polyimide resin (PI), p-phenylene ether resin (PPO), maleimide-styrene resin (MS), polycyanurate resin, polyolefin resin, etc. According to the flame retardancy performance of CCL, they can be divided into flame-retardant type (UL94-V0, UL94-V1) and non-flame-retardant type (UL94-HB) boards.
In recent years, with increasing awareness of environmental protection issues, a new type of CCL variety without brominated compounds has been introduced in flame-retardant CCLs, called "green flame-retardant CCL". As electronic product technology develops rapidly, higher performance requirements are placed on CCL. Therefore, from the performance classification of CCL, they can be further divided into general performance CCL, low dielectric constant CCL, high heat-resistant CCL (L for general boards is above 150℃), low coefficient of thermal expansion CCL (generally used on packaging boards), and other types.
Dielectric Constant of PCB Materials
Research on the dielectric constant of PCB materials is because the speed and signal integrity of the signal transmission on PCB are affected by the dielectric constant. Therefore, this constant is extremely important. The reason why hardware personnel overlook this parameter is that the dielectric constant is determined when the manufacturer chooses different materials to make the PCB board.
Dielectric constant: When a medium is subjected to an external electric field, it will produce an induced charge that weakens the electric field. The ratio of the original applied electric field (in vacuum) to the final electric field in the medium is the relative dielectric constant (or dielectric constant), also known as the dielectric constant, which is related to the frequency.
The dielectric constant is the product of the relative dielectric constant and the absolute dielectric constant of vacuum. If a material with a high dielectric constant is placed in an electric field, the strength of the electric field will experience a significant decrease within the dielectric. The relative dielectric constant of an ideal conductor is infinite.
The polarity of polymer materials can be determined by the material's dielectric constant. Generally, substances with a relative dielectric constant greater than 3.6 are polar substances; substances with a relative dielectric constant in the range of 2.8 to 3.6 are weak polar substances; and substances with a relative dielectric constant less than 2.8 are non-polar substances.
Dielectric Constant of FR4 Materials
The dielectric constant (Dk, ε, Er) determines the speed at which the electrical signal propagates in the medium. The speed of electrical signal propagation is inversely proportional to the square root of the dielectric constant. The lower the dielectric constant, the faster the signal transmission. Let's take an analogy. When you are running on the beach, the depth of the water that covers your ankles represents the viscosity of the water, which is the dielectric constant. The more viscous the water, the higher the dielectric constant, and the slower you run.
The dielectric constant is not easy to measure or define. It is not only related to the characteristics of the medium, but also to the testing method, testing frequency, material state before and during testing. The dielectric constant also changes with temperature, and some special materials take temperature into consideration during development. Humidity is also a significant factor affecting the dielectric constant; as the dielectric constant of water is 70, a small amount of water can cause significant changes.
FR4 Material Dielectric Loss: It is energy loss caused by the dielectric polarization and dielectric conductivity lag effect of the insulation material under the action of electric field. Also known as dielectric loss or simply loss. Under the action of an alternating electric field, the deficiency angle of the cosine of the vector combination between the current passing through the dielectric and the voltage across the dielectric (power factor angle Φ) is called the dielectric loss angle. The dielectric loss of FR4 is generally around 0.02, and the dielectric loss increases as the frequency increases.
FR4 Material TG Value: It is also called the glass transition temperature, which is generally 130℃, 140℃, 150℃, and 170℃.
FR4 Material Standard Thickness
The commonly used thicknesses are 0.3mm, 0.4mm, 0.5mm, 0.6mm, 0.8mm, 1.0mm, 1.2mm, 1.5mm, 1.6mm, 1.8mm, and 2.0mm. The thickness deviation of the board varies with the production capacity of the board factory. The common copper thickness for FR4 copper-clad boards is 0.5oz, 1oz, and 2oz. Other copper thicknesses are also available, and they need to be consulted with the PCB manufacturer to determine.