What are the effects of different PCB substrate characteristics?

PCB substrate is the foundation for building a circuit board and determines the physical, electrical and thermal properties of the board. The substrate is usually a dielectric composite structure made of epoxy resin and bonded to copper foil on one or both sides. Then, the solder mask is coated on the copper layer to provide insulation protection and prevent components from contacting with the copper traces and causing damage.

In multi-layer PCB boards, the substrate is used as a sandwich laminate core and all layers are bonded together through high temperature and high pressure.

The substrate of a PCB directly affects its physical properties. For example, using a rigid substrate will increase the strength and durability of the board, while a flexible substrate allows for the construction of flexible circuits that can bend and twist without interrupting the signal flow.

PCB classification by substrate

According to different reinforcement materials (common classification methods)

Paper substrate (FR-1, FR-2, FR-3): paper substrate, suitable for general electronic applications.

Epoxy glass fiber cloth substrate (FR-4, FR-5): epoxy resin reinforced with glass fiber cloth, with high mechanical strength and heat resistance, is one of the most common types of PCB substrates, and is used in many industries.

Composite substrate (CEM-1, CEM-3): composite materials are used, with specific mechanical and electrical properties.

HDI board (RCC): it is “resin-coated copper”, mainly used for high-density circuits (HDI).

Special substrates (metal substrates, ceramic substrates, thermoplastic substrates, etc.): used for applications that meet special needs. Metal substrates are usually used for applications that require higher heat dissipation performance, ceramic substrates are usually used for high-frequency circuit design, and thermoplastic substrates have higher heat resistance and are suitable for applications in high temperature environments.

Classification by different resins

Phenolic resin board: uses phenolic resin as the base material and has specific chemical properties.

Epoxy resin board: uses epoxy resin and has excellent mechanical properties and heat resistance.

Polyester resin board: uses polyester resin as the base material and is suitable for some general applications.

BT resin board: uses BT resin and is suitable for high-frequency applications and high-speed circuit design.

Polyimide resin board: uses polyimide resin and has excellent high-temperature performance.

Classification by flame retardant performance

Flame retardant type (UL94-VO, UL94-V1): has good flame retardant performance, suitable for high-demand electronic equipment, and can effectively prevent the spread of fire.

Non-flame retardant type (UL94-HB grade): has poor flame retardant performance, usually used for general applications, not suitable for high-demand environments.

Substrate characteristics that need attention when selecting materials

Glass transition temperature (Tg)

When the temperature rises to a certain area, it changes from “glass state” to “rubber state”. The corresponding temperature at this time is called the glass transition temperature (Tg) of the board.

Usually, Tg≥150℃ is called medium Tg board, and Tg≥170℃ is called high Tg board. For high-performance boards with many layers, thick thickness and large area, more heat is required during welding to ensure the reliability of welding. If the welding temperature and welding time of conventional PCB are used, the probability of “false welding” will increase.

Therefore, this type of board should have better heat resistance or higher Tg temperature than conventional PCB. When the Tg of the substrate is increased, the heat resistance, moisture resistance, stability resistance and other characteristics of the printed board will be improved, which is very critical for processing high-density and high-layer boards.

Thermal decomposition temperature (TD)

Refers to the temperature at which thermal decomposition reaction occurs due to heat. TD value is also an important indicator to measure the heat resistance of the board. High TD materials are suitable for high temperature environments and reduce the risk of substrate decomposition.

Coefficient of thermal expansion (CTE)

Describes a percentage rate of expansion or contraction of the board when heated or cooled, and its unit temperature rise causes a linear change in the size of the substrate. The substrate is clamped by glass cloth in the X and Y axis directions, and the CTE is not large. The general expansion coefficient is between 13-17. The main focus is on the Z axis direction of the board thickness. The Z axis CTE is measured by thermomechanical analysis.

a1-CTE: Thermal expansion coefficient below TG, the standard maximum is 60ppm/℃

a2-CTE: Thermal expansion coefficient above TG, the standard maximum is 300ppm/℃

Dielectric constant (DK)
Indicates the conductivity of the material. The PCB medium we commonly use is FR4 material, and its dielectric constant relative to air is 3.8-4.8. This dielectric constant changes with temperature. In the temperature range of 0-70 ℃, its maximum change range can reach 20%. The change in dielectric constant will cause a 10% change in line delay. The higher the temperature, the greater the delay.

The dielectric constant will also change with the signal frequency. The higher the frequency, the smaller the dielectric constant.

For PCBs used in high-frequency (referring to signal transmission frequencies greater than 300MHz) circuits, the lower the dielectric constant Er should be used, the higher the signal transmission speed or the smaller the delay time can be obtained, or the signal transmission delay time is proportional to the square root of DK. The higher the DK, the more serious the signal transmission delay phenomenon.

Dielectric loss (DF)

The energy consumed by dielectric materials due to heat generation under the action of alternating electric fields is called dielectric loss, which is usually expressed as dielectric loss factor tanδ. Er and tanδ are proportional; the lower the Df value, the smaller the energy loss, which is very important for high-frequency signals.

Thermal conductivity

Thermal conductivity is also called thermal conductivity and thermal conductivity. It is a physical quantity that represents the thermal conductivity of a material. It refers to the amount of heat (kcal) that passes through an area of ​​1 square meter in 1 hour due to heat conduction when the vertical distance between isothermal surfaces is 1 meter and the temperature difference is 1 ℃.

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