What is a PCB Substrate?

A PCB Substrate acts as a foundation upon which a printed circuit board is built. The substrate of a PCB defines its physical, electrical, and thermal properties. Substrates are typically dielectric composite structures that comprise of epoxy resin and are used in combination with a copper foil bonded onto one or both sides. A solder mask is then applied over the copper layer to insulate it and prevent contact with any other element that could disrupt the copper traces.

In multi-layer PCB boards, there the substrate is used to sandwich the laminate core and all the layers are bound together using high temperature and pressure.

The substrate of a PCB defines its physical properties which means if a rigid substrate is used it increases its strength and durability, while a flexible substrate allows for building flexible circuits that can be bent and twisted without disrupting signal flow.

Thermal Properties & Electrical properties of PCB Substrates:

The substrate of a PCB also defines the thermal conductivity and electrical conductivity of a printed circuit board.

Key Thermal Properties Include:

Glass Transition temperature (Tg): Tg is the temperature at which the substrate material transforms from a rigid state to a soft state (can be thought of as melting). PCB boards should always operate below the glass transition temperature of the substrate. Click here to learn more about Tg.

Decomposition temperature (Td): Td is the temperature at which the PCB substrate material decomposes chemically.

Coefficient of Thermal Expansion (CTE): CTE is used to determine the rate of expansion of a given material when subjected to heat. The increase in size caused by the expansion will cause potential damage due to internal stress. Click here to learn more about CTE.

Thermal conductivity (k): Thermal conductivity determines the rate at which a material conducts heat. What is its impact?

Electrical Properties of PCB Substrates

Dielectric Constant (Er): The dielectric constant is the ratio of the permittivity of a substance to the permittivity of free space. It is a dimensionless value. Most PCB substrate materials have dielectric constants from 3.5 and 5.5. Click here to learn more about dielectric constant.

Surface Resistivity (ρS): Surface resistivity of a PCB substrate is its ability to resist current leakage along its surface. The ρS of a PCB substrate usually varies between 103 to 109 megaohms. The surface resistivity of a material is affected by high temperature and moisture.

Electrical Strength: A dielectric material’s ability to withstand electrical breakdown is known as its electrical strength. The electrical strength of a material is calculated in Volts.

There are plenty of PCB substrate materials that are available in the market. Choosing the correct PCB Substrate will determine the electrical and thermal performance of the board. FR4 and CEM-1 are the primary options for Cost-effective boards. For high-performance boards, polyimides can be used as they have a high Glass transition temperature (Tg) making them expensive for certain applications.

Types of Substrate Materials :

 FR-4 is a low-cost and versatile substrate material. It offers electric insulation with high dielectric strength. It can be used in low-speed digital circuits and when the design structure is complex and requires multiple layers.

CEM-1 is a cheaper alternative to FR-4. CEM stands for composite epoxy materials. It is used for producing single-sided PCBs.

Teflon or PTFE is used for high-speed, high-frequency applications. It is incredibly flexible, extremely lightweight, and flame resistant making it versatile for several applications.

Polyimide as a substrate offers a glass transition temperature of more than 250˚C making it useful for high-temperature applications. Polyimide is also used to develop flex circuits and rigid-flex boards. They are more expensive to implement when compared to FR-4.