A printed circuit board, commonly known as a PCB, is an assembly that uses copper conductors and electricity to create electrical connections between electrical components. PCB manufacturers use a substrate to make it, which houses the components that form the basis of the PCB. PCBs are made of a variety of substrate materials, depending on the requirements of the circuit. Given below are the types of common substrate materials and their uses.

Characteristics of PCB Substrate

PCB substrates have some common properties, based on which the type of PCB can be determined.

Dielectric Constant
The dielectric constant refers to the ability to store electrons in an electric field. Devices that are subject to high voltages and currents generate electromagnetic fields. The dielectric constant is fundamental to these electronic devices. If the dielectric constant is too high, these strong electromagnetic fields can induce currents in nearby traces.

In addition, to achieve the same capacitance value, PCB substrates with higher dielectric constants must be made thicker than substrates with lower dielectric constants. Therefore, using materials with lower dielectric constants can reduce the size and weight of the substrate. Smaller circuits are also a result of designers being able to shorten the space between traces on substrates with lower dielectric constants.

Thermal Conductivity

The coefficient of thermal expansion (CTE) is a measure of a material’s ability to change dimensions as temperature changes. A substrate with a high CTE expands more than one with a low CTE at the same temperature. In a multilayer PCB, the CTE of the PCB substrate is critical. When the PCB is exposed to temperature cycling, separation may form between the layers if the substrates that make up the layers have different CTEs. A high CTE substrate may expand as temperature rises, with a force greater than its mechanical strength, resulting in cracks, chipping, or other mechanical damage.

Maximum Operating Temperature
Above this temperature threshold, the PCB substrate becomes more susceptible to failure. The maximum operating temperature (MOT) of a substrate is the temperature at which the substrate retains the properties specified by the manufacturer. Substrate manufacturers specify the MOT value and the period of time that the substrate can maintain this temperature without damage.

Electrical Insulation
The electrical insulation of a PCB substrate indicates how poorly it conducts electricity. This property, often called bulk resistivity (BR), is a measure of how much electrons a PCB can transport through a substance. The higher the bulk resistivity of a substrate, the less likely floating currents are to form within the material. The insulating ability of a substrate depends on its thickness. Therefore, a substrate with a high bulk resistivity can be thinner than one with a low bulk resistivity for the same total insulation.

Types
FR-2

FR-2 is a low-performance substrate made of synthetic resin-bonded paper impregnated with plastic resin and used to make PCB boards. FR-2 is a very cheap PCB substrate; however, it has many applications. FR-2 boards are used with copper foil to make low-quality consumer electronics. FR-2 has significantly inferior performance compared to upgraded versions such as FR-4.

In addition, it is much cheaper. This property makes it suitable for installation in car interiors to counteract the effects of vibration. FR-2 protects cars from crack propagation and circuit trace breakage. FR-2 can also be used alone for simple structural shapes and electrical insulation. It can be drilled, sawed, milled, and even used in hot stamping to help the material fit into tight spaces.

FR-4

FR-4 is a name and grade. The name comes from the fiberglass-reinforced epoxy laminate used to make printed circuit boards. FR stands for flame retardant, and 4 is the type of material in the FR class. So, FR 4 is a composite structure of thin glass fibers, which makes FR-4 stable. It contains flame retardant epoxy resin, which makes FR-4 strong. FR-4 forms the base of the PCB, on which the circuits are located. After preparation, the manufacturer laminates the substrate with copper foil, which comprises the final product.

FR-4 is a very effective PCB substrate, and its dielectric properties help determine the mechanical and thermal reliability of the board. The FR-4 dielectric allows the board to operate in harsh environments that require high-temperature resistant materials. FR-4 is made of insulating materials with excellent mechanical and electrical properties.

It is very flexible, and its copper planes have great bending strength. FR-4 is water-resistant and can absorb moisture due to its dielectric constant. The dielectric constant depends on the resin content and the weave of the glass. However, exposing FR-4 to high frequencies weakens the dielectric properties of the material, which makes it lose its water absorption properties.

However, some precautions need to be taken when using FR-4 substrates. The principle is that its dielectric constant varies with area and actual propagation delay is required. To be safe, calculate the accurate dispersion value.

Low Temperature Co-fired Ceramics

Low Temperature Co-fired Ceramics, also known as LTCC, is a multi-layer, high-performance PCB substrate used in electronic packaging and substrate technology. It is well suited for microwave products and products using radio frequencies. It is a low-cost substrate technology used as a ceramic material system in electrical engineering.

In LTCC, material layers are stacked on top of each other to help conduction. The conductor paths are usually made of thick film pastes of gold or silver. The layers are printed layer by layer on a ceramic tape and printed using screen printing technology. LTCC has excellent physical and electrical properties and is relatively low cost compared to other substrates such as laminate substrates.

Low Temperature Co-fired Ceramics has great advantages. Its advantages include:

LTCC can tolerate low dielectric constants. Microelectronic devices and other similar devices require low dielectric constants.

Low Temperature Co-fired Ceramics has excellent thermal conductivity, which means it is able to prevent heat from being transferred outside the circuit board. It reduces energy consumption and makes PCB substrates cost-effective.
The low content of trichloroethylene in the substrate has adverse effects on the human body, and TCE can cause symptoms such as eye discomfort, dizziness, headache, nausea, etc.
Multilayer modules use substrates due to the presence of multilayer ceramic tapes.
LTCC has integrated cavities and passive components such as resistors, inductors, and capacitors.
PCBs using LTCC are very strong and can withstand mechanical and thermal stress.
Since the conductive paths are made of silver, a cheap material, this substrate allows the construction of affordable PCBs.
Due to the low production cost, LTCC substrates are mass-produced.
Metal substrates

Aluminum-based metal PCB substrates have strong dielectric and thermal properties and a small degree of expansion. Aluminum guarantees high frequencies and can easily resist temperatures up to 350°C. Ceramic substrates are generated using an electrochemical technology that deposits a dielectric layer of aluminum oxide crystals directly on the surface of the aluminum substrate and electrically couples it to the aluminum substrate.

This material has a higher dielectric thermal conductivity than traditional dielectric materials currently used for PCBs because the thickness is significantly thinner than ordinary substrates. Aluminum substrates are more expensive than other options. However, it provides excellent high-frequency performance. Applications of metal substrates include PCBs for devices that use radio frequencies, wireless communication base stations, microwave devices, and LED lighting.

Flexible Substrates

Flexible substrates PCB substrates are thin and use heat-resistant materials made from polymers such as polyethylene terephthalate and polyamide. PCBs that transmit signals between control prompts and screens are made from flexible substrates. Flexible substrates for PCBs are easier to insert into narrow and thin places. In addition, the substrate can withstand various stresses and temperatures while providing the best frequency with limited components. Flexible substrates have a wide range of uses, including:

Automotive Industry:
Flexible PCB substrates are widely used in the automotive field to equip new cars with features such as WIFI, voice recognition, sensors, etc. Flexible substrates are cheap, which is an advantage for automobiles.

Electronics:
Electronic products are becoming more and more compact; therefore, they use flexible substrates.

Microwave Ovens:
Flexible substrates can withstand extremely high temperatures, which is why they are used in microwave ovens to withstand high temperatures.

Rigid-Flex Substrates

Rigid-Flex boards are important components used when assembling small products in complex systems. Rigid-Flex boards are made of woven glass fibers with a small amount of epoxy resin. However, industrial applications use polyamide and polyester. Rigid-flex substrates are compact and can be easily fitted into small devices. They are also very lightweight and you can bend them according to the project requirements.

They are considered to have both stability and flexibility. They are ductile and can be stretched without any significant effects. Their flexibility makes them easy to fix as the boards can be folded and bent, so technicians can use them easily. Due to their flexibility, rigid-flex boards are cost-effective as they can fit into small areas.

The above-mentioned PCB substrates are used to build a specific PCB. Depending on the project requirements, the manufacturer uses the appropriate substrate. FR-4 substrate is one of the highest performing substrates and is widely used in industrial applications, while other substrates are used in many different industries.