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Common PCB Dielectric Materials and Their Properties

Key Takeaways

  • PCB dielectric materials provide the non-conductive substrate layer between the conducting copper layers in a circuit board.

  • A stable dielectric constant over a wide range of frequencies is preferred to ensure safe operation. 

  • FR-4 is the universally-accepted standard for single-sided, double-sided, and multilayered PCBs with strict mechanical strength requirements.

PCB

If you open any consumer electronics device today, you will most likely see printed circuit boards (PCBs) of various dimensions. PCBs are the building blocks of consumer and industrial electronics; any feature in an electronic device maps to a PCB, and a single PCB may provide multiple features to the electronic system.

The level of integration and performance of PCBs invariably relies on the PCB’s dielectric materials. The thermal, electrical, chemical, and mechanical properties of PCB dielectric materials are important design parameters to consider to increase the reliability and performance of PCBs, and in turn, the reliability and performance of electronic devices. 

PCB Dielectric Materials 

PCB dielectric materials are the base of any circuit board. They provide the non-conductive substrate layer between the conducting copper layers. These materials are the surface where components are soldered to form the circuit, with traces connecting each other. 

Dielectric Constant

The dielectric constant, or relative permittivity, is an electrical property that characterizes PCB materials. It is the measure of the insulation provided by the PCB material. It also indicates the degree to which an electromagnetic material slows down in the PCB material.

The dielectric constant of a PCB dielectric material is dependent on the frequency of the system embedded in it. A stable dielectric constant over a wide range of frequencies is preferable to ensure safe operation. However, there are other electrical, thermal, chemical, and mechanical properties that are of equal importance in the selection of PCB dielectric material for specific applications.  

Properties to Consider When Selecting PCB Dielectric Materials

The table below gives a list of properties to consider when selecting materials for PCB substrate, apart from the dielectric constant.  

Classification of property

Properties

Electrical

Dielectric loss tangent or dissipation factor

Volume resistivity

Surface resistivity

Electrical strength

Thermal

Glass transition temperature

Decomposition temperature

Coefficient of thermal expansion

Chemical 

Flammability specs (UL94)

Moisture absorption

Methylene chloride resistance

Mechanical

Peel strength

Flexural strength

Density

Time to delamination

Properties of PCB dielectric materials

Common PCB Dielectric Materials 

When choosing PCB dielectric materials, it is difficult to pick one with all the desirable properties. A designer's priorities may change based on the application environment, frequency of operation, power ratings, expected service time, and cost. Here are a few common PCB dielectric materials to consider.

FR-4

FR-4 is a collection of materials that satisfies the National Electrical Manufacturers Association’s (NEMA) standards, and is the most commonly-used PCB dielectric material. FR4 is a woven glass fiber epoxy compound material with fire retardant (FR) properties. The high glass transition temperature of FR-4 makes it suitable for high power density circuit boards. It is accepted universally as the standard for single-sided, double-sided, and multilayered PCBs with strict mechanical strength requirements. 

CEM- 1, CEM-2, and CEM-3

The CEM is a family of PCB materials listed by NEMA. They are suitable for high-density applications. 

  • CEM-1 materials are a combination of paper, woven glass epoxy, and phenol compounds. They are often used in single-sided PCBs. The dielectric property of CEM-1 is closer to that of FR-4.  CEM -1 material is a good alternative for FR-4 material when cost is a constraint. 
  • CEM-2 materials offer higher glass-transition temperatures compared to CEM-1 materials. 
  • CEM-3 materials are the most suitable PCB dielectric materials for double-sided PCBs with plated holes. 

Polytetrafluoroethylene (PTFE)

PTFE is a synthetic polymer consisting of carbon and fluorine. It is well known under the brand name Teflon, which is used to coat non-stick cookware. The glass-transition temperature of PTFE is high and falls between 160-280˚C. This material is excellent for building high speed, high frequency, microwave, and RF high power boards. 

Polyimide

If you are building a flexible PCB, polyimide is the best PCB dielectric material to use. Polyimide material is popular for its electrical properties, tensile strength, chemical resistance, and high flexibility over a wide temperature range. It is usually used in high-power, high-frequency microwave circuit boards.

Choosing the appropriate PCB dielectric material is critical when a circuit is handling high power, high frequency, and high temperatures. Cadence software offers tools to support designers in diagnosing the effects of PCB substrate dielectric constants on the performance of high frequency and high-speed boards. 

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