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The FR4 Substrate Dielectric Constant’s Role in Reliability

Published Date
Author Cadence PCB Solutions

Key Takeaways

  • PCB material or PCB substrate is the backbone of rigid circuits. 

  • FR4 is the standard given by the National Electrical Manufacturers Association (NEMA) for glass-reinforced epoxy resin laminate. 

  • The dielectric properties of FR4 material consist of the dielectric constant and loss factor or dissipation factor. 

FR4 Substrate Dielectric Constant
The FR4 substrate dielectric constant is frequency dependent

FR4 substrate is often used as a laminate for printed circuit boards. Most users are unaware of the influence of laminate material on electrical performance indices such as signal integrity, propagation delays, etc. FR4 substrate dielectric material is essential for posing serious challenges to high-frequency signal applications. Frequency is a significant parameter that causes changes in the FR4 substrate dielectric constant. There is a drop in the FR4 substrate dielectric constant when it is used in high-frequency or high-speed circuit boards due to the polarization of the material. It is important to understand the substrate material properties when designing PCBs for mission-critical applications.

Let’s learn more about FR4 substrate material and its dielectric constant in this article.

PCBs and Dielectric Materials

Printed circuit boards (PCBs) are invariably used in all fields of applications such as healthcare, consumer electronics, automotive, and aerospace industries. They are used to build circuits that can handle low or high frequencies such as RF, microwave, millimeter wave, etc. When being used for various frequency and voltage ranges, it is important to choose the appropriate PCB material.

PCB material or PCB substrate is the backbone of rigid circuits. In rigid PCBs, the insulating material (PCB material) forms the case on which the copper tracks, components, and connections are placed. It is important to carefully choose the PCB dielectric material that is appropriate for the circuit application, as it affects the technical performance of the circuit.

Dielectric Material Selection: What to Look For

When choosing dielectric material, there are certain aspects to look for. Depending on the system requirements, the PCB dielectric material should take into account the following parameters:
 

System Requirements

  • Operating frequency, power, and bandwidth

  • System loss requirements or efficiency requirements

  • Size of the PCB board

  • Temperature class of the circuit laid on the PCB

  • Number of PCB layers

Material Properties

  • Mechanical strength, electromagnetic loss, chemical properties

  • High-frequency power handling capabilities

  • Stability under extreme humidity and temperature conditions

  • Ease of processing and compatibility with hybrid constructions

  • Cost

Electrical Properties

  • Low dissipation factor for maximum power delivery

  • Low dielectric constant for rapid propagation of signal

  • Consistency in dissipation factor and dielectric constant over the operating bandwidth of the circuit of interest to prevent signal dislocation and varying transmission line impedance

  • Consistency in dissipation factor and dielectric constant over the operating temperatures


Most designers end up selecting the dielectric material FR4. Let’s learn why.

What Is So Special About FR4 Substrate Material?

FR4 is the standard given by the National Electrical Manufacturers Association (NEMA) for glass-reinforced epoxy resin laminate. The letters ‘FR’ denote the flame retardant or fire retardant property of PCB material and is compliant with the plastic material inflammability standard UL94V-0. There are rare chances for fire propagation in FR4 material, and in case of fire, it extinguishes rapidly. 

Useful Electrical Properties of FR4

FR4 material is flame retardant and moisture resistant and possesses several advantageous electrical properties.
 

Property

Value

Surface resistivity

105-1010 ohms/square unit

Electrical strength

800-1800 V/mil

Dissipation factor

0.009 at 1 MHz

Dielectric constant

5  at 1 MHz

 

FR4 Substrate Dielectric Constant and Dissipation Factor

The dielectric properties of the FR4 material consist of the dielectric constant and loss factor or dissipation factor. The dielectric constant of the PCB material influences the signal propagation speed, crosstalk, and characteristic impedance. The energy from the propagating pulse is absorbed by the surrounding dielectric materials; this phenomenon depends on the dielectric constant as well as the dissipation factor (loss tangent).

FR4 Substrate Dielectric Constant and Reliability

FR4 substrate dielectric constant variations can cause a few issues. The FR4 dielectric constant varies with the thickness of the substrate, copper foil roughness, resin content, and glass weave style. The stability of the FR4 insulating property is subject to deterioration under the influence of high power, voltage, or heat. As the temperature varies, the FR4 substrate dielectric constant becomes unstable, leading to impedance instabilities in the board. FR4 substrate dielectric constant variations pose serious challenges when used in controlled impedance boards.

The FR4 substrate dielectric constant is frequency dependent and offers non-uniform values in high-speed or high-frequency circuit board applications. The characteristics of the circuit and the performance of the board fluctuate with the dielectric constant variations, which are not preferable in high-frequency applications.

The transmission line or interconnect size is also influenced by FR4 substrate dielectric constant. When designing smaller boards, it is essential to choose a PCB substrate material with a higher dielectric constant. 

Analyzing FR4 Substrate Dielectric Constant

The FR4 substrate dielectric constant is a sensitive parameter that varies with dimensional changes, texture differences, signal frequencies, etc. Cadence’s suite of design and analysis tools can help you analyze the variation in the dielectric constant of PCB material under different conditions. With Cadence’s signal integrity tools, the effects of FR4 substrate dielectric constant variations on high-speed and high-frequency signals can be easily diagnosed.

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