All About FR4 Dielectric Constant

In the modern electronics industry, “FR-4” is a well-known term. It represents a material specification for flame resistance, meaning the resin material can self-extinguish when burning. This material consists primarily of glass fiber cloth and epoxy resin, known for its high strength, good adhesion, and chemical stability, making it widely used in the production of printed circuit boards, circuit protection covers, and electronic components. As a significant electronic manufacturing material, the electrical properties of FR-4, one of the typical representatives is the dielectric constant. In this article by TechSparks, we will explore the importance of the dielectric constant of FR-4 and its influencing factors.

What is the Dielectric Constant

The dielectric constant is an essential property of insulating materials, describing the material’s ability to respond to an electric field relative to free space. It is typically represented by ε (the Greek letter epsilon) or Dk. We can imagine the dielectric constant as the polarization of the material caused by the displacement of electrons in response to an external electric field. A higher dielectric constant indicates a stronger polarization of the material in the electric field, and vice versa.

Why Focus on the Dielectric Constant of FR-4

In the field of printed circuit boards, the dielectric constant is often associated with signal transmission speed. A higher Dk value results in slower signal transmission speed. While signal transmission speed is essential for all electronic devices, it is particularly critical in high-frequency and communication applications. FR-4 is considered a general-purpose electronic material, with a Dk value typically ranging from 4 to 5, making it more suitable for low-frequency applications. Designers often prefer materials with lower dielectric constants, such as PTFE or Rogers, for high-frequency and communication applications. This has led to a misconception that the dielectric constant of FR-4 does not need to be considered.

However, this is clearly a mistaken viewpoint. Although FR-4 PCB are commonly used in general electronic devices, the dielectric constant of the material still needs to be considered during the design phase because it affects more than just signal transmission speed.

For example, the dielectric constant directly affects the characteristic impedance of signals in the PCB, i.e., the resistance or reaction of signals when propagating through the PCB. In high Dk value media, there may be a waveguide effect, meaning signal propagation is constrained, causing signals to propagate along the conductor surface rather than directly through the medium. This ultimately leads to signal loss and distortion, affecting stable signal transmission and performance. Additionally, the dielectric constant affects signal coupling and crosstalk between signals, as well as the coupling capacitance between wires and ground planes on the PCB. Therefore, considering the Dk value of FR-4 during the design phase is crucial.

Factors Affecting the FR4 Substrate Dielectric Constant

If you search online for the Dk value of FR4 substrates in the 1-2 GHz frequency range, you may encounter a problem: different values are specified by different sources, ranging from 3.9 to 4.9 for 1 GHz. This is a normal phenomenon because the Dk of FR-4 is not a fixed value but is influenced by the material composition and processing. Even different batches of products from the same manufacturer may have slight differences.

In addition to material and processing, the dielectric constant of FR4 substrates is also influenced by temperature. As temperature increases, the thermal motion of molecules in the material increases, weakening the polarization of molecules, resulting in a decrease in the dielectric constant of the material. Therefore, it is generally believed that the Dk of FR-4 exhibits a negative temperature coefficient within the range of 0 to 70°C.

Furthermore, because the completeness of the material polarization process is frequency-dependent, frequency also affects the dielectric constant. Specifically, at high frequencies, the response time of the polarization process is shorter, leading to a decrease in polarization degree and, consequently, a decrease in the dielectric constant. At low frequencies, the polarization process has enough time to follow the frequency changes, resulting in an increase in polarization degree and a corresponding increase in the dielectric constant. Typically, within the range of 1 MHz to 10 GHz, the dielectric constant of FR-4 can fluctuate by 5-10%.

Additionally, factors such as moisture absorption rate, laminate thickness, copper thickness, among others, also affect the dielectric constant. Understanding these sources of variation enables designers to consider the potential effects on key parameters. Below are the Dk values of FR4 at different thicknesses for your reference:

IndexTheoretical Thickness (MM)Theoretical Thickness(MIL)Combination MethodDielectric Constant

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