FR4 Dielectric Constant and Material Properties
High speed and high frequency boards need accurate dispersion calculations to ensure signal integrity and accurate propagation delay. FR4 is the most popular type of PCB laminate, and its material properties are well-documented. We’ve compiled the important material properties for FR4, including FR4 dielectric constant, thermal conductivity, and more. Altium Designer includes an accurate wideband Debye model calculation for FR4 dielectric constant and the tools you need for high speed PCB design.
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Your PCB design begins with an insulating substrate, typically made from a material called FR4, as its core. The designation “FR4” refers to the flame-retardant properties of the dielectric material and type-4 woven glass-reinforced epoxy laminate used to build up the substrate. This type of laminate material is highly insulating and rigid, and every manufacturer should know how to work with FR4 laminates as a base material.
An important part of designing PCBs on an FR4 laminate is to understand its material properties, which includes its dielectric properties and thermal properties. With the right design software and materials stackup library, you can define an accurate impedance profile for your high speed PCB and run accurate signal integrity simulations. Altium Designer provides the stackup design tools you need to include FR4 dielectric constant and thermal properties in your simulations, as well as many other features for designing your next high speed PCB.
All materials have some electric permittivity—a characteristic that describes the speed that an electrical signal travels through a material and the amount of electrical charge that material can store in a given volume. The electric permittivity of free space (vacuum) is:
8.854 x 10-12 Farads/meter or Ɛ0
Signal propagation speed occurs relative to the speed of light in a vacuum, which is defined in terms of a relative permittivity, or dielectric constant. The dielectric constant of FR4 ranges from 3.8 to 4.8, depending on the glass weave style, thickness, resin content, and copper foil roughness.
In addition to the dielectric constant of FR4, the arrangement of traces and planes on a PCB laminate determine the effective dielectric constant for signals travelling in an interconnect. Signals travelling on surface microstrip traces or coplanar waveguides will have speed that is determined by some effective dielectric constant, which then depends on the geometry of a trace. In order to determine the appropriate impedance and propagation velocity for signals on a PCB trace, your PCB design software needs to include a stackup design utility with data for a range of standard PCB laminate materials, including FR4.
Dissipation in FR-4 Dielectric Constant
The limitations also become apparent when using standard FR4 with RF applications. On one hand, dielectric stability may suffer across the higher frequencies. Along with a higher dissipation factor (Df) and higher insertion losses at microwave frequencies of 1 GHz to 15GHz, traces used on FR4 have a larger attenuation at RF frequencies. Further, the FR4 thickness affects the effective dielectric constant of the PCB and…the greater impedance matching required for RF circuits. When working with RF circuits, the selection of high-performance FR4 that can handle multiple lamination cycles and has good reliability characteristics works as a solution for those problems.
- Measurements of FR4 dielectric constant can be difficult as the results will vary with respect to the measurement method.
- Impedance calculations for common trace geometries, such as microstrips and striplines, require accurate wideband Debye dispersion models for FR4 dielectric constant.
- Balancing FR4 dielectric constant with PCB laminate thickness and trace width is a difficult problem, but the right stackup manager can help you produce accurate impedance and propagation delay calculations.
Creating an impedance profile with copper roughness models directly from a PCB laminate stackup in Altium Designer.
When selecting a PCB laminate material, the thermal and mechanical properties may be a primary consideration in addition to the dielectric constant. The dielectric properties and the mechanical properties of an FR4 laminate material will also depend on temperature, so temperature stability can be a primary consideration in selecting PCB laminate materials. The table below summarizes the important mechanical and thermal properties of FR4 laminates:
3.3-4.8 (depends on weave style, resin content, and material composition)
Breakdown field strength
Approximately 20 kV/mm
Glass transition temperature
~130 °C (low-Tg laminates) or ~170 °C (high-Tg laminates)