Every new and expert designer should be familiar with the basics of FR4, such as its dielectric properties and thermodynamic behavior. FR4 weaves are the most common materials used for PCB substrates, but they have some disadvantages compared to other materials for very high speed/high frequency devices.
The primary advantage of FR4 laminates is their low cost and the ability to tailor the glass weave/resin content to exert some control over the glass transition temperature, thermal conductivity, and Dk value. For high speed/high frequency products, low-Dk materials are often cited as essential substrates as they have some advantages in the GHz range. Here are some important low-Dk PCB material advantages and disadvantages, and when you should consider using one of these materials in your next PCB.
A low-Dk PCB material has a low dielectric constant (real part). The term “Dk” refers specifically to the real part of the dielectric constant (i.e., the refractive index), while the term “Df” refers to the imaginary part. Note that the imaginary part of the dielectric constant only determines losses, while the real and imaginary parts collectively determine dispersion. Note that the geometry of the system and dielectric constant contrast at the boundary also determine modal wavelengths and dispersion in the system.
In a PCB, it’s important to remember that electromagnetic waves are not confined in traces. In reality, waves are confined in the medium around a trace. The potential of the trace and nearby reference planes will determine the distribution of the electric field strength around the trace. As propagating signals in a PCB are travelling through the substrate, the dielectric constant of the substrate plays an important role in determining signal behavior.
Electric field (blue) and magnetic field (red) lines around a differential pair. This shows how the electromagnetic field exists around a PCB trace and extends through the substrate, thus the dielectric properties of the substrate are important for describing signal propagation.
Since the propagation speed of electromagnetic waves is inversely proportional to the dielectric constant, a low Dk value means the wave propagation speed is larger. This provide some advantages in transmission lines on PCBs as the wave speed is faster:
Parallel nets in a signal bus need to be length matched to ensure signals arrive at the destination component simultaneously. In other words, all parallel traces in a bus should have as close to the same length as possible. The allowed skew between parallel high speed nets in a bus is specified as a length difference or time difference. If the time difference is known, the allowed length mismatch is proportional to product of the signal speed and the time difference. Since a low-Dk PCB material has a larger signal speed, the allowed length mismatch is also larger.
The strength of capacitive coupling between vias and traces is proportional to the dielectric constant of the PCB substrate material. A low-Dk PCB material has smaller capacitance between neighboring conductors, so the strength of capacitive crosstalk will be lower. For a given allowed crosstalk signal strength, the conductors can be placed closer together when the dielectric constant is smaller. Similarly, for a given trace, via, or conductor spacing, the strength of capacitive crosstalk will be smaller.
The critical length of an interconnect is the usually quantified by comparing the rise time (for digital signals) or oscillation period (for analog signals) of the signal with the propagation delay. This is not the best way to determine the critical length; the best way to do this is to examine the allowed difference between the transmission line’s input impedance and the source/load impedances. I’ve written about this extensively in an article on Altium’s PCB design blog.
Although using the signal’s propagation delay is not the most accurate method, it does illustrate how the dielectric constant affects the critical length of an interconnect. It also provides a decent estimate for the critical length/critical rise time. When the rise time is shorter than some fraction (~1/10) of the signal’s propagation delay, a given trace needs to be treated as a transmission line. In this case, impedance matching needs to be enforced on the interconnect. When the signal speed is larger, the time required for a signal to traverse the interconnect is shorter, thus the critical rise time is shorter. When the rise time for a digital signal is longer than the critical rise time, then the transmission line does not need to be impedance matched.
For analog signals, we normally compare the interconnect length with the wavelength of the signal, or the oscillation frequency with the propagation delay. For a broadband analog signal, the wavelength or frequency corresponding to the carrier frequency is normally used. In a low-Dk PCB material, the signal speed is shorter, so the wavelength will be longer for a given frequency. The critical length for analog signals is also specified as some fraction (~1/10) of the wavelength; when the wavelength on the interconnect is longer, the critical length is also longer.
A low-Dk PCB material is not a cure-all for signal integrity problems in a PCB. Problems like dispersion and losses will still affect signal behavior. Even low-Dk PCB materials have dispersion, and a low Dk does not mean the Df value is also lower. The ratio of these two quantities is described as a loss tangent:
While it is true that a smaller Dk value produces a larger loss tangent, this does not mean that losses are larger. The real part of the signal’s propagation constant (i.e., attenuation constant) is proportional to the transmission line’s parasitic capacitance and loss tangent:
The result is that the product of the two factors combine to produce direct proportionality to the Df value only. This matches the classical result from optics.
Although the two quantities are only directly related through Kramers-Kronig relations, PCB substrate materials are not homogeneous, and the dielectric properties are determined by the glass fiber weave and the resin content. Laminate manufacturers have take significant efforts to understand how the resin type and content influence the Dk and Df values in PCB substrate materials. Read this study from Isola Group to learn more about the influence of resin content on the dielectric properties of PCB laminate materials.
This brings us to the primary disadvantage of low-Dk PCB materials: price. These materials are less-commonly used compared to FR4, and they carry a higher price as a result. However, there are some companies that offer a range of PCB material options with low Dk values. The resin content influences the Dk value, mechanical strength, thermal conductivity, glass transition temperature, and moisture uptake. Take a look at the material options from Rogers and Isola if you want to see some popular options for low-Dk PCB substrate materials.
Here at NWES, we have experience designing PCBs on a broad range of PCB substrate materials, including a variety of low-Dk PCB material options. We're here to help electronics companies design modern PCBs and create cutting-edge technology. We've also partnered with a number of advanced PCB manufacturers, and we'll make sure your next layout is fully manufacturable on any substrate. Contact NWES for a consultation.