The Benefits of High-Dk PCB Materials
The terms “high-speed design” and “low-Dk PCB laminate” are often used in the same articles, and often in the same sentence. Low-Dk PCB materials have their place in high speed and high-frequency PCBs, but high-Dk PCB materials provide power integrity. Low-Dk PCBs are typically chosen as they tend to have lower loss tangent. Thus high-DK PCB materials tend to get overlooked for high speed and high-frequency PCBs.
When we look at power integrity for high speed/high-frequency boards, rather than just signal loss or accepting the value provided by a high-speed laminate, you should consider dielectric constant as part of the overall strategy for stable power. This includes the real and imaginary parts of the dielectric constant, as both affect your PCB's power integrity. With this in mind, let’s look at the role played by high-Dk PCB materials in ensuring power integrity.
High-Dk PCB Materials and PCB Power Integrity
First things first, when we look at power integrity, we’re always trying to ensure that the voltage you output from your regulator stages remains constant as power flows throughout the PDN. This brings up two aspects of PDN analysis and power integrity:
- DC analysis: Here, we’re only concerned about IR drop across the conductors that make up the PDN. The dielectric constant doesn’t play a role in DC analysis.
- AC analysis: By AC analysis, we mean the behavior of any time-varying current on the power plane. This is where the impedance of the PDN becomes essential, as the voltage variation seen at a downstream component is the product of PDN impedance and the time-varying voltage (Ohm’s law).
A high-Dk PCB material used as the dielectric between the power and ground plane provides some important power integrity benefits. In particular, a high-Dk value for PCB material between the ground and power planes will provide larger interplanar capacitance, meaning your planes act like a larger decoupling capacitor, and PDN impedance will be lower. Placing the ground and power planes closer together also increases interplanar capacitance. Some example simulation results from a 2006 IEEE paper are shown below.
The other important aspect of dielectric constant is the imaginary part or the Df value. This is usually summarized using the loss tangent, but this is not the only metric to use in examining the usefulness of a particular laminate in high speed/high-frequency boards. Dispersion in the laminate is also quite important for digital signals as it will cause signals to stretch and distort in your board.
For signal integrity, the important parameters are the Dk and Df values individually, rather than just looking at the loss tangent. Note that, for low-loss PCB substrates, the Dk and Df values tend to scale together (e.g., Rogers laminates), but this is not always the case. You can see some examples in popular laminates; for instance, Nelco 4000-13 EP has about 20x lower loss tangent than FR4, but the Dk value is only about 10% lower.
This means you could see more stable power alongside lower signal losses by using a high Dk/low Df laminate. If you want to balance signal integrity and power integrity, you should opt for lower Df and higher Dk value, rather than just looking at the loss tangent. The other option is to create a hybrid PCB stackup, where different laminate materials are used. Depending on the laminate materials involved, you may save some costs by mixing and matching laminates, rather than choosing a single exotic material for the entire stackup.
Hybrid PCB Stackups: The Best of Both Worlds
You can see the benefits of a low-loss dielectric for signal integrity and a high-Dk dielectric for power integrity in a hybrid PCB stackup. In this type of stackup, the high-Dk layer would be a better option for separating power and ground planes in the PDN, which will reduce PDN self-impedance and transfer impedance. You would then want to use a low-Dk material with low loss to support signals on the surface layer and encase stripline geometries on the interior layers.
An example of a 10-layer board is shown below. These stackups can be a bit odd and difficult to create as you want to ensure symmetry. This ensures that any stress created by CTE mismatches is uniform, both during assembly and operation. Note that any of the ground planes could be swapped for a power plane with different voltage, and it could still serve as a reference for an adjacent signal layer.
Before creating a hybrid stackup, be sure to consult with your fabricator regarding their capabilities and which materials they recommend using. If you opt to design a hybrid stackup, your fabricator may recommend some limits on CTE mismatches between different laminate materials, constraining your available options. Although PCB design software will basically allow you to create any stackup you like, it does not mean your manufacturer will be able to produce it. Always check with a fabricator before producing this type of stackup to ensure they know how to handle these boards and prevent delamination during assembly.
Other Important Effects of High-Dk PCB Materials
Here are some of the other important effects of high-Dk PCB materials in your circuit board.
- Slower signal propagation. This means your allowed length mismatch in parallel nets and differential pairs will be smaller (for a given timing mismatch). However, with the right routing and impedance control tools in your PCB design software, this becomes a non-issue.
- Smaller transfer impedance. As I discussed in a recent article, transfer impedance describes how a PDN voltage disturbance created by a switching component affects the voltage fluctuation seen at a different component. If the Dk value for the dielectric is larger, then the transfer impedance is smaller, the voltage fluctuation seen at the other component is smaller. The Df value also plays a role here, in that a lossy substrate will dampen the voltage fluctuation seen at other components (see Fig. 12 in this article).
- Delayed fluctuations between different components. When a fluctuation occurs at one component, it takes some time to propagate along the PDN to other components. When the dielectric’s Dk value is larger, the delay between fluctuations at different components is longer. However, bypass capacitors placed at other components will compensate for any fluctuation, and the correct bypass capacitor makes this a non-issue.
- Interplanar cavity anti-resonances move to lower frequencies. This becomes important out to GHz bandwidths. At a cavity anti-resonance, the impedance peaks at a particular frequency. Using a thinner high-Dk material with a larger loss between ground and power planes dampens these anti-resonances (see Fig. 11 in this article). I’ll discuss this issue with resonances in cavities and waveguides more in a future article.
If you’re building a hybrid stackup for a high speed/high-frequency board, you should use a high-Dk/high-Df dielectric between the ground and power planes. If you’re using the same laminate material throughout the stackup, you can balance power integrity and signal integrity if you use a high Dk/low Df dielectric.
The downside to using high-Dk PCB materials is a stronger capacitive coupling between conductors. This means parasitic capacitances involving the substrate are larger. If this sounds esoteric, your trace capacitance values will be larger; thus, your trace inductance values need to be larger to ensure impedance control. This then means crosstalk will be stronger, so trace separation should be larger to compensate for the larger Dk value.
Your PCB stackup is a major determinant of power integrity and signal integrity. You can ensure your board functions correctly in both aspects when you have access to the right PCB design and analysis tools. The Layer Stack Manager in Altium Designer® gives you access to a library of common and specialized PCB laminates. You can define material parameters for a specialty laminate for your PCB. The integrated 3D field solver from Simberian uses these material parameters to model signal behavior in your PCB as you create your PCB layout.
Altium Designer on Altium 365 delivers an unprecedented amount of integration to the electronics industry until now relegated to the world of software development, allowing designers to work from home and reach unprecedented levels of efficiency.
We have only scratched the surface of what is possible to do with Altium Designer on Altium 365. You can check the product page for a more in-depth feature description or one of the On-Demand Webinars.