We often say that, in high-speed design, decoupling is the primary ingredient that ensures power integrity. While this is true in general, it can drive designers to pile hundreds of discrete capacitors in very small packaging in an attempt to reduce PDN impedance. Eventually you will run out of useful space for capacitors, and the long connections between capacitors and your integrated circuits will create new power integrity problems.
To overcome these problems, there is one type of material designers can incorporate into a PCB stackup. This is a specialty material known as an embedded capacitance material, sometimes referred to as an ECM. While an embedded capacitance material will not ever be the magic bullet that solves all your power integrity problems, it will provide some important functions in certain systems where high decoupling capacitance and plane capacitance are required.
Embedded Capacitance Material Overview
An embedded capacitance material will appear in the PCB layer stack and it should be placed between the main power plane and its adjacent ground plane. When we refer to an embedded capacitance material, we’re not referring to embedded discrete capacitors. While you can certainly mill channels for embedded discretes, embedded capacitance materials play an entirely different role that is related to power integrity.
Layer stackup with an embedded capacitance material. The ECM layer will be very thin, while the other layers can be standard thicknesses and Dk values chosen from off-the-shelf materials.
The image below shows a typical location where an embedded capacitance material might be placed in a PCB stackup. In this example, the PWR layer could be split into multiple rails, or it could be a single large rail.
All embedded capacitance materials have some common characteristics:
- The material is very thin
- They tend to have high loss tangent
- They can have high dielectric constant if desired
The table below illustrates some of the important material properties of embedded capacitance materials. This list is not exhaustive and it only illustrates some possible values that can be found in commercially available products. Comparable values in FR4-grade laminates are also included in this table.
High Dielectric Constant = High Capacitance
Why does an embedded capacitance material work in the location outlined above? The reason for this is simple: a very thin layer of insulator with high dielectric constant will have high capacitance density and thus high energy density. The planar capacitance density of these materials can be 2-3 orders of magnitude larger than the planar capacitance in a standard thickness PCB. This makes embedded capacitance materials ideal for decoupling up to the GHz range of frequencies, which is exactly where on-die capacitance might be deficient and begin to affect power delivery.
High Loss Tangent = Low Ripple
The loss tangent of these materials near 100 MHz to 1 GHz provides damping of electromagnetic waves whenever chips draw power from the PDN in the PCB. This is why, if you look at the impedance vs. frequency of a plane pair in a field solver, the plane will fail to exhibit characteristic resonances seen when only decoupling capacitors are used. In other words, only the
When to Not Use Embedded Capacitance Materials
From the above discussion, it would seem like embedded capacitance materials are exactly what designers need to ensure power integrity. So where does this idea break down, and why would that be the case? There are a few areas where embedded capacitance materials either should not be used, or they provide no advantages:
- As the supporting dielectric for routing transmission lines
- As the supporting dielectric for mmWave interconnects
- On very large boards that already provide a lot of plane capacitance
- On boards with plenty decoupling capacitance already, including on-die capacitance
In the first two points, these materials should not be used because their loss tangents are comparable to FR4, but at lower frequency values. This lossy property is what makes these materials so useful for power integrity, even when there are insufficient decoupling capacitors. Therefore, they damp signal propagation beginning at a lower frequency cutoff and they can reduce the signal level excessively if the core voltage is low and/or the routing channel is long.
In the last two points, the issue is one of cost vs. performance. In these two cases, the board probably already has plenty of capacitance with a standard laminate, so there is no reason to spend the money on additional capacitance. This is especially the case when scaling up to high volume; every bit of unnecessary cost that can be removed should be considered, and you might find you don’t need an embedded capacitance material.
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