How Absorbing Conformal Coatings Affect Your PDN and Radiated EMI

Signal integrity analysis and measurements go hand-in-hand

I remember the first time I—back in my days of working with sensors—pumped nitrogen dioxide into a humid environmental chamber while evaluating a DUT. Over time, the surface of my copper leads slowly corroded as they sat unprotected in such a noxious environment. As my electronics experiment was living a double life as a chemistry experiment, I questioned whether to use a conformal coating on bare electrical leads, given that problems like volatilization and outgassing might interfere with my sensitive gas concentration measurements.

If you’re designing a new board that will be deployed in a harsh environment, then there are a number of conformal coatings on the market which you should consider applying to the surface of your finished board. These coatings provide a number of protective functions that can extend your product’s lifetime. One aspect of conformal coatings that doesn’t receive enough attention is their effect on signal integrity, power integrity, and EMI. Let’s take a look specifically at absorbing conformal coatings and how they influence power integrity and radiated EMI.

Why Use Conformal Coatings in Your PCB?

Conformal coatings are typically used to protect a PCB from specific environmental hazards. As an example, electronics that will be deployed in an overly humid environment or on a boat should be waterproofed; using a conformal coating provides an extra layer of protection as it seals your conductors behind a thin film of water-resistant (usually hydrophobic) material. Other examples include conformal coatings with high breakdown threshold for use in high voltage PCBs. This provides additional protection against discharge. Another application is protection against dust and soil, which can create a short between pins on a component. These coatings are common in various automotive boards and in military systems.

Boards that must run at high frequencies can also benefit from a specialized conformal coating if they are to be deployed in a harsh environment. Some specialized coatings account for their effects on signal integrity as they are intended to protect RF boards from water absorption, noxious gases, and high voltage discharge. These coatings typically have very low loss tangent of up to 10’s of GHz, although most commercially available coatings are not suitable for the Ka band (~40 GHz), as the loss tangent drastically increases, causing signals to quickly degrade.

Thermal deposition in vacuum is one method for depositing high performance conformal polymer coatings, including absorbers.

There is another aspect of conformal coatings that is not usually discussed: their effects on power integrity and radiated EMI. This is a rather important point to consider in high speed and high frequency multilayer boards. We’ve discussed power integrity in various places on this blog. The primary goal of designing to ensure power integrity is to maintain a stable supply voltage to components in the face of switching components; I’ll refer the interested reader to a recent article on impedance analysis as part of power integrity design.

When a transient propagates on a power rail, it can be emitted strongly as radiated EMI. Similarly, when a signal traverses between layers in a board, the burst of current also excites a strong field in the interior of the board. These effects can excite cavity resonances in the interior of the board, which are then guided to the edge of the board where they produce strong radiated EMI from the edge; this is known in the research community as the cavity resonance edge effect (CREE).

How Can a Conformal Coating Reduce Radiated EMI?

Here, a conformal coating that absorbs strongly in the relevant frequency band will suppress radiated EMI from the board edge. In effect, any propagating electromagnetic waves guided by plane layers to the board edges were attenuated as they passed into the conformal coating. This was verified in 3D simulations at frequencies ranging from 10 kHz to 10 GHz.

It was shown in a recent article that applying a commercially-available absorbing coating to the board edge also reduced the impedance of the PDN. As the conformal coating was effectively connected to the PDN in parallel, the coating did not alter the PDN DC resistance, although it did provide an impedance reduction. When decoupling capacitors with high self-resonance frequencies (were added near the edges of the board, the PDN impedance matrix (both the diagonal elements and the off-diagonal coupling elements) also decreased.

PDN impedance matrix element reduction due to application of an absorbing conformal coating and decoupling capacitors. Source: www.jpier.org

Compared to using a via fence along the edge of the board to create an image plane, using a conformal coating might be more cost effective as it reduces the number of required drilling steps. This depends on the exact frequency or range of frequencies you need to block. Using a via fence effectively blocks a single frequency, while using an absorbing coating is a broadband solution.

Determining which steps to take requires balancing cost, available materials, and deposition method, and this must be done while considering your initial EMC tests with your prototype. If you’re exceeding limits at a specific frequency, and you can identify it as a cavity resonance, then adding a via fence that suppresses that specific frequency will help you pass EMC tests. Companies like Parker provide conformal coatings that are specifically designed for radiated EMI suppression, which can be applied at the board edge and over the board surface.

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