Using EMI Shielding to Reduce Radiated Emissions in Electronic Designs

Electronic noise is everywhere. As a designer, you need to have at hand all the tools you can to protect your equipment from it. This article is about EMI shieldings, tools that can be widely used to improve the electromagnetic behavior of your device and to protect it from external interference.

According to Maxwell’s equations, every time there is an electrical current circulating through a conductor, a magnetic field is generated. Additionally, every time a magnetic field exists, an electrical field does, too. Due to the radiated aspect of these electric and magnetic fields, they are widely known as radiated emissions.

These radiated emissions can cause problems in circuits or to an entire printed circuit board (PCB). In an ideal circuit, you will find only the signals, current, and voltages produced by the circuit, itself. In the real world, there will be noise. This occurs when the signal of a circuit is disturbed by any other.

Due to the nature of electromagnetic signals, it is impossible to avoid the presence of noise, but it is possible to considerably reduce its effects.

It is critical that a device does not affect others when functioning, just as it is critical that the device is not affected by others in turn. Electromagnetic susceptibility is the ability of a circuit or a system to keep working even it is impacted by interferences. This susceptibility will be determined by the noise level applied. Depending on the application—automotive, medical, military, etc.—different levels of susceptibility are acceptable.

Every circuit, device, or system has to be properly designed to radiate as little in terms of emissions as possible and to be susceptible only to a high level of electromagnetic fields.

Electromagnetic compatibility (EMC) certification is a necessary step for any product going to market. Every product has to be subjected to EMC testing to ensure that it will not affect any other equipment when being installed (i.e., an emissions test) and that its operation will be normal even with the presence of other systems in the surrounding area (i.e., a susceptibility test).

Electronic designs are usually installed inside enclosures. Metallic enclosures are good at confining electromagnetic fields, but they are not perfect. The union between the PCB and the enclosure presents holes or slots and the electromagnetic fields can pass through them. EMI shielding can conveniently cover these holes.

Furthermore, there is a common problem in many product designs: EMC certification is only taken into account in the latest stages of its design cycle. At this point, electromechanical designs are frozen and then the EMC engineer has almost no room to modify the product to solve electromagnetic issues. Because of this, it's often necessary to have a set of tools to solve radiation problems without modifying a PCB. In these situations, EMI shieldings play a key role.

The global trend in electronics is to be small and fast. PCBs have more and more fast digital circuits with very short rise times. The shorter the rise time, the greater the bandwidth and, thus, the smaller the wavelength. Problems arise when the wavelengths present in the circuits are comparable to the physical dimensions of the PCB. If these wavelengths are small enough, they can reach outside and produce interference with other equipment.

These opening can be closed using EMI shielding, i.e., a magnetic material that helps to cover these small holes and improve the Faraday cage effect of the mechanical enclosure.

There are myriad EMI shieldings, all of them with different materials and shapes. They all, however, have the same common goal: to confine electromagnetic fields.

Shielding elements act as a barrier that blocks electromagnetic emissions. This blocking process is, in reality, a huge attenuation that depends on the frequency of the electromagnetic wave and the material of the shielding element.

When a wave impacts a shielding material, two new waves are generated: a reflected and a transmitted wave. Therefore, the energy of the incident wave is divided into these two waves. The transmitted component is the relevant one since it will go outside, crossing the shielding material. The effectiveness of shielding will determine its aptitude to attenuate this component.

Skin depth is the distance a wave can travel before its amplitude has decreased by a factor of 1/e. It is a factor dependant on the magnetic permeability, frequency, and resistivity of the material. It can be approximated by the following expression:

\[\delta = \sqrt{\frac {2}{\omega \mu \sigma}}\]

Where \(\sigma \) is the electrical conductivity, \(\mu \) is the magnetic permeability, and \(\omega \) is the angular frequency.

The goal of using a shielding material is to reduce the amplitude of the waves as much as possible after they go through it. Therefore it is critical to choose the material type as well as its thickness t to be sure that all the frequencies of the system are attenuated.

How good the shielding material is at this task is given by shielding effectiveness (SE), expressed below:

\[S.E =20log \frac {\eta_o}{4\eta_s}+20loge^{t/\delta } (dB)\]

Here, the first term corresponds to the reflection loss and the second term to the absorption loss.

The type of EMI shielding you select will depend a lot upon the type of product, the electromagnetic requirements, and the environmental conditions. The most common EMI shieldings are the following:

EMI gaskets are used to cover the microholes present due to the irregularities between two mechanical surfaces. They can also serve to improve the grounding connection. They have an adhesive part and many profiles, so they can be easily installed in different types of mechanical joints

When you want to be sure that all the microholes are covered, but there is not a lot of vertical room for options like EMI gaskets, EMC tapes are quite handy. These tapes have highly conductive material on the top, such as nickel or copper, and an adhesive on the other side.

A short, wide, and direct connection to ground is always needed within any device. If this connection is not well done an unwanted monopole is formed, then it will radiate electromagnetic fields. Metal clips improve this connection and reinforce the mechanical connection.

Components such as microprocessors, memory ICs, and radio frequency (RF) stages are common good sources of interference. You may choose to shield them individually by placing a shielding cabinet over them at the PCB level.

All circuits emit electromagnetic radiation and are susceptible to being radiated by others. Attaining the certifications needed to bring your product to market can be a painful testing process.

EMI shielding, in all of its shapes and types, is fundamental to fight EMI issues.

Featured image created using works from Markus Bautsch and Harwin Electronics.