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As an industry, telecommunications includes many networks like Wi-Fi (6, 6E, 7), 5G and open radio access networks (RAN), cellular base stations, edge computing and distributed antenna systems (DAS) to name a few. These systems require sophisticated hardware to operate like base station radios, edge computing servers, telecom switches and routers, radio frequency (RF) amplifiers, antenna control units, ethernet switches and fiber optic transceivers.
For the engineers that design the hardware and circuitry to support telecommunications, electromagnetic compliance (EMC) is a critical and often difficult parameter to meet given the layout of densely integrated RF and high-speed digital electronics. Classically, EMC was achieved simply by housing circuits in aluminum boxes which blocked external RF and prevented internal signals from leaking out.
The aluminum housing units act as a Faraday cage, surrounding the circuitry with an electrical conductor. The metal reflects and absorbs external radiation so the circuit resting inside does not receive (through induction or RF coupling) any signals. Just as important, the Faraday cage reflects and absorbs the electromagnetic fields generated by the housed circuit as well, preventing it from interfering with neighbouring devices. Figure 1 below is a graphical depiction of the Faraday cage effect.
Figure 1: Depiction of how a Faraday shield works
Despite its intrinsic ability to shield against electromagnetic interference (EMI), aluminum is relatively heavy and expensive so designers have moved to alternative materials like ABS and polycarbonate. While these materials are cheaper and lightweight, they are also electrically insulative and allow EMI to simply pass right through the material.
Basic Principles of Electromagnetic Shielding
The objective behind electromagnetic shielding is to prevent electromagnetic energy that’s conducted or radiated from a source reaching a circuit. Shielding is not a binary outcome that’s simply evaluated as shielded or not shielded, rather, it’s a measure of how much reduction there is from the incident radiation by the time it reaches nearby electronics. The degree of radiation reduction is known as attenuation.
Attenuation of EMI is measured in decibels (dB), which is a logarithmic function. Effective shielding ranges from 20 to over 100 dB which translates to the following in terms of signal reduction.
|
Attenuation (dB) |
Signal Reduction (%) |
% Strength of Incident Signal |
|
10 |
90 |
10 |
|
20 |
99 |
1 |
|
30 |
99.9 |
0.1 |
|
40 |
99.99 |
1×10-2 |
|
60 |
99.9999 |
1×10-4 |
|
80 |
99.999999 |
1×10-6 |
Table 1: Signal reduction of EMI for different attenuation values
It is important to note that shielding is highly frequency dependent so it is necessary to identify the operating frequencies or frequency ranges the shielding material must attenuate.
The conductivity of the shielding material is also an important property, especially at higher frequencies beyond 10 MHz. At low frequencies, like the kHz range, materials with conductivities that vary by orders of magnitude can exhibit similar performance in shielding whereas in the MHz and GHz range, conductive materials like copper and silver perform best.
Another variable influencing shielding is the thickness of the material. This variable is difficult to quantify though as thicker shields can reduce porosity and even lower the surface resistance which improves attenuation but the magnitude of this improvement can vary significantly depending on the shield’s composition and operating frequency.
Electrically Conductive Paints as a Shielding Material
The switch from aluminum housing units to plastics like ABS and polycarbonate meant that designers must now metallize these plastics to achieve EMC. While foils and conductive meshes are effective materials for the purpose of EMC, their application is labor-intensive and the materials are prone to tearing and breaks which can allow radiation leakage; especially at high frequencies.
Electrically conductive paints are a practical solution for housing units as the cured films are durable, highly conductive, cost-effective and easy to scale. For an effective shield, the paint must be applied along all 6 sides of the housing interior to completely surround the circuit and create a Faraday cage. While the paint can be applied on the housing’s exterior, it is more practical to apply it to the interior of the housing so as not to expose it to damage from the outside environment.
Electrically conductive paints are formulated using different polymer materials that have their own unique advantages, depending on the application requirements. Common polymers used are acrylics, epoxies and polyurethanes. The radar diagram highlights the benefits and constraints with each polymer type.
Figure 2: Radar diagram comparing the different polymer types used in formulation of electrically conductive paints
As indicated in the diagram, for applications with exposure to especially harsh environments, an epoxy-based paint is most suitable given its chemical resistance and durability whereas in less harsh environments, acrylics may be more practical given how user-friendly they are. Users must also consider the substrate they are applying the paint to as adhesion varies between polymer types. For plastics used in housing units, acrylic systems are the preferred chemistry.
While the polymer type determines the durability and adhesion, the conductive flake of the paint governs the conductivity and shielding attenuation. Carbon, nickel, silver-coated copper and silver are the most common conductive flakes used so the choice of which paint system will work depends on what frequencies need to be shielded and the required target attenuation. Figure 3 below shows attenuation vs. frequency for four acrylic conductive paints tested per IEEE-299.1:2013
Figure 3: Comparison of attenuation for different flakes used in acrylic conductive paints tested per IEEE-299.1:2013
MG Chemicals’ Electrically Conductive Paints
MG Chemicals has formulated a full line of electrically conductive paint to help engineers within telecom achieve EMC. The line includes acrylics, epoxy and water-based polyurethane to accommodate all substrates and environments. While the choice of which product to use might seem overwhelming, MG has a full technical support staff to walk you through your application and help you along the way, from material selection to validation through scale-up.
Meeting EMC is becoming more difficult as designers must balance this alongside other constraints like cost and weight. The switch to plastic housing units requires a method to metallize these parts inexpensively and at scale. Fortunately, electrically conductive coatings offer a turnkey solution, providing durable, thin films that provide adequate shielding across a wide frequency range.
