EMI/RFI shielding of Polycarbonate

Electronics systems can cause problems by emitting electromagnetic radiation or they can fail to perform due to electromagnetic radiation in the environment. This electromagnetic radiation is often a combination of noise and information. Leakage of information can be of great concern in applications requiring secure communication. Emissions of electromagnetic radiation can interfere with other systems and may have health and safety implications.

To protect against problems caused by both emission and receipt of electromagnetic radiation, systems can be shielded; this process is known as Electromagnetic Interference (EMI) or Radio Frequency Interference (RFI) Shielding.

To shield against EMI/RFI it is necessary to install a conductive ground plane, which will ground some of the electromagnetic radiation. In applications requiring shielding for transparent Polycarbonate, such as screens and windows, this conductive ground plane can be either applied as a coating to the surface or laminated between two sheets. In this article we will discuss the merits of these two options.

To apply a ground plane using a coating we would typically use a transparent conductive oxide such as Indium Tin Oxide (ITO) or Index Matched Indium Tin Oxide (IMITO). It is also possible to use a thin metal layer such as Gold. With these products we have the option of varying the resistance by varying the amount of oxide applied to the surface. The lower the resistance achieved, the better the ground plane achieved and therefore the better the shielding of the finished product.

Using a 10 Ohms/square surface resistivity we can typically achieve a 20 dB reduction in EMI/RFI over the frequency range of 30 MHz to 1 GHz. A 20 dB reduction is about 100 times reduction in noise. If using ITO, this reduction in EMI/RFI does have a trade off, the ITO does lower the light transmission of the Polycarbonate from 89% down to 82%. One option to resolve this loss in light transmission is to use the more expensive IMITO, which allows a light transmission of 94% to be achieved.

The other solution for shielding is to laminate a wire mesh between two sheets of Polycarbonate. Obviously the visible appearance of a fine wire mesh may not be suitable in all applications. There are many options for the mesh including material of construction, mesh density and diameter of the wire; all of these properties will influence both the shielding effectiveness, visible appearance and light transmission of the finished product. For full details of the technical options for wire mesh shielding you will need to contact your supplier or HighLine Polycarbonate. For a simple comparison with the ITO option we will give some technical data for a couple of wire mesh structures.

For a stainless steel 50 Mesh using 0.0012” diameter wire, we would expect a light transmission of 82% with a 30-40 dB reduction in EMI/RFI over the range of 30 MHz to 1 GHz. A 30 dB reduction is about 1000 times reduction in noise.

If we are prepared to tolerate a lower light transmission, we can use a blackened copper mesh which would give a 50-60 dB reduction over a range of 30 MHz to 1 GHz, but the light transmission would drop to around 70%.

The following table summarizes the results:

	Shielding 30MHz–1GHz	Light Transmission
ITO	20 dB	82%
IMITO	20 dB	94%
50 Mesh SS Wire	30-40 dB	82%
Blackened Copper Wire	50-60 dB	70%

As with most projects, there are trade offs to be made between different attributes and overall cost. This article is intended to give a basic understanding of what needs to be considered when specifying Polycarbonate in EMI/RFI shielding applications.