Tuesday, March 16, 2010

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


20 dB



20 dB


50 Mesh SS Wire

30-40 dB


Blackened Copper Wire

50-60 dB


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.

Friday, March 5, 2010

Anti-reflective coating options for Polycarbonate

There are several options available for improving anti-reflective performance of Polycarbonate sheet. The correct choice depends on a number of factors including the level of anti-reflection required, the size of the part, the number of parts required and the cost sensitivity of the application. In this blog entry we will discuss how to make the correct choice for the application.

Anti-reflective coatings are typically applied to Polycarbonate that has an abrasion resistant coating applied to the surface. The abrasion resistant coating provides a better surface for the anti-reflective coating to adhere to than the uncoated Polycarbonate. The finished product is therefore more durable. The abrasion resistant coating itself also improves the anti-reflective properties of the Polycarbonate sheet, as discussed in a previous blog post.

There are essentially two broad types of anti-reflective coatings, liquid anti-reflective coatings and vapor deposition anti-reflective coatings. Liquid anti-reflective coatings are applied to the sheet in a solution and are then cured using either ultraviolet light or heat. Vapor deposition coatings are applied using a sputtering process.

Level of anti-reflection achieved.

The following table shows the amount of reflection from each surface of the sheet with each of the anti-reflective options. These figures are over the visible light range of 420-680 nm.

Uncoated Polycarbonate sheet 5.1%

Abrasion resistant coated Polycarbonate sheet 3.9%

Liquid anti reflective on Polycarbonate sheet 2.0%

Vapor deposition anti-reflective on Polycarbonate sheet 0.75%

If a very low level of reflection is required a vapor deposition anti-reflective is normally used. However, it is often possible to use a liquid anti-reflective or even just an abrasion resistant coated sheet for applications not needing such a low level of reflection.

Cost of anti-reflective solutions.

A liquid anti-reflective coated sheet typically sells for about five times the price of a standard abrasion resistant coated Polycarbonate sheet.

A vapor deposition coated anti-reflective sheet would sell for about five times the price of a liquid anti-reflective sheet.

These broad pricing guidelines obviously depend on a number of factors including part size and the number of parts required, but they do give some indication of what you can expect to pay for increasing levels of anti-reflective performance. Often only very high technology applications can justify the cost of a vapor deposition anti-reflective coating.

Part size and minimum order quantity.

One of the problems of vapor deposition technology is the limitation on the size of the part. Parts of up to 14” x 18” can be produced on a standard sputtering machine in reasonably small quantities. However, once you get above this size you need to use a very large sputtering machine that requires large set up costs and thus large production runs. Parts up to 24” x 36” are easily possible but may require production of at least 1000 parts at a time; this makes it very difficult to obtain a couple of parts for a prototype development if parts over 14” x 18” are required. Once you require parts of over 24” x 36” you need very specialized equipment and the cost is extremely high.

For liquid anti-reflective coatings it is possible to easily coat sheets of 48” x 96” or larger and the minimum production size is much smaller. The easier production makes liquid anti-reflective materials much easier to obtain for prototype development. For large parts we typically recommend that liquid anti-reflective coatings are evaluated first, before trying the expensive vapor deposition anti-reflective coatings.