Optically Transparent EMI Shielding for Defense and Aerospace Applications

February 1, 2014

Modern electronic devices and computing equipment are often integrated with wireless capabilities which utilize multi-bands for high speed communication, as well as high speed circuits and high definition graphical displays that operate at a wide range of frequencies. As such, hardware systems are becoming increasingly complex, resulting in products that are highly sensitive to electromagnetic interference (EMI). The vulnerability to EMI is further heightened when multiple components are packed close together within the device due to space, size and weight constraints. As such, traditional shielding methods that employ materials like thick metal Faraday cages are not always practical due to their heavy weight and bulkiness. Despite its complexity, managing this interference and increasing the resilience of components, interconnects and subsystems within the device is crucial to ensuring reliability and functionality.

For aerospace and defense applications, the requirements for EMI shielding will be more stringent as the shielding structure and material should also withstand potential high power EM attacks such as electromagnetic pulse (EMP) and electrostatic discharge (ESD), which can result in electrical short or dielectric breakdown. Today, an increasing range of military equipment and avionics have displays; shielding EM interference through optical paths such as cockpit windows, displays on global positioning systems and vehicles, touchscreens on advanced avionics equipment and computer screens, pose an even greater challenge as the optical transparency of the material is as important as its shielding effectiveness. As such, there is high demand for EMI shielding materials that are resilient to extreme environmental conditions, lightweight, optically transparent and provide military compliant shielding performance.

Transparent conductive films are used for EMI shielding on displays and touchscreens to reduce radiofrequency interference (RFI). Today, Indium Tin Oxide (ITO) is one of the incumbent materials commonly used for this purpose, but shielding effectiveness is limited to 35dB or less, depending on the radiofrequency. The shielding effectiveness of other optically transparent materials, such as conductive polymer, carbon nanotube and graphene, provide less than 20dB attenuation. While shielding results of 20 – 35 dB may be sufficient to prevent interference from low power consumer electronics, this performance does not meet the EMI shielding requirements for defense and aerospace applications.

Figure 1. SANTE® Technology Self-Assembly Process

“Advanced composite materials are replacing metal in a wide range of applications, in particular for aerospace, due to its attractive properties like light weight, high stiffness, dimensional stability, and chemical/temperature resistance,” commented Steve Hanson, President of Precision Gasket Company (PGC), a leading EMI shielding solutions provider for military and global corporations. “Unfortunately, advanced composite materials do not provide shielding levels close to the performance of metal and EMI shielding is becoming one of the major challenges in aircraft designs.”

At present, the most promising optically transparent EMI shielding solution for defense and aerospace is metal mesh, which is a cross-hatched wire network formed by micrometer-sized metal wires. Metal mesh with wire spacing and conductivity that is optimized for the targeted frequency range can achieve shielding effectiveness of more than 60dB. In principle, metal mesh wires can be developed to be thin enough to become invisible to the human eye. This would require the amount of light shaded by the micro wires to be engineered to less than 10%, or more than 90% light-through without substrate. Thus, the total transmittance of metal mesh with a film substrate can be higher than 80% if light reflection from the substrate can be minimized. However, in practice, at large-area, high-volume manufacturing, the micro wires are typically thicker and visible to the human eye. Additional drawbacks of this technology are its complicated, high-cost, precision manufacturing process. Another challenge when using metal mesh for displays is moiré, or the visible interference pattern that results from placing the metal mesh on the LCD panel in a non-optimal position to the liquid crystal pixels.

Next Generation Metal Mesh Solution

An innovative new solution, SANTE^® Technology, provides not only the EMI shielding performance of metal mesh, but it can also be mass produced through a standard wet coating, roll-to-roll manufacturing process for higher throughput and volumes.

SANTE^® Technology is a self-assembling nanoparticle technology for transparent conductors. SANTE^® EMI shielding films are manufactured by adding pure nanoparticles (< 50nm in size) into an emulsion consisting of water, solvents, and chemical binders. A thin layer of this emulsion is wet coated onto a substrate such as PET film. Under ambient conditions, the solvent and water evaporates, and the emulsion self-assembles into a conductive network with a dense array of random nanoscale pores (Figure 1). Within a few seconds, the silver nanoparticles come together into agglomerated, interconnected, micron-size metal wires with line width of approximately 5-8 microns, height of 3- 5 microns and pore size ranging from 250-350 microns. The mesh, also known as the SANTE^® Network, shades 5-10% of light on the coated surface depending on the formulation. Coupled with a film substrate, the total transmittance is between 80% - 88%. To view a video of this self-assembly process, go to: www.techbriefs.com/tv/SANTE.

Figure 2. SANTE Technology Shielding Effectiveness

The SANTE^® Technology formulation used varies for applications depending on the requirements for shielding effectiveness, conductivity and transparency. For applications requiring higher attenuation, the SANTE^® Network can act as a seed layer for additional metal coating. This post-processing step, or electroplating, customizes the optical and electrical properties of SANTE^® Films for targeted applications. Other post-processing steps include sintering or densification, which engineers the RF current flow and the skin depth to improve shielding performance at a wider range of RF frequencies. Adding multiple layers of SANTE^® Film can also further boost its overall shielding performance.

EMI Shielding Performance

EMI shielding effectiveness is heavily dependent on conductivity and surface resistance – lower surface resistance and higher conductivity results in better shielding performance. Figure 2 shows the surface resistance achievable by SANTE^® Technology and the corresponding shielding effectiveness for EMI/EMC frequencies of 30MHz to 1.5GHz, as measured by the ASTM D4935 standard. Typically, an attenuation of 40dB or less is sufficient to meet the EMI/EMC shielding requirements of consumer electronics and industrial applications. Defense and aerospace applications, on the other hand, require shielding of 50dB and above. Cima NanoTech is currently developing a new series of EMI shielding films with surface resistance less than 0.1Ω/sq for applications which require higher shielding effectiveness.

Areas Where Transparent EMI Shielding is Required

Today, advanced defense and aerospace equipment such as digital vision systems, military autonomous vehicles, surveillance devices, handheld GPS systems, tactical radios, and ruggedized computers have displays which emit radiation that may not fulfill the required military standards (MIL-STD-461 and MIL-STD-464); the shielding effectiveness required is 20-30 dB higher than that of consumer and industrial applications, as shown in Figure 2. Without proper shielding, military personnel using such unprotected devices risk sending an unwanted signal which alerts others of their location. In most cases, radiated emission levels can be easily reduced by designing a full enclosure that forms a Gaussian sphere to shield the outside from the inside. However, the challenge comes when EMI shielding is required for the displays on these equipment. Unlike materials used in other parts of the device, displays require a shielding material that is transparent.

Another application which requires a transparent material is shielding for glass doors and windows. Many military bases today build shielded rooms to ensure confidential information discussed within is not communicated to external parties.

This article was written by Chandarasekaran Krishnan, Senior Applications Engineer, Cima NanoTech Pte. Ltd. (Singapore). For more information, Click Here .