Shielding devices from electromagnetic radiation

Electromagnetic radiation can harm the best of electronic devices. Smaller sized electronic devices need better protection from these waves. Understanding that EMI shielding is the best solution for this, Electronics Bazaar takes a look at what’s new in this space  

By Samonway Duttagupta
Most electronic devices have electromagnetic radiation. The waves born out of this radiation can damage the devices. This is where electromagnetic interference shielding or radio frequency electromagnetic radiation (EMI/RF shielding) comes into play. While the electromagnetic radiation can never be stopped, a circuit needs to be designed in such a manner that it minimises the electromagnetic interference. But there are times when the circuit design is not enough to block the effect of these waves. That’s when proper EMI shielding can block the emissions from within the equipment and also the radiation into the environment.

Advancements in electronic devices demand EMI shielding
Electronic devices are getting smaller and smaller. Hence, the challenge is to design smaller enclosures with dense circuits that perform at higher speeds. To simultaneously protect the core of a device from electromagnetic waves is also a challenge. R.S. Madhava Murthy, technical manager, 3M Electronics Division, comments, “These days, product designers can analyse various scenarios regarding such interference and incorporate controls in the circuit design to curtail their effects to the desired limits.”

The way it is done
Efficient EMI shielding begins at the design stage of any electronic device. Murthy says that most of the designers try to design their circuits to include active and passive filters, incorporate effective shielding in printed circuit boards (PCBs) and optimise PCB tracks to reduce the intensity of EMI signals. Based on his expertise in this field, he says, “Higher frequencies require optimised grounding and Faraday cage design. It is possible to achieve such shielding systems with high conductive metals like copper or aluminium. Designs that use plastic enclosures are more challenging in providing an effective Faraday cage. In such scenarios, design engineers need engineered materials to create an effective Faraday shielding process.”

The variety of options available
There is a wide range of products that help in meeting the varied requirements of EMI shielding, which differs from one device to another. Electronics Bazaar, based on its research and industry interactions, has identified some commonly used EMI shielding products and their key features. These are listed below:

1. Electrically conductive foil and fabric tapes: Thin material with electrically conductive adhesive on one or both sides. Easy to cut and bond.
2. Electrically conductive adhesive transfer tape: Provides bonding and electrical conductivity in Z axis or XYZ axes. A variety of thickness options and adhesive properties; the tape is also easy to die-cut.
3. Electrically conductive foam gasket: Very soft, yet highly compressible, thin, highly conductive foams with adhesive on one side. Good for designs that are required to be taken apart for servicing.
4. Electrically conductive gasket: Less compressible than foam, available in thin format with single- or double-sided adhesive. Good for designs that are required to be taken apart for servicing.
5. EMI absorber: Offers flexibility to designers as shielding is possible without grounding. Absorbers are effective in controlling the specific frequency band that is causing interference.
6. Multi-function product :One product for shielding, absorbing, yet very thin and easy to install due to adhesive backing.
7. Flux field directional material: A multi-layer construction material, consisting of a primary inner soft magnetic foil layer with a protective PET cover film and an acrylic pressure-sensitive adhesive. Suitable for RFID, NFC and wireless power applications.
8. Silicon heat transfer compound: These are designed for transferring heat away from electrical and electronic devices such as transistors, power diodes, semiconductors, ballasts and thermocouple wells. Have high thermal conductivity, high dielectric constant and high dissipation factor. When used with heat sinks or metal they will not dry or harden. Contains zinc oxide and polydimethyl siloxane.
9. Thermal grease: This is a low thermal resistance, non-corrosive grease. It uses an extremely thermally stable synthetic oil base that is electrically insulating. It is used to improve the thermal interface contact conductivity between heat sinks, LEDs, motors and heat-generating electronic components such as central processing units (CPUs), graphics processing unit (GPU) chipsets, power components and so on. It improves the thermal interface between irregular and pitted surfaces.
10. Thermal adhesive: A thermal adhesive is an electronically insulating epoxy that combines a moderate curing rate and high thermal conductivity. It has a convenient 1:1 ratio, a workable 45 minute pot life and a moderate curing rate. It may achieve a minimal service cure in seven hours at room temperature. The cured adhesive bonds very well to most substrates used in electronic assemblies, and resists thermal and mechanical shocks.
11. Thermally conductive epoxy: This is formulated with undiluted Bisphenol F epoxy resin for superior physical properties and pigmented with high purity aluminium oxide pigment to provide excellent thermal conductivity at a reasonable cost. Pigmented black for excellent thermal absorption and emission.

The latest options

As mentioned earlier, EMI shielding is something that varies from one device to the other. Naturally, the number of products on offer and the pace at which they are developed and launched is also pretty high. Products that have been launched recently are listed below along with their features and USP:

1. Flux Field Directional Materials (FFDM) particle EM05EC by 3M: This is a permeable multi-layered insulator particulate material consisting of multiple sub-skin-depth magnetic layers alternated with dielectric spacer layers. This material may be used in flux field directional or absorber material constructions.  3M FFDM particle EM05EC is available in the form of a powder. USP:3M EM05EC particles are processes with epoxy or polyurethane to produce custom parts with varying magnetic permeability. The particles are produced using nanotechnology. A typical particle size is <500µm x <500µm x <6µm. This is a new concept to replace ferrite particles in wireless power chargers and RFID readers.
2. 834ATH flame retardant epoxy by MG Chemicals (Progressive Engineers): This is an encapsulating and potting compound, which is a two-part, economical, electronics-grade, self-extinguishing, flame retardant epoxy that provides excellent physical, chemical and electrical protection as well as a degree of thermal conductivity. It protects against static discharges, shocks, vibrations and mechanical impacts. USP: It is extremely resistant to environmental humidity, salt water and harsh chemicals. It also helps to hide and restrict access to intellectual property and is more difficult to remove than standard epoxy encapsulating compounds.
3. 4223F polyurethane conformal coating by MG Chemicals (Progressive Engineers): This is a heat-cured coating that provides an excellent scratch and chemical resistant finish. It meets UL 94-V0, and is free of xylene and other hazardous air pollutants. This one-part coating is easy to use and repair — it does not require special or costly equipment or materials. It is ideal for chemically challenging environments. USP: The 4223F urethane protects electric circuits against aggressive chemicals, moisture, dirt, dust, thermal shocks and scratches. This avoids corrosion and physical damages to electric components. It insulates against high-voltage arcing, shorts and static discharges, allowing for traces to be put closer to one another.
4. 4224 optically clear conformal coating epoxy by MG Chemicals (Progressive Engineers): This offers a very strong and ultraviolet-resistant (UV-resistant) finish. The 4224 epoxy coating protects electric circuits against corrosive chemicals, moisture, dirt, dust, thermal shocks and scratches. This avoids corrosion and physical damage to electronic components. It also insulates against high-voltage arcing, shorts and static discharges. USP: This two-part coating is easy to use: it does not require special or costly equipment to apply.
5. 8330 silver conductive epoxy adhesive by MG Chemicals (Progressive Engineers): Moderate cure/extreme conductivity is an electronic grade epoxy that combines a moderate curing rate and high conductivity with ease of use. It has a convenient 1:1 ratio, a workable 10-minute pot life, and a moderate curing rate. It achieves an operational level in five hours at room temperature. With heat, the 8330 cures in less than 15 minutes and maximises its conductivity. The cured adhesive bonds very well to most substrates used in electronic assemblies, resists thermal and mechanical shocks, and provides the low resistivity needed for many operating conditions.  USP: The 8330 epoxy adhesive provides high conductivity seals and bonds to repair electronic devices.

Products in demand
Industry interactions indicate that the types of EMI shielding products that are in demand include foil tapes of aluminium and copper and the liquid type variants. Muhammed Zubair Quraishi, manager – business development, Progressive Engineers, confirms, “Liquid type EMI shielding products are in demand because of the complexity of enclosure designs in electronics these days.” Murthy explains the importance of foil tapes in this way: “Most customers need foil tapes of copper and aluminium. Where flexibility is needed, conductive fabric tapes are also in demand. Absorbers suitable for RFID applications are also in demand.”

How to select the right product
EMI shielding materials are mostly needed in electronics when the products do not meet target specifications. Murthy is of the opinion that aItering PCB designs to eliminate EMI/RFI is not a viable option in most cases. One should have knowledge on the following while deciding on the right EMI shielding product:

• Frequency of radiation that needs to be controlled
• Location or source of radiation
• How much reduction is needed
• Constraints of space, weight and grounding options, if any

After one has detailed information on the above parameters, the right EMI shielding product can be purchased after understanding what the application requires by checking the following:

• Shielding to form Faraday’s cage
• Flexibility of material needed
• Method to install shielding material and how to extend electrical ground to shielding
• Any specific shape/size needed
• Production volume and the process involved in production