Electromagnetic radiation can harm the best of electronic devices. Smaller electronic devices need higher protection from such waves. Electromagnetic interference (EMI) shielding is the best solution for this and is used in a variety of industries including automotive, electrical, electronics, energy and healthcare.
By Baishakhi Dutta
Today’s product designers face a number of challenges when it comes to designing electronic devices for consumers as well as for the business-to-business (B2B) markets. As these devices become increasingly smaller and more complex in nature, shielding them against electromagnetic interference gets more difficult.
High-frequency radiation, particularly radio waves, emanating from gadgets like cell phones, not only produces cross-talk between devices, but also has a negative impact on the health of people. In an effort to minimise such interference, engineers design the printed circuit boards (PCBs) of the devices in a way that the components remain isolated from one another.
All reputed brands in the industry focus on the careful selection and use of proper EMI shielding materials, as this is not only crucial for effective shielding, but also important in order to meet a wide range of application challenges that today’s designers face.
Application areas
- Defence
- Industrial
- Electronics
- Naval
- Aeronautical
- Space
- Medical
- Automobiles
Types of EMI shielding
Device failure caused by interference or ‘noise’ from electromagnetic energy is increasing due to the growing number of products that contain sensitive electronic components.
- Reflection: The primary mechanism of electromagnetic shielding involves reflection. In order to be able to reflect the radiation, the shield needs to be electrically conductive. Since metals have this property, they are good shielding materials; however, they are bulky. In order to overcome this shortcoming, sheets with metal coatings that are created by electroplating, electroless plating or vacuum deposition, are generally used.
- Absorption: The secondary aspect of shielding involves the absorption of radiated energy. In order to facilitate absorption, the material should have electric and/or magnetic dipoles – the separation of positive and negative charges. Magnetic dipoles can be provided by materials with high levels of magnetic permeability. Super permalloy, mumetal and iron oxide are some of the materials that are excellent for absorption.
- Multiple reflections: This process involves the reflection off various surfaces and interfaces. To be effective, this mechanism requires a large surface area. Shields made of foam or porous materials are good for multiple reflections. Composite materials containing fillers also make excellent EMI shields.
Experts’ views
“Recent advances in EMI shielding have resulted in products that have multiple functions. For example, they work as effective EMI shielding materials as well as good thermal conductors, or shielding and absorbing functions are incorporated in one product, etc. Most products come with proprietary electrically conductive acrylic adhesive systems to enable conductivity in the Z axis and are also comparatively easy to install.”
—R.S. Madhava Murthy, technical services, 3M Electro & Communication India Private Limited
“EMI products are generally made up of plastic enclosures, and are very important in the medical, aerospace, defence and automation sectors. Products made for these sectors contain conductive coating with silver, copper, nickel and graphite. Nowadays, manufacturers are giving a lot of importance to these products. There is no particular emerging trend when it comes to EMI products but it is very important for customers to decide between the use of copper, silver or nickel, depending on the amount of heat emitted. For example, if the product is emitting heat of around 80°C, then one should opt for silver. For temperatures below 40°C, one can go for copper, nickel or graphite.”
—Manjunath, director, Siltech Corporation
The variety of options available
There are a wide range of products that help in meeting the varied requirements of EMI shielding. Some commonly used EMI shielding products and their key features are listed below.
- Electrically conductive foil and fabric tape: These are made from a thin material with electrically conductive adhesive on one or both sides. They are easy to cut and bond.
- Electrically conductive adhesive transfer tape: This provides bonding and electrical conductivity. There are a variety of thickness options and adhesive properties to choose from and the tape is also easy to die-cut.
- Electrically conductive foam gaskets: These are very soft, yet highly compressible, thin and extremely conductive foam gaskets with adhesive on one side.
- Electrically conductive gaskets: These are less compressible than foam, and are available in a thin format with single- or double-sided adhesive.
- EMI absorbers: These offer flexibility to designers as shielding is possible without grounding. Absorbers are effective in controlling the specific frequency band that is causing interference.
- Multi-function products: These are used for shielding and absorbing. They are very thin and easy to install due to their adhesive backing.
- Flux field directional material: This is a multi-layer construction material comprising a primary inner soft magnetic foil layer with a protective PET cover film and an acrylic pressure-sensitive adhesive. It is suitable for RFID, NFC and wireless power applications.
- Silicon heat transfer compound: This is designed to transfer heat away from electrical and electronic devices such as transistors, power diodes, semiconductors, ballasts and thermocouple wells. It has high thermal conductivity, a high dielectric constant and a high dissipation factor. When used with heat sinks or metal, it does not dry or harden. It contains zinc oxide and polydimethyl siloxane.
- Thermal grease: This is a low thermal resistance, non-corrosive grease. It uses an extremely thermally stable synthetic oil base that is electrically insulating. This 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.
- 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 can 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.
- Thermally conductive epoxy: This is formulated with undiluted Bisphenol F epoxy resin for superior physical properties and is mixed with high purity aluminium oxide pigment to provide excellent thermal conductivity at a reasonable cost. It is pigmented black for excellent thermal absorption and emission.
Emerging materials
As technology advances and the signals from electronic equipment grow stronger, more robust shielding, like that provided by gaskets, is needed. An alternative to all these shielding devices is the EMI filter. This is a passive electronic device that suppresses the EMI created and emitted by surrounding electronic devices and equipment.
Flexible graphite is fast emerging as an attractive EMI shielding material. It comprises a flexible sheet made up of a large number of exfoliated graphite flakes or worms. A notable feature of this emerging material is that it is devoid of any binder. A chemical process causes the worms to remain tightly interlocked to each other, eliminating the need for a binder. This material has high thermal and electrical conductivity. It also has a high specific surface area, thanks to exfoliation. Because of these two features, flexible graphite is expected to have a promising future as a highly effective material for electromagnetic shielding.
Tips for buyers
- One should be aware about the following parameters prior to selecting an EMI shielding product:
- The frequency of radiation that needs to be controlled
- The location or source of radiation
- How much reduction is needed
- Constraints of space, weight and grounding options, if any
