What Is an Electromagnetic Wave Absorbing Sheet?
An electromagnetic wave absorbing sheet is a component used in electronic equipment that generates electromagnetic waves or may malfunction due to electromagnetic waves.
EMC (electrode compatibility) measures for electronic equipment include EMI (Electro Magnetic Interference), which is a measure for electromagnetic noise generation, and EMS (Electro Magnetic Susceptibility), which is a measure for the receiving side. The International Electrotechnical Commission (IEC) standards and the Electrical Appliance and Material Safety Law define the methods used.
Electromagnetic wave absorbing sheets are a typical component used for such measures. In recent years, as CPU speeds have increased, the frequency band of electromagnetic waves has shifted to higher frequencies, making noise countermeasures more difficult. By attaching electromagnetic wave absorbing sheets near noise sources, noise in the vicinity can be absorbed and suppressed.
Composites of magnetic materials and rubber and thin plastic laminates are available as electromagnetic wave absorbing sheets, which are easy to cut and affix, inexpensive and lightweight.
Uses of Electromagnetic Wave Absorbing Sheets
The uses of electromagnetic wave absorbing sheets are generally not well known, but they are used in a very wide range of fields. Typical applications are as follows.
1. Electronic Equipment Applications
Electromagnetic wave absorbing sheets are used in equipment with semiconductor substrates inside, such as smartphones and household appliances. By attaching electromagnetic wave absorbing sheets to the inside of housings and to circuit boards, it is possible to reduce the generation of radiated noise, resonance and internal reflections in the equipment.
2. Applications for Countermeasures Against Communication
interference
Electromagnetic wave absorbing sheets are used to reduce noise in aircraft and ship radios, which use electromagnetic waves with a frequency of 30 MHz or lower. Electromagnetic wave absorbing sheets are also used to prevent telephone interference in mobile phones and televisions, which are indispensable in daily life.
3. Automotive Applications
In recent years, automobiles have been equipped with numerous sensors, including anti-collision radar. Radar uses millimetre waves, and electromagnetic wave absorbing sheets are used as a countermeasure against electromagnetic interference and noise.
Principle of Electromagnetic Wave Absorbing Sheets
Electromagnetic wave absorbing sheets convert electrical energy into thermal energy by having the electromagnetic waves absorbed by the magnetic material compounded in the sheet. Different types of magnetic materials are used depending on the absorption band of the electromagnetic waves, for example, to prevent noise in satellite communications, materials that can selectively absorb the frequency bands that are practical there are used.
In recent years, electromagnetic wave absorbing sheets made of plastic or non-woven fabrics, which do not use magnetic materials, have also been distributed. The former type absorbs electromagnetic wave absorbing sheets by generating polarisation through the lamination of plastic sheets with different dielectric constants. The latter uses non-woven fabrics with special surface treatment to convert noise into heat through resistive losses.
Types of Electromagnetic Wave Absorbing Sheets
There are various types of electromagnetic wave absorbing sheets in circulation, depending on the materials they are made of. Typical types are as follows.
1. Electromagnetic Wave Absorbing Rubber Sheets
These sheets are a mixture of magnetic material and rubber. They have excellent flexibility and insulation properties characteristic of rubber and are easy to process. They also have good corrosion resistance and are suitable for harsh operating conditions such as humid environments. Thicknesses range from 0.25 mm for thin sheets to 4 mm for thicker ones.
2. Electromagnetic Wave Absorbing Plastic Sheets
This product is made of laminated plastic sheets moulded from polyester resin. As they do not contain magnetic materials, costs can be kept low, and they have the advantage of being lightweight due to their thin thickness. Transparent products are also available and can be used for adherends that require light transmission, such as windows.
3 Electromagnetic Wave-Absorbing Non-woven Fabrics
Like plastic sheets, these are environmentally friendly products that do not use magnetic materials. The thickness can be reduced to 50 µm at its thinnest point, and its flexibility allows it to follow the unevenness of semiconductor substrates.