Electromagnetic waves propagated from various electrical and telecommunication devices are becoming a problem as a result of the rapid spread in recent years. The electromagnetic waves can interfere with the normal operation of other devices and may cause malfunction or noise. Electromagnetic waves are said to be injurious on human bodies when exposed to them.
For mobile devices such as mobile phones and portable computers in particular, various shielding materials are provided for the prevention of leakage of electromagnetic waves from the inside to the outside and vice versa. Since these shielding materials are often applied on non-flat irregular surfaces, it is desirable for the shielding materials to have flexibility. Although the shielding materials can protect the device to which they are applied by reflecting electromagnetic waves propagated from a source device, the reflected electromagnetic waves, in turn, may cause malfunction of another device. Accordingly, electromagnetic wave absorbers are more effective than the shielding materials for the prevention of malfunction of electrical devices used in a limited space.
As an example of electromagnetic wave-absorption materials having flexibility, rubber sheets incorporating magnetic or electroconductive particles are known. Another example is fabric sheets carrying carbon or other electroconductive particles.
The electromagnetic wave absorber of the rubber or fabric sheets of the above type, however, require to have a varying thickness depending upon a particular frequency band to be absorbed. Therefore, the flexibility of these materials is compromised depending upon the frequency band to be absorbed. Other disadvantages of the rubber type electromagnetic wave absorbers include their ability of absorption of electromagnetic waves only in limited frequency range and not in broadband frequency range. Moreover, they are not necessarily light weighted.
The electromagnetic wave absorbers of fabric type carrying electroconductive particles such as carbon powder are flexible and do not exhibit a frequency-dependent absorption peak. However, it is difficult to firmly fix carbon or other electroconductive particles onto the fabric sheet so that the electroconductive particles are not easily released. Moreover, a satisfactory electromagnetic wave absorption performance per unit volume cannot be achieved unless a sufficient quantity of the conductor particles is uniformly carried by the fibrous substrate.
Other fiber-based electromagnetic wave-shielding materials are known including the type having a metal foil adhered to one side of a fibrous sheet or sandwiched between a pair of the fibrous sheets.
The electromagnetic wave-shielding materials of the above type exhibit a satisfactory shielding performance. However, they are not permeable to air and thus block heat radiation from the source device. Heat radiation may be improved to some extend by the use of perforated metal foils but the effect is not sufficient. In addition, the use of fiber sheet-metal foil composite shielding materials may interfere with thinning the device. Use of perforated metal foils adds the production cost of the shielding material.
Other types of fiber-based electromagnetic wave-shielding materials are known including metal-carrying fabric sheets such as metallic fiber-containing fabric or metal vapor-deposited fibrous sheets.
The fiber-based shielding materials of the above types do not have a sufficient shielding effect to electromagnetic waves and the metal vapor-deposited fabric sheets are expensive because the manufacturing process is complicated.
Still other types of fiber-based electromagnetic wave-shielding materials are known including fabric sheets having electroless metal plating.
The fiber-based electromagnetic wave-shielding materials of the above type have a defect that the shielding effect for electromagnetic waves is not sufficiently high.
Recently, it has been proposed to improve the above defects by applying an electrolytic plating on the surface of fabric sheets having electroless metal plating or electroconductive polymer coating without adversely affecting the flexibility and gas-permeability of fabric sheets.
The fiber-based electromagnetic wave-shielding materials having dual layers of electrolytic plating in addition to electroless plating or conductive polymer coating may enhance the shielding effect to some extent. However, their manufacturing process requires complicated steps making the production thereof in a cost effect manner difficult. Especially, when the manufacturing process comprises electroless plating, several steps of etching and washing the substrate for activation are required before liberating elementary metal by a redox reaction for plating. These additional steps further affect the productibility and add production cost.