Today’s electronic era has resulted in an extreme application of electronic devices across a broad spectrum of military, industrial, commercial and consumer sectors. This boom in application and use of electronic gadgets and equipment has created a new form of pollution known as radio frequency interference (RFI) or electromagnetic interference (EMI). Since EMI/RFI can cause disturbances and even malfunctioning of equipment, the need for the effective shielding of components has grown exponentially.
For shielding devices against EMI/RFI, there are various materials with different shielding characteristics, such as woven, perforated, knitted, and honeycomb ventilation-shielding materials. However, in addition to EMI/RFI shielding, most components of electric equipment also require heat removal.
The method of convection is so far the most effective one to remove heat, but it has a drawback also. The method of convection involves putting holes in the shield which in turn damages the product’s shielding effectiveness. Therefore, to maintain shielding effectiveness, the ventilation-shielding material should be selected by taking into account some factors like the size of the hole, material thickness, and the number of holes.
By and large, the effectiveness of shielding depends on the type of vent materials and how they are bonded into the enclosure. Usually, the ventilation panels are often one of the largest apertures in an enclosure.
Types of Shielding Material
There are basically four types of ventilation materials used to protect large openings and these are- knitted mesh, woven screen, perforated metal, and honeycomb. All these materials have some mechanical differences which result in the different level of shielding effectiveness.
One of the vital factors determining the shielding effectiveness is material homogeneity which is associated with the surface transfer impedance of the shielding material. The efficacy of shielding, particularly in thin materials, depends on the size of the holes in the material. This means for uniform and effective shielding, the size of the hole should be definite and the surrounding material needs to be homogeneous.
Therefore, if we have to rank materials on the basis of their performance, then perforated materials rank higher than woven materials. On the other hand, woven materials provide better shielding than knitted materials. However, perforated materials are typically about 65% open and do not have as good airflow characteristics.
In cases where only moderate shielding is sufficient, it is recommended to use perforated materials or woven or knitted screenings as these are cost-effective materials. However, for the applications which require highly effective shielded ventilation panels and greater airflow with the best attenuation, honeycomb materials are the right choice.
On the whole, the shielding effectiveness depends on the ventilation panels installed in the enclosure. Honeycomb is constructed of small adjacent electrically connected tubes with greater directional characteristics and a stable airflow. The directional characteristics enable honeycomb vent panels to be constructed in drip-proof or visually secure configurations.
When an electromagnetic wave passes through a shield, it results in absorption and reflection. The part of the energy that is neither reflected nor absorbed by the shield is the residual energy. As such, the ratio of impinging energy to the residual energy determines the shielding effectiveness.