Design and Difficulties of Electromagnetic Interference Shielding
Since the seam reduces the shield's conduction rate, the signal jammer power also decreases. It should be noted that the attenuation of radiation below the cutoff frequency depends only on the length-diameter ratio of the slot, for example, 100dB attenuation can be obtained when the length-diameter ratio is 3. When perforation is required, the waveguide properties of the small holes in the thick shield can be used; another way to achieve a higher aspect ratio is to add a small metal shield, such as a suitably sized gasket. The above principle and its generalization in the multi-slot situation constitute the basis for the design of the porous shield.
Porous thin shielding layer: There are many examples of porous, such as ventilation holes on thin metal sheets, etc. When the holes are close together, the planning must be carefully considered. The following is the calculation formula of shielding power under such conditions: SE=[20lg(fc/o/σ)]-10lgn, where fc/o: cut-off frequency n: the number of holes.
Note that this formula is only applicable when the hole distance is less than the hole diameter, and can also be used to calculate the relative shielding power of the metal braid.
Seams and joints: Welding, brazing or soldering are common methods for permanent fixing between sheets. The metal surface of the joint must be cleaned so that the joint can be completely filled with conductive metal. Fixing with screws or rivets is not recommended, because the low-resistance contact conditions at the joints between the fasteners are not easy to maintain for a long time.
The purpose of the conductive backing is to reduce the slots, holes or gaps in the seams or joints so that RF radiation does not escape. An EMI gasket is a conductive medium used to fill voids within a shield and provide for connecting low impedance contacts. Typically EMI gaskets provide a sensitive connection between two conductors, allowing current on one conductor to pass to the other.
The selection of sealing EMI gaskets can refer to the following functional parameters: 1. Shielding power at a specific frequency range 2. Equipment method and sealing strength 3. Compatibility with housing current and corrosion resistance to external environment 4. Operating temperature range.
Most commercial gaskets have adequate shielding capabilities for equipment to meet EMC specifications, and it is critical to properly plan the gasket within the shield.
Gasket Systems: An important factor to consider is compression, which can result in higher electrical conductivity between the gasket and the gasket. If the conductivity between the gasket and the gasket is too poor, the shielding power will be reduced. If there is one missing piece in other joints, a thin slit will appear to form a slot antenna, and the radiation wavelength is about 4 times smaller than the length of the slit.
Ensuring continuity begins with ensuring that the surface of the gasket is smooth, clean, and treated for good conductivity. These surfaces must be masked before bonding; other shielding gasket materials have continued good adhesion to this gasket. Compatibility is also very important. The shrinkable feature of the conductive gasket compensates for any irregularities in the gasket.
All gaskets have an effective working minimum contact resistance, and the designer can tighten the gasket to reduce the contact resistance of multiple gaskets, which of course will increase the seal strength and cause the shield to become more curved. Most liners work better when shrunk to 30% to 70% of their original thickness. The pressure between the two opposing pits should therefore be sufficient to ensure good electrical conductivity between the gasket and the gasket within the claimed minimum contact surface size.
On the other hand, the pressure on the pads should not be so great that the pads are in an abnormally compressed condition, which would result in pad contact failure and possible electromagnetic leakage. The requirement to separate from the gasket is very important to control gasket compression to the manufacturer's stated scale, this design requirement ensures that the gasket has sufficient stiffness to avoid large bending between gasket fasteners. In some cases, additional fasteners may be required to prevent flexing of the housing structure.
Tightness is also an important characteristic of rotating joints, such as in doors or flaps. If the gasket is prone to compress, the shielding function will decrease with each rotation of the door, at which time the gasket requires a higher tightening force to achieve the same shielding function as the new gasket. This is unlikely to be possible in most cases, thus requiring a long-term EMI solution.
If the shield or gasket is made of plastic coated with a conductive layer, adding an EMI gasket will not cause much problem, but planners must consider that many gaskets will wear on the conductive surface, generally metal gaskets The surface of the coating is more prone to wear. Increasing this wear over time reduces the shielding power of the gasket joint and causes trouble for the manufacturers behind.
If the shield or gasket structure is metal, a gasket can be added to wrap the gasket surface before spraying the polishing material, just use conductive film and tape. If tape is used on both sides of the splice gasket, the EMI gasket can be fastened with mechanical fasteners, such as "C-type" gaskets with plastic rivets or pressure sensitive adhesive (PSA). The gasket device is on one side of the gasket to complete the EMI shielding.
Pads and Accessories
A wide variety of shielding and gasketing products are currently available, including beryllium-copper splices, metal mesh wire (with or without elastic core), metal mesh and orientation wire embedded in rubber, conductive rubber, and urethane foam gasketing with metal plating Wait. Most shielding material manufacturers can provide estimates of SE that can be achieved by various gaskets, but keep in mind that SE is a relative value and depends on porosity, gasket size, gasket shrinkage, and material composition. Pads come in a variety of shapes for a variety of specific applications, including wear, sliding, and hinged applications. Many current liners come with adhesive or have fixing devices on the liners, such as kneading piercing, pin piercing or barb devices.
Coated foam pads are one of the newest and most versatile products on the market. These gaskets are available in various shapes and thicknesses greater than 0.5mm, and can also be reduced in thickness to meet UL incineration and environmental sealing specifications. There is another new type of gasket, the environmental/EMI hybrid gasket, which eliminates the need for separate sealing materials, reducing shielding cost and complexity. The outer coating of these liners is UV stable, resistant to moisture, wind, and cleaning solvents, while the inner coating is metallized and highly conductive. Another recent modification is the installation of a plastic clip on the EMI gasket, which is more attractive to the market due to its lower weight, shorter installation time and lower cost than traditional confinement metal gaskets.
Summarize:
Equipment generally needs to be shielded due to the presence of slots and gaps in the structure itself. The required shielding can be confirmed by some basic principles, but there are still differences between theory and practice. For example, it is also necessary to consider the signal strength when calculating the size and distance of the pad at a certain frequency, as is the case when multiple processors are used in a device. Surface treatment and gasket design are key factors in maintaining long-term shielding to complete the EMC function.
