Introduction to the application of absorbing materials in electromagnetic protection

With the development of electronic devices in the direction of miniaturization and higher data rates, the result is that the spacing between components is getting smaller and the wavelength is continuously shortening. When the wavelength of the absorbing material is shortened to close to the physical size of the components and equipment, this will cause the "antenna effect" of noise to increase. Therefore, it is more important to prevent noise from coupling to these "antenna" structures that can radiate or generate coupling fields, because at higher frequencies, it becomes more difficult to achieve electromagnetic protection of products using low-cost methods. .

At the same time, the smaller wavelength will be close to the physical size of many EUTs, resulting in cavity resonance effects. When the size of the closed body is equal to an integer multiple of half the wavelength, the corresponding frequency is a resonance frequency. The wave node (ie, zero amplitude) of the wave generated in the enclosure is located on the conductive wall of the enclosure. This structure functions as a cavity resonator. For example, the resonant frequency of the first-order mode of a 2 inch square by 1/2 inch metal cavity is about 12 GHz. At these very high frequencies, even weak coupling can excite strong oscillations, and then the field can couple to any other point in the cavity or can generate radiation. The danger of cavity resonance is that if a noise source contains frequency components corresponding to the resonance frequency, due to the product or amplification effect produced by the cavity "Q-factor", then a strong field will be excited at the resonance frequency. One way to reduce this phenomenon is to reduce the "Q-factor" of the cavity by means of energy loss (Q-suppression). The usual practice is to place absorbent materials in the cavity.

Reduce the edge scattering of the printed circuit board (PCB)

Through proper use of PCB design techniques, such as routing, stacking distribution, decoupling and termination, the radiation generated by the printed circuit board itself can be minimized. However, printed circuit board assemblies still have several other mechanisms that can become radiation sources. These mechanisms include the component itself, the cavity resonance effect of the power/signal return layer, and the edges of the printed circuit board. Edge effect is a very serious problem, because the edge of the circuit board is very close to the chassis shell, so the generated radiation field can excite current on the chassis structure frame.

There are a lot of research, analysis and discussion of various methods and technologies to reduce the edge radiation effect of printed circuit boards, such as proper termination technology. A problem that arises with the application of these technologies is that additional components may need to be added and take up valuable PCB board space, and the actual effect is often not reducing the radiation energy. These common methods will produce energy reflection, which may produce additional internal resonance effects and internal through-hole coupling, which will lead to increased radiation.

The use of microwave absorbing materials to lay along the edge of the printed circuit board can reduce the edge radiation caused by the edge and does not require additional area of the circuit board. By dissipating energy and preventing the energy from being reflected back to the circuit board, the absorbing material can also reduce the possibility of circuit board resonance problems. The absorbing material can be fixed by opening a U-shaped groove on the edge of the circuit board.

Reduce PCB trace radiation

Placing the absorbing material directly on the top of the microstrip line can eliminate the field radiation from the edge of the trace. If the trace is located on the bottom layer of the circuit board and is close to the bottom of the chassis shell, a particularly tricky coupling mechanism will appear if the trace location board is close to the bottom surface of the case. At this time, the field coupled to the chassis will excite current, and the current will flow into the chassis and form a circulating current. These circulating currents then generate radiation through any slots, seams or apertures in the path through which they flow. Sticking the absorbing material with pressure sensitive adhesive (PSA) on the trace can reduce the field coupling to the chassis. Placing the absorbing material in this way has little effect on the impedance of the trace, because the absorbing material has high impedance characteristics (greater than 10Ω). The absorbing material can also be conveniently placed directly on the top of the trace without any additional installation or mechanical fastening measures. This method has been used in a switch box. When the frequency is 6GHz, the radiation emission can be reduced by about 4~6dB.

Reduce cavity resonance effect

As mentioned earlier, a six-sided conductive housing or cavity can support electromagnetic resonance. The coupling of the cavity is the result of self-resonance of various structures, such as the slot on the PCB, the metal shell, the slot between the PCB board and the metal shell. However, a small-sized housing such as a GBIC (GigaBitInterfaceConverter) module or a single PCB covered by a flat housing, and/or only contains a few components, because most of the space volume is Empty (that is, air), which is more like a real resonant cavity. The danger of resonance is that if a noise source contains frequency components corresponding to the resonance frequency, due to the product or amplification effect produced by the cavity "Q-factor", then a strong field will be excited at the resonance frequency. One way to reduce this effect is to reduce the "Q-factor" of the cavity by taking measures that can consume energy (Q-suppression). The absorbing material added in the cavity acts as a resistive load. Today, the protection concept we see more and more is a multi-level concept. The flat housing will handle lower frequencies, while the inner sandwich of microwave absorbing material will handle higher frequency components. Absorbing materials are a viable method for dealing with these higher frequency resonance frequency problems. Although the absorption effect of the absorbing material at the low frequency end is continuously reduced, the absorbing efficiency is very high in the higher frequency band (that is, greater than 1GHz).

By gradually absorbing energy and converting it into thermal absorbing materials, the radiation is reduced or "protection" is achieved, and the Q factor in a cavity is reduced. It is more convenient to use absorbing materials because it converts electromagnetic energy into heat energy without having to use "grounding" measures. As long as the absorbing material blocks the field or is placed on the propagation path of the field, it can reduce the electromagnetic energy of the field. The additional effect of adding a wave-absorbing material in the cavity is that it changes the effective dielectric constant of the cavity, depending on the amount of material added. As the volume of the material in the cavity increases, it will have a greater impact on the composite dielectric constant. By changing the effective dielectric constant, the position of the resonance frequency point can be shifted. This technology was used in the design of a switch box, and as a result, an energy reduction of about 6dB was achieved at 8.5GHz.
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