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Method For Reducing The Radiation Effect At The Edge Of Printed Circuit Board (PCB)

With the development of electronic devices in the direction of miniaturization and higher rate, the resulting gap between components is getting smaller and the wavelength is shortening. This will result in an increase in the "antenna effect" of the noise as the wavelength is shortened to near the physical dimensions of the components and devices. Therefore, it is more important to prevent noise from coupling to these "antenna" structures that can radiate or create coupled fields, because at higher frequencies, it is more difficult to achieve electromagnetic protection of the product in a low-cost manner.

At the same time, the smaller wavelengths are close to the physical dimensions of many of the devices under test (EUTs), resulting in resonance effects in the cavity. When the closed body size is equal to an integral multiple of a half wavelength, the corresponding frequency is a resonant frequency. Waves generated within the chassis have their wave nodes (ie, zero amplitude) located on the conductive walls of the enclosure. This structure acts as a cavity resonator. For example, a 2-inch square by 1/2-inch metal cavity has a first-order mode resonant frequency of around 12 GHz. At these very high frequencies, even weak coupling can excite strong oscillations, and then the field can be coupled to any other point in the cavity or can generate radiation. The danger of cavity resonance is that if a noise source contains a frequency component corresponding to the resonant frequency, a strong field is excited at the resonant frequency due to the product or amplification effect produced by the cavity "Q-factor". One way to attenuate this phenomenon is to reduce the "Q-factor" of the cavity by means of energy loss (Q-suppression), usually by placing an absorbing material in the cavity.

Reduce edge scattering on printed circuit boards (PCBs)

By properly applying PCB design techniques such as routing, stacking, decoupling and termination, the radiation generated by the printed circuit board itself can be minimized. However, there are still several other mechanisms that can be used as a source of radiation for printed circuit board assemblies. These mechanisms include the component itself, the cavity resonance effects of the power/signal reflow layer, and the edges of the printed circuit board. The edge effect is a serious problem because the edge of the board is very close to the chassis housing, so the resulting radiation field can excite current on the chassis frame.

There has been extensive research and analysis of various methods and techniques for reducing the effects of radiation on printed circuit board edges, such as proper termination techniques. One problem that arises with these technical applications is that additional components may be required and take up valuable PCB space, and the actual effect often does not reduce the radiant energy. These common methods produce energy reflections, which may result in additional internal resonance effects and internal via coupling, which leads to enhanced radiation.

The use of a microwave absorbing material to lay along the edges of the printed circuit board reduces edge radiation caused by the edges and does not require additional board space. By consuming energy that does not allow energy to be reflected back to the board, the absorbing material also reduces the likelihood of board resonance problems. The absorbing material can be fixed by opening a U-shaped groove at the edge of the board.

Reduce the trace radiation of the PCB board

Placing the absorbing material directly on top of the microstrip line eliminates field radiation from the side of the trace. If the trace is on the bottom layer of the board and adjacent to the bottom of the chassis, a particularly tricky coupling mechanism can occur if the trace is located near the bottom of the case. At this point, the field coupled to the chassis will energize the current, which flows into the interior of the chassis and creates 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 to the trace with a pressure sensitive adhesive (PSA) reduces the field coupled to the chassis. This placement of the absorbing material has minimal effect on the impedance of the trace because the absorbing material has a high impedance characteristic (greater than 10 Ω). The absorbing material can also be conveniently placed directly on top of the trace without the need for any additional mounting or mechanical fastening measures. This method has been used on a switch box to reduce radiation emissions by about 4 to 6 dB at 6 GHz.

Reduce cavity resonance effects

As previously mentioned, 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 slots in the PCB, metal housings, slots between the PCB board and the metal casing. However, a small-sized case such as a GBIC (GigaBit Interface Converter) module or a single-panel PCB covered by a flat case has and/or contains only a few components due to most of the space. The volume is empty (ie air), which is more like a real cavity. The danger of resonance is that if a noise source contains a frequency component corresponding to the resonance frequency, a strong field is excited at the resonance frequency due to the product or amplification effect produced by the cavity "Q-factor". One way to attenuate this effect is to reduce the "Q-factor" of the cavity by taking measures that can deplete energy (Q-suppression). The absorbing material applied to the cavity acts as a resistive load. At present, the concept of protection we see is more and more a multi-level concept. The flat housing will handle lower frequencies while the inner interlayer of the microwave absorbing material will handle higher frequency components. Absorbing materials are a viable means of dealing with these higher frequency resonant frequencies. Although the absorption of the absorbing material at the low frequency end is continuously reduced, the absorbing efficiency is very high in the higher frequency band (i.e., greater than 1 GHz).

By gradually absorbing energy and converting it into a thermal absorbing material reduces radiation or "protection" while reducing the Q factor in a cavity. It is more convenient to use absorbing materials because it converts electromagnetic energy into heat without having to use "grounding" measures. As long as the absorbing material blocks the field or the propagation path on the field, it reduces the electromagnetic energy of the field. An additional effect of adding an absorbing material to the cavity is that it changes the effective dielectric constant of the cavity, depending on the amount of material added. As the volume of material increases in the interior of the cavity, the effect on the composite dielectric constant will be greater. By changing the effective dielectric constant, the positional shift of the resonance frequency point can be caused. This technique was used in the design of a switch box, resulting in an energy reduction of approximately 6 dB at 8.5 GHz.

Radiator radiation

In general, the physical and electrical dimensions of the heat sink are larger than those of the high frequency chip device, which is bonded to the high frequency chip device and is therefore an efficient radiator. No matter how well the signal is transmitted on the printed circuit board, if the current of the chip is parasitic coupled to the heat sink, a radiation emission will occur. Each heat sink of the heat sink is equivalent to a monopole antenna structure, and all the heat sinks are equivalent to the antenna array. Depending on the overall shielding effect or the resonant effect of the heat sink, these emissions may or may not exceed the limits specified in the specification. The most common way to control radiated emissions from a heat sink is to connect the ground to the reference ground of the PCBs.

As the frequency increases, the size of the heat sink becomes electrically large and even becomes a more efficient radiator. Therefore, any heat sink grounding scheme designed must also be effective at higher frequencies. The connection between the heat sink and the printed circuit ground will have inductance and the connection must exhibit low impedance characteristics. The greater the number of contact points used, the lower the impedance, which will reduce the amount of radiation emissions more effectively. In general, grounding measures for heat sinks do not effectively reduce electromagnetic radiation at frequencies above 1 GHz. Therefore, other methods must be considered. In order to improve grounding at high frequencies, we must reduce the contact point spacing to within 1/20 to be effective. An example is the continuous grounding of the heat sink with its continuous reference ground through an elastic conductive spring. However, this not only still requires a considerable board area, but it has also proven to be ineffective in reducing radiation emissions above 10 GHz. The use of absorbing materials to reduce the surface current on the heat sink, thereby reducing the radiation effect of the heat sink, has proven to be effective. Thus, absorbing materials can be utilized to reduce potential radiation emissions by reducing surface currents on the heat sink blades. Studies have shown that placing the absorbing material directly under the heat sink, placed between the heat sink and the printed circuit board, also reduces radiation emissions.

RF absorbing materials and microwave absorbing materials have many different names. Some of the most common names include: RF absorbers, microwave absorbers, electromagnetic interference absorbers, radar absorbers or RAM, magnetic radar absorbers or mag-RAM, EMI suppression materials or surface wave absorbers. The magnetic and/or electrical properties of the materials referred to by all of these different terms have changed, but they all absorb or lose energy.

Historically, global military forces have used microwave absorbing materials to reduce reflections on high frequency radars. However, over time, there has been a trend to use microwave absorbing materials in commercial applications. Consumer electronics, notebook computers, wireless LAN devices, network servers and switches, wireless antenna systems, and cellular phone base stations are just a few of the high-frequency devices that use this technology.


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