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In the electromagnetic compatibility design of electronic devices, an easily overlooked fact is that shielding effectiveness itself does not distinguish whether energy is absorbed or reflected. A piece of metal foil can easily provide shielding effectiveness of over 60 dB, but the way it achieves this figure is by reflecting almost all incident electromagnetic waves back to their original location. For the internal noise source of the device, the reflected electromagnetic waves do not disappear, they are only changed in propagation direction and continue to search for leakage paths in the cavity.
This "reflection for shielding" strategy is not a big problem in the low frequency range, but when the frequency enters the GHz range and the wavelength of near-field electromagnetic waves is shortened to be comparable to the package size and connector spacing, the interference and standing wave effects of reflected waves begin to dominate the EMI performance of the system. Even more troublesome is that high reflectivity shields can become a secondary radiation source in dense electronic systems, transforming local noise into a broader interference problem.
Impedance matching: the key to determining whether electromagnetic waves "enter" or "bounce off"
When electromagnetic waves enter any material from air, partial reflection and partial transmission occur at the interface. The proportion of reflection is determined by the impedance difference between the two media. The wave impedance of free space is about 377 Ω. When the input impedance of the material approaches this value, electromagnetic waves can enter the interior of the material to the maximum extent, rather than being bounced back on the surface.
The impedance of metals is extremely low and severely mismatched with free space, so incident electromagnetic waves are almost completely reflected on the metal surface. This is also why high conductivity materials, although having high shielding effectiveness values, essentially belong to the "reflection dominant" shielding type. To truly hope for electromagnetic energy to be dissipated rather than transferred, materials need to meet two conditions simultaneously: matching input impedance with free space, and sufficient electromagnetic loss mechanisms to attenuate waves entering the material's interior.
The design of electromagnetic shielding absorbing wave plates revolves around these two conditions. Unlike pure metal shields, absorbing wave plates convert electromagnetic energy entering the material's interior into thermal energy through magnetic and dielectric losses. Magnetic losses come from the natural resonance, domain wall resonance, and eddy current losses of soft magnetic powders in alternating magnetic fields; The dielectric loss originates from dipole polarization and interface polarization. These mechanisms work together to cause electromagnetic waves to undergo multiple absorptions within the material, rather than simply bouncing back from the surface.
Near Field and Far Field: Same Material, Different Design Logic
The application scenarios of shielding absorbing wave plates can be roughly divided into two categories: near-field suppression and far-field shielding, which have systematic differences in material requirements.
In near-field scenarios, the distance between the noise source and the absorbing plate is less than λ/2 π. Under this condition, the electric and magnetic field components are no longer orthogonal, and the magnetic field component often dominates. Radiation near chips, switching noise of DCDC converters, and crosstalk between connectors are all typical near-field problems. For near-field noise dominated by magnetic fields, the magnetic permeability of the material is the primary parameter, and the geometric shape and mounting position of the shielding body have a much greater impact on the final effect than in far-field scenarios. The AB3000 series absorbing wave plate from 3M is designed for near-field applications ranging from 1 GHz to 10 GHz, with a nominal permeability of 30 and a target frequency range of 3-7 GHz, with a thickness range of 0.1 mm to 0.5 mm.
In far-field scenarios, electromagnetic waves propagate in the form of plane waves, with electric and magnetic field components orthogonal and wave impedance close to 377 Ω. Materials need to have sufficient conductivity or magnetic loss to attenuate incident waves, and impedance matching requirements are more stringent because the thickness of far-field shielding is usually much smaller than the wavelength, and matching cannot be achieved by relying on quarter wavelength interference. For far-field applications, impedance gradient design of multi-layer structures is often more effective than single materials: the outer layer has impedance close to free space to allow waves to enter, and the inner layer gradually increases reflectivity to promote multiple scattering and absorption inside.
The following table summarizes the differences in key design dimensions between near-field and far-field shielding absorption plates:
Engineering implementation path of absorption dominant shielding
There are three main paths in engineering to reduce the reflectivity of the absorbing wave plate while maintaining available shielding effectiveness.
The first type is impedance gradient structure
By designing a resistive film or low conductivity layer on the side of the absorbing plate facing the incident wave, the input impedance gradually transitions from 377 Ω in free space to a lower impedance inside the material. The surface resistance value of the resistive film is usually controlled within the range of -15% to 20% of the vacuum impedance to achieve practical impedance matching. The key to this design is that the surface resistance of the resistive film does not change under bending conditions - if bending causes the resistive film to crack, the impedance matching window shifts, and the absorption performance deteriorates accordingly.
The second type is multi-layer interference structure
Place the high loss layer between two reflective layers and enhance absorption by utilizing the multiple reflections and interference cancellation of electromagnetic waves between the layers. The Fe ₂ O3/CNT/Fe ₂ O3 sandwich structure is a representative example: the rough surface of the outer Fe ₂ O3 improves the impedance matching between free space and composite materials, making it easier for incident waves to enter the film, the inner CNT layer provides conductive loss, and the back Fe ₂ O3 layer further attenuates transmitted waves through magnetic loss. The total shielding effectiveness of this structure reached 56.8 dB, and the absorption to reflection ratio (A/R) increased from 0.69 of pure CNT film to 1.86, indicating a shift in shielding mechanism from reflection dominated to absorption dominated.
The third type is gradient loss structure
Within a single absorbing wave plate, the concentration or type of magnetic loss filler exhibits a gradient distribution from the incident surface to the back, causing electromagnetic waves to gradually attenuate during penetration rather than being reflected at a single interface. The MXene/Fe ∝ O ₄ dual gradient structure achieved a shielding efficiency of 49.98 dB and an absorption coefficient of 0.51 at a thickness of only 180 μ m. Its "absorption reflection reabsorption" process effectively improved the absorption intensity per unit thickness.
Two practical constraints in selection
For engineers, the above design principles need to be applied to specific material parameters and mounting conditions. The following two constraints are easily underestimated during selection.
The implicit impact of bending reliability on impedance matching
If the wave absorbing plate needs to undergo bending during assembly or be installed on a non planar surface, the change in surface resistance after bending must be verified. Microcracks in the resistive film or conductive layer will not significantly change the appearance, but will cause the input impedance to deviate from the matching window, resulting in a decrease in absorption performance in silence. Choosing a wave absorbing plate with a flexible conductive polymer resistance layer is more reliable in deformation scenarios than using a brittle metal thin film version.
Matching of target frequency band with material permeability spectrum
The real and imaginary parts of the magnetic permeability of an absorbing wave plate vary with frequency, and the peak frequency of μ 'determines the strongest absorption band of the material. If the target noise frequency deviates from the peak value by more than one octave, even if the material thickness is increased, it is difficult to achieve the desired absorption effect. When selecting, the material's μ "data in the target frequency band should be confirmed first, rather than just focusing on the nominal" operating frequency range ".
Advanced Institute (Shenzhen) Technology Co., LtdThe research platinum brand absorbing material series covers three models: XJY-MH100, XJY-MH150, and XJY-MH200. The magnetic permeability of the real part covers ≥ 100 to ≥ 200, and the thickness of the series products can be as low as 0.2 mm. The working temperature range is -40 ° C to 125 ° C, and the flame retardant rating reaches UL94 V-0.
This series adopts the orientation dispersion technology of sheet-like nanocrystalline soft magnetic powder, which improves the magnetic permeability while regulating the dielectric constant through surface coating process to maintain the stability of impedance matching window. The product supports flexible mounting, precision die-cutting, and multi-layer composite, and can be customized according to specific frequency bands, spatial constraints, and bending conditions. For more technical information, please visithttp://www.avanzado.cn/.