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There is a long-standing intuition in mobile phone EMI design: adding a metal shielding cover above the noise source to "shut in" electromagnetic energy inside, and the problem is solved.
This intuition generally holds true in the Sub-6 GHz frequency band. The reflection loss of the metal shielding cover is high enough, the cavity size is much smaller than the wavelength, and the resonance problem is not prominent. But in the millimeter wave frequency band above 24 GHz, the situation has fundamentally changed. The size of the shielding cover is comparable to the wavelength, and the enclosed metal cavity becomes a high-quality factor (Q value) resonator. The noise energy is not "shut off", but repeatedly reflected and superimposed inside the cavity, forming a standing wave peak at a specific frequency point, and coupled to adjacent millimeter wave antennas through gaps, openings, and packaging interfaces. At this point, the shielding case itself becomes the amplifier of the problem, rather than the solution.
Limitations of Reflective Shielding in Millimeter Wave Frequency Range
The wavelength of millimeter waves is extremely short. The wavelength of 28 GHz in free space is approximately 10.7 mm, which is further shortened in the medium. The typical size of the internal shielding cover of a mobile phone is in the centimeter to millimeter range, which is on the same order of magnitude as the wavelength. This means that the resonant modes of the cavity are dense, and any small structural changes - such as gaps in the shielding cover, screw holes, FPC wiring - may trigger new resonant peaks.
The metal shielding cover exhibits extremely low surface impedance to incident electromagnetic waves, and energy is reflected almost without loss inside the cavity. The higher the Q value of the cavity, the sharper the resonance peak, and the more concentrated the field strength at a specific frequency point. For millimeter wave antennas, resonant coupling between adjacent cavities can raise the noise floor and compress the dynamic range, resulting in decreased sensitivity or degraded beamforming accuracy. Simple metal shielding transfers energy to other frequencies and directions without consuming it.
The working position of mobile phone absorbing materials: from "sticking a piece" to "becoming a part of the structure"
The engineering role of mobile phone absorbing materials is shifting from the early auxiliary method of "sticking a piece of lossy material on the inner wall of the shielding cover" to structural functional components. Around millimeter wave modules and antennas, absorbing materials need to simultaneously meet several interdependent conditions: thickness thin enough to accommodate millimeter level stacking space, usable absorption depth at specific frequencies of 24-43.5 GHz, ability to withstand deformation during mounting or compression without cracking or detachment, and inability to exert excessive dielectric loading on the radiation characteristics of adjacent antennas.
Fine pore absorbing foam is a typical carrier for this role transformation. Its customizable parameters cover an aperture of 80-300 μ m, a porosity of 60% -90%, and a thickness of 0.3-5.0 mm. The effective frequency band can be designed within 1-40 GHz, with a focus on commonly used mobile frequency bands such as 2.4/5/6 GHz, 24-43.5 GHz, and 76-81 GHz. The impedance gradient and internal scattering formed by the porous structure gradually dissipate electromagnetic waves during penetration, rather than reflecting them at once on the surface. For the narrow cavity inside the shielding cover, this "enter and decay" mechanism can lower the peak of the resonance peak more effectively than "surface reflection".
Magnetic and dielectric losses: division of labor in different frequency bands within mobile phones
The loss mechanism of mobile phone absorbing materials is not the same in the low and high frequency bands.
In the Sub-6 GHz and Wi Fi frequency bands (2.4/5/6 GHz), harmonics of high-speed digital interfaces and near-field coupling of RF modules are the main sources of noise. The absorbing materials in this frequency band are usually dominated by magnetic losses, relying on the natural resonance and eddy current losses of soft magnetic fillers to convert electromagnetic energy into heat. The microsecond of the material in the target frequency band determines the attenuation ability per unit thickness. For the narrow interior space of mobile phones, the thickness is usually compressed to 0.1-0.5 mm, and the strength of magnetic loss directly determines whether this thin layer is "sufficient".
After entering the millimeter wave frequency band, the weight of dielectric loss and porous scattering increases. Carbon loaded polyurethane foam absorbs radio frequency energy through dipole polarization loss and plays a major role in high-frequency interference above 1 GHz. The pore structure of fine pore foam itself also contributes to impedance gradient: when electromagnetic waves enter the foam from air, the effective dielectric constant gradually changes with the porosity, reducing interface reflection and allowing more energy to enter the material and be dissipated. In the 5G millimeter wave frequency band of 24-43.5 GHz, fine pore absorbing foam with a thickness of 1-3 mm can suppress the resonance peak of the inner cavity of the shielding cover by more than 6 dB.
Matching thickness and frequency band: hard boundary under mobile phone space constraints
In the selection of mobile phone absorbing materials, the matching relationship between thickness and frequency band is more engineering meaningful than "how high the absorption peak is".
The absorption depth of millimeter wave frequency band is directly related to the thickness of the material. The higher the frequency, the shorter the wavelength, and the thinner the thickness required to achieve effective interference or impedance gradient - but this does not mean that it can be infinitely reduced. In the vicinity of 24 GHz, if the thickness of the absorbing layer is not sufficient to cover the propagation path of electromagnetic waves in the target frequency band inside the material, the absorption depth will be insufficient and the resonance suppression effect will decrease. In industry engineering practice, 5G millimeter wave applications require absorbing materials to achieve absorption rates of>15 dB at specific frequencies such as 24 GHz, 28 GHz, and 39 GHz with extremely thin thicknesses (usually<0.5 mm). This data point reveals the typical contradiction of mobile phone absorbing materials: the spatial requirement is "thin", and the performance requirement is "still effective within a thin layer".
For millimeter wave cavity resonance suppression, fine pore foam with a thickness of 1-3 mm is a more common engineering choice because it requires sufficient scattering and gradient paths to be formed inside the cavity. For near-field noise at the chip or module level, thin absorbing plates or nanocrystalline absorbing films with a thickness of 0.1-0.5 mm are more suitable. The first step in selection is not to compare "who absorbs stronger", but to confirm whether the target frequency band and available thickness fall within the intersection of effective material operation.
Integrated thermal conductivity: absorbing materials cannot become a new thermal bottleneck
Any newly added materials inside the phone must answer hot questions. Absorbing materials attached to shielding covers, chips, or antennas may become a "thermal insulation layer" for local hotspots if their own thermal conductivity is insufficient.
Composite gaskets that combine thermal conductivity and absorption functions are becoming a common option in highly integrated designs for mobile phones. The heat dissipation and absorption layer can be made of graphene material, with a planar thermal conductivity of up to 500 W/mK, while maintaining an electromagnetic wave absorption effect of over 60 dB. It has a thickness of 0.1 mm and is bonded with 0.03 mm pressure-sensitive adhesive. The typical thermal conductivity of a thermally conductive absorbing pad is on the order of 1.5 W/m · K (ASTM D5470), and the thermal impedance is approximately 0.324 ° C-in ²/W at 10 psi. This type of data provides engineers with a verifiable starting point: whether the absorbing layer solves EMI while at least not deteriorating the original thermal path.
Assembly adaptability: compression deformation and long-term fit
Mobile phone absorbent foam typically undergoes compression during assembly. The range of compression deformation that can be designed is 10% -50%. When the compression is insufficient, there is an air gap between the foam and the cavity wall, and the reflection of electromagnetic waves in the gap will weaken the absorption effect; When the compression amount is too large, the pore structure of the foam may be crushed, resulting in synchronous loss of effective thickness and impedance gradient path.
Long term adhesion reliability also needs attention. The internal temperature cycle and vibration of the mobile phone may cause fatigue in the adhesive layer between the absorbing material and the shielding cover or structural components. For foam materials, the resilience of compression deformation determines whether they can still maintain close contact with the wall after temperature cycling. When selecting, the supplier's compression permanent deformation data should be confirmed, not just the initial compression rate.
Selection and judgment of mobile phone absorbing materials
Firstly, confirm what needs to be suppressed
If it is a cavity resonance inside the shielding cover, the selection starting point is the reflectivity or attenuation of the absorbing material in the target resonance frequency band, and whether the thickness is sufficient to form effective suppression under the cavity size. If it is near-field coupling between the chip or FPC and the antenna, the selection starting point is the material's μ "and thin layer attenuation ability in the target frequency band. The evaluation indicators of the two are different and cannot be mixed.
Secondly, use thickness frequency band matching as the first sieve, rather than absorbing peak values
Require suppliers to provide reflectance curves or attenuation data for target frequency bands (such as 24-28 GHz, 37-43.5 GHz) and actual available thickness. If the peak absorption rate is measured under conditions of mismatched thickness, its reference value is limited.
Thirdly, for the absorbing material attached near the heat source or inside the shielding cover, confirm the thermal conductivity data or at least confirm that it will not become a thermal bottleneck
The thermal conductivity, thermal impedance, and thickness of the integrated material for heat conduction and absorption need to meet the requirements of both thermal management and EMI suppression. If the material itself has low thermal conductivity, the mounting position should avoid the main heat dissipation path or leave a bypass for heat through structural design.
Advanced Institute (Shenzhen) Technology Co., LtdThe product line of mobile phone absorbing materials covers fine pore absorbing foam, thin absorbing wave plates, and thermal conductive absorbing composite pads.
Fine pore absorbing foam with a pore size of 80-300 μ m can be customized, with a thickness of 0.3-5.0 mm. The effective frequency range covers 1-40 GHz, with a focus on millimeter wave cavity resonance and antenna decoupling at 24-43.5 GHz; Thin wave absorbing plates are designed for chip level and module level near-field noise, with a thickness that can be reduced to the order of 0.1 mm; The thermal conductive composite solution supports integrated design of absorption and thermal management.
The product supports precision die-cutting, irregular cutting, and multi-layer composite, and can be matched according to the actual cavity size, frequency band, and assembly compression conditions inside the phone. For more technical information, please visithttp://www.avanzado.cn/.