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There is a recurring confusion in the selection of electromagnetic materials inside mobile phones: treating "magnetic separators" and "absorbing plates" as the same type of material and using the same standard of "higher magnetic permeability, better" to measure them.
The engineering goals of the two are exactly opposite. The magnetic separator should allow the magnetic flux to "pass" through itself and avoid entering the metal backplate; Wave absorbing plates should make electromagnetic waves "disappear" inside themselves, no longer reflecting back into the cavity or coupling to the antenna. One pursues low loss magnetic flux guidance, while the other pursues controllable loss dissipation. The materials that are effective in the NFC and wireless charging frequency bands may not be thin enough and the loss mechanism may not be correct in the face of 5G millimeter wave or high-speed digital noise.
Magnetic isolation and absorption: the same word, two sets of physical logic
The application scenario of magnetic separators is near-field magnetic field coupling. NFC operates at 13.56 MHz, while wireless charging operates at 110-205 kHz. The distance between the antenna and the metal backplane is much smaller than the wavelength, and the field is mainly composed of magnetic field components. The metal backplate induces eddy currents in an alternating magnetic field, and the reverse phase magnetic field of the eddy currents cancels out the antenna magnetic flux, resulting in a decrease in inductance and a collapse of Q value. The function of the magnetic separator is to insert a high μ 'and low μ' magnetic flux bypass between the antenna and the metal, pulling the magnetic field lines away from the metal surface rather than consuming them. Therefore, the core indicator of the magnetic separator is that μ 'is high enough to guide the magnetic flux, while μ' is low enough to avoid dissipating energy during the guidance process.
The absorption plate faces another type of problem. The 5G millimeter wave antenna operates at 24-43.5 GHz, and the noise harmonics of high-speed digital interfaces may extend to several GHz. The cavity formed by the shielding cover and metal frame will form standing wave resonance at specific frequencies. At this point, electromagnetic waves have already reflected multiple times within the cavity in the form of radiated or quasi radiated fields. Simple metal shielding will only reflect energy to other areas, raising the Q value of the cavity and intensifying the coupling between antennas. The goal of a waveplate is to convert electromagnetic energy into heat within a limited thickness, reducing reflection and coupling. Its evaluation index is the attenuation or reflectivity of the target frequency band, rather than the height of μ '.
In other words, the magnetic separator is concerned with whether the magnetic flux is guided to the correct position, while the absorbing plate is concerned with whether the noise energy is dissipated. Attaching a high μ 'magnetic separator to the vicinity of a millimeter wave antenna cannot effectively absorb noise above 24 GHz, and may also alter the antenna's matching conditions due to its magnetic loading.
Three typical scenarios inside mobile phones, corresponding to three material requirements
The first type is the magnetic isolation between NFC and wireless charging receivers.The typical range of μ 'for ferrite magnetic separators at 13.56 MHz is 180-660, with a thickness of 0.06-0.1 mm and a temperature resistance of approximately -40~90 ℃. Some ultra-thin absorbing materials can achieve a thickness of 0.03 mm and a diameter of approximately 250 μ ', which is used to provide magnetic flux circuits and reduce coil resistance in extremely narrow spaces. The key constraint in this scenario is not "how strong the absorption is", but whether the "μ" is low enough - if the "μ" is high enough, it will directly consume the limited magnetic field energy of the NFC antenna, manifested as shortened read distance or resonance detuning.
The second type is noise suppression and decoupling near 5G millimeter wave antennas.The cavity resonance and surface wave coupling in the millimeter wave frequency band require materials with effective electromagnetic losses in the 24-43.5 GHz or even higher frequency range. The customizable parameters of fine pore absorbing foam in this scenario include pore size of approximately 80-300 μ m, porosity of 60% -90%, thickness of 0.3-5.0 mm, effective frequency range covering 1-40 GHz, and typical reflectivity of -8~-20 dB. This type of material dissipates energy through impedance gradient and internal scattering of porous structure, rather than relying on high μ 'to "guide" magnetic flux.
The third type is antenna coupling suppression in metal framed mobile phones.The metal frame itself becomes a coupling path for internal noise, transmitting the noise to the top and bottom RF antennas. The ferrite absorbing plate can be mounted on the inside of the frame to reduce the transmission coefficient of this coupling path. The material selection here needs to consider both the surface fitting of the frame, the influence of thickness on the overall size, and whether the absorbing frequency band covers the operating frequency of the interfered antenna.
The following table summarizes the different material requirements for three types of scenarios inside mobile phones:
Mismatch of thickness and frequency: why "one piece universal" doesn't work in mobile phones
The tolerance of thickness for different materials in the internal space of mobile phones is completely different. NFC magnetic separators are installed above batteries or near cameras, with a thickness budget typically in the range of 0.1 mm. Ferrite sheets with a thickness of 0.06-0.1 mm are a typical solution under this constraint. Millimeter wave absorbing foam requires a thickness of 0.3-5 mm to form an effective impedance gradient and porous scattering path. If it is too thin, the absorption depth at the low-frequency end will be insufficient. If the thickness logic of millimeter wave foam is moved to NFC magnetic isolation scenarios, space is not allowed; If the thin logic of NFC magnetic separators is applied to millimeter wave absorption scenarios, the loss mechanism cannot be established in the target frequency band.
Frequency mismatch is equally critical. The high μ 'design of the magnetic separator is focused on the low frequency range (kHz to 13.56 MHz), and its magnetic loss mechanism does not contribute to effective broadband absorption in the millimeter wave frequency band. The porous or magnetic filler system of the absorbing wave plate is designed for radiation fields above GHz. When placed in the near-field magnetic field environment of NFC, it cannot effectively guide magnetic flux, and its dielectric loss may also change the loading conditions of the antenna. The working frequency band and field type of the material system determine that it cannot be "translated across scenes".
Selection and judgment of magnetic separators and absorbing materials for mobile phones
Firstly, determine whether to address "flux guidance" or "energy dissipation"
If the problem is short NFC reading distance, low wireless charging efficiency, or Q value collapse caused by metal backplate, the starting point for selection is the magnetic separator, and the core indicators are the μ 'and μ "at the target frequency, rather than reflection loss. If the problem is a decrease in sensitivity of millimeter wave antennas, an increase in resonance peak inside the shielding cover, or insufficient isolation between multiple antennas, the starting point for selection is the absorption plate, and the core indicator is the attenuation or reflectivity of the target frequency band, rather than the height of μ '.
Secondly, in the selection of magnetic separators, place the absolute value of μ "before μ '
The magnetic field energy of NFC antenna itself is limited, and a high μ "will directly consume the signal budget. At 13.56 MHz, tan δ should not exceed 0.05-0.1, and μ "should be controlled at the single digit level as much as possible. A thin sheet with a medium μ 'but extremely low μ' may perform better in NFC scenarios than a solution with a high μ 'but also a high μ'.
Thirdly, in the selection of absorbing wave plates, confirm the boundary conditions between the target frequency band and the actual installation position
The reflectivity data of millimeter wave absorbing materials varies with frequency band, thickness, and incident angle. The peak value on the specification sheet should not be directly equivalent to the effect inside the entire cavity. The compression deformation, adhesive type, and mounting position of fine pore foam will all change its effective absorption path. When selecting, suppliers should be required to provide reflectance curves or attenuation data that are close to the actual installation conditions, rather than just looking at the peak value of a single frequency point.
Advanced Institute (Shenzhen) Technology Co., LtdThe mobile phone magnetic isolation sheet and absorbing material cover multiple product lines such as ferrite magnetic isolation sheet, nanocrystalline magnetic isolation sheet, fine pore absorbing foam, and ultra-thin absorbing plate.
The ferrite magnetic separator has a typical range of 180-660 μ 'at 13.56 MHz, with a thickness of 0.06-0.1 mm, and faces the magnetic flux guidance of NFC and wireless charging receiving ends; Fine pore absorbing foam with a thickness of 0.3-5.0 mm and an effective frequency range of 1-40 GHz, used for millimeter wave cavity resonance suppression and antenna decoupling; Ultra thin wave absorbing plates can be used in conjunction with near-field noise control in narrow spaces.
The product supports rolls, sheets, and precision die-cutting, and can be matched according to the specific frequency band, space, and assembly conditions inside the phone. For more technical information, please visithttp://www.avanzado.cn/.