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When searching for "anti magnetic stickers for mobile phones" on e-commerce platforms, the descriptions usually include "anti demagnetization", "anti metal interference", and "allowing bus card stickers to be swiped on mobile phones". These descriptions point to a real engineering problem, but the use of the word 'anti magnetic' can easily lead people to have a wrong physical intuition: thinking that anti magnetic stickers are a layer of 'barrier to block magnetic fields'.
If it is really 'blocking', a piece of aluminum foil or copper foil is enough. But the actual test data is exactly the opposite: when aluminum foil is attached to the back of an NFC card, the card reading distance drops sharply from 50mm to 8mm, and the communication failure rate increases to 92%. Metal foil reflects electric fields and does not absorb magnetic fields at low frequencies, instead forming parasitic resonance loops that disrupt the coupling conditions of the antenna itself. The real character of anti magnetic stickers in mobile phones is not "blocking", but "attracting".
The actual meaning of "anti magnetism" is to repair a low magnetic resistance bypass for magnetic field lines
NFC and wireless charging both operate in the near-field magnetic field coupling region. NFC operates at 13.56 MHz, while the Qi standard for wireless charging is between 100-300 kHz. The distance between the antenna and the metal frame, battery shielding layer, and metal back plate inside the phone is much smaller than the wavelength. The magnetic field lines should have passed through the air from the reader/writer or charging coil to reach the phone antenna, but the metal parts induce eddy currents in the alternating magnetic field. The reverse magnetic field generated by the eddy currents cancels out some of the incident magnetic flux, resulting in a decrease in antenna inductance, deterioration of Q value, shortened reading distance, and decreased charging efficiency.
The function of anti magnetic tape is to insert a layer of high magnetic permeability material between the antenna and the metal, providing a low magnetic resistance path for magnetic field lines. Magnetic field lines preferentially pass through this layer of material, rather than entering the metal surface to excite eddy currents. After the eddy current is suppressed, the coupling conditions of the antenna are restored, and the reading distance and charging efficiency also increase accordingly.
This mechanism determines that the core indicators of anti magnetic tape are not "shielding effectiveness" or "reflection loss", but whether the real part of magnetic permeability μ 'is high enough and whether the magnetic loss tangent tan δ is low enough. μ 'determines how much magnetic flux it can' pull away ', and tan δ determines whether the energy will be dissipated in vain during the' pulling away 'process.
Magnetic permeability and absorption: there are actually two different functional layers inside the anti magnetic patch
The "anti magnetic tape" on the market is not a single material. It may only serve the function of magnetic permeability, or it may be compounded with a layer of absorbing material outside the magnetic permeability layer.
Essentially, it is a magnetic separator that only serves the function of magnetic conduction
It operates in the low frequency range of 100 kHz to 13.56 MHz, with the goal of high μ 'and low tan δ. The typical design window for NFC scenarios is μ'120-180, tan δ ≤ 0.05, and thickness 0.06-0.12 mm; the typical range of μ 'for wireless charging scenarios is 80-180 tanδ ≤0.03， Thickness 0.10-0.30 mm.
Undertaking the function of absorbing waves, dealing with a different type of problem
The millimeter wave module, high-speed digital circuit, and shielding enclosure inside the mobile phone generate radiated noise and resonance in the GHz frequency band, and this energy needs to be absorbed rather than guided. Absorbing materials work in the range of 100 MHz to 40 GHz, and the evaluation criteria are the reflection loss or attenuation of the target frequency band, rather than the level of μ '.
The composite anti magnetic tape stacks the two together with the aim of simultaneously solving "low-frequency magnetic flux guidance" and "high-frequency noise absorption" in the same space. But the two functions do not always have the same direction of material requirements: the magnetic conducting layer requires low loss, and the absorbing layer requires controllable loss. Simple stacking may cause the NFC antenna to be additionally loaded by the absorbing layer's μ "near 13.56 MHz, resulting in a decrease in read distance instead of an increase. The rationality of the composite scheme depends on whether the two functional layers are truly separated in frequency and whether the impedance after superposition still matches.
Where is the boundary between thickness and temperature rise between ferrite and nanocrystals
The magnetic conductive layer of anti magnetic tape mainly consists of two material systems: ferrite and nanocrystals.
The advantage of ferrite in the NFC frequency band is its "natural low loss". Its resistivity is high, eddy current losses can be ignored, tan δ can be made low, and μ 'is in the range of 100-150. The disadvantage is that the saturation magnetic induction Bs is only 0.4-0.5 T, which makes it easy to enter saturation under high-power wireless charging; Moreover, ferrites are hard and brittle, and are prone to cracking during curved surface mounting.
The Bs of nanocrystals are about 1.2 T or more, which is more than twice that of ferrites. The Curie temperature can reach 570-620 ℃, much higher than the 200 ℃ or less of ferrites. This means that at the same power, the working point of the nanocrystals is further away from the saturation region, and the thickness can be thinner without triggering saturation. The measured data also supports this point: under the same efficiency, the typical thickness of nanocrystalline magnetic separators is 0.11 mm, while ferrite requires 0.27 mm; the charging efficiency of the nanocrystalline scheme is about 2.5 percentage points higher than that of ferrite, and the temperature rise is 7-8 ℃ lower. But nanocrystals have a process cost: their electrical resistivity is extremely low, and without magnetic fragmentation treatment, eddy current losses will become dominant, resulting in high tan δ. The effective μ 'of the finished product after magnetic fragmentation will decrease from the initial value of the strip to the range of 150-800, and the filling method of the fragment gap directly affects the long-term stability. Therefore, the selection of nanocrystal anti magnetic patches cannot only focus on the "tape material", but must also consider the finished product status, effective μ 'and tan δ after being attached to the antenna structure.
Thickness is not 'thicker is better': 0.3 mm is an easily overlooked boundary
A common misconception in the purchase of anti magnetic stickers on the consumer side is that "thicker stickers have better anti magnetic effects". But the boundaries given by actual measurements and experience are exactly opposite: when the thickness exceeds 0.3 mm, the probability of wireless charging disconnection or slow induction increases significantly. Thick patches can widen the distance between the coil and the receiving end, alter the coupling conditions, and even cause the phone case buckle to fail.
From an electromagnetic perspective, the 0.15 mm ferrite layer at 13.56 MHz is close to the skin depth of the material in this frequency band, and the attenuation of the interference magnetic field can reach 22 ± 3 dB. As the thickness continues to increase to 0.5 mm, the loss tangent is not significantly optimized, and the marginal gain decreases. In NFC scenarios, the thickness of anti magnetic patches is usually controlled at 0.06-0.12 mm, rather than using thickness to exchange for "stronger shielding".
What really needs to be wary of is using "anti magnetic stickers" as "anti radiation stickers". Mobile phone radiation is mainly electromagnetic wave radiation, and communication itself requires continuous transmission and reception of electromagnetic waves. If a patch really "blocks" most of the electromagnetic waves, communication cannot be completed. Using metal patches to interfere with antenna signals may also force the phone to increase its transmission power, resulting in a brief increase in radiation levels. Radiation protection is not the functional positioning of anti magnetic stickers inside mobile phones, and such marketing tactics should not be used as engineering basis when selecting.
Selection judgment based on target frequency band and mounting interface
Firstly, confirm whether the problem to be solved is "weakening of low-frequency magnetic flux by metal" or "coupling of high-frequency noise"
If the NFC reading distance is short, wireless charging efficiency is low, or the metal backplate causes a decrease in Q value, the starting point for selection is the magnetic isolation performance: whether the μ 'is high enough, whether the tan δ is low enough, and whether the thickness is within 0.3 mm. If the sensitivity of the millimeter wave antenna decreases or the resonance peak inside the shielding case rises, the starting point for selection is the absorption performance: whether the reflection loss or attenuation of the target frequency band is sufficient, rather than the height of μ '. The evaluation indicators of the two cannot be mixed.
Secondly, in the selection of the magnetic permeability layer, place tan δ before μ '
The magnetic field energy of NFC antenna itself is limited, and a high tan δ will directly consume the signal budget. Tan δ ≤ 0.05 at 13.56 MHz is a referenceable engineering boundary; If the supplier only provides μ 'without providing the tan δ of the target frequency band, the reference value of their data for NFC scenarios is limited. The recommended tan δ for wireless charging scenarios is ≤ 0.03, and it is necessary to confirm whether Bs is sufficient to cover the working magnetic flux density at the target power.
Thirdly, control the thickness within the allowable range of the mounting structure, rather than compensating for performance with thickness
0.3 mm is a practical upper limit boundary. For ultra-thin models, the typical thickness of NFC anti magnetic patches is 0.06-0.12 mm, and wireless charging is 0.10-0.30 mm. If the performance does not meet the standard within this thickness, the correct approach is to replace the material system or adjust the magnetic circuit design, rather than thickening the patch.
Advanced Institute (Shenzhen) Technology Co., LtdThe product line of anti magnetic stickers for mobile phones covers ferrite composite sheets, nanocrystalline composite sheets, and magnetic isolation absorption composite structures. The effective magnetic permeability of the nanocrystalline series products can be customized from 150-2000, with a typical control of 150 ± 10 μ 'at 13.56 MHz, tanδ ≤0.05， Thickness tolerance ± 3 μ m, covering the range of 0.03-0.3mm; The wireless charging series μ 'is typically 850-1200, optimized for the main frequency range of 100-300 kHz.
The product supports coiled material, sheet material and precision die-cutting, and can match the material system according to the actual mounting interface and metal backplane conditions of NFC/wireless charging. For more technical information, please visithttp://www.avanzado.cn/.