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One of the most familiar problems for hardware engineers is that NFC tags cannot be read when attached to metal surfaces. The common explanation is that 'metal blocks the magnetic field', but this description is not accurate. Metal does not "block" the magnetic field, it induces eddy currents in the alternating magnetic field, and the reverse magnetic field generated by the eddy currents cancels out the magnetic flux sent by the reader/writer. If it's just 'blocked', thickening the magnetic separator can solve it; But the actual failure is much more complex than that.
The impact of metal backplate on NFC antennas is ultimately reflected in one parameter: quality factor Q.
Why does the Q value under the metal backplate collapse
NFC operates at 13.56 MHz and belongs to near-field magnetic field coupling. The antenna itself is an inductive coil, with a quality factor Q=ω L/R, which measures the ratio of energy storage to energy consumption of the coil. The higher the Q value, the stronger the frequency selection ability of the antenna, and the higher the energy transmission efficiency between the reader and the tag.
When the NFC antenna is close to a good conductor such as copper or aluminum, an alternating magnetic field induces eddy currents on the metal surface. At 13.56 MHz, the skin depth of copper and aluminum is only about 18-25 μ m, and the eddy current height is concentrated on the metal surface, forming a magnetic field opposite to the direction of the antenna magnetic field. This reverse magnetic field brings three consequences: the equivalent inductance L of the antenna decreases, the equivalent series resistance R increases, and the resonant frequency shifts simultaneously. When the three factors are combined, the Q value drops sharply from 20-50 in free space to 5-10 or even lower, and the reading distance drops from 3-8 cm to 0-1 cm.
In other words, the metal backplate does not simply "weaken" the magnetic field, but simultaneously destroys both the inductance and resistance parameters of the antenna. If the magnetic separator is optimized for only one parameter, it often cannot restore the Q value to a usable level.
The job of a magnetic separator is not to 'shield', but to reconstruct the magnetic flux path
The core mechanism of the magnetic separator is to insert a high magnetic permeability material between the antenna coil and the metal backplate, providing a low magnetic resistance "bypass" for the magnetic flux. Magnetic field lines preferentially pass along the high μ 'path of the magnetic separator, rather than entering the metal surface to excite eddy currents. After the eddy current is suppressed, the reverse magnetic field weakens, and both the decrease in L and the increase in R of the antenna are alleviated, resulting in the recovery of Q value.
This mechanism has an easily misunderstood aspect: the magnetic separator does not "seal" the magnetic field on one side of the antenna, but changes the spatial distribution of the magnetic flux. The ideal magnetic separator should allow as much magnetic flux as possible to pass through itself and as little as possible to enter the metal, while minimizing its own loss of magnetic flux (μ ") to avoid consuming energy during the" guiding "process.
Therefore, the performance of NFC magnetic separators can be divided into two independent dimensions: μ 'determines how much magnetic flux can be' pulled away 'from the metal surface, and μ' 'determines how much energy is lost during the pulling process. The two jointly determine the degree of Q value recovery, but their directions of action are not consistent.
Engineering benchmark for Q-value recovery: Triangular constraints of μ ′, μ ″, and tan δ
TDK's IFQ06 series provides a clear engineering reference: at 13.56 MHz, μ′≈56，μ″≈2， The corresponding Q=μ ′/μ ″ ≈ 28. This Q value is close to the level that NFC systems can use, and its μ 'is not high - only 56, far below the hundreds or even thousands of nominal values of many wireless charging separators.
This data illustrates an important fact: NFC magnetic separators do not require extremely high μ ', they require sufficiently low μ' '. μ '=56 can already provide effective magnetic flux guidance, but if μ' increases from 2 to 20, the Q value will drop to less than 3, and the antenna will still not function properly.
This is also the watershed in the selection logic of NFC magnetic separators and wireless charging magnetic separators. Wireless charging operates at 110-205 kHz, where the weights of hysteresis loss and eddy current loss are relatively balanced. The magnetic flux constraint benefits brought by high μ 'can partially offset the increase in losses; NFC operates at 13.56 MHz, and the weight of eddy current losses sharply increases. Any approach that sacrifices μ "for μ 'will result in disproportionate costs in terms of Q value.
Actual measurement data: marginal benefits of thickness and Q value recovery
Under the testing conditions of 13.56 MHz NFC tag and aluminum plate surface, the 0.05 mm magnetic separator increased the antenna Q value by 18% and the reading distance by 32%; 0.10 mm increases Q value by 41% and read distance by 50%; 0.20 mm increases Q value by 52% and reading distance by 65%; 0.30mm increases Q value by 55% and read distance by 68%; 0.50 mm increases Q value by 56% and read distance by 70%. The marginal benefit of Q-value recovery significantly narrows after 0.2 mm, while further increase in thickness sacrifices flexibility and assembly adaptability. For most HF/NFC applications, 0.10-0.20 mm is a better balance point between performance and structure.
The division of material systems: the "natural low loss" of ferrite and the "process compensation" of nanocrystals
Ferrite occupies the mainstream position of NFC magnetic separators at 13.56 MHz, fundamentally due to its extremely high resistivity (on the order of 10 Ω· cm), and eddy current losses inside the material can be almost ignored. The main sources of μ "are hysteresis loss and residual loss, which can be achieved at a very low level. The microsecond ≈ 2 of IFQ06 is a typical performance of ferrite systems in this frequency band.
However, the μ 'of ferrite is constrained by the Snoek limit and typically does not exceed 250 at 13.56 MHz, with limited adjustable magnetic flux guidance capability. For scenarios that require a higher μ 'to cope with thicker metal backplates or smaller antenna areas, the adjustment space of ferrite will reach its upper limit.
The μ 'of nanocrystalline materials can be much higher than that of ferrites, and the Bs is also higher (about 1.2 T), but it faces a structural obstacle: extremely low resistivity (about 130 μ Ω· cm), and if left untreated at 13.56 MHz, eddy current loss will become the dominant term for μ', and the Q value cannot meet NFC requirements. Therefore, the prerequisite for nanocrystals to enter the NFC scene is not that "μ 'is high enough", but that "the eddy current is first pressed down".
Engineering methods include magnetic fragmentation and high-temperature micro oxidation. Magnetic fragmentation divides the continuous strip into small, mutually insulating fragments, cutting off the closed path of eddy currents over a large area; High temperature micro oxidation forms a dense oxide film on the surface of the strip, further increasing the surface resistance. The cost of these two processes is the decrease in effective μ '- the fragment gap forms the equivalent air gap in the magnetic circuit, and the effective μ' of the finished nanocrystalline magnetic separator usually falls in the range of 150-800, much lower than the initial value of the strip.
This means that the selection of nanocrystalline NFC magnetic separators cannot only be based on how high the "tape" is, but must also consider whether the fragmented finished product has been compressed to a usable level at 13.56 MHz. A poorly designed nano chip with broken magnetic parameters may have a strip size of tens of thousands, but the finished product size is still high, and the Q value recovery effect is not as good as a simple ferrite sheet.
The following table compares the engineering characteristics of two material systems in NFC magnetic separators:
Selection and judgment of NFC magnetic separators based on Q value
Firstly, use the μ "and Q values at 13.56 MHz as the first screening criteria, rather than μ '
Require suppliers to provide data on the target frequency points of μ ', μ', and tan δ, rather than just providing magnetic permeability at 100 kHz or 1 MHz. The engineering boundary that can be referred to is: μ "should be controlled at the single digit level as much as possible, and tan δ should not exceed 0.1. A scheme with μ '=200 and μ'=4 may outperform the scheme with μ '=600 and μ'=40 in terms of Q value recovery. If the supplier cannot provide loss data at 13.56 MHz, its applicability to NFC scenarios cannot be determined.
Secondly, distinguish between "tape μ '" and "effective μ' and Q values after bonding with the antenna"
The inductance design of NFC antennas relies on the actual loading effect after the magnetic separator is attached to the back of the antenna. When selecting, the supplier should be required to provide the finished product status, Q-value changes measured on the NFC antenna structure, and reading distance data, rather than the initial μ 'on the tape specification sheet. The measured data shows that there is a strong correlation between Q-value improvement and read distance improvement, and Q-value recovery is a more direct engineering indicator than "magnetic permeability value".
Thirdly, according to the conditions and space constraints of the metal backplane, make system level trade-offs between ferrite and nanocrystals
If the metal backplate is thin, the antenna area is sufficient, and the space allows for flat mounting, ferrite magnetic separators with natural low μ "and mature cost structures are usually a more reliable choice. If the metal backplate is thick, the antenna area is limited, or it needs to be mounted on curved/narrow frame structures, the high μ 'and flexibility advantages of nanocrystals become the decisive factors - but only if the supplier's fragmentation and insulation process can compress the μ' at 13.56 MHz to a Q-value usable level. When selecting, it should be confirmed whether the supplier provides process correspondence data between fragment size, oxidation treatment, and finished product μ ", rather than just the final nominal value.
Advanced Institute (Shenzhen) Technology Co., LtdThe NFC magnetic isolation sheet covers two routes: ferrite composite sheet and nanocrystalline composite sheet. Ferrite composite sheets can achieve a microsecond ≥ 120 at 13.56 MHz with a thickness of 0.1 mm; The nanocrystalline composite sheet controls eddy current loss through magnetic fragmentation and insulation treatment, and the effective μ 'of the finished product can be adjusted within the range of 150-800, supporting 0.03-0.2 mm thickness and curved surface mounting.
The product can provide Q value recovery and reading distance improvement data after fitting the NFC antenna structure, and match the material system according to the metal backplane conditions and space constraints. For more technical information, please visithttp://www.avanzado.cn/.