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In the selection review of NFC anti metal magnetic separators, engineers usually present a material specification sheet to compare μ ', μ', and tan δ. This set of parameters determines how much magnetic flux the material can guide and how much energy it consumes at 13.56 MHz. But when applying these data directly to specific models, a common confusion is that the same magnetic separator has good read distance recovery on the aluminum frame of Project A, but is almost ineffective in front of the stainless steel battery compartment of Project B.
The problem is not with the magnetic separator itself, but with the material difference of the metal backplate. The conductivity and skin depth of different metals at 13.56 MHz differ by an order of magnitude, leading to changes in the intensity and distribution of eddy currents. As a result, the magnitude of the "reverse magnetic field" that the magnetic separator needs to counteract also varies.
Conductivity and skin depth: two determining variables of eddy current intensity
NFC operates in the near-field magnetic field coupling region at 13.56 MHz. The metal backplate induces eddy currents in an alternating magnetic field, and the reverse magnetic field generated by the eddy currents cancels out the magnetic flux emitted by the reader/writer. The strength of eddy currents depends on two material properties: conductivity and skin depth.
The conductivity of copper is about 58 MS/m, and the skin depth at 13.56 MHz is only about 17 μ m; The conductivity of aluminum is about 37.7 MS/m, and the skin depth is about 22 μ m. Both belong to good conductors, with eddy currents highly concentrated on the metal surface and strong reverse magnetic fields. Their "impact level" on NFC is classified as Severe. The conductivity of stainless steel (304) is about 1.4 MS/m, the skin depth is about 113 μ m, the eddy current density is low, and the impact level is High rather than Severe.
This means that in front of the copper or aluminum backplate, the magnetic isolation plate faces a "strong eddy current source" that requires a sufficiently high μ 'to pull the magnetic field lines away from the metal surface, while requiring an extremely low μ "to avoid consuming the limited energy of the NFC antenna during the guidance process. In front of the stainless steel backplate, the eddy current intensity itself is relatively low, and the absolute value of μ 'required for the magnetic isolation sheet can be appropriately reduced, but the control requirements for losses have not been relaxed - because the magnetic field energy budget of NFC antennas is still limited, any additional μ' 'will directly erode the reading distance. The impact level of carbon steel and titanium is also High, with skin depths of approximately 42 μ m and 88 μ m, respectively. This set of data indicates that the "NFC threat level" of metal backplates is not solely determined by whether they are metal or not. The conductivity is one order of magnitude lower, the skin depth is several times greater, and the magnitude of eddy currents and the design window of magnetic separators will also shift accordingly.
Why does the same magnetic separator exhibit differentiation in front of different metals
The working state of the magnetic separator in front of the copper/aluminum backplate can be understood as a "strong adversarial" mode: strong eddy currents and strong reverse magnetic fields require the magnetic separator to provide a sufficiently high μ 'to maintain the magnetic flux path, while using an extremely low μ "to maintain the Q value. If the μ 'of the magnetic separator is at a moderate level (such as μ' 50-80), it may be just enough in front of the aluminum plate, but there may be a waste of "excessive design" in front of the stainless steel plate - the eddy current itself is not strong, but the thickness and coverage area of the magnetic separator occupy the stacking space.
On the other hand, if a magnetic isolation sheet is optimized for stainless steel backplate (with medium μ 'and thin thickness) and placed in front of a copper or aluminum backplate, its magnetic flux guiding ability may not be sufficient to suppress strong eddy currents, and the recovery effect of the reading distance may be compromised. This is not a matter of whether the material is good or bad, but rather a mismatch between the magnetic flux capacity of the material and the eddy current intensity of the metal backplate.
Industry data provides clear indications for this. In the typical design window of NFC magnetic separators, μ′ 80–150、tanδ≤0.05、 A common starting point is a thickness of 0.05-0.20 mm. However, the "starting point" of this window is established under specific metal backplane conditions. If the backplate is made of high conductivity materials such as copper or aluminum, it may need to be biased towards the upper edge of the window in terms of μ 'and thickness; If the back panel is made of stainless steel or titanium, the lower edge of the window may already be sufficient. When selecting, do not confirm the material of the metal backplate, only compare the μ 'value of the magnetic separator, which is equivalent to choosing a weapon without knowing the strength of the opponent.
NFC and wireless charging share: metal backplane is only the first constraint, frequency band separation is the second
Inside the phone, the NFC antenna often shares the same space in front of the metal back panel as the wireless charging coil. The wireless charging Qi standard operates at 100-300 kHz, while NFC operates at 13.56 MHz, with a difference of about two orders of magnitude between the two frequency bands. The magnetic separator needs to provide magnetic flux guidance for wireless charging in the low frequency range, while maintaining low loss at 13.56 MHz to protect NFC signals.
The magnetic sheet design disclosed in the patent literature provides a reference for balancing multiple frequency bands: approximately 100-300 μ 'at 3 MHz, approximately 80-270 at 6.78 MHz, and approximately 60-250 at 13.56 MHz. This set of data indicates that the μ 'of the same magnetic material decreases step by step at different frequencies, and the μ' at 13.56 MHz is naturally lower than the low frequency range. If the magnetic separator is excessively designed for the low-frequency high μ 'requirements of wireless charging (thick magnetic layer, high filling), at 13.56 MHz, its μ' may synchronously rise, creating an additional burden on the Q value of NFC.
When NFC and wireless charging share a magnetic separator, the influence of the metal backplate material will be amplified. Copper/aluminum backplates are more sensitive to both low-frequency and high-frequency eddy currents, and magnetic separators need to manage eddy currents and losses in both frequency bands simultaneously; The eddy current pressure of stainless steel backplate is relatively low, but the low loss requirement of NFC frequency band has not been reduced. When selecting, "metal backplate material" and "whether it is shared with wireless charging" should be confirmed as two independent constraints, rather than assuming that "a single magnetic separator can access all metals and all frequency bands".
Thickness and coverage area: 0.3 mm and engineering boundary exceeding the antenna by 5 mm
The material of the metal backplate determines the strength of eddy currents, while the thickness and coverage area determine how much "resistance" the magnetic separator can provide.
The reference boundary given by industry practice is that a thickness of 0.1-0.3 mm for the magnetic separator is sufficient in most NFC anti metal scenarios, and a multi-layer structure of 0.5-1 mm can be used in harsh metal environments; The coverage area should be at least as large as the NFC antenna, and exceeding 5mm on each side can help improve the shielding effect. These two boundaries are not a monotonic relationship of 'bigger is better'. An increase in thickness will occupy the stacking space, and an increase in coverage area will change the distribution of the medium around the antenna, which may pull the resonant frequency. In front of the copper/aluminum backplate, a deviation of 0.2-0.3 mm and moderate outward expansion are reasonable starting points; Before the stainless steel backplate, 0.1-0.15 mm may already be sufficient, and further thickening may not necessarily result in a proportional improvement in reading distance.
Type selection judgment based on the condition of metal back plate
Firstly, confirm the material and conductivity of the metal backplate, and then discuss the μ 'and thickness of the magnetic separator
The skin depth of copper, aluminum, stainless steel, carbon steel, and titanium at 13.56 MHz varies from 17 μ m to 113 μ m, and their impact on NFC ranges from Severe to High. When selecting, the actual material of the backplate (including grade, significant difference in conductivity between 304 stainless steel and carbon steel) should be confirmed with the structural team, and this should be used as the starting point for the magnetic flux capacity of the magnetic separator, rather than using the word "metal" to generalize.
Secondly, in front of the high conductivity backplate (copper/aluminum), priority should be given to ensuring that the μ ″ is sufficiently low before pursuing the μ ′
In a strong eddy current environment, the magnetic separator requires sufficient μ 'to guide the magnetic flux, but if μ' 'is too high, the guided energy will be consumed inside the material, and the Q value cannot be restored. Tan δ ≤ 0.05 is a reference engineering boundary, and based on this, select μ 'and thickness.
Thirdly, in the scenario where NFC and wireless charging share a magnetic separator, confirm the loss data at 13.56 MHz, rather than just looking at low-frequency performance
The optimized magnetic separator for wireless charging may perform well at 100-300 kHz, but the μ "at 13.56 MHz may not be suppressed to the level suitable for NFC. When selecting, the supplier should be required to provide both low-frequency and 13.56 MHz microsecond data, and confirm the superimposed effect of the metal backplate material on the eddy current conditions in both frequency bands.
Fourth, use thickness and coverage area as variables for "matching metal backplate strength" rather than fixed values
A reasonable starting point for copper/aluminum backplate is 0.2-0.3 mm and an outward expansion of 5 mm on each side; A thinner solution may already be sufficient for the stainless steel backplate. The final determination of thickness and coverage area should be based on the measured data of the NFC reading distance and Q value of the entire machine, rather than just the peak reflection loss on the material specification sheet.
Advanced Institute (Shenzhen) Technology Co., LtdThe NFC anti metal magnetic isolation sheet product line covers ferrite composite sheets and nanocrystalline composite sheets. The material system can be matched according to the material (copper, aluminum, stainless steel, etc.) and conductivity conditions of the metal backplate, providing microsecond and microsecond frequency band data at 13.56 MHz, as well as read distance recovery and Q value change data under different thickness and coverage area schemes.
The product supports a thickness range of 0.03-0.5mm and precision die-cutting, suitable for multi band loss assessment in NFC and wireless charging scenarios. For more technical information, please visithttp://www.avanzado.cn/.