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A common misconception in the selection of flexible ferrite absorbing plates is to treat "flexibility" as a purely additional property: the material is still the same ferrite, but it is made into a bendable shape.
Actually, it's not like that. The transformation of ferrite from sintered blocks to flexible sheets involves a fundamental change in material morphology. NiCuZn ferrite particles are dispersed into the epoxy or rubber matrix, and there are no longer continuous grain boundaries formed by sintering between particles. The transfer of magnetic flux between particles depends on the particle contact and the magnetic dilution effect of the matrix. This change not only endows the material with bendability, but also redefines its loss mechanism and frequency response. Understanding the cost of flexibility can better predict actual performance than simply comparing magnetic permeability values.
How Flexibility Changes the Path of Magnetic Loss
The magnetic loss of sintered ferrite mainly comes from hysteresis loss and natural resonance, and the high resistivity makes eddy current loss negligible. After being made into flexible composite sheets, the particles are isolated by polymers, and the magnetic coupling between particles is weakened. The effective magnetic permeability is usually lower than that of sintered bodies with the same composition. On the other hand, the introduction of polymer matrix changes the dielectric constant and eddy current path of the sheet, making the composition of losses more complex.
Taking the NiCuZn ferrite epoxy composite system as an example, its electromagnetic response is attributed to the combined effect of macroscopic magnetic loss and dielectric loss, involving domain wall resonance, spin relaxation, and dipole polarization. This means that the loss of flexible ferrite sheets is no longer a single magnetic loss channel, but a mixture of magnetic loss and dielectric loss. In the near-field frequency band of 13.56 MHz, magnetic loss remains the dominant mechanism, but in higher frequency bands, the weight of dielectric loss may increase and the real part of magnetic permeability may rapidly decay.
The proportion of fillers directly affects this balance. At a ferrite/epoxy volume ratio of 0.55, an 8mm thick sample achieved a maximum return loss of -48 dB at 4.6 GHz. This data indicates that flexible ferrites can still achieve considerable absorption in the GHz frequency band, but it also reveals a constraint: effective absorption often requires a thickness that matches the frequency, and in space limited scenarios such as mobile phones, the thickness is usually compressed to the order of 0.1-0.5 mm, at which point the magnetic loss strength of the material at the low frequency end is the decisive variable.
Advantages and Costs of Flexible Sheet Material Loss: Referring to 13.56 MHz
In the 13.56 MHz scenario of NFC and RFID, the mainstream product form of flexible ferrite sheets is thin sintered ferrite sealed between PET film and pressure-sensitive adhesive layer, with a typical thickness as low as about 0.05 mm and a magnetic permeability of 13.56 MHz covering the range of 130-200. The advantage of this type of material lies in its high Q-factor and low magnetic loss, which can provide a low magnetic resistance path for magnetic flux to bypass the metal backplate without significantly consuming the limited magnetic field energy of the NFC antenna.
But the other side of 'low loss' is' low absorption'. The main function of flexible ferrite sheets at 13.56 MHz is magnetic conductivity and resistance to metal interference, rather than converting electromagnetic energy into heat. TDK's IFL04 series is clearly positioned as a "high-Q low loss magnetic shielding material" with extremely low magnetic loss, used to stabilize antenna resonance and isolate metal interference, rather than broadband absorption. This means that if engineers misinterpret the low μ "in the NFC frequency band as" insufficient absorption capacity "and instead pursue higher loss formulas, it will actually consume the NFC signal itself, leading to a decrease in read distance.
True loss type flexible ferrites are more commonly found in EMI suppression scenarios in the GHz frequency band. In this frequency band, the design goal of materials has shifted from "low loss magnetic conductivity" to "controllable loss absorption". The PVDF composite flexible film reinforced with BaCo ₂ Fe ₁₆ O ₂₇ nanosheets achieved a shielding efficiency of 35.94 dB at a thickness of 0.2 mm, with an absorption contribution of 29.85 dB, much higher than reflection. The dielectric loss tangent reached 1.63 and the magnetic loss tangent was 0.26. This set of data indicates that the absorption capacity of flexible ferrite composite systems in the GHz frequency band can be established through dielectric magnetic synergistic losses, but its loss composition is fundamentally different from that of low loss magnetic plates at 13.56 MHz.
Thickness: The most easily misused variable in flexible sheets
There is no monotonic "thicker, better" relationship between the thickness and performance of flexible ferrite absorbing plates. In a single-layer metal backplate structure, the return loss exhibits frequency dependent interference characteristics as the thickness changes. The C-band flexible sandwich composite film achieves a minimum reflection loss of -54.64 dB at 6.144 GHz with a total thickness of 1.13 mm, but the effective absorption bandwidth of the same material at a thickness of 1.62 mm can be extended to 9.268 GHz, covering the entire C-band and extending to the S and X bands. The increase in thickness expands the bandwidth, but both peak frequency and peak intensity change simultaneously.
In the low-frequency near-field scenarios of NFC and wireless charging, the direction of thickness is different. Thin flexible ferrite sheets (on the order of approximately 0.05 mm) are already able to provide sufficient magnetic flux guidance, and the marginal benefits of increasing thickness rapidly decrease while occupying the already tight stacking space. Translating the "thickness for bandwidth" logic of the GHz absorption scenario to the 13.56 MHz magnetic field scenario is one of the most common misalignments in the selection of flexible ferrites.
Selection judgment based on triangular constraints
The selection of flexible ferrite absorbing plates essentially involves finding matching points between frequency bands, loss mechanisms, and flexibility costs.
Firstly, determine whether the target frequency band corresponds to "magnetic permeability" or "absorption"
The core requirement for 13.56 MHz NFC and low-power wireless charging (5 W and below) is low μ ", high-Q magnetic flux guidance, rather than high loss absorption. In this scenario, the evaluation criteria should be the consistency of μ ', the low level of μ' ', and the thickness tolerance, rather than the peak reflection loss. EMI suppression in the GHz frequency band is the opposite of radar absorption, requiring materials to have a usable loss tangent in the target frequency band. The absorption component in reflection loss or shielding effectiveness is the core indicator.
Secondly, confirm the magnetic dilution boundary of the filler under flexible constraints
The introduction of flexible matrix inevitably reduces the effective magnetic permeability. Ordinary ferrite has a μ 'of about 80-120 in the low frequency range, which has decreased to 15-30 at 2-6 GHz; The nanocrystalline composite system can maintain higher μ 'in the same frequency band, but the cost and process control difficulty increase. The selection of flexible ferrite sheets should not be based on the magnetic permeability of the sintered body, but on the effective magnetic permeability of the finished sheet at the target frequency band and actual thickness, and the corresponding relationship between the filler volume ratio and μ 'should be confirmed with the supplier.
Thirdly, consider bending reliability as an independent verification term, rather than assuming that "flexibility equals no cost"
Flexible ferrite sheets undergo tension and compression when adhering to curved surfaces, and the interface between particles and matrix may experience micro damage. Whether the magnetic properties are maintained after repeated bending depends on the matrix modulus, filler ratio, and sheet thickness. When selecting, suppliers should be required to provide data on magnetic permeability retention or Q-value changes under specific bending radii, rather than only accepting qualitative descriptions of "flexibility".
Advanced Institute (Shenzhen) Technology Co., LtdThe flexible ferrite absorption plate product line covers two routes: 13.56 MHz NFC/RFID anti metal magnetic plates and GHz band EMI absorption plates.
The NFC series is mainly composed of thin ferrite polymer composite structures, with a thickness that can cover the order of 0.05 mm. It can provide μ 'and μ "data at 13.56 MHz and maintain performance after bending; The GHz absorption series can adjust the filling system and thickness according to the target frequency band, supporting roll, sheet, and precision die-cutting. The product can be matched with a material system based on specific frequency bands and flexible constraints. For more technical information, please visithttp://www.avanzado.cn/.