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When using RFID tags or NFC devices on metal surfaces, "metal resistant magnetic plates" are a common solution. The common logic in procurement and design review is to select a magnetic sheet with sufficiently high magnetic permeability, stick it between the label and the metal, and the reading distance should be restored.
The results of sample testing sometimes do not match this logic. The magnetic permeability of the magnetic sheet meets the standard, and the thickness also meets the design value. Even after being attached, the label still cannot be read, or the reading distance is much lower than expected. Re test the magnetic properties of the magnetic sheet, and there are no issues with the data. The problem lies on another level: the magnetic flux guiding path of the magnetic plate does not form a closed loop.
'Anti metal' is not 'blocking metal', it is' repairing a winding path for magnetic field lines'
The mechanism by which metal causes tag failure is eddy currents: alternating magnetic fields induce eddy currents on the metal surface, and the reverse magnetic field of the eddy currents cancels out the magnetic flux emitted by the reader, weakening the effective magnetic field obtained by the tag antenna.
The working mechanism of magnetic plates
The function of the magnetic flux sheet is not to "block" the reverse magnetic field, but to provide a low magnetic resistance bypass for the magnetic field lines. The magnetic field lines start from the tag antenna, enter the magnetic plate, follow the high magnetic permeability path of the magnetic plate to bypass the metal surface, and then "wrap back" from the other end of the magnetic plate to enter the other side of the tag antenna. The magnetic resistance of this path is much lower than that of magnetic field lines directly penetrating the metal, so the magnetic field lines preferentially pass through the magnetic sheet, weakening the eddy currents in the metal.
The key to this mechanism lies in "closure": magnetic field lines must have a complete path from the antenna back to the antenna. If this path breaks at any point - the edge of the magnetic flux sheet is raised, there is an air gap between the magnetic flux sheet and the metal, or the coverage area of the magnetic flux sheet is not enough for the magnetic field lines to "go out and go back" - the closed loop will not be established, and the magnetic permeability of the magnetic flux sheet, no matter how high, will not be able to function.
Edge lifting: the most concealed circuit breakpoint
When the magnetic sheet is attached to the metal surface or the back of the label, the edge is the most prone to warping. The reason for the uplift may be:
Three common reasons for warping
① The rigidity of the magnetic sheet itself is relatively high, and the edges cannot fully conform to the curved surface when fitting;
② The initial adhesion of the backing adhesive is insufficient, and the edges slowly rebound after bonding;
③ The metal surface has oil stains or uneven roughness, and local adhesion is poor.
After the edge is raised, a wedge-shaped air gap is formed between the magnetic plate and the metal. The magnetic resistance of this gap is much higher than that of the magnetic plate itself. When the magnetic field lines reach the edge, they "discover" a sudden increase in the magnetic resistance of the winding path. Some magnetic field lines choose to "leak" back to the metal from the gap, and the continuity of the closed circuit is disrupted. The edge curling may not be obvious during visual inspection - the height of the curling may only be a few tens of microns, which is not easy to judge with the naked eye. But at the magnetic circuit level, this air gap of several tens of microns is already sufficient to significantly increase local magnetic resistance and weaken the effective guiding ability of the magnetic plate.
Air gap: hidden cost of poor fit
In addition to edge curling, the overall bonding state between the magnetic sheet and the metal is also crucial. If the magnetic sheet is bonded to the metal surface through adhesive, and the adhesive layer itself is a polymer material with a magnetic permeability close to 1, it is equivalent to connecting a "gap" in the magnetic circuit. The thicker the adhesive layer, the greater the magnetic resistance of this air gap, and the lower the effective magnetic permeability of the magnetic sheet.
The contradiction between adhesive layer thickness and thinning
That's why the effective magnetic permeability of "double-sided adhesive bonding" magnetic sheets is usually lower than that of "direct pressing" magnetic sheets. The adhesive layer is not a good conductor of magnetic flux, it raises the interface magnetic resistance between the magnetic sheet and the metal. For thin magnetic plates (0.05-0.1 mm), the adhesive layer thickness may account for a considerable proportion of the overall thickness, and the influence of interface magnetic resistance cannot be ignored.
If there are oil stains, dust, or bubbles on the bonding surface, the thickness of the local air gap may be much greater than the thickness of the adhesive layer, resulting in higher magnetic resistance. These local gaps may appear only slightly 'not solid' in appearance, but they are significant breakpoints in the magnetic circuit.
Coverage boundary: Magnetic field lines should be able to 'go out and then go back'
The coverage area of the magnetic flux determines whether the magnetic field lines can complete the path of "going out and then going back". If the size of the magnetic sheet only covers the projection area of the tag antenna, the magnetic field lines have no path to wrap around at the edge of the magnetic sheet and can only enter the metal directly from the edge, and the closed loop is interrupted at the edge.
The empirical rule in engineering is that the area of the magnetic field should have a coverage rate (Sb/Sa) of over 90%, and each side should extend outward by 1.0-1.5 mm. The physical meaning of this margin is to leave space for the magnetic field lines to "detour" - the magnetic field lines start from the edge of the antenna and need a short distance to "turn" into the magnetic field, and then "turn" back to the antenna from the other side of the magnetic field. If the edge of the magnetic plate coincides with the edge of the antenna, this turning space does not exist, and the magnetic field lines are forced to enter the metal directly at the edge, weakening the guiding effect of the magnetic plate.
Selection and assembly judgment of anti metal magnetic plates from the perspective of "closed circuit"
Firstly, manage the "bonding state" as a part of the performance of the magnetic flux sheet, rather than assuming that it "closes when attached"
The effective magnetic permeability of a magnetic plate depends not only on the material itself, but also on the interface state between it and the metal. When selecting, it is necessary to confirm the thickness and modulus of the backing adhesive, as well as whether the bonding process can ensure continuous contact between the magnetic sheet and the metal. For curved surface mounting or magnetic plates with high rigidity, special attention should be paid to the adhesion of the edges.
Secondly, check the raised edges and gaps after assembly, rather than just checking if they have been attached
A few tens of microns of edge lifting is enough to break the closed circuit, but it is not easy to judge with the naked eye. You can lightly press the edge with your fingers to feel if there is any rebound, or use a thin feeler gauge to check the edge gap. For high demand scenarios, the local magnetic performance at the edge of the magnetic flux sheet can be measured after bonding to confirm whether the guiding ability of the edge area meets the standard.
Thirdly, the coverage area should leave a "detour margin" instead of just covering the antenna
The rule of a coverage rate of 90% and a margin of 1.0-1.5 mm on each side essentially leaves room for magnetic field lines to turn. If space permits, the margin can be biased towards 1.5 mm; if space is extremely limited, at least 90% coverage should be ensured, and it should be confirmed whether the magnetic lines at the edges can still complete the detour.
Fourthly, try to avoid thick adhesive layers and air gaps between the magnetic sheet and the metal as much as possible
If the application allows, priority should be given to using thin adhesive layers or direct compression to reduce interface magnetic resistance. Clean the metal surface before fitting, remove oil stains and dust; Roll from the center to the edge during fitting to eliminate bubbles. For the double-sided tape scheme, the influence of the adhesive layer thickness on the effective magnetic permeability should be confirmed, and if necessary, the interface magnetic resistance can be compensated by increasing the thickness of the magnetic conductive sheet.
Advanced Institute (Shenzhen) Technology Co., LtdThe anti metal magnetic sheet product line covers ferrite composite sheets and nanocrystalline composite sheets, providing effective magnetic permeability data at different back adhesive thicknesses, as well as evaluating the impact of edge adhesion on guiding performance.
The product supports matching the coverage area with the backing adhesive system based on the size of the tag antenna and the metal surface conditions, helping engineers convert "material permeability" into "effective guidance capability under closed loop". For more technical information, please visithttp://www.avanzado.cn/.