Design of anti-metal NFC tags

Why design anti-metal NFC tags?

Due to the limitations of the usage scenario, we need a small NFC tag that can be used on metal surfaces,

and the card reading distance must be more than 30mm when the size of the read head antenna and tag is less than 40 mm*25 mm.

After searching the market and purchasing several samples, tests revealed that none of the products met the requirements. In this case, you can only consider designing it yourself.

Why can’t NFC tags be used on metal surfaces?

NFC tags work at a frequency of 13.56MHz and mainly rely on electromagnetic field coupling to transmit energy. This process is like a transformer,

in which the reader antenna coil is equivalent to the primary winding,

and the antenna coil of the NFC tag is equivalent to the secondary winding.

NFC tag working diagram

When the tag antenna is close to a metal surface, an eddy current will be induced because the magnetic flux of the antenna passes through the metal surface.

According to Lenz’s law, the eddy current will generate a reverse electromagnetic field,

which will offset the electromagnetic field of the antenna, causing the magnetic field on the metal surface to rapidly Attenuation,

so that the data reading distance between the reader and the electronic tag will be seriously affected, and misreading or reading failure may even occur.

How to solve this problem?

1) The usual approach is to stick a high-permeability magnetic sheet made of ferrite alloy powder and a binder where the label contacts the metal to isolate the electromagnetic field from being transmitted to the metal surface, thereby reducing the eddy current generated on the metal surface.

Antenna interference; however, this approach will significantly change the inductance of the tag antenna,

causing the frequency point to shift, and the parameters of the tag antenna need to be readjusted.

NFC tag structure design diagram

2) The tag antenna is isolated from the metal surface by a certain distance through the design of the mechanical structure to reduce the impact of the chaotic electromagnetic waves generated by tiny eddy currents on the metal surface of the antenna. Because this eddy current energy is usually very small, it attenuates very much in space. Fast, usually, a distance of 10mm will produce good results.

In actual testing, we found that a single improvement measure could not meet the design specifications, so we designed a label shell to combine them.

The picture below is our 3D-printed test sample. We designed an 8mm thick double-layer packaging box to seal the label in.

The label antenna is on a layer far away from the metal surface, and the magnetic shielding material is on a layer close to the metal;

in practical applications. , we will use epoxy resin to fill the gaps to achieve a certain degree of anti-oil and anti-wear effects.

Anti-metal NFC tags testing sample

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