Why don't NFC tags work on metal?

NFC tags do not contain any power source. They get their power from the reader - such as a mobile - phone using electromagnetic induction. Electrical current inside a coil or loop of wire inside the reader/phone generates a magnetic field. This magnetic field radiates from the back of the phone and generates a current in the coil or loop of wire - the 'antenna' - of the NFC tag.

If you place tags onto metal, then the metal acts as a ground plane. It dissipates the electromagnetic field preventing any current to be created in the NFC antenna.

To get an NFC tag to work on metal, you need to either create a large enough physical gap or use a high permeability material such as ferrite between the NFC tag and the metal. This ferrite sheet effectively reflects the magnetic field back around the antenna giving the effect of physical distance between the tag and the metal. However, it's not completely effective and all on-metal tags perform worse than standard tags. Not placing a tag on metal is usually the best option.
Seritag - 04 Mar 2023