What is an NFC tag physically made of?
Three parts. A silicon chip about the size of a grain of sand. A flat coil of conductor forming the antenna, which occupies nearly the whole tag. And a carrier holding both, called an inlay before it is encased. Every physical failure a tag suffers follows from this.
Three parts
The chip is tiny. It holds the memory, the logic that answers a reader, and nothing else. It has no battery and no clock of its own.
The antenna is a flat spiral of conductor with several turns, etched from foil or printed with conductive ink. It occupies almost the entire area of the tag, which is why the outline of a tag is effectively the size of its antenna.
The carrier is a thin sheet holding both and keeping the coil flat. It contributes nothing electrically and everything mechanically.
The inlay and what encases it
Those three parts together are an inlay, and an inlay is a finished component sold as such. Everything you recognise as a tag is an inlay with something around it.
A sticker is an inlay with a face sheet and an adhesive back. A card is an inlay laminated between two sheets of plastic. A keyfob is an inlay moulded into a shell. A wooden card is an inlay embedded in a thin panel.
The encasing decides durability, printability and thickness. It does not decide compatibility or memory, which belong to the chip, or read distance, which belongs to the antenna.
Where a tag actually fails
Two places, and neither is the chip.
The connection between chip and antenna is the first. It is a bond across a very small area, and flexing the tag works it loose. A tag that has been repeatedly bent can fail here with nothing visible on the outside.
The antenna itself is the second. It is a thin conductor, and a crease across it breaks the loop. A broken loop collects no energy, so the tag is dead rather than weak, and there is no repair.
The chip almost never fails on its own. When a tag stops reading, the cause is mechanical far more often than electronic.
What this tells you before buying
That rigidity is protection. A card or a moulded fob keeps the antenna flat, and flat antennas survive.
That a tag which will be bent should be built to be bent, with a flexible carrier and an encapsulation that spreads the stress rather than concentrating it at the chip.
And that cutting a tag is not resizing it. The antenna reaches nearly to the edge, so a cut almost anywhere severs the loop and ends the tag permanently.
Frequently asked questions
Can a broken antenna be repaired?
Not practically. The conductor is thin and the geometry is tuned, so a patch changes the tuning even when it restores continuity. A tag with a broken antenna is replaced.
Why is the chip so small?
Because it does very little: store a few hundred bytes and answer a reader. Almost all of a tag's area is antenna, and the chip is a speck at one point on the loop.
Does the carrier material affect reading?
Ordinary plastics and paper are transparent to the field and change nothing. Metal in or near the tag changes everything, which is why on metal tags exist.
How can I tell if a tag is physically damaged?
Often you cannot by eye, since the break is internal. A tag that reads nothing on any phone, when an identical one reads fine, has almost certainly failed mechanically.