Find out if your phone has NFC built in.
Why are on-metal NFC tags different ?
Because NFC Tags work by electromagnetic induction, metal can easily interfere with their operation. NFC Tags can never work behind metal and if placed directly onto metal, then you need to use a special 'on-metal' tag.
The technical bit
The reason NFC Tags don't work on metal is that the metal surface behind the tag acts as a ground plane and reduces the performance of the tag's antenna, effectively stopping it creating the current needed to power the chip.
In reality, you can often get the tags to work with just a small gap (0.5 - 1 cm) and it's possible to simply use thick foam type stickers to create a gap. However, to do the job properly, you should use on-metal tags which have a ferrite foil barrier between the tag and the adhesive layer.
The ferrite foil (or similar) barrier effectively creates a 'gap' (not an actual gap, but a 'fake' physical gap in terms of electromagnetic induction) so that the antenna can function properly. The ferrite foil is only a fraction of a millimetre thick.
Can on-metal NFC tags be used on non-metal surfaces ?
The downside is that on-metal tags have a signficant and noticable performance disadvantage to their normal counterparts. It's not unusual for the scan distance to be half that of a comparable 'normal' tag placed on a non-metal surface. It's worth noting that there will be a reduction in performance of on-metal tags even when they are not placed on metal. This means that if you purchase a batch of on-metal tags for both on-metal and not on-metal use, you will get a performance reduction across all your tags.
How big a gap is a gap ?
A common question is how big a gap is OK. Under test conditions, NFC.Today tested the relative scan performance of a 29mm NTAG203 standard tag against a 29mm NTAG203 tag designed for metal surfaces.
The tests show that although a normal NFC tag doesn't function at all on a metal surface, the performance of the tag with a gap of just 3mm performs as well as an on-metal tag. With a gap of 6mm (0.6cm), the normal tag starts to perform better than an on-metal tag.
It's important to note that the performance will vary depending on the phone, the tag and the metal itself. Please use these tests as an indicator only !
What this means is that if you can create even a very small gap between the metal and the tag, then you will see an equal or even improved performance just by using a standard NFC tag. And you'll save money as well !
Metal to the sides
Generally, metal 'columns' on a single side of the tag will not affect performance providing you leave a small few millimetre gap. Metal all around the tag can affect performance - for example where a tag is placed inside a circular cut-out on a metal plate.