ST25TN01K-AFH5: can I connect a wire coil directly to the AC0/AC1 pads without a PCB?

[Adityaaaaaaa]

Can I attach a hand-wound enamelled copper wire coil (about 0.05 to 0.1 mm wire) directly to the AC0 and AC1 pads of the UFDFPN5 package, with the chip mounted pads-up and the wire joined using solder or conductive epoxy, and no PCB? Is there any ST guidance or reliability concern with this?

[ozammo]

There seems to be no documented guidance or method to do this without the use of a PCB. I would strongly recommend only doing this method if you absolutely have to or if it is purely for prototyping purposes.

 

If you really do have to connect the coil without a PCB, make sure the  wires, coil , everything is anchored and held in place as you introduce a mechanical risk by not using a PCB. 

 

Could you please provide more light on why you want to do this?

 

 

[Adityaaaaaaa]

I want to develop an ultra-small NFC tag for jewellery, such as solid gold rings, necklaces, and pendants. I am considering the ST25TN01K-AFH5, which has a chip package of approximately (1.7 × 1.4 mm).

My goal is to minimize the size of both the chip and antenna so the total size of the device will be around 1-2mm only . I want the antenna to be as compact as possible, with minimal wire length, and integrated directly into the jewellery's shape—for example, following a ring's circumference or a pendant's contours. I am not restricted to spiral, circular, or rectangular designs and am open to any antenna geometry that works.

Can I connect a custom antenna directly to the chip's AC0 and AC1 pads without a PCB? How small can the antenna realistically be while remaining readable by a standard NFC-enabled smartphone, and how would solid gold affect its performance?

[ozammo]

Intriguing project. My first thought is you could design a super compact flexible PCB, JLCPCB is a good supplier of this. Secondly, your questions about gold affecting performance of an antenna would likely be better suited on a dedicated RF/NFC forum however here is the extent of my knowledge.

 

Gold is conductive so yes I am certain it would affect performance somehow, it will attenuate or possibly nullify your signal. However a quick google search lets me find some “on-metal” NFC tags/antenna that has built-in barriers to prevent signal issues when in close proximity to metal. 

 

My suggestion is, design a flexible PCB very small and connect the AC0 and AC1 pads to the PCB and in turn a dedicated “on metal” antenna. This way means you can follow usual guidance, keep it small and flexible and have it easily manufacturable and repeatable if your end goal is a commercial product. 

 

I would not want to give you advice on antenna design as that is not my ball park and I expect not something relevant to this forum, I suggest asking those questions on a specific RF forum. However have a look at SMD antennas as you can get super small ones for use on metal: https://uk.rs-online.com/web/p/rfid-antennas/0690916

 

I personally would go down the small flexible PCB route and design the antenna on the PCB itself, lots of tools around to help with this especially in KiCad. Though that small antenna I found on RF does look intriguing.