I'm a high school student in China,half an electronics enthusiast,and I pretty intersted in your project.About 2 years ago I have seen this project.Recently, I began to refer to your project to make a water card that can flash.I have little knowledge to done it well.The school do not allow me to carry electronic devices, so I printed some official documents of NXP(About LPC8N04 DataSheet and NTAG Antenna Design Guide) and went to the school to read them in free time,sometimes in class.I wish to get your advice in some simple questions.The antenna input capacitor is 1.2pf in your schematics,and I refer to antenna desgin guide about LC equivalent circuit part is recommand use the 50pf one.I had did some calculations,results is 2-3uH inductance 1.2pf capacitor the resonance frequency is up to 100MHz,is much over the NFC resonance frequency which is 13.56MHz.I feel confused here. I hope you can reply.Thanks~

I was playing with this design and found the NFC powered stabilization capacitors are recommended to be put on "high-drive" pins.  They are 0_3, 0_7, 0_10, 0_11.  The are mentioned in the data sheet but called high-source pins therein (confusing). The Users Manual has them identified and called out consistently as high-drive.  Don't know if your board layout will accommodate a pin swap 0_1 for 0_3 or 0_7.  (10 and 11 you have committed to the break out)  You may get longer on times for your LEDs when pulled from the NFC field.

Yes, I'm aware of these pins and recommendation in datasheet. I have been experimenting with the development board before designing the PCB, and I have made a conclusion that for this application it didn't matter. The MCU is running at 500kHz with about 150uA@1.8V power consumption. Two LEDs being on consume about 1mA@1.8V, which gives about 2.1mW total power consumption. It is possible to to harvest 10 times of that with no issues. I have also prioritized aesthetically pleasing routing and breakout pins.

Nevertheless, it should be possible to swap the pins, like you have suggested. However, to store enough energy for the contnued operation after pulled away from NFC reader, the capacitance would need to be in the range of milli-Farads. It would take a lot of space/volume, and I don't know if it would work, since the capacitor of that capacitance would appear as a short for relatively long time during startup.

You may want to look at Kingbright LEDs their ultra-thin APG1608 series is 0.25mm thick in the 0603 package.  More color options too.  Don't know if voltage and current specs will fit the bill.

Hope you can get your hands on some. 

Thank you! I thought I found the thinnest. I will probably replace with these in the next version. The real challenge though is the MCU, it is 1 mm thick. 

Can the card be powered simply by your phone being near it?

Yes, if a phone has the NFC

what's the product description more exactly?  can two cards tap each other and exchange information? 

The card is passive and doesn't have any energy source. So, no, it is not possible. Works only with active NFC reader.