Self-alignment of RFID dies on four-pad patterns with water droplet for sparse self-assembly

This paper reports an in-depth study of a water-droplet-assisted self-alignment technique that self-aligns radio frequency identification (RFID) dies on four-pad patterns. The segmented structure of four hydrophilic pads on a hydrophobic substrate brings freedom to the design of the electrical functionality and the surface functionality. The paper investigates the influence of the key parameters that may affect the self-alignment in theory and experiment. The theoretical model justifies that RFID dies can be reliably aligned on the segmented four-pad pattern even when the initial placement error is as large as 50% of the size of the die and the gap between the four pads is about 10% of the size of the die. A method has been introduced to estimate the sufficient droplet volume for self-alignment. A series of experiments have been carried out to verify the results of the model. The experiments indicate that the self-alignment between the 730 x 730 mu m RFID dies and the pattern occurs reliably when the releasing bias between the RFID die and antenna is less than 400 mu m for patterns with 50 and 100 mu m gaps, and successful self-alignment is possible even with greater bias of 500 mu m.