Analysis of Flip Chip Bonding for Performance Stability of UHF RFID Tags

Thermocompression bonding is one of the most important procedures in radio frequency identification tag fabrication because the electrical and mechanical properties of the joint greatly influence the overall tag performances. The characteristics of the joint are determined by the alignment between the microchip and antenna pads, the adhesives used, and various bonding parameters (temperature, pressure, and time). This paper uses an electrical equivalent circuit analysis, electromagnetic full-wave simulations, and environmental tests to investigate the factors that influence the reading performance of the flip chip bonding process. First, based on an equivalent circuit analysis of the bonded tag, the effect of the parasitic impedance on impedance matching and power transfer is analyzed. The parasitic impedance is then estimated by comparison with the measured impedance of the bonded chips. Second, we investigate the variation in the reading performance of the tag when various geometrical errors are generated at the joint between the microchip and the antenna. Finally, the bonding parameters for stable reading performance are studied by completing environmental tests. The results provide useful information for achieving stable reading performance of tags in flip chip bonding.