Research on the contact resistance and reliability of flexible RFID tag inlays packaged by anisotropic conductive paste

In this work, ACA was prepared by mixing micro-sized spherical Ag particles and latent curing agent into thermo-set epoxy resin, and RFID flip chips were assembled on the Al/PET, printed Ag/PET and printed Ag/Paper antennae through hot-press bonding process. The contact resistance and shear bonding strength before and after the reliability tests (hot humidity test, 85 °C, RH 85%), and degradation mechanism of ACA interconnection for flip-chip-on-flex (FCOF) assembly were studied using modified RFIC and the three kinds of antennae mentioned above. It was found that the contact resistance changed after the reliability test, it was caused by the total results of oxidation of Al/PET antennae and conductive particles, mismatch of coefficient of thermal expansion (CTE) between the ACA adhesive, antennae and flip-chips and post curing of resin. And the bonding strength also affect by the further curing of paste, strain release accumulated in resin and the microstructure change caused by moisture absorption during the reliability test. It was concluded that it was benefit to improve the reliability of FCOF assembly packaged by ACA by introducing the post curing process. And it was suggested that selecting the anti-oxidation conductor and the anti-heat substrate of antennae could decrease the shift of contact resistance, which was especially favored for ultra-high frequency RFID tag. Therefore, the printed Ag/Paper antenna was preferred to large scale, cheap and rapid manufacturing RFID tags.