| This paper involves the materials, process and reliability of the flip chip packaging of the passitive RFID (radio frequency identification) tag inlays. The work is a part of the 863 project—The Packaging Equipments and Technologies for RFID Tag Inlays Manufacture. The main contents include the controllable preparation of silver powders and the study of their formation mechanism, the development of the formulation of the anisotropic conductive adhesive (ACA) for RFID flip chip package, the package processing of the RFID tag inlays, and the reliability study and the failure mechanism analysis of the RFID tag inlays.Ag powders with different morphologies were prepared by the wet-chemical methods. Using the silver nitrate as the Ag source and the iron (Ⅱ) sulfate heptahydrate as the reductant, adding the appropriate additives, such as sulfuric acid, maleic acid, citric acid and polymer stabilizer, with one or more of them, or not add any additives, and giving several sets of the reaction parameters, the micro-sized Ag particles were prepared and their sizes were controllable. The shapes of the Ag particles were various. It included the spherical, the hexagonal with a bump on the center, the flat hexagonal and flat triangular and the flowerlike. Besides, a kind of uniform ball Ag powders were also prepared by mixing the ascorbic acid solution (reductant), the diluted sulfuric acid (stabilizer) and the silver nitrate solution (Ag source). It is concluded that:The silver sulfate is a suitable Ag resource to prepare the micro-sized spherical Ag particles by wet-chemical methods; the residual anionic groups of the inorganic and organic polybasic acids which the H ions are dissociated can attach on the (111) facet of Ag crystals preferentially. It promotes the anisotropic growth of Ag crystals. The Ag dendritic crystals were prepared by mixing the silver nitrate solution with the fresh uniform Cu microcrystals, which are synthesized by reducing the copper (Ⅱ) sulfate pentahydrate with the ascorbic acid at the presence of maleic acid at the room temperature. It is considered that the growth process of dendritic crystal is as follow:The diffusion process of Ag nuclei is controlled by the kinetic factors in the non-equilibrium state and the Ag nuclei aggregate spontaneously to form the dendritic crystals.A type of intermediate temperate curing ACA was prepared by mixing the spherical micro-sized Ag powder and the epoxy-based resin. The optimal formula is:Bisphenol A diglycidyl ether,100 part, Ag powder,44.2 part, plasticizer,10 part, filler,4 part and latent curing agent,36.6 part. It has a capability of curing at 150-180℃in 15-20 sec, and is tailored for the RFID flip chip packaging.The kinetics equation of the cure reaction of ACA was accessed by using the differential scanning calorimetry (DSC) curves at the different heat rates. The relationships between the curing time, curing temperature and curing degree were forecasted by the curing kinetic model of ACA. The optimal parameters of the automatic packaging equipment for manufacturing the RFID tag inlays with the flexible Al/PET antennae are:Curing time,12.5 sec, curing temperature,170℃, hot press pressure, 4.5MPa, the temperature difference between the upper and lower hot-press heads,60℃(The temperatures of the upper and lower hot-press heads are 200℃and 140℃respectively) and the amount of dispersed adhesive is 1.0 sec (The dispersing pressure, 0.04 MPa)The reliability of the RFID tag inlays packaged with the above ACA was studied. After the high-temperature and humidity(HHT,85℃,85%RH) aging test for 336 hours, 80% of the RFID tag inlays with Al/PET antennae can be read. The maximum response distance of them were about 80% of the initial value, although the DC resistance of bonding joints shifted. However, the RFID tag inlays by assembling the chips on the rigid modules (134.2 KHz) can be read after the same aging test. This implies that the RFID tag inlays packaged on the rigid modules had a higher reliability. The high-temperature and humidity test had a great impact on the contact resistance of ACA bonding joints of the RFID tag inlays with Al/PET antennae. The contact resistance increased with the increase of the aging time. It is inferred that the failure mechanism of the RFID tag inlays with Al/PET antennae is the oxidation of Al pads. It is also found that the cure degree of ACA increased with the increase of the aging time. This postcure process is beneficial to the reliability improvement of RFID tag inlays. In a word, the prepared ACA can be applied in the packaging of RFID tag inlays with flexible and rigid antenna substrates.
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