A technology enabling improved properties of polymer conductive pastes

The use of conductive pastes in polymer thick-film technology shows a continual increase since 1970. This technology offers the possibility to work at low temperatures on low-cost substrates. Main applications are the mass production of membrane touch keys, the via-fill of PCB's and the electromagnetic shielding. A relatively new market are smart labels. These RFID tags have the potential to replace the common barcode systems in future. So, the today's manufacturing costs have to be reduced by increasing the throughput and the use of low-cost materials. The electrical performance and therefore also the reading distance of such smart labels is mainly determined by the resistance of its antenna. This paper shows our investigations in manufacturing smart labels by screen-printed polymer conductive pastes. Compared to the common metal-etched antennae (copper, aluminum) printed antennae have only poor electrical performance. So it was necessary to increase the particle density of the conductive silver paste. Applying a DoE we have tested the effect of a compression process. The results show a substantial improving of the paste properties. Not only the area resistance is reduced by more than 70 percent. Also the reliability has been improved; the adhesion strength is increased by 2.4 times. Due to compression process at temperatures around glass transition temperature of epoxy resin the conductive particles form a closer contact to each other and are stronger interlocked within the substrate material, especially in the case of paper. In consequence the wide range of commercial silver pastes can be used to buildup antenna coils with comparable performances as metal etched smart labels.