Ohmic Curing of Three-Dimensional Printed Silver Interconnects for Structural Electronics

Ohmic curing was utilized as a method to improve the conductivity of three-dimensional (3D) interconnects printed from silver-loaded conductive inks and pastes. The goal was to increase conductivity of the conductive path without inducing damage to the substrate. The 3D via/interconnect structure was routed within 3D polymeric substrates and had external and internal sections. The 3D structures were created by the additive manufacturing (AM) process of stereolithography (SL) and were designed to replicate manufacturing situations which are common in the fabrication of 3D structural electronics that involve a combination of AM and direct write (DW) processing steps. The photocurable resins the 3D substrates were made of possessed glass transition temperatures of 75 °C and 42 °C meaning that a nonthermal method to increase the conductivity of the printed traces was needed as the conductive inks tested in this study required oven cure temperatures greater than 100 °C to perform properly. Ohmic curing was shown to decrease the measured resistance of the via/interconnect structure without harming the substrate. Substrate damage was observed on thermally cured samples and was characterized by discoloration and scaling of the substrate. Resistance measurements of the via/interconnect structures revealed samples cured by the ohmic curing process performed equal or better than samples subjected to thermal curing. The work presented here demonstrates a method to overcome the thermal cure temperature limitations of polymeric substrates imposed on the processing parameters of conductive inks during the fabrication of 3D structural electronics and presents an example of overcoming a manufacturing process problem associated with this emerging technology. An ink selection process involving characterization of the compatibility of inks with the substrate material and the use of different inks for the via and interconnect sections was also discussed.