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

Abstract

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.

Combined Inkjet Printing and Infrared Sintering of Silver Nanoparticles using a Swathe-by-Swathe and Layer-by-Layer Approach for 3-Dimensional Structures

“…As a consequence, to facilitate the sintering process, the printed pattern is typically transferred to another system equipped with an energy source, such as an oven, infrared (IR), electrical, , laser, microwave, , and photonic sintering . Transferring the printed pattern to another system for sintering can be time-consuming and complicated when multiple layers of the same pattern are required to be printed onto each other (i.e., building in the vertical, Z direction).…”

Printable Materials for the Realization of High Performance RF Components: Challenges and Opportunities

“…Additionally, the adhesion of DW materials on AM surfaces postsintering is also not well understood and warrants further research [24]. As buried metal on dielectric applications, such as through lines and 3D vias in multilayer/multimaterial printing, becomes more relevant, then nonthermal curing methods will also need to be further developed such as electrical sintering to obtain near-bulk σ [25].…”

Sintering strategies for inkjet printed metallic traces in 3D printed electronics

“…Ohmic sintering is principally an exploitation of electrical current across a conductive trace to produce ohmic heating to cure selectively printed traces. An essential processing step prior to ohmic curing is a pretreatment step to achieve a relatively conductive state [14,15]. The results reveal that for our ink/substrate combination an initial conductivity of at least 0.15% of bulk silver was required, which was facilitated by either IPL pre-sintering or thermal pre-heating.…”

“…as a pre-sintering step for ohmic and microwave sintering. Moreover, implementing multiple sintering steps as suggested before [15,21] revealed promising improvement for rapid sintering techniques, in which a gradual evaporation of the solvent and successively sintering of the NPs is promoted. The results of the current study also indicate that by applying multiple sintering steps with gradual increase in the intensity of input energy, higher conductivity and less failure were enabled.…”