Laser curing of silver-based conductive inks for in situ 3D structural electronics fabrication in stereolithography (2014)

A hybrid stereolithography (SL)/direct print (DP) system has been developed and previously described that fabricates three-dimensional (3D) structural electronic devices in which component placement, interconnect routing, and system boundaries are not confined to two dimensions as is the case with traditional printed circuit boards (PCBs). The resulting increased level of design and fabrication freedom provides potential for a reduction in both volume and weight as well as the capability of fabricating systems in arbitrary and complex shapes as required to conform to unique application requirements (e.g., human anatomy, airframe structures, and other volumetrically-constrained mechanical systems). The fabrication of these devices without intermediate part removal between SL and DP processes requires in situ curing of DP-dispensed silver-based inks to sufficiently cure the inks prior to continuing additional SL fabrication. This paper describes investigations into the laser curing process using the two laser wavelengths, 355 nm and 325 nm, which have been used in commercial SL machines. Various laser curing parameters, including energy (laser power and scan speed), scanning location, and laser wavelength were investigated. The trace resistances and structural changes in the SL substrate and printed trace were compared for each experiment to determine the most preferred laser ink curing method. Furthermore, oven curing of partially laser cured ink traces was investigated as a means for minimizing the number of in situ laser passes required to embed ink traces during SL fabrication. The laser curing process was repeated for a wide variety of conductive inks, having different structure, composition, and curing properties to determine if certain inks were more responsive to laser curing and if the ink curing results could be generalized. A statistical study was conducted under the hypothesis that laser curing of inks at 325 nm wavelength would be better, due to lower silver reflectance, as compared to 355 nm wavelength. Results indicated that particulate silver based conductive inks can be successfully cured in situ using SL lasers with various laser curing parameters. Curing ink traces at high laser power and slow scan speeds with the laser beam located on the substrate adjacent to the ink channel resulted in the most effective ink curing but resulted in discoloration of the ink and/or charring of the SL substrate. Comparatively, when laser power was reduced sufficiently to eliminate the charring, lower effective ink curing was achieved. Irradiating the laser directly on the ink did not damage the ink or the substrate, while providing low trace resistances, and represents the most viable laser curing alternative to achieve acceptable trace resistance without charring the SL substrate. The results further indicated that partial laser curing of ink using a reduced number of laser passes with subsequent oven cure seem to work effectively and may decrease overall manufacturing time. SL lasers with low power (<100 mW) may not be effective for curing conductive inks that have high viscosities and require high curing temperatures. Finally, a statistical study determined that 325 nm is more effective for direct curing of the ink as compared to the 355 nm laser.