Synthesis, formulation and application of copper-based conductive pastes and inks

by Sze Kee Tam

Institution: Hong Kong University of Science and Technology
Year: 2016
Keywords: Printing ink; Materials; Printed electronics; Flexible printed circuits; Nanoparticles
Posted: 02/05/2017
Record ID: 2131845
Full text PDF: http://dx.doi.org/10.14711/thesis-b1584428


Conductive inks and pastes are widely used for fabricating printed electronic components and devices. Conductive tracks are formed by screen-printing and inkjet printing. Copper-based conductive inks are poised to replace the traditional silver inks in some applications because of its low cost and high conductivity. This thesis aims at developing copper-based inks and pastes for rigid and flexible substrates. There are four achievements. First, a synthesis method has been developed to prepare high-quality copper nanoparticles in large quantities. The reaction concentration was as high as 1 M and the recovery yield was over 95%. The size of the copper nanoparticles could be controlled within the range of 12 to 99 nm. The surface polarity of the particles could be modified from hydrophilic to hydrophobic. Next, the synthesized copper nanoparticles in combination with micron-sized copper flakes were formulated as a new bimodal copper particle paste. A film printed with this paste could be sintered at a temperature as low as 120ºC. The formation of cracks caused by sintering could be suppressed. The optimal composition of the screen-print paste was determined to be 20 wt% copper flakes and 80 wt% copper nanoparticles, leading to a volume resistivity of 28 μΩ cm for a sintered film. In addition to the bimodal paste with copper particles, a paste was developed using copper precursors as active ingredients, which was subsequently reduced to copper metal after printing. Such a paste can print a film with a sheet resistance of 39 mΩ/sq after sintering without hydrogen gas. This is equivalent to a volume resistivity of 21 μΩ cm, which is compatible with the copper particle paste. Finally, a systematic procedure for developing conductive inkjet ink for use in different applications has been formulated. First, product attributes of the ink are defined. The qualitative product attributes are then converted to quantitative product specifications for the specified printing conditions, the so-called product use conditions. Next, the ingredients are selected using the models and methods in the procedure to meet the required product specifications and product use conditions. Then, the ink is prepared and printed to evaluate the product performance. An aqueous copper nanoparticle inkjet ink was developed to illustrate the procedure.