Microwave Sintering of Silver Nanoink for Radio Frequency Applications.

Microwave sintering is a promising method for low-temperature processes, as it provides advantages such as uniform, fast, and volumetric heating. In this study, we investigated the electrical characteristics of inkjet-printed silver (Ag) circuits sintered by microwaves. The microstructural evolutions of inkjet-printed Ag circuits sintered at various temperatures for different durations were observed with a field emission scanning electron microscope. The electrical properties of the inkjet-printed Ag circuits were analysed by electrical resistivity measurements and radio frequency properties including scattering-parameters in the frequency range of 20 MHz to 20 GHz. The experimental results show that the signal losses of the Ag circuits sintered by microwave heating were lower than those sintered by conventional heating as microwave heating led to granular films which were nearly fully sintered without pores on the surfaces. When the inkjet-printed Ag circuits were sintered by microwaves at 300 °C for 4 min, their electrical resistivity was 5.1 µΩ cm, which is 3.2 times larger than that of bulk Ag. Furthermore, microwave sintering at 150 °C for 4 min achieved much lower signal losses (1.1 dB at 20 GHz) than conventional sintering under the same conditions.

Electrical and electrochemical migration characteristics of Ag/Cu nanopaste patterns.

Since direct printing technology has developed intensively, low-cost fabrication and reliability have become critical challenges for mass production of printed electronic devices. The silver/copper (Ag/Cu) nanopaste was manufactured by Ag nanopaste mixed with different proportions of Cu nanoparticles ranging from 0 to 5 vol.% in order to investigate the influences of Cu content on the electrical properties and electrochemical migration (ECM) characteristics. The patterns were constructed on a glass wafer via screen printing with the Ag/Cu nanopaste. They were then annealed through debinding for 30 min in air followed by sintering for 30 min in a hydrogen atmosphere at various temperatures (150, 200, 250, and 300 degrees C). The electrical resistivity of printed patterns that were sintered at 150 degrees C grew with increases in the percentage of Cu content in the Ag/Cu nanopaste, while printed patterns that were sintered at 300 degrees C show similar electrical resistivity values of around 2-3 µΩ cm regardless of Cu content. The ECM characteristics of the printed patterns were evaluated by performing a water drop test. The printed patterns that were sintered at higher temperatures showed longer ECM times. At 300 degrees C, the ECM time was considerably lengthened when the Cu content was over 2 vol.%, and the 5 vol.% Cu pattern showed the longest ECM time of 305 s, which was around 1.65 times that of the Ag pattern.

Effect of atmospheric-pressure plasma treatment on the adhesion characteristics of screen-printed Ag nanoparticles on polyimide.

We investigated the adhesion characteristics of screen-printed silver (Ag) tracks on polyimide (PI) treated by atmospheric-pressure plasma (APP). Oxygen plasma was applied to the PI surface, and the APP-treated surface was exposed to air for various periods of time in order to evaluate the sustainability of the APP treatment. The adhesion of the Ag/PI interface was measured using a roll-type 90 degrees peel test. The peel strength was dramatically increased by the APP treatment, but the strength decreased by around 62.7% when the APP-treated PI surface was exposed to air for 2 h. The peeled PI surface showed ductile fracture immediately after the APP treatment; however, after 2 h of exposure to air, the fracture behavior returned what was observed before the APP treatment. To analyze the deterioration of adhesion, the interface between the printed Ag track and the APP-treated PI was investigated physically and chemically. The surface morphology became rougher after the APP treatment, but the roughness slightly decreased after being exposed to air for 2 h. X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical bonding of the printed Ag and the PI interface. XPS analyses show that the concentration of oxygen-containing groups decreased as the exposure time to air increased.

Screen-printed Cu circuit for low-Cost fabrication and its electrochemical migration characteristics.

Circuit pitch has decreased due to the demand for high-performance and multi-functional electronic devices. This trend has increased the risk of short-circuit failures by electrochemical migration (ECM), which is the transportation of ions between the cathode and anode under electrical potential. While direct printing has emerged as a promising technology in terms of manufacturing cost and environmental issues, there are few studies about ECM in directly printed copper (Cu) nanopaste. We prepared screen-printed comb-type Cu patterns on a Si wafer with various sintering temperatures (200, 250, 300, 350 degrees C). ECM characteristics of the printed Cu were determined by water drop testing under various electrical potentials (3, 6, 9 V). The microstructures and the roughness profiles of the pattern surfaces were identified with field emission scanning electron microscopy (FE-SEM) and a three-dimensional surface profiler, respectively. While the electrical potential increased from 3 V to 9 V, the time to failure (ECM time) required for dendrites to grow from the cathode to the adjacent anode decreased by 63.0%. On the other hand, the ECM time increased by 205.1% when the sintering temperature increased from 200 degrees C to 350 degrees C. FE-SEM micrographs and energy-dispersive X-ray spectroscopy analysis of dendrites showed a mixture of trunk and lace types, which were mainly composed of Cu.

Characterization of Ag-Pd nanocomposite paste for electrochemical migration resistance.

Direct printing such as inkjet, gravure, and screen printing is an attractive approach for achieving low-cost circuitry in the printed circuit board industry. One of the challenges for direct printing technology, however, is the poor resistance to electrochemical migration (ECM), especially for silver (Ag) which has been widely used in printed electronics. We demonstrate improved resistance to Ag electrochemical migration by adding palladium (Pd) nanoparticles to the Ag nanopaste. Conductive comb-type patterns were fabricated on a bismaleimide-triazine substrate via screen printing. Their ECM characteristics were assessed by water drop test with deionized water. These results showed that the ECM time required for dendritic growth from cathode to anode to cause short-circuit failure was affected by the Pd content and applied voltages: the ECM time of Ag-15wt.% Pd nanopaste was nearly threefold that of Ag nanopaste, and the ECM time decreased by 94.22%, on average, while the applied voltage increased from 3 V to 9 V.