Comparing The Effect of Single and Dual Sintering Methods Using Silver and Copper Nanoparticle Patterns for Flexible Electronics Applications

The utilization of metallic nanoparticle (NP) ink has gained significant attention in the fabrication of cost-effective and mechanically flexible printed electronic devices, including wearables, displays, sensors, and solar cells. The choice of metallic NPs is a crucial determinant of the electrical, material, and mechanical properties of the printed patterns. Commonly used NPs for ink formulation include silver (Ag), gold (Au), and copper (Cu). Ag NP ink is highly favored for its exceptional oxidation stability and electrical conductivity. Although Au NPs also offer good electrical conductivity and oxidation resistance, their high cost makes them less desirable. Cu NPs have good electrical conductivity and cost-effectiveness, but they have lower oxidation stability. The metallic NP ink is not electrically conductive after the printing process due to steric repulsion forces acting between the particles. These repulsion forces are caused by the introduction of organic additives and stabilizing agents to the ink, preventing the NPs from agglomerating due to Van Der Waals interactions between them. To achieve electrical conductivity, a post-processing step, known as the sintering process, is necessary to decompose the additives and stabilizing agents.<br/><br/>In this study, Ag NP ink (PSI-211, NovaCentrix) and Cu NP ink (CP-008, NovaCentrix) were chosen for printing and characterizing the conductive patterns. Flexible Kapton polyimide (PI) sheets with a thickness of 0.102 mm served as the substrate. The study explored the single and dual sintering methods on the printed Ag and Cu NP patterns. In the single sintering methods, the printed NP patterns were exposed to either laser irradiation (LO) or thermal treatment (TO). For the LO sintering condition, the Ag and Cu NPs underwent Nd: YAG laser irradiation at 600 mJ for 15 s and 800 mJ for 30 s, respectively. For the TO sintering condition, the Ag and Cu NPs were placed in a formic acid (FA) environment at 140 °C for 1.5 min and 260 °C for 15 min, respectively. In the case of the dual sintering methods, one approach is to subject the printed metal patterns to thermal treatment followed by laser irradiation (TL), while the other method involves exposing the patterns to laser irradiation followed by thermal treatment (LT). The sintering parameters for TL and LT followed the same sintering conditions as LO and TO.<br/><br/>After analyzing the microstructure using the scanning electron microscope, the Ag NP patterns sintered using LT showed enhanced agglomeration and increased networking of particles through necking compared to the Ag NP patterns sintered using the other conditions. This pattern also displayed the highest roughness of 48 nm from atomic force microscopy (AFM) analysis, indicative of superior grain growth. Due to this fact, the Ag NP pattern sintered using LT demonstrated the lowest electrical sheet resistance with a value of 0.0031 Ω/Sq and the lowest resistance ratio (R/R₀) of 1.75 after the folding test. The highest hardness was found for the Ag NP pattern sintered using LT with a value of 4.28 N/mm², which contributed to a better result in the adhesion test. On the other hand, the Cu NP pattern sintered using TO showed the most uniform grain growth through agglomeration and coalescence. Additionally, due to increased connectivity between NPs, the patterns sintered under TO showed the highest mechanical hardness of 55.36 N/mm² and the lowest R/R₀ value of 10.58 after the folding test. The lowest electrical sheet resistance was observed for the Cu NP pattern sintered using LT with a value of 0.0117 Ω/Sq, which thermogravimetric analysis (TGA) showed to be due to the complete removal of organic residue from the pattern. Additionally, the Cu NP pattern sintered under LT showed the highest roughness value of 51.36 nm.