Session: 04-23-01: Mechanics and Materials of Soft/Flexible/Stretchable Electronics

Paper Number: 111946

111946 - Fabrication of Conductive Patterns by Laser Irradiation and Thermal Treatment of Silver Nanoparticle Inks for Flexible Printed Electronics 

Printed electronics have gained significant attention in various fields due to their cost-effective and eco-friendly fabrication process and unique mechanical properties, such as being lightweight, flexible, and foldable. This technology involves the direct additive deposition of an ink material onto various flexible substrates, simplifying the fabrication process of electronic devices. Conductive ink is a critical component for printed electronics and can be patterned through printing processes to achieve robust channels with high conductivity. Different types of conductive inks, including those based on silver (Ag), copper (Cu), gold (Au), graphene, carbon, and conductive polymers, have been explored for printed flexible electronics. Among them, Ag-based inks are preferred due to their cost-effectiveness over Au-based inks, superior stability over Cu-based inks, and higher conductivity than polymer or carbon-based inks. This technology finds application in wearable devices, microheaters, solar cells, implantable medical devices, batteries, and radio frequency identification (RFID) tags.

In this study, the conductive patterns were fabricated and characterized using Ag nanoparticle (NP) ink (PSI-211, NovaCentrix). The ink had the following compositional properties: a solids content of 42±2 wt%, a viscosity of 3500-6000 cP at 10 s^-1, an acidity of 5.80±0.05 pH and a density of 1.6 g/ml. Kapton polyimide (PI) sheets with a thickness of 0.102 mm were used as the flexible substrate. Metal ink comprises of metal NPs, capping molecules, additive organic materials, and solvents. In order to achieve high electric conductance of the Ag NP patterns, organic substances must be removed, and diffusion of particles leading to neck development and grain boundary is necessary. Thus, the sintering process is required with optimal sintering conditions such as atmosphere, temperature, and time. Six different carboxylic acid vapor groups including mono-, di-, and tri-carboxylic acids as well as air and N₂ were used as sintering atmospheres. The sintering temperature and time were 140 °C and 1.5 min, respectively. Prior to thermal sintering, laser irradiation using Nd:YAG laser was performed at a wavelength of 1064 nm with a laser power of 600 mJ for 15s.

The microstructure of the sintered Ag NP patterns was analyzed using scanning electron microscopy (SEM). Agglomeration and coalescence were observed in all atmospheres, but the formic acid vapor atmosphere chosen from monocarboxylic acids showed less porosity and a high amount of necking. Thermogravimetric analysis (TGA) revealed a maximum weight loss of 7.62% for the Ag NP patterns sintered in the air. The chemical bonds of the samples were investigated using Fourier-transform infrared spectroscopy (FTIR), which identified four chemical bonds: C-H stretching, C-O stretching, and C-H bending modes assigned at wavenumbers of 2961, 1255 and 1052, and 788 cm^-1 respectively. A four-point probe was utilized to measure the sheet resistance of the Ag patterns, and the Ag NP pattern sintered in formic acid vapor atmosphere demonstrated the lowest sheet resistance of 0.0015 Ω/sq. The patterns sintered in formic acid vapor and air atmospheres showed the highest and lowest hardness with their respective values of 4.38 and 0.64 N/mm². Finally, the adhesion and flexing or bending capabilities of the Ag patterns were tested using ASTM-D3359-09 standard cross-cut tape test and folding test, respectively. Adhesion strength was measured as 4B for the Ag NP pattern sintered in formic acid vapor atmosphere. The folding test was performed up to 50 cycles, and the electrical resistance ratio before and after the folding test was compared for all sintering conditions. The Ag NP patterns sintered in formic acid vapor and air atmospheres exhibited the lowest and highest resistance ratio of 1.565 and 8.578, respectively.

Presenting Author: Rajib Chowdhury University of Louisiana at Lafayette

Presenting Author Biography: Rajib Chowdhury is currently enrolled for a Ph.D. in System Engineering with a concentration in mechanical engineering at University of Louisiana at Lafayette. He completed his bachelor degree in mechanical engineering from Khulna University of Engineering and Technology, Bangladesh in 2015. In 2021, he joined the Multifunctional Materials and Devices Lab at ULL. After joining the lab, he worked with printed and flexible electronics along with applications. He also did research in deposition and analysis of dielectric thin films.

Authors:

Rajib Chowdhury University of Louisiana at Lafayette
Justin Courville University of Louisiana at Lafayette
Seonhee Jang University of Louisiana at Lafayette