무연계 도전성 페이스트의 미세구조와 전기저항에 미치는 실버 및 프릿 입자 크기의 영향

Abstract

Particle size effect of silver and glass-frit on the microstructure and electrical resistivity of lead-free conductive paste

Han, Hyun Geun Advisor : Prof. Lee, Jong-Kook, Ph.D. Dept. of Advanced Materials Engineering, Graduate School of Chosun University.

Thick film technology has been used for various electronic devices such as HIC (Hybrid Integrated Circuit) and MLCC (Multilayer ceramic capacitor). Thick film of metal-glass-organic composite i.e, conductive paste has been used to produce printed circuits because of repeatability in the electronic industry. Conductive paste consists of metal filler, glass-frit and organic binder. Silver powder as a metal filler has been attracted recently because silver has a good electrical conductivity, a relatively high melting point, a high resistance from oxidation, excellent solderability and a relatively low price. Thus, the silver powder has been used as raw materials of conductive ink and paste, and adhesive for various electronic devices. PbO-based glass-frit has been used to improve solderability of metal filler. However, PbO is dangerous to health and environment. Thus, Pb-free glass-frits for examples, Bi2O3, B2O3 and P2O5 glass-based frits have replaced PbO glass-frit. In this study, the effects of silver and glass-frit particle size on the microstructure and electrical resistivity of lead-free conductive paste were investigated. Pb-free silver paste was prepared from two commercial silver powders (1.60 μm and 0.80 μm in size) and synthetic powders (0.20 μm and 0.05 μm) by a chemical reduction method. The silver paste was prepared by mixing the monomodal silver powder, lead-free glass-frit and organic binder in the weight ratio of 70 : 3 : 27, respectively. Silver thick films were fabricated by a screen printing on the alumina substrate, and the films were sintered from 400°C to 500°C for 15 min. The characteristics of silver thick film was changed by the particle size of starting silver powder. As silver particle decreases in size, packing density of the thick films increases. But, in the case of using nano-sized particles (0.05 μm), sintered density was reduced by the agglomeration of silver nano-particles. High networking and density of silver particles are very important to obtain low electrical resistivity of silver paste. In this experiment, silver paste with bimodal silver powders was prepared in order to improve packing density at low sintering temperature, which consists of the mixture of micro- and nano-sized particle (1.6 μm and 0.20 μm in size). In order to fit nano-Ag particle into the interstices of the micro-Ag particles (1.6 μm in size), the size of small silver particles have to control at the range approximately below 360 nm. The silver paste prepared by mixing of the bimodal silver powders, Pb-free glass-frit and vehicle. As increasing the amount of nano-Ag particles and sintering temperature, the sintered silver thick film show higher packing density and electrical conductivity. The nano-Ag particles between the micro-Ag particles, may be contribute to improve interconnectivity and densification of the silver thick films. The effects of glass-frit size on the microstructural evolution and electrical resistance of conductive silver paste were investigated. Silver paste was prepared by mixing of 70 wt% commercial silver powder with 1.6 μm, 3 wt% Bi based glass-frit and 27 wt% organic vehicle. Glass-frit changed from 3.0 to 0.5 μm in size. The small glass-frit was easily melted at low sintering temperature and then, melted glass-frit was rapidly spread between the silver particles, which induced the dense networking among silver particles and strong adhesiveness to substrate. The silver film with small glass-frit sintered at 500°C has low electrical resistivity because of small pore-size and low porosity. Consequently, improvement of microstructural connectivity by densification resulted in excellent conductivity of the sintered thick films. We obtained the silver film having minimum electrical resistivity of 2.5 μΩ.cm by using 0.20 μm of silver particle, and this result can be useful for applications in various electronic devices and electrodes..