Pang Lin

Effects of Metallo-Organic Decomposition Agents on Thermal Decomposition and Electrical Conductivity of Low-Temperature-Curing Silver Paste

Six low-temperature-curing silver pastes were prepared from silver flake, α-terpineol and various metallo-organic decomposition (MOD) compounds. The thermal decomposition behaviors of the pastes were determined. The microstructures and resistivities of screen-printed films on alumina substrate after thermal treatment were characterized and discussed. Results indicated that 2-ethylhexanoate possesses the lowest decomposition temperature (190.3 °C) among the MOD agents studied, and it forms silver particles to promote the linking of silver flake powders and thus reduces the resistivity to <13 µΩcm at a temperature as low as 200 °C.

Characterization of the low-curing-temperature silver paste with silver 2-ethylhexanoate addition

In this study, the effects of the solvent and atmosphere on the thermal decomposition behaviors of silver 2-ethylhexanoate and α-terpenol are investigated. Low-curing-temperature silver pastes made from Ag flakes, α-terpineol and various amounts of silver 2-ethylhexanoate, were prepared and characterized. The microstructures and resistivities of cured films screen-printed from the pastes were examined. The results of thermal analysis in oxidizing and reducing atmospheres revealed that thermal decomposition is the dominating reaction during the heating process of silver 2-ethylhexanoate, even though the differential scanning calorimetry (DSC) result revealed an exothermic reaction for silver 2-ethylhexanoate heated in air due to oxidation. Thermal decomposition left almost pure Ag particles, which is beneficial for bridging between silver flake particles in the films. On the basis of the rheological behavior, microstructural evolution and electrical evaluation, it can be concluded that a low-curing-temperature silver paste with 5 wt % silver 2-ethylhexanoate addition is the best formulation, which possesses shear-thinning and thixotropic properties and a resistivity of 7.8×10-6 Ω cm after being cured at 250 °C, which is relatively close to the bulk resistivity of Ag.

Effects of Silver Oxide Addition on the Electrical Resistivity and Microstructure of Low-Temperature-Curing Metallo-Organic Decomposition Silver Pastes

The thermal decomposition of silver paste with the addition of a metallo-organic decomposition (MOD) compound generally requires a curing time of greater than 10 min and a curing temperature greater than 250 °C, which does not meet the requirement for high-speed production in flexible substrates. In this study, attempts to modify the curing conditions of MOD silver pastes through the substitutions of silver flakes with silver(I) oxide (Ag2O) and silver(II) oxide (AgO) were performed. Differential thermal analysis (DTA), derivative thermogravimetric analysis (DTG), and X-ray diffraction (XRD) results indicated that the presence of residual silver oxide, which effectively catalyzes the evaporation of α-terpineol and the decomposition of silver 2-ethylhexanoate, decreases the curing temperature and shortens the soaking time. The reduced silver and the remaining Ag2O enhance the connectivity and packing density of the silver flakes, and thus increase the electric conductivity of the films. For films prepared from pastes with 20 wt % Ag2O or AgO, resistivities of 14×10-6 and 19×10-6 µΩcm, respectively, were successfully achieved after being cured at 200 °C for 5 min.

Microstructure and Electrical Resistivity of Low-Temperature-Cured Silver Films Prepared Using Silver Oxide and Silver Stearate Pastes

In this study, paste formulations containing silver oxide coated with a metallo-organic decomposition (MOD) agent of silver stearate were prepared without using any silver powders or silver flakes. Results indicate that all pastes appear to have a pseudoplastic flow property that is acceptable for roll-to-roll printing and screen printing. The pastes were screen-printed on an alumina substrate and then thermally treated in a range of temperatures. The lowest electrical resistivity of 13.2 ×10-6 Ωcm was obtained for the film prepared from paste with a Ag2O/silver stearate ratio of 100:5 at a solid loading of 80 wt % in the solvent α-terpineol, after being cured at 160 °C for 5 min, which meets the requirements of low-temperature and high-speed manufacturing for practical applications. The low resistivity of the film is facilitated by the combination of Ag2O and silver stearate added to the paste. Ag2O produces a high density of silver matrix after being reduced at low temperatures, and the presence of silver stearate contributes to the rheological behavior of the paste after dissolution in the solvent. Coexistence of Ag2O and silver stearate induces their simultaneous transformation to the silver form at temperatures below 160 °C.

High-Frequency Electrical Properties of Silver Thick Films Measured by Dielectric Resonator Method

The electrical properties of silver films, prepared using a low-curing-temperature metallo-organic-decomposition (MOD) paste and a high-temperature silver paste screen-printed on polished and nonpolished alumina substrates, at microwave frequency were characterized in this study. Surface resistance and effective conductivity of the silver films at microwave frequency (approximately 4 GHz) were evaluated using the TE011 mode of the resonator cavities method. Devices of T-type resonator circuits were fabricated to determine the simulated and measured Q-values and to evaluate the effects of silver films and the surrounding substrate. The surface roughness of the fired films printed on nonpolished Al2O3 substrate is slightly less than those on polished substrate, because the surface energy of the nonpolished alumina (29.81 mN/m) is slightly less than that of the polished alumina (36.69 mN/m). The calculated effective conductivities at 4.3 GHz are slightly less than the DC conductivities of the films. Moreover, the films prepared using the high temperature silver paste have higher electrical conductivity ranging from 4.08 ×107 to 4.13 ×107 S/m, since the high-temperature firing process leads to an improved connectivity of the silver particles. The results indicate that the films screen-printed on the polished substrate have a higher Q and a lower ΔQ value than those of films that are screen-printed on the nonpolished substrate. For the silver films prepared using the high-temperature silver paste, both the Q and ΔQ values were the highest among the films studied, which is consistent with the observation of the dense microstructure of the silver film and the interfacial reaction between the glass in the film and the substrate as a result of high firing temperature.