No abstract

Low-temperature air-fireable glass-free metallic thick-film electrical conductor materials were developed for interconnections in electronic packaging. The thick film with composition (by weight) 96.60%Ag, 1.38%Cu, 0.28%Al, 0.35%Ti, and 1.39%Sn used Ti-Al as the active binder. After firing in air at 500∞C it exhibited low electrical resistivity (6.2 ¥ 10 -6 W·cm), good scratch resistance and strong bonding with the alumina substrate, with no pinholes. The firing caused complete melting of the particles in the film. Firing in argon rather than air degraded both electrical and bonding properties, due to the absence of oxygen, which helped to burn out the vehicle. The use of Ti rather than Ti-Al as the active binder resulted in holes in the thick film due to incomplete melting of the Ti-rich particles and also resulted in poor scratch resistance and weaker bonding to the substrate. Tin in the composition was important for promoting melting and protecting the active particles from oxidation during firing.

The burnout of the organic vehicle in a silver-particle, glass-free, electrically conductive, thick-film paste during firing in air was studied. For a vehicle consisting of ethyl cellulose dissolved in ether, burnout primarily involves the thermal decomposition of ethyl cellulose. The presence of ether with dissolved ethyl cellulose facilitates the burnout of ethyl cellulose. Excessive ethyl cellulose hinders the burnout. A high heating rate results in more residue after burnout. By interrupting the heating at 160°C for 15 min, the residue after subsequent burnout is diminished probably because of reduced temporal overlap of the processes of organic burnout and silver particle necking. By interrupting the heating at either 300°C or 385°C for 30 min, the temperature required for complete burnout is reduced. The addition of silver particles facilitates drying at room temperature and burnout upon heating.

An air-fireable, glass-free, electrically conductive thick-film material (96.6% Ag, 1.38% Cu, 0.28% Al, 0.35% Ti, and 1.39% Sn by weight) and a conventional glass-containing, electrically conductive thick-film material (96.6% Ag and 3.4% glass frit by weight), both on alumina substrates, were studied by electrical, mechanical, thermal, and microscopic methods. The volume electrical resistivity of the glass-free thick film (2.5 ϫ 10 Ϫ6 Ω·cm, 30-µm thick) is lower than that of the glass-containing thick film (3.9 ϫ 10 Ϫ6 Ω·cm, 19-µm thick), with each film processed at its optimum firing temperature. The optimum firing temperature is 930°C and 850°C for glass-free and glass-containing thick films, respectively, as indicated by the criteria of low resistivity and high scratch resistance. The glass-free thick film has a higher scratch resistance than the glass-containing thick film, both fired at their respective optimum temperatures, suggesting that the former has higher bond strength to the alumina substrate. The formation process of the glass-free and glass-containing thick films is similar. The process involves solid-state diffusion of silver, which results in a silver network and grain boundaries. However, the sintering of silver particulates in the glass-containing thick film is enhanced by the viscous flow of glass.

Air-firable glass-free metal electrically conductive thick film pastes of different compositions were developed by using a titanium alloy component, and tin and zinc metal substitutes for glass frit used in traditional thick film pastes. The effect of different components on the electrical resistivity and bonding between the thick film and the alumina substrate was investigated. Thick films with low electrical resistivity and good bonding to the alumina substrate were obtained by using silver, zinc, tin, and TiCu alloy powders in the pastes. The addition of zinc at a small proportion (<0.5 wt.%) to a thick film paste enhanced the adhesion between the thick film and the alumina substrate with negligible increase in the electrical resistivity. The use of titanium alloy powder instead of pure titanium powder is preferred. Better composition distribution, and consequently, better wetting and bonding are expected by using active metal particles of a smaller size.