Thick film material is a coating or slurry formed by uniformly suspending one or more solid particles (0.2 ~ 10 micron) in a carrier. In order to facilitate printing and forming, the slurry must have proper viscosity and thixotropy (the property that viscosity changes with external force). Solid particles are the components of thick films, which determine the properties and uses of thin films. The carrier decomposes and escapes during sintering. The carrier contains at least three components, resin or polymer binder, solvent and surfactant. Binders provide basic rheological properties for mud; The solvent dilutes the resin and then volatilizes to dry the printed pattern; The activator makes the solid particles wet by the carrier and disperse properly in the carrier.
According to the properties and uses of thick film, there are five kinds of pastes used: conductor, resistor, dielectric, insulation and packaging paste.
The conductor paste is used for manufacturing thick-film conductors, and interconnects, multilayer wiring, microstrip lines, welding areas, thick-film resistor terminals, thick-film capacitor plates and low-resistance resistors are formed in thick-film circuits. The welding area is used to weld or paste discrete components, devices and external leads, and sometimes it is also used to weld a metal cover to realize the packaging of the whole substrate. Thick film conductors have different uses, and no paste can meet the requirements of all these uses, so a variety of conductor pastes should be used. The same requirements for conductor paste are high conductivity, firm adhesion, aging resistance, low cost and easy welding. The metal components in common conductor paste are gold or gold-platinum, palladium-gold, palladium-silver, platinum-silver and palladium-copper-silver.
In thick film conductor paste, there are not only metal powders or metal organic compounds with appropriate particle sizes, but also glass powders or metal oxides with appropriate particle sizes and shapes, and organic carriers for suspending solid particles. Glass can firmly bond metal powder to the substrate, forming a thick film conductor. Commonly used alkali-free glass, such as borosilicate lead glass.
Thick film resistor is a kind of thick film component with the earliest development and the highest manufacturing level in thick film integrated circuits, which can manufacture all kinds of resistors. The main requirements for thick film resistors are high resistivity, low temperature coefficient of resistance and good stability.
Like conductor paste, resistor paste has three components: conductor, glass and carrier. But its conductor is usually not a metal element, but a compound of a metal element or a compound of a metal element and its oxide. Commonly used pastes are platinum-based, ruthenium-based and palladium-based resistor pastes.
Thick film dielectric is used to manufacture miniature thick film capacitors. Its basic requirements are high dielectric constant, low loss tangent, high insulation resistance, high voltage resistance, stability and reliability.
The dielectric paste is made of low melting point glass and ceramic powder particles uniformly suspended in an organic carrier. The commonly used ceramics are titanate ceramics of barium, strontium and calcium. By changing the relative content of glass and ceramics or the composition of ceramics, dielectric thick films with various properties can be obtained to meet the needs of manufacturing various thick film capacitors.