Wednesday, May 23, 2007

Thick Film Materials

The range of materials available for thick-film technology is determined by their capacity to be both printed and fired. Established thick-film technology is based on three classes of material supplied in the form of printing inks. Their classification and function are determined by the resistivity, and the three original classes of conductors, resistors and dielectrics. Emerging sensor technology prescribes a whole new range of materials with desirable properties of interaction with the external world. These new materials are naturally less developed and optimized than the established ones, and they therefore tend to lack some of the more desirable properties.

An important property in manufacture is adhesion, and in commercial inks this tends to be optimized so that structure bond strongly to the substrate. In specially developed sensor inks, we sometimes have to compromise in order to achieve the desired interaction properties. The other major property is the coefficient of thermal expansion, which needs to be matched to the substrate to prevent large thermally induced stresses during manufacture.

In general, thick film structure are created by printing patterns in inks composed of an organic carrier, a low softening temperature glass frit in the form of a finely divided powder and a characterizing ingredient. For conductors, the characterizing ingredient is a finely divided noble metallic power (gold, silver, platinum or Ag/Pd). After processing, the metallic particles fuse to form continuous electrical paths through the carrier glass. Sheet resistivities of the order of 10mΩ/cm^2 are typical. In sensor applications, conducting inks have an important function in the formation of electrode patterns, which range from simple rectangular structures to interdigitated pairs. Platinum is also used for resistance thermometry and for combined heaters and thermometers in areas such as gas sensing, where control at a variety of temperatures is an important technique.

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