3 must-know advances in thin-film resistor technology

Thin film resistor technology has come a long way since metallised glass and ruptured carbon films were first offered over eighty years ago as an alternative to wirewound and composite materials. Metal oxides emerged in the 1950s as a more stable film and were widely used until they were milled into metal films for precision (metal film resistors) and thick films for high power applications (high power chip resistors). Both technologies were then adopted in the emerging SMD chip format. This was achieved over thirty years ago and is greatly reflected in today's thin film resistor products. However, it would be a mistake to assume that nothing has changed for thin film resistors.

This paper identifies three drivers for continued development and outlines some of the responses that are being made. The primary driver is the reduction of environmental impact, which is imperative through legislative regulation and indirect consumer pressure. Thereafter, the continued expansion of safe operating areas has pushed back the electrical rating poles* that limit the miniaturisation of analogue circuits. Afterwards, the growing demand for higher levels of stability and reliability in industrial and industrial applications can be met by the transition of technologies developed for space and military applications.

Two responses to environmental drivers are identified. The first is the reduction in overall material usage reflected in miniaturisation and the second is the elimination of hazardous substances. In the area of thick film resistors this can be seen in the removal of lead oxide from the glass used to formulate the film material. Consideration was given to the extent to which reliance on ongoing updates of the relevant RoHS exemptions was reduced.

For the second driver, the three electrical ratings that define the safe operating area of the resistor were reviewed. Continuous Power Rating, Limiting Component Voltage and Pulse Energy Limit. Each is defined and the results and failure mechanisms for moderate and extreme overloads are presented with practical examples. The dependence of these three ratings on the material, size and geometry of the resistive element is discussed with reference to the continuing trend towards converting designs to SMD format and *minimising the footprint. The current state of commercial thick film chip resistors relative to these ratings is then given, and multi-component solutions and their hidden costs are discussed. Existing and new technologies are then presented so that the three ratings can be extended.

End drivers related to stability and reliability are driving tantalum nitride technology from its high reliability origins into the mainstream of precision resistor applications. The differences associated with alternative materials will be examined, as well as the performance advantages indicated by highly accelerated lifetime testing.

Although the pace of change may seem slow compared to that of semiconductors or even other passive components, the development of thin-film resistor technology continues in the 21st century.