Passive components are largely ignored in highfrequency circuits, until they become a problem. They are the fundamental building blocks of electronic circuits and, like bricks and lumber in the building industry, are assumed to perform as advertised.

It is not by accident that this issue features a special report on materials by Editor Nancy Friedrich (see p. 28). The focus of that story is on numerous refinements made to composites to increase their heat-bearing or heat-dissipating capabilities. And improvements in passive components are not possible without enhancements in materials. Thermal management is of increasing concern for advancing the state of the art in both materials and passive components because of the increasing power densities of new transistor technologies.

The adage of "where there's smoke, there's fire" can be adapted for modern electronics to "where there's heat, there's a problem." Heat can rapidly age circuits and components. The heat is generated by such factors as inefficiencies in solid-state devices and thermal resistance in passive elements. When not properly managed, few things can shut down an electronic device as quickly.

Materials suppliers are aware of the growing need for higher power densities and are developing new formulations. Some component suppliers have long understood the connection between insertion loss and the generation of heat from high power levels. Companies such as Werlatone, Narda Microwave, RLC Electronics, and Labtech Microwave build passive components for high power levels. The key in building such components is to minimize any cause of heat from the power, such as insertion loss. Some loss is unavoidable, due to fabrication tolerances and dissipative materials losses. But materials suppliers fully understand this connection between power and heat, and the need to "get the heat out" from modern, high-speed, high-frequency circuits. They have invested in considerable research on different composites and combinations of materials, searching for the best compromises in mechanical and electrical stability, loss, and thermal dissipation capabilities.

There will always be a limit to the amount of power (and heat) that can be contained within a given area or volume of a printedcircuit board. But with the ever-improving thermal characteristics of circuit-board and other related materials, those limits are being stretched, allowing for further miniaturization of passive components even at high power levels.