Thermal Resistances Impede Heat Transfers
The majority of cooling techniques employed today are utilized in power, semiconductor, and microprocessor devices, many of which are being introduced on a daily basis. Each of these active devices is specified to operate safely within a certain temperature range that must not be exceeded. The maximum operating temperature is commonly based on maintaining the semiconductor's internal junction temperatures below its maximum. Since the internal junction temperatures cannot be measured directly, devices such as MOSFETs, DC-DC converters, power bricks, etc., are specified to operate safely as long as its metal case, mounting tab, or baseplate temperature is kept below the specified maximum. To achieve this, a network of thermal resistances must be considered and taken into account. Thermal resistances are analogous to electronic resistances. They represent the mechanical interfaces between the various layers of materials which impede the flow of heat from one level to the next. A diagram of these series-connected thermal resistances beginning with the internal semiconductor junctions and concluding with the ambient air is shown in Figure 2.
Current and Future Trends
The ongoing trend is to provide as many features and benefits in the smallest package possible. This trend presents increased demands and challenges to the product development and packaging engineers. With greater power densities comes the need to find improved techniques of dissipating the unwanted heat from the end-product to insure its long life and reliability. There are now many "green" initiatives and policies which are demanding vast improvements in electronic product efficiencies. There is no doubt that these improved products' efficiencies will reduce the amount of heat and energy that is now wasted. However, since achieving 100% efficiency is akin to developing a perpetual motion machine, within the foreseeable future there remains a need for designers to keep abreast of the latest techniques for cooling power and other electronic devices.