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Basic Discussion

Semiconductor components are not 100% efficient. Consequently, the electrical energy input to a semiconductor is not entirely passed through to serve some other function in the circuit. Some of the input energy is dissipated at the atomic level as the chip performs its function and this energy is manifest as a rise in temperature (internal energy) of the semiconductor.

This associated temperature rise increases the chip failure rate and the general rule is for an approximately 1-degree C rise in the die temperature above about 100 degrees C, the chip failure rate increases about 5%. Because of this physical relationship, the design goal is to limit the juncture temperature to a maximum of about 100-120 degrees C for power semiconductors, and about 90 degrees C for microprocessors.

Most electronic equipment is designed for and used in the room ambient environment where the temperature is about 25 degrees C. However, circuits are generally packaged in enclosures where the ambient air may be 30-40 degrees C depending upon how the enclosure is vented.

Analysis

Typically, the steady-state thermal analyses of semiconductor components are approached using the electrical analogy which consists of three resistors in series as shown below:

The energy dissipated internal to the semiconductor chip must flow to the ambient air otherwise the chip temperature will continue rising. Since heat flows “downhill” due to a temperature potential this potential is the difference between the die juncture temperature (Tj) and the local ambient air temperature (Ta) i.e. (Tj-Ta).

The thermal resistance between the temperature potential consists of three series resistors as shown:

The thermal resistance from die-to-case (Rj-c) is a measured quantity that depends upon the package design and is supplied by the component manufacturer. The thermal resistance from the component case to the heatsink (Rc-s) is the interface resistance of the joint where the component contacts the heatsink. This resistance depends on the joint area, pressure, flatness, and any material that may be sandwiched in the joint to electrically insulate the component and/or enhance the heat transfer across the joint. This resistance should be a measured quantity since it is not readily amenable to analysis methods.

The thermal resistance for the heatsink to ambient (Rs-a) is also a measured quantity that should be supplied by the heatsink manufacturer in his catalog. This resistance is the “lumped” resistance for the energy to conduct through the heatsink structure and flow to the ambient environment by convection and radiation.

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